94th ANNUAL MEETING

Minneapolis, MinnesotaMay 6-10, 1998

MINNESOTA GEOLOGICAL SURVEYUNIVERSITY OF MINNESOTA

Program and Abstracts

Organized by

Program and Abstracts

Minneapolis, Minnesota May 6-10,1998

Organized by

MINNESOTA GEOLOGICAL SURVEY UNIVERSITY OF MINNESOTA

44th Annual MeetingINSTITUTE ON LAKE SUPERIOR GEOLOGY

Volume 44 contains the following parts:Part 1: Program and Abstracts

Part 2: Field Trip Guidebook

i—Early Proterozoic intrusive rocks of east-central Minnesota2—Geology of the southeastern portion of the Midcontinent Rift

System, eastern Minnesota and western Wisconsin3—Glacial exotica of the Twin Cities area4—Stratigraphy and hydrogeology of Paleozoic rocks of southeastern

Minnesota5—Minnesota River Valley and vicinity, southwestern Minnesota

Reference to the material in this volume should follow the example below:

Sims, P.K., Neymark, L.A., Peterman, Z.E., and Kotov, A.B., 1998, Nd isotope evidencefor Middle and Early Archean crust in the Wawa subprovince of the Superior Province,Michigan, U.S.A., [abstract]: Institute on Lake Superior logy Proceedings, 44thAnnual Meeting, Minneapolis, MN, 1998; v. 44, Part 1, p. :

Volume 44 is published by the Institute on Lake Superior Geology anddistributed by the Institute Secretary-Treasurer:

Mark Jima (term 1994-2000)Minnesota Geological Survey2642 University AvenueSt. Paul, MN USA 55114-1057(612)-627-4780email: jirsa001@tc.umn.eduILSG website http://www.geo.mtu.edu/great_lakes/ilsgl

ISSN 1042-9964

44th Annual Meeting INSTITUTE ON LAKE SUPERIOR GEOLOGY

Volume 44 contains the following parts: Part 1: Program and Abstracts

Part 2: Field Trip Guidebook

1-Early Proterozoic intrusive rocks of east-central Minnesota 2-Geology of the southeastern portion of the Midcontinent Rift

System, eastern Minnesota and western Wisconsin 3-Glacial exotica of the Twin Cities area 4ÑStratigraph and hydrogeology of Paleozoic rocks of southeastern

Minnesota 5-Minnesota River Valley and vicinity, southwestern Minnesota

Reference to the material in this volume should follow the example below:

Sims, P.K., Neymark, L.A., Peterman, Z.E., and Kotov, A.B., 1998, Nd isotope evidence for Middle and Early Archean crust in the Wawa subprovince of the Superior Province, Michigan, U.S.A., [abstract]: Institute on Lake Superior Geology Proceedings, 44th Annual Meeting, Minneapolis, MN, 1998; v. 44, Part 1, p. .

Volume 44 is published by the Institute on Lake Superior Geology and distributed by the Institute Secretary-Treasurer:

Mark Jirsa (term 1994-2000) Minnesota Geological Survey 2642 University Avenue St. Paul, MN USA 55114-1057 (6 12)-627-4780 email: jirsa001 @tc.umn.edu ILSG website http://www.geo.mtu.eddgreat_lakes/ilsg/

ISSN 1042-9964

INSTITUTE ON LA SUPERIOR GEOLOGY

44th Annual MeetingMay 6-10, 1998

Minneapolis, Minnesota

Sponsored by:Minnesota Geological Survey

University of Minnesota

PROCEEDINGSVolume 44

Part 1 Program and Abstracts

4th Annual Meeting

Minneapolis, Minnesota

Sponsored by: Minnesota Geological Survey

University of Minnesota

PROCEEDINGS Volume 44

Part 1-Program and Abstracts

CONTENTSProceedings Volume 44Part 1—Program and Abstracts

Institutes on Lake Superior Geology, 1955-1998 iv

Constitution of the Institute on Lake Superior Geology v

By-Laws of the Institute on Lake Superior Geology vii

Goldich Medal Guidelines viii

Past Goldich Medalists x

Goldich Medal Committee x

Citation for 1998 Goldich Medal Recipient xi

Student Travel Awards xii

Student Paper Awards xiii

Student Paper Committee xiii

Board of Directors xiv

Local Planning Committee xiv

Session Chairs xv

Banquet Speaker xv

Report of the Chair of the 43rd Annual Institute Meeting xvi

Program xviii

Abstracts for Special Session:

Geolo2ical Overview of the Lake Superior Region 1

Card, Ken D.Archean geology of the Great Lakes region of North America 3

Ojakangas, Richard W.Generalized Early Proterozoic history, Lake Superior region 5

Cannon, William F.Understanding the Middle Proterozoic history of the Lake Superior region:What's new? What's next? 19

Runkel, Anthony C.Paleozoic rocks in the northern part of the central midcontinent of North America 25

Patterson, Carrie J.Models for interpreting the Quaternary history of the Lake Superior region 33

Southwick, David L.What's next for geology in the Lake Superior area9 37

Abstracts for General Technical Sessions 39

Ill

CONTENTS Proceedings Volume 44 Part 1-Program and Abstracts

............................................... Institutes on Lake Superior Geology, 1955-1998 iv

Constitution of the Institute on Lake Superior Geology .......................................... v . . ......................................... By-Laws of the Institute on Lake Superior Geology.. vu ... ........................................................................ Goldich Medal Guidelines vm

............................................................................. Past Goldich Medalists. .x

.......................................................................... Goldich Medal Committee .x

.................................................... Citation for 1998 Goldich Medal Recipient.. .xi . . ........................................................................... Student Travel Awards.. xu ... ............................................................................. Student Paper Awards xm ... ......................................................................... Student Paper Committee xm

................................................................................. Board of Directors xiv

...................................................................... Local Planning Committee.. xiv

...................................................................................... Session Chairs xv

.................................................................................. Banquet Speaker.. xv

................................... Report of the Chair of the 43rd Annual Institute Meeting.. xvi ... .......................................................................................... Program.. xvm

Abstracts for Special Session: ......................................... Geolo~ical Overview of the Lake Superior Region .1

Card, Ken D. Archean geology of the Great Lakes region of North America ................................ 3

Ojakangas, Richard W. ................................ Generalized Early Proterozoic history, Lake Superior region .5

Cannon, William F. Understanding the Middle Proterozoic history of the Lake Superior region: ............................................................... What's new? What's next?. 19

Runkel, Anthony C. ........ Paleozoic rocks in the northern part of the central midcontinent of North America .25

Patterson, Carrie J. ................. Models for interpreting the Quaternary history of the Lake Superior region 33

Southwick, David L. ...................................... What's next for geology in the Lake Superior area? .37

...................................................... Abstracts for General Technical Sessions .39

INSTITUTES ON LAKE SUPERIOR GEOLOGY, 1955-1998# DATE PLACE CHAIRS1 1955 Minneapolis, Minnesota C.E. Dutton2 1956 Houghton, Michigan A.K. Sneigrove3 1957 East Lansing, Michigan B.T. Sandefur

4 1958 Duluth, Minnesota R.W. Marsden

5 1959 Minneapolis, Minnesota G.M. Schwartz & C. Craddock6 1960 Madison, Wisconsin E.N. Cameron7 1961 Port Arthur, Ontario E.G. Pye8 1962 Houghton, Michigan A.K. Sneigrove9 1963 Duluth, Minnesota H. Lepp

1 0 1964 Ishpeming, Michigan A.T. Broderick

1 1 1965 St. Paul, Minnesota P.K. Sims & R.K. Hogberg12 1966 Sault Ste. Marie, Michigan R.W. White13 1967 East Lansing, Michigan WJ. Hinze14 1968 Superior, Wisconsin A.B. Dickas

1 5 1969 Oshkosh, Wisconsin G.L. LaBerge

16 1970 Thunder Bay, Ontario M.W. Bartley & E. Mercy1 7 1971 Duluth, Minnesota D.M. Davidson

1 8 1972 Houghton, Michigan J. Kalliokoski19 1973 Madison, Wisconsin M.E. Ostrom20 1974 Sault Ste. Marie, Ontario P.E. Giblin2 1 1975 Marquette, Michigan J.D. Hughes

22 1976 St. Paul, Minnesota M. Walton

23 1977 Thunder Bay, Ontario M.M. Kehienbeck24 1978 Milwaukee, Wisconsin G. Mursky

25 1979 Duluth, Minnesota D.M. Davidson

26 1980 Eau Claire, Wisconsin P.E. Myers27 1981 East Lansing, Michigan W.C. Cambray

28 1982 International Falls, Minnesota D.L. Southwick

29 1983 Houghton, Michigan T.J. Bornhorst

30 1984 Wausau, Wisconsin G.L. LaBerge

3 1 1985 Kenora, Ontario C.E. Blackburn

32 1986 Wisconsin Rapids, Wisconsin J.K. Greenberg

33 1987 Wawa, Ontario E.D. Frey & R.P. Sage34 1988 Marquette, Michigan J. S. Klasner

35 1989 Duluth, Minnesota J.C. Green

36 1990 Thunder Bay, Ontario M.M. Kehlenbeck

37 1991 Eau Claire, Wisconsin P.E. Myers

38 1992 Hurley, Wisconsin A.B. Dickas

39 1993 Eveleth, Minnesota D.L. Southwick

40 1994 Houghton, Michigan T.J. Bornhorst

4 1 1995 Marathon, Ontario M.C. Smyk42 1996 Cable, Wisconsin L.G. Woodruff

43 1997 Sudbury, Ontario R.P. Sage, W. Meyer

44 1998 Minneapolis, Minnesota J.D. Miller, M.A. Jirsa

iv

INSTITUTES ON LAKE SUPERIOR GEOLOGY, 1955-1998 # DATE P L A C E C H A I R S

Minneapolis, Minnesota Houghton, Michigan East Lansing, Michigan Duluth, Minnesota Minneapolis, Minnesota Madison, Wisconsin Port Arthur, Ontario Houghton, Michigan Duluth, Minnesota Ishpeming, Michigan St. Paul, Minnesota Sault Ste. Marie, Michigan East Lansing, Michigan Superior, Wisconsin Oshkosh, Wisconsin Thunder Bay, Ontario Duluth, Minnesota Houghton, Michigan Madison, Wisconsin Sault Ste. Marie, Ontario Marquette, Michigan St. Paul, Minnesota Thunder Bay, Ontario Milwaukee, Wisconsin Duluth, Minnesota Eau Claire, Wisconsin East Lansing, Michigan International Falls, Minnesota Houghton, Michigan Wausau, Wisconsin Kenora, Ontario Wisconsin Rapids, Wisconsin Wawa, Ontario Marquette, Michigan Duluth, Minnesota Thunder Bay, Ontario Eau Claire, Wisconsin Hurley, Wisconsin Eveleth, Minnesota Houghton, Michigan Marathon, Ontario Cable, Wisconsin Sudbury, Ontario Minneapolis, Minnesota

C.E. Dutton A.K. Snelgrove B.T. Sandefur R.W. Marsden G.M. Schwartz & C. Craddock E.N. Cameron E.G. Pye A.K. Snelgrove

H- L ~ P P A.T. Broderick P.K. Sims & R.K. Hogberg R.W. White W.J. Hinze A.B. Dickas G.L. LaBerge M.W. Bartley & E. Mercy D.M. Davidson J. Kalliokoski M.E. Ostrom P.E. Giblin J.D. Hughes M. Walton M.M. Kehlenbeck G. Mursky D.M. Davidson P.E. Myers W.C. Cambray D.L. Southwick T.J. Bornhorst G.L. LaBerge C.E. Blackburn J.K. Greenberg E.D. Frey & R.P. Sage J. S. Klasner J.C. Green M.M. Kehlenbeck P.E. Myers A.B. Dickas D.L. Southwick T.J. Bornhorst M.C. Smyk L.G. Woodruff R.P. Sage, W. Meyer J.D. Miller, M.A. Jirsa

CONSTITUTIONOF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY

(Last amended by the Board—May 8, 1997)

Article I NameThe name of the organization shall be the "Institute on LakeSuperior Geology".

Article II ObjectivesThe objectives of this organization are:

A. To provide a means whereby geologists in the Great Lakes region mayexchange ideas and scientific data.B. To promote better understanding of the geology of the Lake Superiorregion.C. To plan and conduct geological field trips.

Article ifi StatusNo part of the income of the organization shall insure to the benefit of anymember or individual. In the event of dissolution, the assets of theorganization shall be distributed to

_________(some

tax free organization).

To avoid Federal and State income taxes, the organization should be notonly "scientific" or "educational, but also "non-profit".

Minn. Stat. Anno. 290.01, subd. 4Minn. Stat. Anno. 290.05(9)1954 Internal Revenue Code s.501(c)(3)

Article IV MembershipThe membership of the organization shall consist of persons who haveregistered for an annual meeting within the past three years, and those whoindicate interest in being a member according to guidelines approved by theBoard of Directors.

Article V MeetingsThe organization shall meet once a year, preferably during the month ofApril. The place and exact date of each meeting will be designated by theBoard of Directors.

Article VI DirectorsThe Board of Directors shall consist of the Chair, Secretary-Treasurer, andthe last three past Chairs; but if the board should at any time consist of fewerthan five persons, by reason of unwillingness or inability of any of theabove persons to serve as directors, the vacancies on the board may be filledby the Chair so as to bring the membership of the board to five members.

V

Article I

Article II

Article III

Article IV

Article V

Article VI

(Last amended by the Board-May 8,1997)

Name The name of the organization shall be the "Institute on Lake Superior Geology".

Objectives The objectives of this organization are: A. To provide a means whereby geologists in the Great Lakes region may exchange ideas and scientific data. B. To promote better understanding of the geology of the Lake Superior region. C. To plan and conduct geological field trips.

Status No part of the income of the organization shall insure to the benefit of any member or individual. In the event of dissolution, the assets of the organization shall be distributed to (some tax free organization).

To avoid Federal and State income taxes, the organization should be not only "scientific" or "educational, but also "non-profit".

Minn. Stat. Anno. 290.01, subd. 4 Mim. Stat. Anno. 290.05(9) 1954 Internal Revenue Code s.501 (c)(3)

Membership The membership of the organization shall consist of persons who have registered for an annual meeting within the past three years, and those who indicate interest in being a member according to guidelines approved by the Board of Directors.

Meetings The organization shall meet once a year, preferably during the month of April. The place and exact date of each meeting will be designated by the Board of Directors.

Directors The Board of Directors shall consist of the Chair, Secretary-Treasurer, and the last three past Chairs; but if the board should at any time consist of fewer than five persons, by reason of unwillingness or inability of any of the above persons to serve as directors, the vacancies on the board may be filled by the Chair so as to bring the membership of the board to five members.

Article VII OfficersThe officers of this organization shall be a Chair and Secretary-Treasurer.

A. The Chair shall be elected each year by the Board of Directors, whoshall give due consideration to the wishes of any group that may bepromoting the next annual meeting. His/her term of office as Chair willterminate at the close of the annual meeting over which he/she presides, orwhen his/her successor shall have been appointed. He/she will then servefor a period of three years as a member of the Board of Directors.B. The Secretary-Treasurer shall be elected at the annual meeting. His/herterm of office shall be four years, or until his/her successor shall have beenappointed.

Article Vifi AmendmentsThis constitution may be amended by a majority vote of the membership ofthe organization.

vi

Article VII Officers The officers of this organization shall be a Chair and Secretary-Treasurer.

A. The Chair shall be elected each year by the Board of Directors, who shall give due consideration to the wishes of any group that may be promoting the next annual meeting. Hisher term of office as Chair will terminate at the close of the annual meeting over which helshe presides, or when hisher successor shall have been appointed. Helshe will then serve for a period of three years as a member of the Board of Directors. B. The Secretary-Treasurer shall be elected at the annual meeting. Hisher term of office shall be four years, or until hislher successor shall have been appointed.

Article VIXI Amendments This constitution may be amended by a majority vote of the membership of the organization.

BY-LAWSOF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY

I. Duties of the Officers and Directors

A. It shall be the duty of the Annual Chairman to:1. Preside at the annual meeting.2. Appoint all committees needed for the organization of the annual meeting.3. Assume complete responsibility for the organization and financing of the

annual meeting over which he/she presides.

B. It shall be the duty of the Secretary-Treasurer to:1. Keep accurate attendance records of all annual meetings.2. Keep accurate records of all meetings of, and correspondence between, the

Board of Directors.3. Hold all funds that may accrue as profits from annual meetings or field trips

and to make these funds available for the organization and operation offuture meetings as required.

C. It shall be the duty of the Board of Directors to plan locations of annualmeetings and to advise on the organization and fmancing of all meetings.

II. Duties and Exoenses

A. There shall be no regular membership dues.

B. Registration fees for the annual meetings shall be determined by the Chair inconsultation with the Board of Directors. The registration fees can includeexpenses to cover operations outside of the annual meeting as determined bythe Board of Directors. It is strongly recommended that registration fees bekept at a minimum to encourage attendance of graduate students.

ifi. Rules of Order

The rules contained in Robert's Rules of Order shall govern this organization in allcases to which they are applicable.

IV. Amendments

These by-laws may be amended by a majority vote of the membership of theorganization; provided that such modifications shall not conifict with theconstitution as presently adopted or subsequently amended.

vu

BY-LAWS OF THE INSTITUTE ON LAKE SUPERIOR GEOLOGY

I. Duties of the Officers and Directors

A. It shall be the duty of the Annual Chairman to: 1. Preside at the annual meeting. 2. Appoint all committees needed for the organization of the annual meeting. 3. Assume complete responsibility for the organization and financing of the

annual meeting over which helshe presides.

B. It shall be the duty of the Secretary-Treasurer to: 1. Keep accurate attendance records of all annual meetings. 2. Keep accurate records of all meetings of, and correspondence between, the

Board of Directors. 3. Hold all funds that may accrue as profits from annual meetings or field trips

and to make these funds available for the organization and operation of future meetings as required.

C. It shall be the duty of the Board of Directors to plan locations of annual meetings and to advise on the organization and financing of all meetings.

11. Duties and Expenses

A. There shall be no regular membership dues.

B. Registration fees for the annual meetings shall be determined by the Chair in consultation with the Board of Directors. The registration fees can include expenses to cover operations outside of the annual meeting as determined by the Board of Directors. It is strongly recommended that registration fees be kept at a minimum to encourage attendance of graduate students.

In. Rules of Order

The rules contained in Robert's Rules of Order shall govern this organization in all cases to which they are applicable.

IV. Amendments

These by-laws may be amended by a majority vote of the membership of the organization; provided that such modifications shall not conflict with the constitution as presently adopted or subsequently amended.

vii

GOLDICH MEDAL GUIDELINES(Adopted by the Board of Directors, 1981; amended 1997)

PreambleThe Institute on Lake Superior Geology was born in 1955, as documented by the fact thatthe 27th annual meeting was held in 1981. The Institute's continuing objectives are to dealwith those aspects of geology that are related geographically to Lake Superior; to encouragethe discussion of subjects and sponsoring field trips that will bring together geologists fromacademia, government surveys, and industiy; and to maintain an informal but highlyeffective mode of operation.

During the course of its existence, the membership of the Institute (that is, those geologistswho indicate an interest in the objectives of the ILSG by attending) has become aware ofthe fact that certain of their colleagues have made particularly noteworthy and meritoriouscontributions to the understanding of Lake Superior geology and mineral deposits.

The first award was made by ILSG to Sam Goldich in 1979 for his many contributions tothe geology of the region extending over about 50 years. Subsequent medalists and thisyear's recipient are listed in the table below.

Award Guidelines1) The medal shall be awarded annually by the ILSG Board of Directors to a geologistwhose name is associated with a substantial interest in, and contribution to, the geology ofthe Lake Superior region.

2) The Board of Directors shall appoint the Goldich Medal Committee. The initialappointment will be of three members, one to serve for three years, one for two years, andone for one year. The member with the briefest incumbency shall be chair of theNominating Committee. After the first year, the Board of Directors shall appoint at eachspring meeting one new member who will serve for three years. In his/her third year thismember shall be the chair. The Committee membership should reflect the main fields ofinterest and geographic distribution of ILSG membership.

3) By the end ofNovember , the Goldich Medal Committee shall make its recommendationto the Chair of the Board of Directors, who will then inform the Board of the nominee.

4) The Board of Directors normally will accept the nominee of the Committee, will informthe medalist immediately, and will have one medal engraved appropriately for presentationat the next meeting of the Institute.

5) It is recommended that the Institute set aside annually from whatever sources, suchfunds as will be required to support the continuing costs of this award.

Nominating Procedures1) Nominations shall be taken at any time by the Goldich Medal Committee. Committeemembers may themselves nominate candidates. The deadline for nominations is November1.

2) Nominations must be in writing and supported by appropriate documentation such asletters of recommendation, lists of publications, curriculum vita's, and evidence ofcontributions to Lake Superior geology and to the Institute.

vifi

GOLDICH MEDAL GUIDELINES (Adopted by the Board of Directors, 198 1 ; amended 1997)

Preamble The Institute on Lake Superior Geology was born in 1955, as documented by the fact that the 27th annual meeting was held in 1981. The Institute's continuing objectives are to deal with those aspects of geology that are related geographically to Lake Superior; to encourage the discussion of subjects and sponsoring field trips that will bring together geologists from academia, government surveys, and industry; and to maintain an informal but highly effective mode of operation.

During the course of its existence, the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the ILSG by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the understanding of Lake Superior geology and mineral deposits.

The first award was made by ILSG to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years. Subsequent medalists and this year's recipient are listed in the table below.

Award Guidelines 1) The medal shall be awarded annually by the ILSG Board of Directors to a geologist whose name is associated with a substantial interest in, and contribution to, the geology of the Lake Superior region.

2) The Board of Directors shall appoint the Goldich Medal Committee. The initial appointment will be of three members, one to serve for three years, one for two years, and one for one year. The member with the briefest incumbency shall be chair of the Nominating Committee. After the first year, the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In hisfher third year this member shall be the chair. The Committee membership should reflect the main fields of interest and geographic distribution of ILSG membership.

3) By the end of November, the Goldich Medal Committee shall make its recommendation to the Chair of the Board of Directors, who will then inform the Board of the nominee.

4) The Board of Directors normally will accept the nominee of the Committee, will inform the medalist immediately, and will have one medal engraved appropriately for presentation at the next meeting of the Institute.

5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award.

Nominatin~ Procedures 1) Nominations shall be taken at any time by the Goldich Medal Committee. Committee members may themselves nominate candidates. The deadline for nominations is November 1.

2) Nominations must be in writing and supported by appropriate documentation such as letters of recommendation, lists of publications, curriculum vita's, and evidence of contributions to Lake Superior geology and to the Institute.

3) Nominations are not restricted to Institute attendees, but are open to anyone who hasworked on and contributed to the understanding of Lake Superior geology.

Selection Guidelines1) Nominees are to be evaluated on the basis of their contributions to Lake Superiorgeology (sensu lato) including:

a) importance of relevant publications;b) promotion of discovery and utilization of natural resources;c) contributions to understanding of the natural history and environment of the

region;d) generation of new ideas and concepts; ande) contributions to the training and education of geoscientists and the public.

2) Nominees are to be evaluated on their contributions to the Institute as demonstrated byattendance at Institute meetings, presentation of talks and posters, and service on Instituteboards, committees, and field trips.

3) The relative weights given to each of the foregoing criteria must remain flexible and atthe discretion of the Committee members.

4) There are several points to be considered by the Goldich Medal Committee:a) An attempt should be made to maintain a balance of medal recipients from each of

the three estates—industry, academia, and government.b) It must be noted that industry geoscientists are at a disadvantage in that much of

their work in not published.

5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedlyone of the Institute's great strengths and should be nurtured by equitable recognition ofexcellence in both countries.

ix

3) Nominations are not restricted to Institute attendees, but arc open to anyone who has worked on and contributed to the understanding of Lake Superior geology.

Selection Guidelines 1) Nominees are to be evaluated on the basis of their contributions to Lake Superior geology (sensu lato) including:

a) importance of relevant publications; b) promotion of discovery and utilization of natural resources; c) contributions to understanding of the natural history and environment of the

region; d) generation of new ideas and concepts; and e) contributions to the training and education of geoscientists and the public.

2) Nominees are to be evaluated on their contributions to the Institute as demonstrated by attendance at Institute meetings, presentation of talks and posters, and service on Institute boards, committees, and field trips.

3) The relative weights given to each of the foregoing criteria must remain flexible and at the discretion of the Committee members.

4) There are several points to be considered by the Goldich Medal Committee: a) An attempt should be made to maintain a balance of medal recipients from each of

the three estates-industry, academia, and government. b) It must be noted that industry geoscientists arc at a disadvantage in that much of

their work in not published.

5) Lake Superior has two sides, one the U.S., and the other Canada. This is undoubtedly one of the Institute's great strengths and should be nurtured by equitable recognition of excellence in both countries.

GOLDICH MEDALISTS

1979 Samuel S. Goldich

1980 not awarded 1989 Jorma Kalliokoski

1981 Carl E. Dutton, Jr. 1990 Kenneth C. Card

1982 Ralph W. Marsden 1991 William Hinze

1983 Burton Boyum 1992 William F. Cannon

1984 Richard W. Ojakangas 1993 Donald W. Davis

1985 Paul K. Sims 1994 Cedric Iverson

1986 G.B. Morey 1995 Gene LaBerge

1987 Hemy H. Halls 1996 David L. Southwick

1988 Walter S. White 1997 Ronald P. Sage

1998 Zell Peterman

GOLDICH MEDAL COMMITTEE—1997-1998(Committee membership through the meeting year shown)

Dan England (1998)Eveleth Fee Office, Eveleth, Minnesota

John Kiasner (1999)Western Illinois University, Macomb, Illinois

Mark Smyk (2000)Ontario Geological Survey, Thunder Bay

x

GOLDICH MEDALISTS

1979 Samuel S. Goldich

1980 not awarded

1981 Carl E. Dutton, Jr.

1982 Ralph W. Marsden

1983 Burton Boyum

1984 Richard W. Ojakangas

1985 Paul K. Sims

1986 G.B. Morey

1987 Henry H. Halls

1988 Walter S. White

1989 Jorrna Kalliokoski

1990 Kenneth C. Card

1991 William Hinze

1992 William F. Cannon

1993 Donald W. Davis

1994 Cedric Iverson

1995 Gene LaBerge

1996 David L. Southwick

1997 Ronald P. Sage

1998 Zell Peterman

GOLDICH MEDAL COMMITTEE-1997-1998 (Committee membership through the meeting year shown)

Dan England (1998) Eveleth Fee Office, Eveleth, Minnesota

John Klasner (1 999)

Western Illinois University, Macomb, Illinois

Mark Smyk (2000)

Ontario Geological Survey, Thunder Bay

CITATIONZell E. Peterman

1998 Goldich Medal Recipient

It's my personal pleasure to introduce Zell E. Peterman, this years Goldich medalist. Zellrichly deserves this highest honor of the Institute on Lake Superior Geology because of hismany outstanding contributions to Precambrian geology and geochronology of the LakeSuperior region.

Zell received a Geologic Engineering degree from the Colorado School of Mines, a mastersin Geology from the University of Minnesota, Minneapolis, and a Ph.D. in Geology in1962 from the University of Alberta. At Minnesota, Zell was a student of Sam Goldich.

Zell has spent all of his career—except for the years since 1994—with the GeologicDivision of the U.S. Geological Survey, and it was in this capacity that he carried out mostof his geochronologic research that we acknowledge today.

Over a period of several years, working with field geologists, Zell has been mainlyresponsible for the isotopic time framework for the pre-Keweenawan that we accept today.

A major contribution was a study of Archean rocks across the Great Lakes tectonic zone inthe Marenisco-Watersmeet area in Michigan. This study established that gneisses in theMinnesota River Valley subprovince are as old as 3,550 Ma and flanking metavolcanic-metasedimentary rocks in the Wawa subprovince (to the north) are in the range 2.6-2.8 Ga.This pattern has proved to be characteristic of Michigan-Wisconsin.

In the late 1980's, in an innovative study utilizing Rb-Sr ages of biotite, Zell outlined theuplift history of the Goodwin Swell in northeastern Wisconsin—a lithospheric flexurecaused by crustal loading along the Midcontinent Rift System. This is the sort of thing thathas been characteristic of Zell. More than most people, he has the ability to use variousgeochronologic techniques to solve problems most of us mortals don't know even exist.

Since 1994, Zell has been involved in DOE's Yucca Mountain Project, and some of thesuccessful techniques used to solve projects just blow your mind.

I should mention Zell's administrative duties and skills. From 1971 to 1976, he wasbranch chief of the USGS's Branch of Isotope Geology, a time when the Isotope Branchwas flourishing. Since 1994, he has assembled an isotope hydrology team under theYucca Mountain Project that is doing many marvelous things, particularly using Rb-Srtracers.

P.K. Sims

xi

CITATION Zell E. Peterman

1998 Goldich Medal Recipient

It's my personal pleasure to introduce Zell E. Peterman, this years Goldich medalist. Zell richly deserves this highest honor of the Institute on Lake Superior Geology because of his many outstanding contributions to Precambrian geology and geochronology of the Lake Superior region.

Zell received a Geologic Engineering degree from the Colorado School of Mines, a masters in Geology from the University of Minnesota, Minneapolis, and a Ph.D. in Geology in 1962 from the University of Alberta. At Minnesota, Zell was a student of Sam Goldich.

Zell has spent all of his career-except for the years since 1994Ñwit the Geologic Division of the U.S. Geological Survey, and it was in this capacity that he carried out most of his geochronologic research that we acknowledge today.

Over a period of several years, working with field geologists, Zell has been mainly responsible for the isotopic time framework for the pre-Keweenawan that we accept today.

A major contribution was a study of Archean rocks across the Great Lakes tectonic zone in the Marenisco-Watersmeet area in Michigan. This study established that gneisses in the Minnesota River Valley subprovince are as old as 3,550 Ma and flanking metavolcanic- metasedimentary rocks in the Wawa subprovince (to the north) are in the range 2.6-2.8 Ga. This pattern has proved to be characteristic of Michigan-Wisconsin.

In the late 1980's, in an innovative study utilizing Rb-Sr ages of biotite, Zell outlined the uplift history of the Goodwin Swell in northeastern Wisconsin-a lithospheric flexure caused by crustal loading along the Midcontinent Rift System. This is the sort of thing that has been characteristic of Zell. More than most people, he has the ability to use various geochronologic techniques to solve problems most of us mortals don't know even exist.

Since 1994, Zell has been involved in DOE'S Yucca Mountain Project, and some of the successful techniques used to solve projects just blow your mind.

I should mention Zell's administrative duties and skills. From 1971 to 1976, he was branch chief of the USGS's Branch of Isotope Geology, a time when the Isotope Branch was flourishing. Since 1994, he has assembled an isotope hydrology team under the Yucca Mountain Project that is doing many marvelous things, particularly using Rb-Sr tracers.

P.K. Sims

STUDENT TRAVEL AWARDS

The 1986 Board of Directors established the ILSG Student Travel Awards to support

student participation at the annual meeting of the Institute. The awards will be made from a

special fund set up for this purpose. These awards are intended to help defray some of the

direct travel costs of attending Institute meetings, and include a waiver of registration fees,

but exclude expenses for meals, lodging, and field trip registration. The number of awards

and value are determined by the annual Chair in consultation with the Secretary-Treasurer.

Recipients will be announced at the annual banquet.

The following general criteria will be considered by the annual Chair, who is responsible

for the selection:

1) The applicants must have active resident (undergraduate or graduate) student status at

the time of the annual meeting of the Institute, certified by the department head.

2) Students who are the senior author on either an oral or poster paper will be given

favored consideration.

3) It is desirable for two or more students to jointly request travel assistance.

4) In general, priority will be given to those in the Institute region who are farthest away

from the meeting location.

5) Each travel award request shall be made in writing to the annual Chair, and should

explain need, student and author status, and other significant details.

Successful applicants will receive their awards at the time of registration for the meeting.

xli

STUDENT TRAVEL AWARDS

The 1986 Board of Directors established the ILSG Student Travel Awards to support

student participation at the annual meeting of the Institute. The awards will be made from a

special fond set up for this purpose. These awards are intended to help defray some of the

direct travel costs of attending Institute meetings, and include a waiver of registration fees,

but exclude expenses for meals, lodging, and field trip registration. The number of awards

and value are determined by the annual Chair in consultation with the Secretary-Treasurer.

Recipients will be announced at the annual banquet.

The following general criteria will be considered by the annual Chair, who is responsible

for the selection:

1) The applicants must have active resident (undergraduate or graduate) student status at

the time of the annual meeting of the Institute, certified by the department head.

2) Students who arc the senior author on either an oral or poster paper will be given

favored consideration.

3) It is desirable for two or more students to jointly request travel assistance.

4) In general, priority will be given to those in the Institute region who are farthest away

from the meeting location.

5) Each travel award request shall be made in writing to the annual Chair, and should

explain need, student and author status, and other significant details.

Successful applicants will receive their awards at the time of registration for the meeting.

STUDENT PAPER AWARDS

Each year, the Institute selects the best of the student presentations and honors presenterswith a monetary award. Funding for the award is generated from registrations of theannual meeting. The Student Paper Committee is appointed by the annual meeting Chair insuch a manner as to represent a broad range of professional and geologic expertise.Criteria for best student paper—last modified by the Board in 1997—follow:

1) The contribution must be demonstrably the work of the student.

2) The student must present the contribution in-person.

3) The Student Paper Committee shall decide how many awards to grant, and whether ornot to give separate awards for poster vs. oral presentations.

4) In cases of multiple student authors, the award will be made to the senior author, or theaward will be shared equally by all authors of the contribution.

5) The total amount of the awards is left to the discretion of the meeting Chair andSecretary-Treasurer, but typically is in the amount of about $300 US.

6) The Secretary-Treasurer maintains, and will supply to the Committee, a form for thenumerical ranking of presentations. This form was created and modified by Student PaperCommittees over several years in an effort to reduce the difficulties that may arise fromselection by raters of diverse background. The use of the form is not required, but is left tothe discretion of the Committee.

7) The names of award recipients shall be included as part of the annual Chair's report thatappears in the next volume of the Institute.

1998 STUDENT PAPER COMMITTEE

Nancy Nelson—Committee ChairNorth Shore Technical Communications

Randy Van SchmusUniversity of Kansas

Peter WhelanUniversity of Minnesota—Morris

ll

STUDENT PAPER AWARDS

Each year, the Institute selects the best of the student presentations and honors presenters with a monetary award. Funding for the award is generated from registrations of the annual meeting. The Student Paper Committee is appointed by the annual meeting Chair in such a manner as to represent a broad range of professional and geologic expertise. Criteria for best student paper-last modified by the Board in 1997-follow:

1) The contribution must be demonstrably the work of the student.

2) The student must present the contribution in-person.

3) The Student Paper Committee shall decide how many awards to grant, and whether or not to give separate awards for poster vs. oral presentations.

4) In cases of multiple student authors, the award will be made to the senior author, or the award will be shared equally by all authors of the contribution.

5) The total amount of the awards is left to the discretion of the meeting Chair and Secretary-Treasurer, but typically is in the amount of about $300 US.

6) The Secretary-Treasurer maintains, and will supply to the Committee, a form for the numerical ranking of presentations. This form was created and modified by Student Paper Committees over several years in an effort to reduce the difficulties that may arise from selection by raters of diverse background. The use of the form is not required, but is left to the discretion of the Committee.

7) The names of award recipients shall be included as part of the annual Chair's report that appears in the next volume of the Institute.

1998 STUDENT PAPER COMMITTEE

Nancy Nelson-Committee Chair North Shore Technical Communications

Randy Van Schmus University of Kansas

Peter Whelan University of Minnesota-Morris

1998 BOARD OF DIRECTORS(Board membership through the close of the meeting year shown)

James D. Miller, Jr., Co-chair (2001)

Mark A. Jirsa, Co-chair and Institute Secretary-Treasurer (2000)Minnesota Geological Survey

Ronald P. Sage (2000)Ontario Geological Survey—Sudbury, Ontario

Laurel G. Woodruff (1999)U.S. Geological Survey—Mounds View, Minnesota

Mark C. Smyk (1998)Ontario Geological Survey—Thunder Bay, Ontario

1998 LOCAL PLANNING COMMITTEE

James D. Miller, Jr.—Co-chair

Mark A. Jirsa—Co-chair and meeting Treasurer

Terrence J. Boerboom—Field Trip Coordinator

Lori Day—Meeting Coordinator

Assistance to the local Committee was provided by the following individuals from theMinnesota Geological Survey:

David L. SouthwickDirector of host organization

G.B. MoreyManuscript review

Alan KnaebleField and meeting preparation

Barb LusardiManuscript preparation

xiv

1998 BOARD OF DIRECTORS

(Board membership through the close of the meeting year shown)

James D. Miller, Jr., Co-chair (200 1)

Mark A. Jirsa, Co-chair and Institute Secretary-Treasurer (2000)

Minnesota Geological Survey

Ronald P. Sage (2000) Ontario Geological Survey-Sudbury, Ontario

Laurel G. Woodruff (1999)

U.S. Geological Survey-Mounds View, Minnesota

Mark C. Smyk (1998)

Ontario Geological Survey-Thunder Bay, Ontario

1998 LOCAL PLANNING COMMITTEE

James D. Miller, Jr.-Co-chair

Mark A. Jirsa-Co-chair and meeting Treasurer

Terrence J. Boerboom-Field Trip Coordinator

Lori Day-Meeting Coordinator

Assistance to the local Committee was provided by the following individuals from the Minnesota Geological Survey:

David L. Southwick Director of host organization

G.B. Morey Manuscript review

Alan Knaeble Field and meeting preparation

Barb Lusardi Manuscript preparation

1998 SESSION CHAIRS(In order of appearance)

P.K. SimsU.S. Geological Survey—Denver

G.B. MoreyMinnesota Geological Survey

Jim LundyMinnesota Pollution Control Agency

Laurel WoodruffU.S. Geological Survey—Mounds View

Rod JohnsonBitterroot Resources Ltd.

Mark SmykOntario Geological Survey—Thunder Bay

John GreenUniversity of Minnesota—Duluth

Paul WeiblenUniversity of Minnesota—Twin Cities

1998 BANQUET SPEAKER

Bevan FrenchSmithsonian Institution

"When the sky falls: Large meteorite impacts and

the history of earth and other worlds"

xv

1998 SESSION CHAIRS (In order of appearance)

P.K. Sims U.S. Geological Survey-Denver

G.B. Morey Minnesota Geological Survey

Jim Lundy Minnesota Pollution Control Agency

Laurel Woodruff U.S. Geological Survey-Mounds View

Rod Johnson Bitterroot Resources Ltd.

Mark Smyk Ontario Geological Survey-Thunder Bay

John Green University of Minnesota-Duluth

Paul Weiblen University of Minnesota-Twin Cities

1998 BANQUET SPEAKER

Bevan French Smithsonian Institution

'When the sky falls: Large meteorite impacts and

the history of earth and other worlds"

43rd ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGYMEETING

SUDBURY, ONTARIO

The 43rd annual meeting of the Institute on Lake Superior Geology was held on thecampus of Laurentian University in Sudbury, Ontario from May 6, through May 11, 1997.The meeting was organized by the Precambrian Section and the Resident Geologists Officeof the Ontario Geological Survey. Ron Sage, Precambrian Section, and Wilf Meyer,Resident Geologist, Sudbury, co-chaired the event. Mrs. Tracy Livingstone, ResidentGeologist's Office, helped organize and coordinate all activities relating to the meeting. Allthe guidebooks and individual abstracts were reviewed by W. Meyer, R.P. Sage, and T.Livingstone.

The Proceedings of the 43rd ILSG were published in 7 parts:Part 1: Program andAbstracts (edited by Ron Sage and Wilf Meyer);Part2: The Huronian Supergroup between Sault Ste. Marie and Elliot Lake: Evidence

for the Early Proterozoic atmosphere, climate, and tectonics (G. Bennett,K.D. Card, and K.Y. Tomlinson);

Part 3: New information on the Grenville Front near Sudbury (A. Davidson);Part 4: The Sudbury Structure, with emphasis on the Whitewater Group (S.F.M.

Gibbins);Part 5: Magmatic ore deposits of the Sudbury Igneous Complex (S.A. Prevec);Part 6: Alkalic rocks of the Sudbury region (R.P. Sage); andPart 7: The greening of Sudbury (K. Winterhalder).

A total of 101 people registered for the meeting in Sudbury and most of the field trips werewell attended. There were 13 paid participants for the Huronian Supergroup field trip, 22for the Grenville Front, 15 for the Sudbury Structure, 36 for the Magmatic ore deposits, 6for the Alkalic rocks, and 3 for the greening of Sudbury field trip. Access to local miningsites was granted by INCO Limited and Falconbridge Limited. Coffee breaks weresupported by Laurentian University, Falconbridge, and the Ministry of NorthernDevelopment and Mines.

The annual banquet was attended by 83 diners and held in the Cavern of Science North onRamsey Lake. The Goldich Medal was awarded to Ron Sage of the Ontario GeologicalSurvey for his contributions to geology in the Lake Superior region. The award waspresented by Wilf Meyer, Resident Geologist, Sudbury, Ministry of NorthernDevelopment and Mines. The banquet address was given by Dr. Peter Lightfoot, INCOLimited. The title of the talk was "Origin of the Sudbury Structure and its mineral wealth".

The best Student Paper Award, consisting of $270 (Can), was won by Craig Mancuso forhis paper "Initial results of Ar/Ar mineral dating from the Peevy node area of northernMichigan and Dunbar dome area of northeastern Wisconsin". Zachary Naiman won $150for his poster "Petrogenesis of Chengwatana volcanics, Minnesota and Wisconsin". Threestudent travel awards of $150 each were granted to Terry Arcuri, Zachary Naiman, andDean Peterson.

The Board of Directors of the Institute on Lake Superior Geology met in Sudbury on May8, 1997. Members of the Board in attendance were Mark Smyk, Jim Miller (representingLaurel Woodruff), Mark Jirsa (Secretary-Treasurer), Ted Bornhorst, Ron Sage, and WilfMeyer. Invited guests were Tracy Livingstone and Gene LaBerge. Actions taken were:

xvi

43rd ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY MEETING

SUDBURY, ONTARIO

The 43rd annual meeting of the Institute on Lake Superior Geology was held on the campus of Laurentian University in Sudbury, Ontario from May 6, through May 11, 1997. The meeting was organized by the Precambrian Section and the Resident Geologists Office of the Ontario Geological Survey. Ron Sage, Precambrian Section, and Wilf Meyer, Resident Geologist, Sudbury, co-chaired the event. Mrs. Tracy Livingstone, Resident Geologist's Office, helped organize and coordinate all activities relating to the meeting. All the guidebooks and individual abstracts were reviewed by W. Meyer, R.P. Sage, and T. Livingstone.

The Proceedings of the 43rd ILSG were published in 7 parts: Part 1: Program and Abstracts (edited by Ron Sage and Wilf Meyer); Parti: The Huronian Supergroup between Sault Ste. Marie and Elliot Lake: Evidence

for the Early Proterozoic atmosphere, climate, and tectonics (G. Bennett, K.D. Card, and K.Y. Tomlinson);

Part 3: New information on the Grenville Front near Sudbury (A. Davidson); Part 4: The Sudbury Structure, with emphasis on the Whitewater Group (S.F.M.

Gibbins); Part 5: Magmatic ore deposits of the Sudbury Igneous Complex (S.A. Prevec); Part 6: Alkalic rocks of the Sudbury region (R.P. Sage); and Part 7: The greening of Sudbury (K. Winterhalder).

A total of 101 people registered for the meeting in Sudbury and most of the field trips were well attended. There were 13 paid participants for the Huronian Supergroup field trip, 22 for the Grenville Front, 15 for the Sudbury Structure, 36 for the Magmatic ore deposits, 6 for the Alkalic rocks, and 3 for the greening of Sudbury field trip. Access to local mining sites was granted by INCO Limited and Falconbridge Limited. Coffee breaks were supported by Laurentian University, Falconbridge, and the Ministry of Northern Development and Mines.

The annual banquet was attended by 83 diners and held in the Cavern of Science North on Ramsey Lake. The Goldich Medal was awarded to Ron Sage of the Ontario Geological Survey for his contributions to geology in the Lake Superior region. The award was presented by Wilf Meyer, Resident Geologist, Sudbury, Ministry of Northern Development and Mines. The banquet address was given by Dr. Peter Lightfoot, INCO Limited. The title of the talk was "Origin of the Sudbury Structure and its mineral wealth".

The best Student Paper Award, consisting of $270 (Can), was won by Craig Mancuso for his paper "Initial results of ArIAr mineral dating from the Peevy node area of northern Michigan and Dunbar dome area of northeastern Wisconsin". Zachary Naiman won $150 for his poster "Petrogenesis of Chengwatana volcanics, Minnesota and Wisconsin". Three student travel awards of $150 each were granted to Terry Arcuri, Zachary Naiman, and Dean Peterson.

The Board of Directors of the Institute on Lake Superior Geology met in Sudbury on May 8, 1997. Members of the Board in attendance were Mark Smyk, Jim Miller (representing Laurel Woodruff), Mark Jirsa (Secretary-Treasurer), Ted Bornhorst, Ron Sage, and Wilf Meyer. Invited guests were Tracy Livingstone and Gene LaBerge. Actions taken were:

xvi

1) Accepted the report of the Chair, 42nd ILSG, Laurel Woodruff, including the minutesof the last board meeting.

2) Approved an offer by the Minnesota Geological Survey to host the 1998 meeting. JimMiller and Mark Jirsa will co-chair the meeting. The offer by Charles Blackburn to chairthe meeting in Kenora in 1998 had to be withdrawn due to circumstances beyond hiscontrol. The 1998 meeting will be held in Minneapolis, Minnesota on May 6-10.

3) Approved the 1996-1997 financial report presented by Mark Jirsa.4) Approved Ron Sage as the Goldich Medal winner for 1997.5) Approved the Goldich Medal guidelines prepared by Ken Card.6) Approved the replacement of Ken Card by Mark Smyk on the Goldich Medal

Committee.7) Discussed the ILSG newsletter and web site. The newsletter has been well received and

will be continued. We may eliminate the annual publication of the Constitution and placea general preamble on the web site (http://www.geo.mtu.edu/great_lakes/ilsg). The website includes information about the organization and its constitution, award guidelines,list of publications by the Institute and Goldich Medal recipients.

8) Approved the membership guidelines for the ILSG prepared by Mark Jirsa and TedBornhorst. Mailing list will be revised in an effort to reduce its size and lower mailingcosts.

9) Discussed the Eisenbrey Fund and action on this fund was tabled for further review. Acommittee consisting of Gene LaBerge, Mark Jirsa, and Mark Smyk was formed tostudy and make recommendations on how the monies in the Eisenbrey Fund are to beused. Interest earned from this fund may be used to support student travel to the annualInstitute meeting, and corporations and individuals may donate to the fund. Criteria toreceive funding from the Eisenbrey Fund need to be established.

10) Discussed the ILSG mailing list. Mark Jirsa is to be in charge of the list.11) Discussed US-Canada currency exchange. It was agreed that prices for Canadian

meetings should always list a US currency price because many US participants havedifficulty obtaining Canadian funds.

Budgeting for the 43rd ISLG was done with an attempt to keep cost down and not losemoney. This was successful, in part, due to logistical support provided by the OntarioGeological Survey.

We enjoyed serving as Co-chairs of the 43rd Institute on Lake Superior Geology meeting.Organizing the meeting was a challenge, as the Ontario Geological Survey was goingthrough a difficult period of reorganization, and two of the three individuals planning themeeting received surplusing notices at the height of the planning process. A special thanksis due to Wilf Meyer and Tracy Livingstone for staying with the planning and organizationof the meeting while their lives were being upset due to reorganization. Without theirefforts, the meeting could not have been organized.

Ronald P. SageWilf MeyerCo-chairs, 43rd ILSGSudbury, Ontario

xvii

1) Accepted the report of the Chair, 42nd ILSG, Laurel Woodruff, including the minutes of the last board meeting.

2) Approved an offer by the Minnesota Geological Survey to host the 1998 meeting. Jim Miller and Mark Jirsa will co-chair the meeting. The offer by Charles Blackburn to chair the meeting in Kenora in 1998 had to be withdrawn due to circumstances beyond his control. The 1998 meeting will be held in Minneapolis, Minnesota on May 6-10.

3) Approved the 1996-1997 financial report presented by Mark Jirsa. 4) Approved Ron Sage as the Goldich Medal winner for 1997. 5) Approved the Goldich Medal guidelines prepared by Ken Card. 6) Approved the replacement of Ken Card by Mark Smyk on the Goldich Medal

Committee. 7) Discussed the ILSG newsletter and web site. The newsletter has been well received and

will be continued. We may eliminate the annual publication of the Constitution and place a general preamble on the web site (http://www.geo.mtu.edu/great-lakeslilsg). The web site includes information about the organization and its constitution, award guidelines, list of publications by the Institute and Goldich Medal recipients.

8) Approved the membership guidelines for the ILSG prepared by Mark Jirsa and Ted Bornhorst. Mailing list will be revised in an effort to reduce its size and lower mailing costs.

9) Discussed the Eisenbrey Fund and action on this fund was tabled for further review. A committee consisting of Gene LaBerge, Mark Jirsa, and Mark Smyk was formed to study and make recommendations on how the monies in the Eisenbrey Fund are to be used. Interest earned from this fund may be used to support student travel to the annual Institute meeting, and corporations and individuals may donate to the fund. Criteria to receive funding from the Eisenbrey Fund need to be established.

10) Discussed the ILSG mailing list. Mark Jirsa is to be in charge of the list. 11) Discussed US-Canada currency exchange. It was agreed that prices for Canadian

meetings should always list a US currency price because many US participants have difficulty obtaining Canadian funds.

Budgeting for the 43rd ISLG was done with an attempt to keep cost down and not lose money. This was successful, in part, due to logistical support provided by the Ontario Geological Survey.

We enjoyed serving as Co-chairs of the 43rd Institute on Lake Superior Geology meeting. Organizing the meeting was a challenge, as the Ontario Geological Survey was going through a difficult period of reorganization, and two of the three individuals planning the meeting received surplusing notices at the height of the planning process. A special thanks is due to Wilf Meyer and Tracy Livingstone for staying with the planning and organization of the meeting while their lives were being upset due to reorganization. Without their efforts, the meeting could not have been organized.

Ronald P. Sage Wilf Meyer Co-chairs, 43rd ILSG Sudbury, Ontario

xvii

PROGRAM

CALENDAR OF EVENTS AND PROGRAM

WEDNESDAY. MAY 6

0730-0800 REGISTRATIONIPRE-REGISTRATION PACKET PICK-UPHoliday Inn Pre -Function Area

0800-1800 PRE-MEETING FIELD TRIPSDepart from: Holiday Inn Pre-Function Area

1) EARLY PROTEROZOIC INTRUSIVE ROCKS OF EAST-CENTRALMINNESOTA

Leaders: Terry Boerboom, Mark Jima, and Daniel Hoim

2) GEOLOGY OF THE SOUTHEASTERN PORTION OF THEMIDCONTINENT RJFF SYSTEM, EASTERN MINNESOTA ANDWESTERN WISCONSIN

Leaders: Karl Wirth, Bill Cordua, Bill Kean, Mike Middleton, andZach Naiman

3) GLACIAL EXOTICA OF THE TWIN CITIES AREA

Leaders: Howard Hobbs, Al Knaeble, and Gary Meyer

1600-2200 REGISTRATION/PRE-REGISTRATION PACKET PICK-UPPUBLICATIONS FOR SALE

Holiday Inn Pre-Function Area

1700-2200 POSTER SET-UPHoliday Inn Pre-Function Area

1800-2200 WELCOMING RECEPTION/POSTER SESSIONFree Beer and Hors d'oeuvres; Cash BarHoliday Inn Pre-Function Area /Aragon Ballroom A-B

xd

CALENDAR OF EVENTS AND PROGRAM

0730-0800 REGISTRATIONIPRE-REGISTRATTON PACKET PICK-UP Holiday Inn Pre-Function Area

0800- 1800 PRE-MEETING FIELD TRIPS Depart from: Holiday Inn Pre-Function Area

1 ) EARLY PROTEROZOIC INTRUSIVE ROCKS OF EAST-CENTRAL MINNESOTA

Leaders: Terry Boerboom, Mark J i m , and Daniel Holm

2) GEOLOGY OF THE SOUTHEASTERN PORTION OF THE MIDCONTINENT RIFT SYSTEM, EASTERN MINNESOTA AND WESTERN WISCONSIN

Leaders: Karl Wirth, Bill Cordua, Bill Kean, Mike Middleton, and Zach Naiman

3) GLACIAL EXOTICA OF THE TWIN CITIES AREA

Leaders: Howard Hobbs, A1 Knaeble, and Gary Meyer

REGISTRATIONPRE-REGISTRATION PACKET PICK-UP PUBLICATIONS FOR SALE

Holiday Inn Pre-Function Area

POSTER SET-UP Holiday Inn Pre-Function Area

WELCOMING RECEPTIONPOSTER SESSION Free Beer and Hors d'oeuvres; Cash Bar Holiday Inn Pre-Function Area /Aragon Ballroom A-B

XXI

THURSDAY, MAY 7

0730-1630 REGISTRATION/PRE-REGISTRATION PACKET PICK-UPPUBLICATIONS FOR SALE

Holiday Inn Pre-Function Area

0730-0820 COFFEE AND TEAHoliday Inn Pre-Function Area

SESSION I: GEOLOGICAL OVERVIEW OF THE LAKE SUPERIOR REGIONAragon Ballroom D-E-F

Session Chairs: P.K. Sims (U.S. Geological Survey)G.B. Morey (Minnesota Geological Survey)

0820-0830 WELCOME (Jim Miller—Meeting Co-chair, Minnesota Geological Survey)

0830-0900 Card, Ken D. (Card and Associates Geosearch)Archean geology of the Great Lakes region of North America

0900-0930 Ojakangas, Richard W. (University of Minnesota—Duluth)Generalized Early Proterozoic history, Lake Superior region

0930-1000 Cannon, William F. (U.S. Geological Survey)Understanding the Middle Proterozoic history of the Lake Superior region:What's new? What's next?

1000-1030 COFFEE BREAK AND POSTER SESSION (Holiday Inn Pre-Function Area)

1030-1100 Runkel, Anthony C. (Minnesota Geological Survey)Paleozoic rocks in the northern part of the central midcontinent of NorthAmerica

1100-1130 Patterson, Carrie J. (Minnesota Geological Survey)Models for interpreting the Quatemary history of the Lake Superior region.

1130-1200 Southwick, David L. (Minnesota Geological Survey)What's next for geology in the Lake Superior area?

1200-1400: LUNCH BREAKILSG BOARD MEETING (Grill Room Restaurant, Holiday Inn)POSTERS (Holiday Inn Pre -Function Area)

xdi

0730- 1630 REGISTRATIONIPRE-REGISTRATION PACKET PICK-UP PUBLICATIONS FOR SALE

Holiday Inn Pre-Function Area

0730-0820 COFFEE AND TEA Holiday Inn Pre-Function Area

SESSION 1: GEOLOGICAL OVERVIEW OF THE LAKE SUPERIOR REGION Aragon Ballroom D-E-F

Session Chairs: P.K. Sims (U.S. Geological Survey) G.B. Morey (Minnesota Geological Survey)

0820-0830 WELCOME (Jim Miller-Meeting Co-chair, Minnesota Geological Survey)

0830-0900 Card, Ken D. (Card and Associates Geosearch) Archean geology of the Great Lakes region of North America

0900-0930 Ojakangas, Richard W. (University of Minnesota-Duluth) Generalized Early Proterozoic history, Lake Superior region

0930-1000 Cannon, William F. (U.S. Geological Survey) Understanding the Middle Proterozoic history of the Lake Superior region: What's new? What's next?

1000-1030 COFFEE BREAK AND POSTER SESSION (Holiday Inn Pre-Function Area)

1030-1 100 Runkel, Anthony C. (Minnesota Geological Survey) Paleozoic rocks in the northern part of the central midcontinent of North America

1100-1 130 Patterson, Carrie J. (Minnesota Geological Survey) Models for interpreting the Quaternary history of the Lake Superior region.

1 130-1200 Southwick, David L. (Minnesota Geological Survey) What's next for geology in the Lake Superior area?

1200-1400: LUNCH BREAK ILSG BOARD MEETING (Grill Room Restaurant, Holiday Inn) POSTERS (Holiday Inn Pre-Function Area)

SESSION IIAragon Ballroom D-E-F

Session Chairs: Jim Lundy (Minnesota Pollution Control Agency)Laurel Woodruff (U.S. Geological Survey)

1400-1420 Pfannkuch, H.O, and Paulson, Richard A.Improved geologic sensitivity and vulnerability assessments of groundwaterpollution potential through application of fuzzy logic

1420-1440 Campion, MoiraGround water recharge, discharge and residence time in Rice County,Minnesota: Implications for land use planning

1440-1500 Cannon, W.F., Kolker, Alan, and Westjohn, D.B.The geological source of arsenic in ground water in southeastern Michigan

1500-1520 COFFEE BREAK AND POSTER SESSION (Holiday Inn Pre -Function Area)

1520-1540 Fralick, P.W., and Kissin, S.A.The age and provenance of the Gunflint lapilli tuff

1540-1600 Medaris, L.G., Jr., and Fournelle, J.H.Svanbergite in the Bamboo Quartzite: Significance for diagenetic processesand phosphorus flux in Precambrian oceans

1600-1620 Medaris, L.G., Jr., Brown, P.B., and Bunge, R.J.PostT 1.76 low-grade metamorphism of the Bamboo Quartzite

1620-1640 Mudrey, M.G., Jr.Use of high-resolution aeromagnetic data for regional geologyinvestigations, southeastern Wisconsin (Where's the kimberlite!)

1830-1900 MJXER(Holiday Inn Pre-Function Area)Cash Bar

1900-2200 ANNUAL BANQUET AND AWARDS PRESENTATIONAragon Ballroom D-E-F

• Announcement of 45th Annual Meeting location

• 1996 Goldich Award presentation to Zell Peterman• Banquet speaker: Bevan French, Smithsonian Institution

"When the sky falls: Large meteorite impactsand the history of Earth and other worlds"

xll

SESSION I1 Aragon Ballroom D-E-F

Session Chairs: Jim Lundy (Minnesota Pollution Control Agency) Laurel Woodruff (U.S. Geological Survey)

1400-1420 Pfannkuch, H.0, and Paulson, Richard A. Improved geologic sensitivity and vulnerability assessments of groundwater pollution potential through application of fuzzy logic

1420-1440 Campion, Moira Ground water recharge, discharge and residence time in Rice County, Minnesota: Implications for land use planning

1440-1500 Cannon, W.F., Kolker, Alan, and Westjohn, D.B. The geological source of arsenic in ground water in southeastern Michigan

1500-1520 COFFEE BREAK AND POSTER SESSION (Holiday Inn Pre-Function Area)

1520-1540 Fralick, P.W., and Kissin, S.A. The age and provenance of the Gunflint lapilli tuff

1540-1600 Medaris, L.G., Jr., and Fournelle, J.H. Svanbergite in the Baraboo Quartzite: Significance for diagenetic processes and phosphorus flux in Precambrian oceans

1600-1620 Medaris, L.G., Jr., Brown, P.B., and Bunge, R.J. Post-1.76 low-grade metamorphism of the Baraboo Quartzite

1620-1640 Mudrey, M.G., Jr. Use of high-resolution aeromagnetic data for regional geology investigations, southeastern Wisconsin (Where's the kimberlite!)

1830-1 900 MSXER(Ho1iday Inn Pre-Function Area) Cash Bar

1900-2200 ANNUAL BANQUET AND AWARDS PRESENTATION Aragon Ballroom D-E-F

Announcement of 45th Annual Meeting location 1996 Goldich Award presentation to Zell Peterman Banquet speaker: Bevan French, Smithsonian Institution

"When the sky/alls: Large meteorite impacts and the history of Earth and other worlds"

FRIDAY, MAY 8

0730-1200 REGISTRATION/PRE-REGISTRATION PACKET PICK-UPPUBLICATIONS FOR SALE (coffee and tea 0730-0800)

Holiday Inn Pre -Function AreaSESSION IIIAragon Ballroom D-E-F

Session Chairs: Rod Johnson (Bitterroot Resources Ltd.)Mark Smyk (Ontario Geological Survey)

0820-0840 ursa, Mark A., Boerboom, Terrence J., and Chandler, Val W.Geologic setting of subeconomic gold deposits in the Virginia Horn,northeastern Minnesota: A horn of plenty or a hornswoggle?

0840-0900 Chandler, Val W., Jirsa, Mark A., and Lively, Richard L.Gravity and magnetic studies in the Virginia Horn area, northeasternMinnesota

0900-0920 Saini-Eidukat, Bernhardt, and Bernatchez, RaymondPreliminary ore mineralogy of the Herontrack silver-zinc-copperoccurrence, Lumby Lake area, Ontario, Canada

0920-0940 *Salo, R.W., and Kissin, S.A.Alteration and metamorphism in an Archean lode gold deposit, Kremzarmine, Goudreau-Lochalsh gold camp, Ontario

0940-1000 Hudak, George J., Morton, Ron L., and Franklin, James M.The recognition of a lava dome complex and its relationship to the ArcheanSturgeon Lake caldera, northwestern Ontario

1000-1020 COFFEE BREAK/FINAL POSTER SESSIONHoliday Inn Pre-Function Area

1020-1040 Sims, P.K., Neymark, L.A., and Peterman, Z.E.Nd isotope evidence for Middle and Early Archean crust in the WawaSubprovince of the Superior Province, Michigan, USA

1040-1100 Schulz, Klaus J., and Ayuso, Robert A.Crustal recycling in the evolution of the Penokean Orogen: Isotopicevidence for Archean contributions to crustal growth in the Pembine-Wausau terrane, northern Wisconsin

1100-1 120 Hoim, Daniel K., Schneider, David, and Coath, Chris D.Age and deformation of Early Proterozoic quartzites in the southern LakeSuperior region: Implications for extent of foreland deformation during fmalassembly of Laurentia

1120-1 140 *Czeck, Dyanna M., and Hudleston, Peter J.Kinematic fabrics near Mine Centre, Ontario: Evidence for a modifiedtranspression model

1140-1200 Corfu, F., and Easton, R.M.Extension of the Huronian magmatic suite inside the Grenville Province:New zircon U-Pb evidence from the Grenville Front Tectonic Zone in StreetTownship, Sudbury region, Ontario

1200-1400 LUNCH BREAK (posters to be removed; publication sales end)

xxiv

0730- 1200 REGISTRATION/PRE-REGISTRATION PACKET PICK-UP PUBLICATIONS FOR SALE (coffee and tea 0730-0800)

Holiday Inn Pre-Function Area SESSION 111 Aragon Ballroom D-E-F

Session Chairs: Rod Johnson (Bitterroot Resources Ltd.) Mark Smyk (Ontario Geological Survey)

Jirsa, Mark A., Boerboom, Terrence J., and Chandler, Val W. Geologic setting of subeconomic gold deposits in the Virginia Horn, northeastern Minnesota: A horn of plenty or a hornswoggle?

Chandler, Val W., Jirsa, Mark A., and Lively, Richard L. Gravity and magnetic studies in the Virginia Horn area, northeastern Minnesota

Saini-Eidukat, Bernhardt, and Bernatchez, Raymond Preliminary ore mineralogy of the Herontrack silver-zinc-copper occurrence, Lumby Lake area, Ontario, Canada

*Salo, R.W., and Kissin, S.A. Alteration and metamorphism in an Archean lode gold deposit, Kremzar mine, Goudreau-Lochalsh gold camp, Ontario

Hudak, George J., Morton, Ron L., and Franklin, James M. The recognition of a lava dome complex and its relationship to the Archean Sturgeon Lake caldera, northwestern Ontario

1000-1020 COFFEE BREAKfFINAL POSTER SESSION Holiday Inn Pre-Function Area

Sims, P.K., Neymark, L.A., and Peterman, Z.E. Nd isotope evidence for Middle and Early Archean crust in the Wawa Subprovince of the Superior Province, Michigan, USA

Schulz, Klaus J., and Ayuso, Robert A. Crustal recycling in the evolution of the Penokean Orogen: Isotopic evidence for Archean contributions to crustal growth in the Pembine- Wausau terrane, northern Wisconsin

Holm, Daniel K., Schneider, David, and Coath, Chris D. Age and deformation of Early Proterozoic quartzites in the southern Lake Superior region: Implications for extent of foreland deformation during final assembly of Laurentia

*Czeck, Dyanna M., and Hudleston, Peter J. Kinematic fabrics near Mine Centre, Ontario: Evidence for a modified transpression model

Corfu, F., and Easton, R.M. Extension of the Huronian magmatic suite inside the Grenville Province: New zircon U-Pb evidence from the Grenville Front Tectonic Zone in Street Township, Sudbury region, Ontario

1200-1400 LUNCH BREAK (posters to be removed; publication sales end)

XXIv

SESSION IVAragon Ballroom D-E-F

Session Chairs: John C. Green (University of Minnesota—Duluth)Paul W. Weiblen (University of Minnesota—Twin Cities)

1400-1420 Nicholson, Suzanne W., and Woodruff, Laurel G.The Powder Mill Group revisited: Basal volcanic rocks of the MidcontinentRift System on the south shore of Lake Superior

1420-1440 *Najman, Zachary J., and Wirth, Karl R.Composition and source(s) of Midcontinent Rift lavas (ChengwatanaVolcanics) near Clam Falls, Wisconsin

1440-1500 *Vislova, TatianaThe relevance of the geology of the mid-ocean ridges and ophiolites to theunderstanding of layered intrusions in the Midcontinent Rift: Part I.Geometry; Part II. Internal structure and petrology

1500-1520 *Maki, John C., and Bornhorst, Theodore J.New field observations of the Clarksburg Volcanics, Upper Peninsula ofMichigan

1520-1540 COFFEE BREAK(Holiday Inn P re-Function Area)

1540-1600 Tikoff, Basil, Bauer, Bob, Vigneresse, Jean-Louis, andHaggeman, NickA gravity, magnetic, and structural study of the Wakemup Bay Tonalite,Minnesota

1600-1620 Myers, Paul E.Xenolithologies as indicators of intrusion mechanisms in the WausauSyenite Complex, Wisconsin

1620-1640 BEST STUDENT PAPER AWARDSCLOSING

1905 BASEBALL: TWINS vs. YANKEES (HHHMetrodome)

xxv

SESSION IV Aragon Ballroom D-E-F

Session Chairs: John C. Green (University of Minnesota-Duluth) Paul W. ~ e i b l e n (university of ~innesota- win Cities)

Nicholson, Suzanne W., and Woodruff, Laurel G. The Powder Mill Group revisited: Basal volcanic rocks of the Midcontinent Rift System on the south shore of Lake Superior

*Naiman, Zachary J., and Wirth, Karl R. Composition and source(s) of Midcontinent Rift lavas (Chengwatana Volcanics) near Clam Falls, Wisconsin

*Vislova, Tatiana The relevance of the geology of the mid-ocean ridges and ophiolites to the understanding of layered intrusions in the Midcontinent Rift: Part I. Geometry; Part 11. Internal structure and petrology

*Maki, John C., and Bornhorst, Theodore J. New field observations of the Clarksburg Volcanics, Upper Peninsula of Michigan

1520- 1540 COFFEE BREAK(Ho1iday Inn Pre-Function Area)

Tikoff, Basil, Bauer, Bob, Vigneresse, Jean-Louis, and Haggeman, Nick A gravity, magnetic, and structural study of the Wakemup Bay Tonalite, Minnesota

Myers, Paul E. Xenolithologies as indicators of intrusion mechanisms in the Wausau Syenite Complex, Wisconsin

BEST STUDENT PAPER AWARDS CLOSING

BASEBALL: TWINS vs. YANKEES (HHH Metrodome)

XXV

SATURDAY. MAY 90800-1800 POST-MEETING FIELD TRIPS

Depart from: Holiday Inn Pre -Function Area

4) STRATIGRAPHY AND HYDROGEOLOGY OF PALEOZOIC ROCKS OFSOUTHEASTERN MINNESOTA

Leaders: Tony Runkel and Bob Tipping

5) MINNESOTA RIVER VALLEY AND VICINITY, SOUTHWESTERNMINNESOTA

Leaders: Dave Southwick and Carrie Patterson

SUNDAY, MAY 10

0800-1800 FIELD TRIP 5, DAY 2

Returns to Holiday Inn Metrodome

xxvi

SATURDAY. MAY 9

0800-1 800 POST-MEETING FIELD TRIPS Depart from: Holiday Inn Pre-Function Area

4) STRATIGRAPHY AND HYDROGEOLOGY OF PALEOZOIC ROCKS OF SOUTHEASTERN MINNESOTA

Leaders: Tony Runkel and Bob Tipping

5) MINNESOTA RIVER VALLEY AND VICINITY, SOUTHWESTERN MINNESOTA

Leaders: Dave Southwick and Carrie Patterson

SUNDAY, MAY 10

0800- 1 800 FIELD TRIP 5, DAY 2

Returns to Holiday Inn Metrodome

POSTER PRESENTATIONS

1700 hrs. Wednesday, May 6 through 1400 hrs. Friday, May 8

*Abbott, Kathleen M., Thole, Jeffrey T., and Wirth, Karl R.Petrography and geochemistry of Midcontinent Rift rhyolite (ChengwatanaVolcanics) near Clam Falls, Wisconsin

Bernatchez, Raymond A.A preliminary detailed geological description of the new high grade silver-basemetal discovery in the Lumby Lake metavolcanic belt northeast of Atikokan,Ontario, Canada

Boerboom, Terry, and Oberhelman, MattDimension stone products of Minnesota

Chandler, V.W., Jirsa, M.A., Morey, G.B., and Lawler, T.Mineral potential assesment of northern St. Louis County, southeasternKoochiching County, and northeastern Itasca County, Minnesota

Cannon, W.F., Laberge, G.L., Kiasner, J.S., and Schulz, K.J.Reinterpretation of the Penokean continental margin in part of northern Wisconsinand Michigan

Cannon, W.F., McRae, M.E., and Nicholson, S.W.Geographic information system on the geology and copper deposits of theKeweenaw Peninsula

Daniels, D. L., Snyder, S. L., Nicholson, S. W., and Cannon, W.F.New aeromagnetic surveys in Wisconsin by the U.S. Geological Survey

*Deangelis, M.T., Peck, W.H., and Valley, J.W.Polymetamorphism of skarns related to the Morin Anorthosite Complex, GrenvilleProvince, Quebec

*Gramstad, Sally D.Pre-Wisconsinan gray till in the Mankato area of the Minnesota River Valley

Han, T.M.A mineralographic study of magnetite in the Biwabik lion Formation, MesabiRange, Minnesota

*Hensel, Erin, *Joslin, Rick, and Lehrmann, DanFacies and depositional environments of the Early Proterozoic Ironwood lionFormation, Mt. Whittlesey, Wisconsin

Kjerland, Dean W., and Kjerland, MarcEffects of electric-pulse disaggregation on microfossil-bearing argillaceouslimestone of the Middle Ordovician Decorah Shale

xxvii

POSTER PRESENTATIONS

1700 hrs. Wednesday, May 6 through 1400 hrs. Friday, May 8

*Abbott, Kathleen M., Thole, Jeffrey T., and Wirth, Karl R. Petrography and geochemistry of Midcontinent Rift rhyolite (Chengwatana Volcanics) near Clam Falls, Wisconsin

Bernatchez, Raymond A. A preliminary detailed geological description of the new high grade silver-base metal discovery in the Lumby Lake metavolcanic belt northeast of Atikokan, Ontario, Canada

Boerboom, Terry, and Oberhelman, Matt Dimension stone products of Minnesota

Chandler, V.W., Jirsa, M.A., Morey, G.B., and Lawler, T. Mineral potential assesment of northern St. Louis County, southeastern Koochiching County, and northeastern Itasca County, Minnesota

Cannon, W.F., Laberge, G.L., Klasner, J.S., and Schulz, K.J. Reinterpretation of the Penokean continental margin in part of northern Wisconsin and Michigan

Cannon, W.F., McRae, M.E., and Nicholson, S.W. Geographic information system on the geology and copper deposits of the Keweenaw Peninsula

Daniels, D. L., Snyder, S. L., Nicholson, S. W., and Cannon, W.F. New aeromagnetic surveys m Wisconsin by the U.S. Geological Survey

*Deangelis, M.T., Peck, W.H., and Valley, J.W. Polymetamorphism of skarns related to the Morin Anorthosite Complex, Grenville Province, Quebec

*Gramstad, Sally D. Pre-Wisconsinan gray till in the Mankato area of the Minnesota River Valley

Han, T.M. A mineralographic study of magnetite in the Biwabik Iron Formation, Mesabi Range, Minnesota

*Hensel, Erin, *Joslin, Rick, and Lehrmann, Dan Facies and depositional environments of the Early Proterozoic Ironwood Iron Formation, Mt. Whittlesey, Wisconsin

Kjerland, Dean W., and Kjerland, Marc Effects of electric-pulse disaggregation on microfossil-bearing argillaceous limestone of the Middle Ordovician Decorah Shale

*Loofboro, Jeff, and Holm, DanielResults of modeling Proterozoic thermal histories: Evaluating the possible effects ofWolf River Batholith reheating on thermochronologic data from northern Wisconsin

Luther, Frank R.An Archean subaqueous heterolithic debris flow, Irwin, Pifher, and MeaderTownships, Lake Nipigon region, Ontario

McRae, M.E., Cannon, W.F., and Woodruff, L.G.Post-glacial shorelines of Isle Royale - Where are they now?

McSwiggen, Peter L.Electron microprobe study of the Pt-Pd and related mineralization in theMinnamax/Babbitt Cu-Ni deposit

*peterson, Dean M., and Morton, Ronald L.GIS-based mineral potential analysis for lode-gold and massive sulfide deposits inan Archean terrane of northern Minnesota

*Rausch, Deborah E., and Wattrus, Nigel J.Seismic evidence of pre-Nickerson Quaternary sediments in western Lake Superior

*Schaper, D., *Suess, W., *Katzer, L., and Kean, W.Additional paleomagnetic results for a 1500 Ma mafic dike at Waterloo, Wisconsin

Schmidt, Susanne Th., and Seifert, KarlMetamorphism, hydrothermal alteration, and lateritic weathering of drilled MRSvolcanic rocks in Iowa

Schulz, Klaus J., and Ayuso, Robert A.Crustal recycling in the evolution of the Penokean Orogen: isotopic evidence forArchean contributions to crustal growth in the Pembine-Wausau terrane, northernWisconsin

*Soofi, M.A., and King, S.D.A thin viscous sheet approach to investigate the post-rift evolution of theMidcontinent Rift System under the influence of Grenville Orogeny

*Thomas, C., Kean, W., and Luther, F.Paleomagnetic studies of a Proterozoic porphyritic diabase dike, Pither and IrwinTownships, Lake Nipigon district, Ontario

Wirth, Karl R., and Gehrels, George E.Precise U-Pb zircon ages of Midcontinent Rift rhyolite (Chengwatana Volcanics),Clam Falls, WI

xxviii

*Loofboro, Jeff, and Holm, Daniel Results of modeling Proterozoic thermal histories: Evaluating the possible effects of Wolf River Batholith reheating on therrnochronologic data from northern Wisconsin

Luther, Frank R. An Archean subaqueous heterolithic debris flow, Irwin, Pifher, and Meader Townships, Lake Nipigon region, Ontario

McRae, M.E., Cannon, W.F., and Woodruff, L.G. Post-glacial shorelines of Isle Royale - Where are they now?

McSwiggen, Peter L. Electron microprobe study of the Pt-Pd and related mineralization in the MinnamaxIBabbitt Cu-Ni deposit

*Peterson, Dean M., and Morton, Ronald L. GIs-based mineral potential analysis for lode-gold and massive sulfide deposits in an Archean terrane of northern Minnesota

*Rausch, Deborah E., and Wattrus, Nigel J. Seismic evidence of pre-Nickerson Quaternary sediments in western Lake Superior

*Schaper, D., *Suess, W., *Katzer, L., and Kean, W. Additional paleomagnetic results for a 1500 Ma mafic dike at Waterloo, Wisconsin

Schmidt, Susanne Th., and Seifert, Karl Metamorphism, hydrothermal alteration, and lateritic weathering of drilled MRS volcanic rocks in Iowa

Schulz, Klaus J., and Ayuso, Robert A. Crustal recycling in the evolution of the Penokean Orogen: isotopic evidence for Archean contributions to crustal growth in the Pembine-Wausau terrane, northern Wisconsin

*Soofi, M.A., and King, S.D. A thin viscous sheet approach to investigate the post-rift evolution of the Midcontinent Rift System under the influence of Grenville Orogeny

*Thomas, C., Kean, W., and Luther, F. Paleomagnetic studies of a Proterozoic porphyritic diabase dike, Pifher and Irwin Townships, Lake Nipigon district, Ontario

Wirth, Karl R., and Gehrels, George E. Precise U-Pb zircon ages of Midcontinent Rift rhyolite (Chengwatana Volcanics), Clam Falls, WI

ABSTRACTSSPECIAL SESSION:

Geological Overviewof the

Lake Superior Region

The 1998 ILSG planning committee asked six leadinggeologists for a summary of their special subfields of LakeSuperior geology, including what's new and what's next.

The following is their response.

1

SPECIAL SESSION:

Geological Overview of the

Lake Superior Region

The 1998 ILSG planning committee asked six leading geologists for a summary of their special subfields of Lake Superior geology, including what's new and what's next.

The following is their response.

ARCHEAN GEOLOGY OF THE GREAT LAKES REGIONOF NORTH AMERICA

CARD, K.D., Card and Associates' Geosearch, Kanata, ON, K2K 1M1, emailculau2ag3@aol.com

Archean rocks of the southern Canadian Shield in the Great Lakes region form the world'slargest (2 million sq. km.) Archean craton, the Superior Province. These ancient rockswere formed during several major cycles of volcanism, sedimentation, plutonism, andtectonism—mainly in the Lake Archean (2.5-2.8 Ga), but also in the Middle (2.8-3.4 Ga)and Early Archean (>3.4 Ga). Superior Province consists of northern and southern high-grade gneiss subprovinces and a broad central region of alternating granite-greenstone andsedimentary belts (Figure X). Some of the high-grade rocks represent exposures of lowercrust of Superior Province uplifted in the Early Proterozoic; others, notably the MinnesotaRiver Valley gneisses, are exotic blocks of Early Archean crust. The volcanic and relatedplutonic rocks are similar to those of modern island and continental arcs formed alongconvergent margins. The metasedimentaiy belts may mark Archean oceans. MiddleArchean rocks, including plutonic gneisses approximately 3.0 Ga old are overlainunconformably by ca 2.8 Ga platformal sequences including quartzite, quartz pebbleconglomerate and komatiitic volcanic rocks. In contrast, the Late Archean volcanic andsedimentary sequences are mainly juvenile, derived directly from the mantle or from crustalsources less than 200 million years older, and do not have stratigraphic bases; manygreenstone belts may be allochthonous.

Superior Province is rich in minerals including copper, zinc, and nickel massive sulfideand lode gold deposits. Most of the mineral deposits occur in the granite-greenstone beltswhere they were formed in a period of 100 million years or less by processes related to themajor volcanism, plutonism, and tectonism that marked the end of the Archean. Theseprocesses also led to the formation of a large, stable craton called Kenorland. Kenorlandmay have been part of a supercontinent—one of the first of Earth's supercontinents. It wasbroken up in the Early Proterozoic, parts of it migrating westward to Wyoming, and otherparts going back to the old country to form the Baltic Shield.

Archean Earth was significantly different from modern Earth in many ways. TheArchean atmosphere, for example, lacked free oxygen and was rich in ammonia, carbondioxide, and methane. Meteorite impacts were much more frequent, and until about 4.0 Gaprobably kept the crust well-stirred. This early period of Earth history is called theHadean, probably with justification. Heat flow was also higher, resulting in a thickermantle and a volcanically active crust. However, by the Late Archean, the rocks, mineraldeposits, and structures being formed and preserved are remarkably similar to those ofmodern accretionary orogens, notably those around the Pacific Rim. Although currentlythe subject of debate, it would appear that the Archean Superior Province was formed bysubduction-driven orogenic processes similar in most respects to those operating today.

Additional Reviews:

Card, K.D., A review of the Superior Province of the Canadian Shield, a product of Archean accretion:Precambrian Research, v. 48, p. 99-156.

Hamilton, W.B., Evolution of Archean mantle and crust, p. 597-614 in Reed, J.C. Jr., Ball, T.T., Farmer,G.L., and Hamilton, W.B., 1993, A broader view, in Reed, J.C.Jr., Bickford, M.E., Houston, R.S.,Link, P.K., Ranking, D.W., Sims, P.K., and Van Schmus, W.R., eds., Precambrian: ConterminousU.S, Geological Society of America, The Geology of North America, v. C-2, p.597-636.

3

ARCHEAN GEOLOGY OF THE GREAT LAKES REGION OF NORTH AMERICA

CARD, K.D., Card and Associates' Geosearch, Kanata, ON, K2K 1M1, email cu 1au2ag3@aol.com

Archean rocks of the southern Canadian Shield in the Great Lakes region form the world's largest (2 million sq. km.) Archean craton, the Superior Province. These ancient rocks were formed during several major cycles of volcanism, sedimentation, plutonism, and tectonism-mainly in the Lake Archean (2.5-2.8 Ga), but also in the Middle (2.8-3.4 Ga) and Early Archean (>3.4 Ga). Superior Province consists of northern and southern high- grade gneiss subprovinces and a broad central region of alternating granite-greenstone and sedimentary belts (Figure X). Some of the high-grade rocks represent exposures of lower crust of Superior Province uplifted in the Early Proterozoic; others, notably the Minnesota River Valley gneisses, are exotic blocks of Early Archean crust. The volcanic and related plutonic rocks are similar to those of modem island and continental arcs formed along convergent margins. The metasedimentary belts may mark Archean oceans. Middle Archean rocks, including plutonic gneisses approximately 3.0 Ga old are overlain unconformably by ca 2.8 Ga platformal sequences including quartzite, quartz pebble conglomerate and komatiitic volcanic rocks. In contrast, the Late Archean volcanic and sedimentary sequences are mainly juvenile, derived directly from the mantle or from crustal sources less than 200 million years older, and do not have stratigraphic bases; many greenstone belts may be allochthonous.

Superior Province is rich in minerals including copper, zinc, and nickel massive sulfide and lode gold deposits. Most of the mineral deposits occur in the granite-greenstone belts where they were formed in a period of 100 million years or less by processes related to the major volcanism, plutonism, and tectonism that marked the end of the Archean. These processes also led to the formation of a large, stable craton called Kenorland. Kenorland may have been part of a supercontinent~one of the f i r t of Earth's supercontinents. It was broken up in the Early Proterozoic, parts of it migrating westward to Wyoming, and other parts going back to the old country to form the Baltic Shield.

Archean Earth was significantly different from modem Earth in many ways. The Archean atmosphere, for example, lacked free oxygen and was rich in ammonia, carbon dioxide, and methane. Meteorite impacts were much more frequent, and until about 4.0 Ga probably kept the crust well-stirred. This early period of Earth history is called the Hadean, probably with justification. Heat flow was also higher, resulting in a thicker mantle and a volcanically active crust. However, by the Late Archean, the rocks, mineral deposits, and structures being formed and preserved are remarkably similar to those of modem accretionary orogens, notably those around the Pacific Rim. Although currently the subject of debate, it would appear that the Archean Superior Province was formed by subduction-driven orogenic processes similar in most respects to those operating today.

Additional Reviews:

Card, K.D., A review of the Superior Province of the Canadian Shield, a product of Archean accretion: Precambrian Research, v. 48, p. 99-156.

Hamilton, W.B., Evolution of Archean mantle and crust, p. 597-614 in Reed, J.C. Jr., Ball, T.T., Farmer, G.L., and Hamilton, W.B., 1993, A broader view, in Reed, J.C.Jr., Bickford, M.E., Houston, R.S., Link, P.K., Ranking, D.W., Sims, P.K., and Van Schmus, W.R., eds., Precambrian: Conterminous U.S, Geological Society of America, The Geology of North America, v. C-2, p.597-636.

Figure x. Subprovinces of the Superior Province

4

Figure x. Subprovinces of the Superior Province

GENERALIZED EARLY PROTEROZOIC HISTORY, LAKE SUPERIORREGION

Ojakangas, Richard W., Department of Geology,University of Minnesota-Duluth, Duluth, MN 55812

rojakang@d.umn.edu

INTRODUCTIONOver the last century, countless geologists have worked todecipher the fascinating Early Proterozoic geological history ofthis region. Unfortunately, it is impossible in a short review toacknowledge all the important works, but each cited referencecontains a bibliography. Two compendiums are especially valuableand include more than 500 and 700 Precambrian references,respectively (Sims et al, 1993; Sims and Carter, 1996). As in anylimited review, the bias of the reviewer will be present. Theintegration of plate tectonic theory permeates this history.

HURONIANAfter the supercontinent of Kenorland was assembled about 2.7 Ga(Card, 1990; Williams et al, 1991) during the Algoman (Keewatin)orogeny, a long period of weathering, erosion, and sedimentationoccurred on this stable and thick crustal platform. The oldestsedimentary sequence, the Huronian Supergroup, was depositedbetween 2.45 Ga and 2.2 Ga, mainly north of Lake Huron. As thickas 12 km and thinning northward, this is a sequence quite unlikeArchean sequences in that it contains "uncommon" glacial deposits,paleosols, carbonates, and abundant arkosic to quartzosesandstones. An upper part of this supergroup is a sequence, fromthe bottom up, of glaciogenic rocks, paleosol, quartz sandstone,and carbonate rocks with sabkha minerals (evaporites). A similarsequence is present as erosional remnants in the Upper Peninsulaof Michigan, 200 km to the west (Chocolay Group, Marquette RangeSupergroup). An even more complete similar sequence is found insoutheastern Wyoming, the Snowy Pass Supergroup. The glacial unitsof these three sequences were first interpreted by Young (1970) asremnants of a major continental glaciation, and additional work(e.g., Ojakangas, 1984, 1985, 1988) further strengthens this idea.As an interesting aside, Williams and Schmidt (1997) stated thatpaleomagnetic data indicate that this Huronian glaciation occurredwithin 4-11° of the equator. Further evidence of the correlationof these three sequences was provided by Bekker and Karhu (1996)and Bekker (1998), who found similar high values of 13C_enrichmentand 'BO—depletion in carbonate units.

Supercontinent Kenorland began to undergo extension at 2.45 Ga, asindicated by mafic igneous rock units at the base of the HuronianSupergroup (e.g., Heaman, 1997; Cheney, 1998; Balls, 1998). Actualbreakup occurred at 2.2 to 2.1 Ga with emplacement of Nipissingdiabase sills and major dike swarms (Roscoe and Card, 1993). TheKenora-Kabetogama mafic dikes (2145 +1- 45 Ma) in northernMinnesota (Southwick and Day, 1983) that show up so well onaeromagnetic maps (Chandler, 1991; Chandler et al, 1984) appear tobe products of this final extensional event. The breakup of

5

GENERALIZED EARLY

0 j akangas , University

INTRODUCTION Over the' last centuryf

PROTEROZOIC HISTORY, LAKE SUPERIOR REGION

Richard W , Department of Geologyf of Minnesota-Duluthf Duluthf MN 55812

rojakang@d*m*edu

countless ueolouists have worked to decipher the fascinating Early ~r&eroGoic geological history of this region* Unfortunatelyf it is impossible in a s h o ~ review to acknowledge all the important worksf but each cited reference contains a bibliography* Two compendiums are especially valuable and include more than 500 and 700 Precambrian referencesf respectively (Sims et alf 1993; Situs and Carterf 1996). As in any limited reviewf the bias of the reviewer will be present* The integration of plate tectonic theory permeates this history*

HURONIAN After the supercontinent of Kenorland was assembled about 2.7 Ga (Cardf 1990; Williams et alf 1991) during the Algoman (Keewatin) orogenyf a long period of weatheringf erosionf and sedimentation occurred on this stable and thick crustal platform* The oldest sedimentary sequencef the Huronian Supergroupf was deposited between 2-45 Ga and 2.2 Gaf mainly north of Lake Huron* As thick as 12 km and thinning northwardf this is a sequence quite unlike Archean sequences in that it contains ffuncommon'f glacial depositsf paleosolsf carbonatesf and abundant arkosic to quartzose sandstones* An upper part of this supergroup is a sequencef from the bottom upf of glaciogenic rocksf paleosolf quartz sandstonef and carbonate rocks with sabkha minerals (evaporites). A similar sequence is present as erosional remnants in the Upper Peninsula of Michiganf 200 km to the west (Chocolay Groupf Marquette Range Supergroup)* An even more complete similar sequence is found in southeastern Wyomingf the Snowy Pass Supergroup* The glacial units of these three sequences were first interpreted by Young (1970) as remnants of a major continental glaciationf and additional work (e-gOf Ojakangasf 1984f l98Sf 1988) further strengthens this idea* As an interesting asidef Williams and Schmidt (1997) stated that paleomagnetic data indicate that this Huronian glaciation occurred within 4-110 of the equator* E'urther evidence of the correlation of these three sequences was provided by Bekker and Karhu (1996) and Bekker (1998)f who found similar high values of 13C-enrichment and 180-depletion in carbonate units*

Supercontinent Kenorland began to undergo extension at 2.45 Gat as indicated by mafic igneous rock units at the base of the Huronian Supergroup (e*gOf Heamant 1997; Cheneyf 1998; Hallsf 1998). Actual breakup occurred at 2.2 to 2.1 Ga with emplacement of Nipissing diabase sills and major dike s w a m (Roscoe and Cardf 1993)- The Kenora-Kabetogama mafic dikes (2145 +/- 45 Ma) in northern Minnesota (Southwick and Dayf 1983) that show up so well on aeromagnetic maps (Chandlerf 1991; Chandler et alf 1984) appear to be products of this final extensional event* The breakup of

Kenorland has an even broader intercontinental significance; asimilar sedimentary sequence with similar igneous activity is alsopresent in Finland and adjacent Russia (Marmo and Ojakangas,1984), and correlation seems likely (e.g., Ojakangas, 1984, 1988;Ojakangas, et al, 1991). In summary, Kenorland, which formed inthe Late Archean, apparently broke up in the Early Proterozoic,with the Wyoming craton heading west and the Svecofennian cratonheading east.

CORRELATIONCorrelation of Early Proterozoic rock units of Michigan andOntario has long been a problem. Glacial units have been used asmarker units (e.g., Young, 1983; Ojakangas, 1988); on this basis,the lower portion of the Marquette Range Supergroup (the ChocolayGroup) is correlated with the upper part of the HuronianSupergroup, including the Gowganda Formation and overlying units.

Correlation within the Lake Superior region itself has also longbeen controversial (Morey and Van Schmus, 1988; Morey, 1996).Correlations have long been made on lithostratigraphic grounds;for example, it had been assumed that the various iron-formationsof the Lake Superior region were correlative, but this is nowquestioned. Correlations are complicated by deformation (boththin-and thick-skinned tectonics) and the lack of exposuresbecause of extensive glacial cover. New approaches, suchas C—isotopic studies of carbonate units, have been interpreted asevidence that not all of the carbonate units are correlative, aslong assumed (Bekker and Karhu, 1996; Bekker, 1998). Unless theC-ratios have been affected by diagenesis/metamorphism, theyindicate that the Kona Dolomite may be younger than the Bad RiverDolomite (WI and MI), the Saunders Formation (MI), the RandvilleDolomite (WI), and the Rabbit Lake (MN) units that have all been"correlative units". An excellent summary of the sedimentation andcorrelation of the continental margin assemblage is by Morey(1996).

IRON-FORMATIONNaturally enriched iron ores of the Lake Superior region made theU.S.A. an industrial giant, but only two of the many ranges, theMesabi and the Marquette, are still producing iron ore (taconitepellets). "Lake Superior-type" siliceous iron-formation is knownworld-wide, and similar units have been described from othercontinents. For nearly a century, geologists had thought theywere all about 2.0 Ga old. However, in recent years it has beensuggested that there was more than one period of deposition. InMinnesota, there appear to be three ages ranging from about 2.2 Gato 1 • 9 Ga (Southwick and Morey, 1991; Morey and Southwick, 1995);the oldest two are laminated and fine-grained (deposited belowwave—base in extensional basins?), in contrast to the youngerBiwabik Iron-formation that contains sand-sized grains of chertand iron minerals, is cross-bedded, and contains two stromatolitehorizons indicative of deposition on a shallow shelf. InMichigan-Wisconsin, there may be two major periods of deposition(e.g., Ojakangas, 1994; Morey, 1996). The older Negaunee Iron-

6

Kenorland has an even broader intercontinental significance; a similar sedimentary sequence with similar igneous activity is also present in Finland and adjacent Russia (Manno and Ojakangasf 1984)f and correlation seems likely (e.gOf Ojakangasf 1984f 1988; Ojakangasf et alf 1991). In summaryf Kenorland# which formed in the Late Archean, apparently broke up in the Early Proterozoicf with the Wyoming craton heading west and the Svecofennian craton heading east*

CORRELATION Correlation of Early Proterozoic rock units of Michigan and Ontario has long been a problem* Glacial units have been used as marker units (e-gof Youngf 1983; Ojakangasf 1988); on this basisf the lower portion of the Marquette Range Supergroup (the Chocolay Group) is correlated with the upper part of the Huronian Supergroupf including the Gowganda Formation and overlying units.

Correlation within the Lake Superior region itself has also long been controversial (Morey and Van Schmusf 1988; Moreyf 1996). Correlations have long been made on lithostratigraphic grounds; for example! it had been assumed that the various iron-formations of the Lake Superior region were correlativef but this is now questioned* Correlations are complicated by deformation (both thin-and thick-skinned tectonics) and the lack of exposures because of extensive glacial cover* New approachesf such-as C- isotopic studies of carbonate unitsf have been interpreted as evidence that not all of the carbonate units are correlativef as long assumed (Bekker and Karhuf 1996; Bekkerf1998). Unless the C-ratios have been affected by diagenesis/metmrphismf they indicate that the Kona Dolomite may be younger than the Bad River Dolomite (WI and MI)! the Saunders Formation (MI)f the Randville Dolomite (WI)# and the Rabbit Lake (MN) units that have all been "correlative unitsw* An excellent summary of the sedimentation and correlation of the continental margin assemblage is by Morey (1996).

IRON-FORMATION Naturally enriched iron ores of the Lake Superior region made the U-S.A. an industrial giantf but only two of the many rangesf the Mesabi and the Marquettef are still producing iron ore (taconite pellets). "Lake Superior-typew siliceous iron-formation is known world-widef and similar units have been described from other continents* For nearly a centuryf geologists had thought they were all about 2.0 Ga old. Howeverf in recent years it has been suggested that there was more than one period of deposition* In Minnesotaf there appear to be three ages ranging from about 2.2 Ga to 1.9 Ga (Southwick and Moreyf 1991; Morey and Southwickf 1995); the oldest two are laminated and fine-grained (deposited below wave-base in extensional basin^?)^ in contrast to the younger Biwabik Iron-formation that contains sand-sized grains of chert and iron mineralsf is cross-beddedf and contains two stromatolite horizons indicative of deposition on a shallow shelf* In Michigan-Wisconsinf there may be two major periods of deposition (e*gof Ojakangas! 1994; Moreyf 1996)- The older Negaunee Iron-

formation of the Marquette district appears to have been depositedin deeper waters of a down-faulted graben (with the addition ofterrigenous material by turbidity currents from the south side asnoted by LaRue, 1981), whereas the Ironwood Iron-formation of theGogebic range (Schmidt, 1980) and the Bijiki Iron-formation memberof the Michigamme Formation appear to have been deposited on astable marine shelf. Interesting spirally coiled, megascopiceukaryotic algae have been found in the Negaunee (Han andRunnegar, 1992).

The Biwabik Iron-formation (as thick as 225 m) on the Mesabi range(Morey, 1992) and the Ironwood Iron-formation on the Gogebic rangeappear to have a common origin; part of the evidence is found inthe underlying Pokeama and Palms formations which have beeninterpreted as having formed in a tidal environment (Ojakangas,1983; Ojakangas, G. W., 1996). These studies indicate that theiron-formations were deposited on a marine tidally—influencedshelf, seaward of the land-derived sand and muds of the Pokegamaand Palms formations. Lougheed (1983) also proposed a tidalenvironment, with siderite-rich limestone the primary pre-diagenetic rock type. Deposition appears to have been related to amajor marine transgression (Simonson and Hassler, 1996).

There are two oft-cited sources for the iron and the silica—-hydrothermal activity associated with volcanism iii the basin

, Carrigan and Cameron, 1991; Isley, 1995) and weathering ofthe Archean craton , Lepp, 1987). The Ironwood Iron-formationon the Gogebic is interbedded with volcanics (Klasner et al,1998), the Gunf lint Iron-formation (once continuous with theBiwabik Iron-formation prior to intrusion of the 1.1 Ga DuluthComplex) contains tuff beds, and there is a prominent ashy bed inthe Biwabik. Hoffman (1987) tied the iron-formations of the LakeSuperior region to foredeep volcanism. C and 0 isotopes of theGunflint indicate a hydrothermal source for the Fe+2 and thesilica, with siderite being the primary iron mineral in abasinward facies (Winter and Knauth, 1992). The lower part ofthe overlying Virginia Formation contains numerous ash beds(Lucente and Morey, 1983). Thus there is a growing body ofevidence indicating that volcanism was the main source of the ironand silica, but continental weathering may indeed have beenanother source (e.g., Drever, 1974). Biota have long been toutedas important in the precipitation of the iron minerals (e.g., LaBerge, 1967; Lougheed, 1983; LaBerge et al, 1987).

When was the Biwabik Iron-formation, the largest and probably theyoungest iron-formation in the region, deposited? Part of theevidence comes from the less-metamorphosed and correlativeGunf lint Iron-formation that is on strike with and was oncecontinuous with the Biwabik. Faure and Kovach (1969) reported awhole-rock Rb-Sr isochron age of 1.64 +1- 0.2 Ga for thedeposition or diagenesis of the Gunf lint. A Sm-Nd isochron ageonargillites in the Gunf lint was determined at 2.08 +1- 0.25 Ga(Stille and Clauer, 1986). A Sm-Nd whole-rock isochron of 2100 +1-52 Ma was reported for "slaty" portions of the Biwabik, but this

7

formation of t h e Marquette d i s t r i c t appears t o have been deposi ted i n deeper w a t e r s of a down-faulted graben (with t h e add i t ion of te r r igenous material by t u r b i d i t y currents from t h e south side as noted by LaRuef 1981)! whereas t h e Ironwood Iron-formation of t h e Gogebic range (Schmidtf 1980) and t h e B i j i k i Iron-formation member of t h e Michigamme Formation appear t o have been deposi ted on a stable marine s h e l f - I n t e r e s t i n g s p i r a l l y coiled! megascopic eukaryot ic a lgae have been found i n t h e Negaunee (Han and Runnegar, 1992)-

The Biwabik Iron-formation (as t h i c k as 225 m) on t h e M e s a b i range (Morey! 1992) and t h e Ironwood Iron-formation on t h e Gogebic range appear t o have a common or ig in ; p a r t of t h e evidence i s found i n t h e underlying Pokegama and Palms formations which have been i n t e r p r e t e d as having formed i n a t i d a l environment (Ojakangas! 1983; Ojakangas! Go W O f 1996)- These s tud ies i n d i c a t e t h a t t h e iron-formations were deposited on a marine t idal ly- inf luenced shelf! seaward of t h e land-derived sand and muds of t h e Pokegama and Palms formations- Lougheed (1983) a l s o proposed a t ida l environment! with s i d e r i t e - r i c h limestone t h e primary pre- d iagenet ic rock type- Deposition appears t o have been related to a major marine t ransgress ion (Shonson and Hassler! 1996).

There are two o f t - c i t ed sources f o r t h e i r o n and t h e silica-- hydrothermal a c t i v i t y associated with volcanism ih t h e bas in ( e - g e t Carrigan and Cameron! 1991; Is leyf1995) and weathering of t h e Archean c ra ton ( e m g o t Lepp, 1987)- The Ironwood Iron-formation on t h e Gogebic is interbedded with volcanics (Klasner et a l f 1998)! t h e Gunfl int Iron-formation (once continuous with t h e Biwabik Iron-formation p r i o r t o in t rus ion of t h e 1.1 G a Duluth Complex) conta ins t u f f beds, and t h e r e is a prominent ashy bed i n t h e Biwabik- Hoffman (1987) t ied t h e iron-formations of t h e Lake Superior region t o foredeep volcanism- C and 0 iso topes of t h e Gunfl int i n d i c a t e a hydrothermal source f o r t h e Fe+2 and t h e silica! with siderite being t h e primary i r o n mineral i n a basinward f a c i e s (Winter and Knauth! 1992)- The lower p a r t of t h e overlying Virg in ia Formation contains numerous ash beds (Lucente and Morey! 1983)- Thus t h e r e is a growing body of evidence i n d i c a t i n g t h a t volcanism w a s t h e main source of t h e i r o n and silica! bu t con t inen ta l weathering may indeed have been another source ( e - g o , Drevet! 1974). Biota have long been tou ted as important i n t h e p rec ip i t a t ion of t h e i r o n minerals ( e - g O f La Berge! 1967; Lougheed! 1983; LaBerge et a l f 1987)-

When was t h e Biwabik Iron-formationf t h e l a r g e s t and probably t h e youngest iron-formation i n t h e region! deposited? P a r t of t h e evidence comes from t h e less-metamorphosed and correlative Gunfl int Iron-formation t h a t i s on s t r i k e with and was once continuous with t h e Biwabik. Faure and Kovach (1969) repor ted a whole-rock Rb-Sr isochron age of 1-64 +/- 0.2 G a f o r t h e depos i t ion or diagenesis of t h e Gunflint- A Sm-Nd isochron age-on a r g i l l i t e s i n t h e Gunfl int was determined a t 2-08 +/- 0.25 G a ( S t i l l e and Clauer! 1986)- A Sm-Nd whole-rock isochron of 2100 +/- 52 M a was repor ted f o r 88slatyw port ions of t h e Biwabik! bu t t h i s

is probably a mixing age of Archean and Proterozoic components(Gerlach et al, 1988). Quartz veins in the underlying Pokegamawere dated at 1930 +1- 25 Ma (Hemming et al, 1990). Probably mostindicative of the age is a new U-Pb date of 1876 +1- 2 Ma oneuhedral zircons from a reworked tuff bed in the lower Gunf lint(Fralick and Kissen,1998). This date has major tectonicimplications, for it indicates that these two iron-formations weredeposited on the peripheral foreland bulge of the Animikieforeland basin, rather than on the continental margin prior to thedevelopment of the foreland basin. In this scenario of anorthward-migrating foreland basin, the terrigenous clastic PalmsFormation and the Ironwood Iron-formation on the Gogebic range ofWisconsin-Michigan may be continuous with but older (i.e.,diachronous) than the more northerly (by 100 km) terrigenousclastic Pokegama Formation and the Biwabik Iron-formation on theMesabi range of Minnesota (Ojakangas, 1994).

PENOKEAN OROGENThe Archean ended with the formation of a big supercontinent,Kenorland, the product of the amalgamation of volcanic arcs andgranitic intrusive bodies, and history repeats itself. ThePenokean (Hudsonian) orogeny occurred on the southern edge of theSuperior craton, extending over a distance of 1100 km from theGrenville Front near Sudbury, ON, westward to the area just westof Lake Superior, deforming the sedimentary units deposited on thepassive continental margin when volcanic arcs (the Wisconsinmagmatic terranes) and microcontinents (that included Archeanrocks) collided from the south with northward-directed thrustingover a southward-dipping subduction zone (e.g., Morey andSouthwick, 1995; Sims, 1996).

The Penokean has long been an enigma for it seems to have beenlong-lived, lasting from at least 1982 Ma to 1770 Ma, withtheearliest Proterozoic sedimentary units in Minnesota depositedabout 2200 Ma (Morey and Southwick, 1995). Volcanic rocks of theorogen range in age from 1880-1840 Ma, plutonic rocks from 1980-1770 Ma, and deformation from 1982-1770 Ma (e.g., Southwick andMorey, 1991; Sims et al, 1993). Volcanogenic massive sulfidedeposits and occurrences of Cu, Zn, and Pb, about 1860-1840 Ma,are abundant based on extensive exploration drilling ofgeophysical anomalies; more than 13 bodies have been identified(DeMatties, 1994), although only one (Flambeau) has been mined todate. Post-Penokean rhyolites and granites occur in Wisconsin(Sims et al, 1989) and post-orogenic metamorphism and coevalplutons in east-central Minnesota have been dated at 1770-1760 Mawith rapid cooling at 1760-1750 Ma (Holm et al, 1998). Twodeformations in the southern portion of the Thomson Formation havebeen described, with F1 isoclinal recumbent folds and F2 uprightfolds that refolded the earlier folds (Holst, 1982, 1984).Excellent interpretations of Penokean deformation can be found,e.g., in Holst (1991), Klasner et al (1991), Kiasner and Sims-(1993), Gregg (1993), and Sims and Carter (1996).

8

is probably a mixing age of Archean and Proterozoic components (Gerlach et alf 1988). Quartz veins in the underlying Pokegama were dated at 1930 +/- 25 Ma (Hemming et alf 1990). Probably most indicative of the age is a new U-Pb date of 1876 +/- 2 Ma on euhedral zircons from a reworked tuff bed in the lower Gunflint (Fralick and Kissenf1998)* This date has major tectonic implicationsf for it indicates that these two iron-formations were deposited on the peripheral foreland bulge of the AnMkie foreland basinf rather than on the continental margin prior to the development of the foreland basin. In this scenario of a northward-migrating foreland basinf the terrigenous clastic Palms Formation and the Ironwood Iron-formation on the Gogebic range of Wisconsin-Michigan may be continuous with but older (i.eOf diachronous) than the more northerly (by 100 km) terrigenous clastic Pokegama Formation and the Biwabfi Iron-formation on the Mesabi range of Minnesota (Ojakangasf 1994).

PENOKEAN OROGEN The Archean ended with the formation of a big supercontinentf Kenorlandf the product of the amalgamation of volcanic arcs and granitic intrusive bodies! and history repeats itself* The Penokean (Hudsonian) orogeny occurred on the southern edge of the Superior cratonI extending over a distance of 1100 km from the Grenville Front near Sudburyf ONf westward to the area just west of Lake Superior! deforming the sedimentary units deposited on the passive continental margin when volcanic arcs (the Wisconsin magmatic terranes) and microcontinents (that included Archean rocks) collided from the south with northward-directed thrusting over a southward-dipping subduction zone (e.gOf Morey and Southwickf 1995; S h I 1996).

The Penokean has long been an enigma for it seems to have been long-lived! lasting from at least 1982 Ma to 1770 Maf with the earliest Proterozoic sedimentary units in Minnesota deposited about 2200 Ma (Morey and Southwickf 1995). Volcanic rocks of the orogen range in age from 1880-1840 MaI plutonic rocks from 1980- 1770 Maf and deformation from 1982-1770 Ma (e.gOf Southwick and Moreyf 1991; S h s et alf 1993). Volcanogenic massive sulfide deposits and occurrences of Cut ZnI and Pbf about 1860-1840 Maf are abundant based on extensive exploration drilling of geophysical anomalies; more than 13 bodies have been identified (Mattiesf 1994)f although only one (Flambeau) has been mined to date. Post-Penokean rhyolites and granites occur in Wisconsin ( S d et alf 1989) and post-orogenic metamorphism and coeval plutons in east-central Minnesota have been dated at 1770-1760 Ma with rapid cooling at 1760-1750 Ma (Holm et alf 1998). !lWo deformations in the southern portion of the Thomson Formation have been describedf with Fl isoclinal recumbent folds and F2 upright folds that refolded the earlier folds (Holstf 1982f 1984)- Excellent interpretations of Penokean deformation can be found, e.gOf in Holst (1991)f Klasner et a1 (1991)f Klasner and S h - (1993)f Gregg (1993)! and S h and Carter (1996).

By removing the 60 km-wide Middle Proterozoic Midcontinent Riftsystem from a regional geological map, thereby bringing Minnesotaand Wisconsin-Michigan into juxtaposition, the fold-and-thrustbelts of the two areas become one continuous zone. Southwick andMorey (1991) correlated the Niagara fault zone/suture (WI) andthe Malmo discontinuity (MN). Similarly, the various turbidite-shale units (Thomson, Rove, Virginia, Tyler, Copps, and MichigammeFormations) then become one contiguous unit filling the AnimikieBasin. This, however, does not imply that all of these formationsare the same age, for the basin was migrating northward duringdeposition of these units; some sediment deposited in the earlierstages of basin development was likely cannibalized andredeposited.

The Animikie basin has been interpreted as a northward-migratingforeland basin that developed in front of (on the north side) ofthe fold-and-thrust belt due to the weighting down of the crust bythis folded and thrusted mountain range that then became a majorsource of sediment for the basin (e.g., Hoffman, 1987; Southwicket al, 1988; Barovich et al, 1989; Southwick and Morey, 1991;Ojakangas, 1994; Morey and Southwick, 1995).

Nd isotope data determined by Barovich et al (1989) indicate thatsome Michigainme samples had an Archean provenance to the north andsome had an Early Proterozoic provenance to the south.Paleocurrent data for the Michigamme and the Tyler indicate anorthward transport of sediment from the south side of theAnimikie Basin and a southward transport of sediment from thenorth side of the basin as summarized in Ojakangas (1994). Hemminget al (1995), on gochemical and isotopic evidence, described theprovenance of the Virginia Formation as a young differentiatedvolcanic arc to the south of the basin.

The Wisconsin magmatic terranes were located just to the south ofthe fold-and-thrust belt. The Pembine-Wausau terrane is comprisedof older (1889-1860 Ma) magmatic rocks that formed in an islandarc and/or back-arc basins above a south—dipping subduction zone(the Niagara fault zone or suture) and younger magmatic rocks(1845-1835 Ma) that formed above a north-dipping subduction zone,the Eau Pleine shear zone (Sims et al, 1989, 1993). Deep seismicprofiling indicates that both the Niagara fault and the Eau Pleineshear zone may penetrate the entire crust (Cannon et al, 1991). Anophiolite sequence in the Quinnesec Formation of northeasternWisconsin (Schulz, 1987) indicates the closure of an ocean basin.In the southerly Marshfield terrane, 1860 Ma rocks were depositedon Archean crust and amalgamation of the two terranes along theEau Pleine suture occurred at about 1840 Ma (Sims et al, 1993).Three episodes of post-orogenic magmatism followed, dated at 1835Ma, 1760 Ma, and the 1469+/- 28 Ma Wolf River batholith (Sims etal, 1989). Nd isotopic data of Barovich et al (1989) indicated themajor growth of new crust from the mantle.

The Penokean (Hudsonian) Orogen was part of the reassembly(Hoffman, 1988) of a new supercontinent that Williams et al (1991)

9

By removing t h e 60 kxu-wide Middle Proterozoic Midcontinent R i f t System from a regional geological map, thereby br inging Minnesota and Wisconsin-Michigan i n t o juxtaposit ion, t h e fold-and-thrust belts of t h e two areas become one continuous zone- Southwick and Morey (1991) co r re la t ed t h e Niagara f a u l t zone/suture ( W I ) and t h e Malmo d i scon t inu i ty (MN). Similar ly, t h e var ious t u r b i d i t e - s h a l e u n i t s (Thomson, Rove, Virginia, Tyler, Copps, and Michigamme Formations) then become one contiguous u n i t f i l l i n g t h e Animikie Basin- This, however, does not imply t h a t a l l of these formations are t h e same age, f o r t h e basin was migrating northward during depos i t ion of t h e s e u n i t s ; some sediment deposited i n t h e earlier s t a g e s of basin development was l i k e l y cannibalized and redeposi ted-

The Animikie bas in has been in te rp re ted as a northward-migrating foreland bas in t h a t developed in f r o n t of (on t h e nor th s i d e ) of t h e fold-and-thrust belt due t o t h e weighting down of t h e crust by t h i s folded and t h r u s t e d mountain range t h a t then became a major source of sediment f o r t h e basin ( e - g o , Hoffman, 1987; Southwick e t al , 1988; Barovich e t a l , 1989; Southwick and Morey, 1991; Ojakangas, 1994; Morey and Southwick, 1995)-

N d isotope data determined by Barovich et a1 (1989) i n d i c a t e t h a t some Michigamme samples had an Archean provenance t o t h e nor th and some had an Early Proterozoic provenance t o t h e south- P a l e m u r r e n t data f o r t h e Michigamme and t h e Tyler i n d i c a t e a northward t r a n s p o r t of sediment from t h e south side of t h e Animikie Basin and a southward t r anspor t of sediment from t h e nor th side of t h e basin as summarized i n Ojakangas (1994). H e d n g e t a1 (19951, on geochemical and i so top ic evidence, descr ibed t h e provenance of t h e Virg in ia Formation as a young d i f f e r e n t i a t e d volcanic arc t o t h e south of t h e basin-

The Wisconsin magmatic t e r ranes were loca ted j u s t t o t h e south of t h e fold-and-thrust belt- The Pembine-Wausau t e r r a n e is comprised of older (1889-1860 M a ) magmatic rocks t h a t formed i n an i s l a n d arc and/or back-arc bas ins above a south-dipping subduction zone ( t h e Niagara f a u l t zone o r su ture) and younger magmatic rocks (1845-1835 M a ) t h a t formed above a north-dipping subduction zone, t h e Eau P le ine shear zone ( S h et al , 1989, 1993)- Deep seismic p r o f i l i n g ind ica tes t h a t both t h e Niagara f a u l t and t h e Eau P le ine shea r zone may pene t ra te t h e e n t i r e c r u s t (Cannon et a l , 1991)- ~n o p h i o l i t e sequence i n t h e Quinnesec Formation of nor theas tern Wisconsin (Schulz, 1987) ind ica tes t h e c losure of an ocean bas in- I n t h e souther ly Marshfield te r rane , 1860 M a rocks were deposited on Archean crust and amalgamation of t h e two t e r r a n e s along t h e Eau P le ine s u t u r e occurred a t about 1840 M a ( S h e t al, 1993)- Three episodes of post-orogenic m a g m a t i s m followed, dated a t 1835 M a , 1760 ~ a , and t h e 1469+/- 28 Ma Wolf Mver b a t h o l i t h ( S h e t a l , 1989)- N d i s o t o p i c data of Barovich et a1 (1989) ind ica ted t h e major growth of new crust from t h e mantle-

The Penokean (Hudsonian) Orogen was p a r t of t h e reassembly (Hoffman, 1988) of a new supercontinent t h a t W i l l i a m s et a1 (1991)

dubbed Hudsonland. This supercontinent also included theFennoscandian Shield (as did the Archean supercontinent ofKenorland). The Svecokarelian orogeny of that shield is comparableto the Penokean Orogen in many aspects.

QUARTZ ITESThe Penokean Orogen and adjacent rock units continued to beweathered (Southwick and Mossler, 1984) and eroded for about 100Ma following the mountain-building culmination at about 1850 (?)Ma. Several quartzite units were deposited to the south of theorogen, including the Sioux Quartzite of Minnesota, South Dakota,and Iowa , Morey, 1984; Ojakangas and Weber, 1984; Southwicket al, 1986) and the Baraboo (Dott, 1983), Barron (Rozacky, 1987),Flambeau (Campbell, 1986), and McCaslin (Olson, 1984) Quartzitesof Wisconsin. Their ages and correlation have long been a problem(e.g., Brown, 1986; LaBerge et al., 1991; Chandler and Morey,1992; Ojakangas, 1993), especially because the Baraboo, Flambeau,and McCaslin are deformed whereas the Sioux and the Barron arerelatively horizontal. These "Baraboo Interval" quartzites posethe "Proterozoic red quartzite enigma" of Dott (1983). Dott (1983)and Van Schmus et al (1993) suggested that a plate suture existsbeneath the Paleozoic cover to the south, and that a southerlyterrane collided with the then-passive margin of the continentover a southerly dipping subduction zone. Chandler and Morey(1992) interpreted paleomagnetic data to indicate that there aretwo ages of quartzite bodies, one late Penokean and the otherpost-1760 Ma. Recent U-Pb detrital zircon ages of 1730—1710 Ma forthe Baraboo and the McCaslin and Pb/Pb ages on detrital zircons of1730—1850 for the Sioux, 1745—1880 for the Flainbeau, and 1750—1880for the Barron (Schneider et al, 1998) and similar ages fordetrital zircons in the Baraboo and McCaslin quartzites (Van Wyck,1995; Dott et al, 1997) can be interpreted to indicate that these"Baraboo Interval" quartzites may be correlative. Holm et al(1997) have located a 1630 Ma thermal front in northwest Wisconsinbased on cooling ages in Ar/Ar dates on mica; this front coincideswith the deformational boundary between the Barron and theFlambeau.

If one assumes that the quartzites are erosional remnants of alarger sheet of quartz sand locally >1500 m thick and depositedon the continental margin, a tremendously large and deeplyweathered quartz-bearing source terrane must be assumed. evenwithout the assumption of a single large basin, the volume ofquartz is enormous. Wheras a fluvial origin is most likely formost of the quartzites (e.g., Southwick et al, 1986), a marineorigin for the upper Sioux (Ojakangas and Weber, 1984) and forpart of the Baraboo (Dott, 1983) has been proposed.

REFERENCES

Barovich, K. 14., Patchett, P. J., Peterman, Z. E., and Sims, P. K., 1989, Ndisotopes and the origin of 1.9-1.7 Ga Penokean continental crust of the LakeSuperior region: Geological Society of america Bulletin, v. 101, p. 333—338.

10

dubbed Hudsonland. This supercontinent a l s o included t h e Fennoscandian Shield (as d id t h e Archean supercontinent of Kenorland). The Svecokarelian orogeny of t h a t s h i e l d is comparable t o t h e Penokean Orogen i n many aspects.

QUARTZITES The Penokean Orogen and adjacent rock u n i t s continued t o be weathered (Southwick and Mossier, 1984) and eroded f o r about 100 M a following t h e mountain-building culmination a t about 1850 ( ? ) M a . Severa l q u a r t z i t e u n i t s w e r e deposited t o t h e south of t h e orogen, including t h e Sioux Q u a r t z i t e of Minnesota, South Dakota, and Iowa (e.g., Morey, 1984; Ojakangas and W e b e r , 1984; Southwick et al , 1986) and t h e Baraboo (Dott, 1983), Barron (Rozacky, 1987), Flambeau (Campbell, 1986), and McCaslin (Olson, 1984) Q u a r t z i t e s of Wisconsin. Their ages and cor re la t ion have long been a problem (e.g., Brown, 1986; LaBerge et a l e , 1991; Chandler and Morey, 1992; Ojakangas, 1993), e spec ia l ly because t h e Baraboo, Flambeau, and McCaslin are deformed whereas t h e Sioux and t h e Barron are r e l a t i v e l y horizontal . These "Baraboo In terva lw q u a r t z i t e s pose t h e "Proterozoic red q u a r t z i t e enigmaw of Dott (1983). Dott (1983) and Van Schmus et a1 (1993) suggested t h a t a p l a t e s u t u r e e x i s t s beneath t h e Paleozoic cover t o t h e south, and t h a t a souther ly t e r r a n e c o l l i d e d with t h e then-passive margin of t h e cont inent over a souther ly dipping subduction zone. Chandler and Morey (1992) i n t e r p r e t e d paleomagnetic d a t a t o ind ica te t h a t t h e r e are two ages of q u a r t z i t e bodies, one late Penokean and t h e o t h e r post-1760 M a . Recent U-Pb d e t r i t a l z i rcon ages of 1730-1710 M a f o r t h e Baraboo and t h e McCaslin and Pb/Pb ages on detrital z i rcons of 1730-1850 f o r t h e Sioux, 1745-1880 f o r t h e Flambeau, and 1750-1880 f o r t h e Barron (Schneider et a l , 1998) and similar ages f o r d e t r i t a l z i rcons i n t h e Baraboo and McCaslin q u a r t z i t e s (Van Wyck, 1995; D o t t e t a l , 1997) can be in terpre ted to i n d i c a t e t h a t t h e s e "Baraboo In te rva l f f q u a r t z i t e s may be corre la t ive . H o l m et a 1 (1997) have loca ted a 1630 M a thermal f r o n t i n northwest Wisconsin based on cool ing ages i n A r / A r da tes on mica; t h i s f r o n t coincides with t h e deformational boundary between t h e Barron and t h e Flambeau . I f one assumes t h a t t h e q u a r t z i t e s are erosional remnants of a l a r g e r s h e e t of quar tz sand l o c a l l y >I500 m t h i c k and deposi ted on t h e con t inen ta l margin, a tremendously l a r g e and deeply weathered quartz-bearing source t e r r a n e must be assumed. Even without t h e assumption of a s i n g l e l a r g e basin, t h e volume of quar tz i s enormous. Wheras a f l u v i a l o r i g i n is most l i k e l y f o r most of t h e q u a r t z i t e s (e.g., Southwick et al , 1986), a marine o r i g i n f o r t h e upper Sioux (Ojakangas and Weber, 1984) and f o r p a r t of t h e Baraboo (Dot t , 1983) has been proposed.

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Bekker, A., and Karhu, J. A., 1996, Study of carbon isotope ratios incarbonates of the Paleoproterozoic Snowy Pass Supergroup, WY, and itsapplication for correlation with the Chocolay Group, MI, and the BuronianSupergroup, ON (Abs.): Institute on Lake Superior Geology Proceedings, 42ndAnnual Meeting, Cable, WI, v. 42, P. 4—5.

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Campbell, F. K., 1986, Petrology and sedimentation of the Flainbeau Quartzite:Geoscience Wisconsin, v. 10, p. 38—45.

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Chandler, V. W., Norstrand, E., and Anderson, S., 1984, Shaded—reliefaeromagnetic anomaly map of northeastern and east—central Minnesota:Minnesota Geological Survey Miscellaneous Map M—53, scale 1:1,000,000.

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11

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Carrigan, W. J. and Cameron, E. M., 1991, Petrological and stable isotope studies of carbonate and sulfide minerals from the Gunflint Formation, Ontario: evidence for the origin of early Proterozoic iron-formation: Precambrian Research, v. 52, p. 347-380.

Chandler, V. W., 1991, Aeromagnetic anomaly map of Minnesota: Minnesota Geological Survey State Map Series Map S-17, Scale 1:500,000.

Chandler, V. W., and Morey, G. B., 1992, Paleomagnetism of the Early, Proterozoic Sioux Quartzite, southwestern Minnesota--Implications for correlating quartzites of the Baraboo interval: U.S. Geological Survey Bulletin 1904-N, 14 p.

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12

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H d n g , S., McLennan, S. M., Hanson, G. N., Krogstad, E. J., and Mezger, KO, 1990, Pb isotope systematics in quartz (Abs.): EOS, v. 71, no. 17, p. 654-655.

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Lucente, M. E., and Morey, G. B. , 1983, Stratigraphy and sedimentology of the lower Proterozoic Virginia Formation, northern Minnesota: Minnesota Geological Survey Report of Investigations 28, 28 p.

Marmo, J.. S., and Ojakangas, R. W., 1984, Lower Proterozoic glaciogenic deposits, eastern Finland: Bulletin Geological Society of America, v. 95, p. 1055-1062.

Morey, G. B., 1984, Sedimentology of the Sioux Quartzite in the Fulda basin, Pipestone County, Southwestern Minnesota: in Southwick, D. L. (ed.), Shorter Contributions to the Geology of the Sioux Quartzite (Early Proterozoic), Southwestern Minnesota: Minnesota Geological Survey Report of Investigations 32, p. 59-74.

, 1992, Chemical composition of the Eastern Biwabik iron-formation (Early Proterozoic), Mesabi Range, Minnesota: Economic Geology, v. 87. p. 1649-1658.

, 1996, Continental margin assemblage: in Sims, P. K., and Carter, L. M. H.(eds.), Archean and Proterozoic geology of the Lake Superior region, U.S.A., 1993: U.S. Geological Survey Professional Paper 1556, p. 30-44.

Morey, G. B., and Southwick, D. L., 1995, Allostratigraphic relationships of Early Proterozoic iron-formations in the Lake superior region: Economic Geology, V. 90, p. 1983-1993.

Morey, G. B., and Van Schus, 1988, Correlation of Precambrian rocks of the Lake Superior region, United States: U. S. Geological Survey Professional Paper 1241-F, 31 p.

Ojakangas, G. W., 1996, Cyclic tidal laminations in the Early Proterozoic Pokegama Formation: Digital image analysis and cmputer modeling (Abs.): 42nd Institute on Lake Superior Geology Proceedings v. 42, Cable, WI, p. 44- 45.

Ojakangas, R. W., 1983, Tidal deposits in the early Proterozoic basin of the Lake Superior region--The Palms and the Pokegama Formations: Evidence for subtidal-shelf deposition of Superior-type banded iron-formation: in Medaris, L.G., Jr., (ed.), Early Proterozoic Geology of the Great Lakes Region: Geological Society of America Memoir 160, p. 49-66.

, 1984, Basal Lower Proterozoic glaciogenic formations, Marquette Supergroup, Upper Peninsula, Michigan (Abs.): 30th Institute of Lake Superior Geology Proceedings, Wausau, WI, v. 30, p. 43.

, 1985, Evidence for Early Proterozoic glaciation: the dropstone unit- diamictite association: Geological Survey of Finland Bull., 331, p. 51-72.

, 1988, Glaciation: an uncomnon Nmega-eventn as a key to intracontinental and intercontinental correlation of Early Proterozoic basin fill, North

American and Baltic cratons: in Kleinspehn, K. L., and Paoia, C. (eds.), NewPerspectives in Basin Analysis, Springer—Verlag, p. 431—444.

____

1993, Quartzites: in Sims, P. K. (ed.), The Lake Superior region andTrans—Hudson orogen (Sims and 15 others): in Reed, J. C. Jr. and 6 others(eds.), The Geology of North America, Geological Society of America v C-2,Decade of North American Geology Series, p. 67-69.

_____,

1994, Sedimentology and provenance of the Early Proterozoic MichiganuneFormation and Goodrich Quartzite, northern Michigan--Regional stratigraphicimplications and suggested correlations: U. S. Geological Survey Bulletin1904—R, 31 p.

Ojakangas, R. W., Heiskanen, K., and Marmo, J. S., 1991, Early Proterozoicgiaciogenic deposits: A North America-Baltic connection? in Ojakangas, R. W.(ed.), Precambrian Geology of the Southern Canadian Shield and the EasternBaltic Shield: Minnesota Geological Survey Information Circular 34, p. 83—91.

Ojakangas, R. W. and Weber, R. E., 1984, Petrography and paleocurrents of theLower Proterozoic Sioux Quartzite, Minnesota and South Dakota: in Southwick,D. L. (ed.), Shorter Contributions of the Geology of the Sioux Quartzite(Early Proterozoic), Southwestern Minnesota: Minnesota Geological SurveyReport of Investigations 32, p. 1-15.

Olson, J. M., 1984, The geology of the lower Proterozoic McCasiin Formation,northeastern Wisconsin: Geoscience Wisconsin, v. 9, p. 1—48.

Roscoe, S. M., and Card, K. D., 1993, The reappearance of the Huronian inWyoming: rifting and drifting of ancient continents: Canadian Journal ofEarth Sciences, v. 30, p. 2475—2480.

Rozacky, W. J., 1987, The petrology and sedimentation of the lower ProterozoicBarron Quartzite, northwestern Wisconsin: University of Minnesota—DuluthM.S. Thesis, 94 p.

Schmidt, R. G., 1980, The Marquette Range Supergroup in the Gogebic irondistrict, Michigan and Wisconsin: U. S. Geological Survey Bulletin 1460, 96 p.

Schneider, D., Holm, D.K., and Coath, C., 1998, Initial results of ionmicroprobe U-Pb dating of detrital zircons from Proterozoic red quartzites innorthwest Wisconsin and southwest Minnesota (Abs.): Geological Society ofAmerica regional meeting, Columbus, OH.

Schulz, K. J., 1987, An Early Proterozoic ophiolite in the Penokean Orogen(Abs.): Geological Association of Canada Program and Abstracts, v. 12, p. 87.

Simonson, B. M., and Sassier, S. W., 1996, Was the deposition of largePrecambrian iron—formations linked to major marine transgressions? Journal ofGeology, v. 104, p. 665—676.

Sims, P. K., and Carter, L. K. H., (eds.), 1996, Archean and ProterozoicGeology of the Lake Superior region, U.S.A., 1993: U.S. Geological SurveyProfessional Paper 1556, 115 p.

15

American and Baltic cratons: in Kleinspehn, K. L., and Paola, C. (eds.), New Perspectives in Basin Analysis, Springer-Verlag, p. 431-444.

, 1993, Quartzites:.in Sims, P. K. (ed.), The Lake Superior region and Trans-Hudson orogen (Sims and 15 others): in Reed, J. C. Jr. and 6 others (eds.), The Geology of North America, Geological Society of America v. C-2, Decade of North American Geology Series, p. 67-69.

, 1994, Sedimentology and provenance of the Early Proterozoic Michigamme Formation and Goodrich Quartzite, northern Michigan--Regional stratigraphic implications and suggested correlations: U. S. Geological Survey Bulletin 1904-R, 31 p.

Ojakangas, R. W., Heiskanen, K., and Marmo, J. S., 1991, Early Proterozoic glaciogenic deposits: A North America-Baltic connection? in Ojakangas, R. W. (ed.), Precambrian Geology of the Southern Canadian Shield and the Eastern Baltic Shield: Minnesota Geological Survey Information Circular 34, p. 83-91.

Ojakangas, R. W. and Weber, R. E., 1984, Petrography and paleocurrents of the Lower Proterozoic Sioux Quartzite, Minnesota and South Dakota: in Southwick, D. L. (ed.), Shorter Contributions of the Geology of the Sioux Quartzite (Early Proterozoic), Southwestern Minnesota: Minnesota Geological Survey Report of Investigations.32, p. 1-15.

Olson, J. M., 1984, The geology of the lower Proterozoic McCaslin Formation, northeastern Wisconsin: Geoscience Wisconsin, v. 9, p. 1-48.

Roscoe, S. M., and Card, K. D., 1993, The reappearance of the Huronian in waning: rifting and drifting of ancient continents: Canadian Journal of Earth Sciences, v. 30, p. 2475-2480.

Rozacky, W. J., 1987, The petrology and sedimentation of the lower Proterozoic Barron Quartzite, northwestern Wisconsin: University of Minnesota-Duluth M.S. Thesis, 94 p.

Schmidt, R. G., 1980, The Marquette Range Supergroup in the Gogebic iron district, Michigan and Wisconsin: U. S. Geological Survey Bulletin 1460, 96 p.

Schneider, D., Holm, D.K., and Coath, C., 1998, Initial results of ion microprobe U-Pb dating of detrital zircons from Proterozoic red quartzites in northwest Wisconsin and southwest Minnesota (Abs.): Geological Society of America regional meeting, Columbus, OH.

Schulz, K. J., 1987, An Early Proterozoic ophiolite in the Penokean Orogen (Abs.): Geological Association of Canada Program and Abstracts, v. 12, p. 87.

Simonson, B. M., and Hassler, S. W., 1996, Was the deposition of large Precambrian iron-formations linked to major marine transgressi.ons? Journal of Geology, V. 104, p. 665-676.

Sims, P. K., and Carter, L. M. H., (eds.), 1996, Archean and Proterozoic Geology of the Lake Superior region, U.S.A., 1993: U.S. Geological Survey Professional Paper 1556, 115 p.

Sims, P.K. (ed.) and fifteen others, 1993, The Lake Superior region and Trana-Hudson orogen: in Reed, 3. C. Jr. and six others (eds.), Precambrian:Conterminous U.S., The Geology of North America v. C—2, Geological Society ofAmerica Decade of North American Geology Project series, p. 11—120.

Sims, P. K., Schulz, K. 3., DeWitt, E., and Brasaemle, B., 1993, Petrographyand geochemistry of Early Proterozoic granitoid rocks in Wisconsin magmaticterranes of Penokean Orogen, northern Wisconsin: U.S. Geological SurveyBulletin 1904—3, 31 p.

Sims, P. K., Van Schmus, W. R., Schulz, K. 3., and Peterman, Z. E., 1989,Tectono—stratigraphic evolution of the Early Proterozoic Wisconsin magmaticterranes of the Penokean Orogen: Canadian Journal of Earth Sciences, v. 26, p.2145—2158.

Southwick, D. L., and Day, W. C., 1983, Geology and petrology of Proterozoicmafic dikes, north—central Minnesota and western Ontario: Canadian Journal ofEarth Sciences, v. 20, p. 622—638.

Southwick, D. L., and Morey, G. B., 1991, Tectonic imbrication and foredeepdevelopment in the Penokean Orogen, east—central Minnesota——An interpretationbased on regional geophysics and the results of test—drilling: U. S.Geological Survey Bulletin 1904-C, 17 p.

Southwick, D. L., Morey, G. B., and McSwiggen, P. L., 1988, Geologic map(scale 1:250,000) of the Penokean orogen, central and eastern Minnesota, andaccompanying Minnesota Geological Survey Report of Investigations 37, 25 p.

Southwick, D. L.. Morey, G. B., and Mossier, 3. H., 1986, Fluvial origin ofthe lower Proterozoic Sioux Quartzite, southwestern Minnesota: GeologicalSociety of America Bulletin v. 97, p. 1432—1441.

Southwick, D. L., and Moasler, 3. B., 1984, The Sioux Quartzite and subjacentregolith in the Cottonwood County basin, Minnesota: in Southwick, D. L.,(ed.), Shorter Contributions to the Geology of the Sioux Quartzite (EarlyProterozoic), Southwestern Minnesota: Minnesota Geological Survey Report ofInvestigations 32, p. 17—44.

Stille, P. and Clauer, N., 1986, Sin-Nd isochron-age and provenance of theargillites of the Gunf lint Iron Formation in Ontario, Canada: Geochimica etCosmochemica Acta, v. 50, p. 1141—1146.

Van Schmus, W. R., Bickford, N.E., and Condie, K. C., 1993, Early Proterozoiccrustal evolution: in Reed, J.C., Jr., and six others, (eda.), Precambrian:Conterminous U.S., Geology of North America, v. C—2, p. 270-281.

Van Wyck, N., 1995, Major and trace element, conunon Pb, Sm—Nd, and zircongeochronology constraints on petrogenesis and tectonic Betting of pre— andEarly Proterozoic rocks in Wisconsin: Ph. D. thesis, University of Wisconsin—Madison, p. 47—280.

Williams, G. E., and Schmidt, P. W., 1997, Paleomagnetiam of thePaleoproterozoic Gowganda and Lorrain formations, Ontario: low paleolatitudefor Buronian glaciation: Earth and Planetary Science Letters, v. 153, p. 157-

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Sims, P.K. (ed.) and fifteen others, 1993, The Lake Superior region and Trane- Hud~on orogen: in Reed, J. C. Jr. and six others (eds.), Precambrian: conterminous U.s.8 The Geology of North America v. C-2, Geological Society of America Decade of North American Geology Project series, p. 11-120.

Sims, P. K., Schulz, K. J.8 DeWitt8 E., and Brasaemle, B., 1993, Petrography and geochemistry of Early Proterozoic granitoid rocks in Wisconsin magmatic terranes of Penokean Orogen, northern Wisconsin: U.S. Geological Survey Bulletin 1904-J8 31 p.

Sims8 P. K., Van Schmus8 w. R., Schulz, K. J., and Peterman# Z. E., 1989, Tectono-stratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Pen0kea.n Orogen: Canadian Journal of Earth Sciences, v. 26, p. 2145-2158.

Southwick, D. L., and Day, W. C., 1983, Geology and petrology of Proterozoic mafic dikes, north-central Minnesota and western Ontario: Canadian Journal of Earth Sciences, v. 20, p. 622-638.

Southwick8 D. L., and Morey, G. B., 19918 Tectonic imbrication and foredeep development in the Penokean Orogen, east-central Minnesota--An interpretation based on regional geophysics and the results of test-drilling: U. S. Geological Survey Bulletin 1904-C8 17 p.

Southwick8 D. L., Morey, G. B., and McSwiggen, P. L., 1988# Geologic map (scale 1:2508000) of the Penokean orogen, central and eastern Minnesota8 and accompanying MinneS0ta Geological Survey Report of Investigations 3T8 25 p.

Southwick, D. L.. Morey, G. Be8 and Mossler, J. He, 1986, Fluvial origin of the lower Proterozoic Sioux Quartzite, southwestern Minnesota: Geological Society of America Bulletin v. 97, p. 1432-1441.

Southwick, D. L.8 and MoSSler, J. He8 1984, The Sioux Quartzite and subjacent regolith in the Cottonwood County basin8 Minnesota: in Southwick, D. L., (ed.)# Shorter Contributions to the Geology of the Sioux Quartzite (Early Proterozoic), Southwestern Minnesota: Minnesota Geological Survey Report of Inveatigations 32, p. 17-44.

Stille, P- and Clauer, N., 1986, Sm-Nd isochron-age and provenance of the argillites of the Gunflint Iron Formation in Ontario, Canada: Geochimica et Comochemica Acta, v. 50, p. 1141-1146.

Van Schmus8 W- R., Bickford8 M.E., and Condiet K. CO8 19938 Early Proterozoic crustal evolution: in Reed, J-C., Jr., and six others, (eds.), Precambrian: Conterminous U.Se8 Geology of North America, v. C-2, p. 270-281-

Van W c k 8 Na8 1995, Major and trace element, common Pb, Sm-Nd, and zircon geochronology constraints on petrogenesis and tectonic setting of pre- and Early Proterozoic rocks in Wisconsin: Ph. D. thesis, University of Wit3consin- Madison, p. 47-280.

Willims, G. E., and Schmidt, P. W., 1997, Paleoxnagnetism of the Paleoproterozoic Gowganda and Lorrain formations8 Ontario: low paleolatitude for Huronian glaciation: Earth and Planetary Science Letters8 v. 1538 p. 157-

169.

Williams, H., Hoffman, P.F., Lewry, J. F., Monger, J. W. H, and Rivers, T.,1991, Anatomy of North America: thematic geologic portrayals of thecontinent: in Hilde, T. W. C., and Carison, R. L. (eds.), Silver Anniversaryof Plate Tectonics: Tectonophysics, v. 187, P. 117—134.

Winter, B. L., and Knauth, L. P., 1992, Stable iBotope geochemistry of chertaand carbonates from the 2.0 Ga Gunf lint Iron Formation: implications for thedepositional setting, and the effects of diageneBis and metamorphism:Precambrian Research, v. 59, p. 283-313.

Young, G. M., 1970, An extensive early Proterozoic glaciation in Northamerica:? Palaeography, Palaeoclimatology, Palaeoecology, v. 7, p. 85—101.

_____

1983, Tectono—aedimentary history of early Proterozoic rocks of thenorthern Great Lakes region: in Medaris, L.G., Jr., (ed.), Early ProterozoicGeology of the Great Lakes Region: Geological Society of inerica Memoir 160,p. 15—32

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Williams, H., Hoffman, P.F., Lewry, J. F., Monger, J. W. H., and Rivers, T., 1991, Anatomy of North America: thematic geologic portrayals of the continent: in Hilde, T. W. C., and Carlson, R. L. (eds.), Silver Anniversary of Plate Tectonics: Tectonophysics, v. 187, p. 117-134.

Winter, B. L., and Knauth, L. P., 1992, Stable isotope geochemistry of cherts and carbonates from the 2.0 Ga Gunflint Iron Formation: implications for the depositional setting, and the effects of diagenesis and metamorphism: Precambrian Research, v. 59, p. 283-313.

Young, G. M., 1970, An extensive early Proterozoic glaciation in North America:? Palaeography, Palaeoclimatology, Palaeoecology, v. 7, p. 85-101.

, 1983, Tectono-sedimentary history of early Proterozoic rocks of the northern Great Lakes region: in Medaris, L.G., Jr., (ed.), Early Proterozoic Geology of the Great Lakes Region: Geological Society of America Memoir 160, p.15-32.

UNDERSTANDING THE MIDDLE PROTEROZOIC HISTORY OF THE LAKE SUPERIORREGION: WHAT'S NEW? WHAT'S NEXT?

W.F. Cannon, US Geological Survey, Reston, VA

Middle Proterozoic time encompasses 700 million years (1600-900 Ma) of geologic history. In theLake Superior region only a small part of that time is recorded in the rock record. At about 1470 Malarge anorogenic granitic plutons were emplaced in northern Wisconsin. Between 1108 and about1060 Ma the Midcontinent rift, a 2200 km-long volcanic and subsequent sedimentary basin, formedand was structurally modified. A brief summary of the current understanding of these two events ispresented below emphasizing findings during the past 15 years. An extended list of references for theMidcontinent Rift presents some of the principal publications since the detailed papers in the 1982Geological Society of America Memoir 156, "Geology and Tectonics of the Lake Superior Basin".For anorogenic plutons, references include original research from the 1970's and more recentsummary papers.

Anorogenic plutons A suite of large granitic plutons was emplaced in a broad belt spanning most ofthe North American continent from Labrador to Arizona between about 1500 and 1450 Ma. Theplutons intruded older continental crust during a period generally devoid of major orogeny, hence thename anorogenic plutons. That suite of plutons is represented in the Lake Superior region by theWolf River batholith and Waussau and Stettin plutons in Wisconsin. These plutons were studiedextensively in the 1970's but have received relatively little attention since, with the exception ofbeing discussed in regional summary papers. The Wolf River batholith, with an exposed area ofabout 10,000 km2, and the smaller plutons were emplaced at about 1470 Ma. The Wisconsin plutonsshare characteristics with the other members of the suite in being emplaced at shallow depths in EarlyProterozoic crust and consisting of large volumes of granite with rapakivi affinities, and lesservolumes of more mafic rocks, largely mangerite and anorthosite. The granites of the suite areinterpreted to have formed by relatively low percentage of partial melting (20±10%) of Proterozoicgranitic rocks in the lower crust. Anorthosite may be derived from basaltic magma produced, in part,by partial melting of mantle lithosphere. Studies of the Wolf River batholith indicate that mangeritemagma was derived at a depth of 21-32 km and a temperature of 950±90° C. Crystallization of thegranite now exposed occurred at temperatures of 65 0-840°C and at shallow depths of 2-4 km. Thecause of widespread melting of lower crust in a belt of continental dimensions remains obscure.

Midcontinent rift The Midcontinent rift extends in an arc from Kansas, through the Lake Superiorbasin, into southern Michigan. The physical character of the rift (rock types, geometry, structure,approximate age) have been well known for many decades. Research in the past 15 years has addedcritical information to allow the kinematic and dynamic aspects of evolution of the rift to bedeciphered. Deep crustal seismic surveys, precise age determinations, petrochemical research, andexperimental and theoretical studies of mantle and lithospheric processes can be integrated tounderstand the rift in the broad context of the evolution of the North American continent and global-scale processes.

Seismic surveys have shown that the rift is very deep, in places comprising the entire crustalthickness. Subaerial basalt flows form the lower part of the rift-fill and are as much as 20 km thickalong the rift axis. Continental clastic sedimentary rocks overlie the basalt and are as much as 10 kmthick. Basalt eruption and coeval rift subsidence was rapid and of relatively short duration. Thevolcanic phase of the rift lasted about 14 my (1108-1094 Ma). Rates of volcanism and subsidencewere not entirely uniform and appear to have been greatest near the close of the volcanic phase.Subsidence of volcanic basins, as inferred from seismic profiles, appears to have been by acombination of normal faulting and broad flexure. Large volumes of mafic magma were also

19

UNDERSTANDING THE MIDDLE PROTEROZOIC HISTORY OF THE LAKE SUPERIOR REGION: WHAT'S NEW? WHAT'S NEXT?

W.F. Cannon, US Geological Survey, Reston, VA

Middle Proterozoic time encompasses 700 million years (1600-900 Ma) of geologic history. In the Lake Superior region only a small part of that time is recorded in the rock record. At about 1470 Ma large anorogenic granitic plutons were emplaced in northern Wisconsin. Between 1 108 and about 1060 Ma the Midcontinent rift, a 2200 km-long volcanic and subsequent sedimentary basin, formed and was structurally modified. A brief summary of the current understanding of these two events is presented below emphasizing findings during the past 15 years. An extended list of references for the Midcontinent Rift presents some of the principal publications since the detailed papers in the 1982 Geological Society of America Memoir 156, "Geology and Tectonics of the Lake Superior Basin". For anorogenic plutons, references include original research from the 1970's and more recent summary papers.

Anorogenic plutons A suite of large granitic plutons was emplaced in a broad belt spanning most of the North American continent from Labrador to Arizona between about 1500 and 1450 Ma. The plutons intruded older continental crust during a period generally devoid of major orogeny, hence the name anorogenic plutons. That suite of plutons is represented in the Lake Superior region by the Wolf River batholith and Waussau and Stettin plutons in Wisconsin. These plutons were studied extensively in the 1970's but have received relatively little attention since, with the exception of being discussed in regional summary papers. The Wolf River batholith, with an exposed area of about 10,000 km2, and the smaller plutons were emplaced at about 1470 Ma. The Wisconsin plutons share characteristics with the other members of the suite in being emplaced at shallow depths in Early Proterozoic crust and consisting of large volumes of granite with rapakivi affinities, and lesser volumes of more mafic rocks, largely mangerite and anorthosite. The granites of the suite are interpreted to have formed by relatively low percentage of partial melting (20±10% of Proterozoic granitic rocks in the lower crust. Anorthosite may be derived from basaltic magma produced, in part, by partial melting of mantle lithosphere. Studies of the Wolf River batholith indicate that mangerite magma was derived at a depth of 21-32 km and a temperature of 950±90 C. Crystallization of the granite now exposed occurred at temperatures of 650-840° and at shallow depths of 2-4 km. The cause of widespread melting of lower crust in a belt of continental dimensions remains obscure.

Midcontinent rift The Midcontinent rift extends in an arc from Kansas, through the Lake Superior basin, into southern Michigan. The physical character of the rift (rock types, geometry, structure, approximate age) have been well known for many decades. Research in the past 15 years has added critical information to allow the kinematic and dynamic aspects of evolution of the rift to be deciphered. Deep crustal seismic surveys, precise age determinations, petrochemical research, and experimental and theoretical studies of mantle and lithospheric processes can be integrated to understand the rift in the broad context of the evolution of the North American continent and global- scale processes.

Seismic surveys have shown that the rift is very deep, in places comprising the entire crustal thickness. Subaerial basalt flows form the lower part of the rift-fill and are as much as 20 km thick along the rift axis. Continental clastic sedimentary rocks overlie the basalt and are as much as 10 km thick. Basalt eruption and coeval rift subsidence was rapid and of relatively short duration. The volcanic phase of the rift lasted about 14 my (1 108-1094 Ma). Rates of volcanism and subsidence were not entirely uniform and appear to have been greatest near the close of the volcanic phase. Subsidence of volcanic basins, as inferred from seismic profiles, appears to have been by a combination of normal faulting and broad flexure. Large volumes of mafic magma were also

emplaced both within the crust and as crustal underplating during this same period. Basalt eruptionceased rather abruptly at about 1094 Ma and subsidence rates declined correspondingly. Furtherfilling of the rift was with continental sedimentary rocks, mostly red sandstone, and minor reducedlacustrine rocks. The duration of sedimentation is not well constrained but most likely continued,along with subsidence, at declining rates for at least several tens of millions of years. Duringsedimentation, inversion of the central portions of the rift took place along a set of major reversefaults. These events combined to form the present geometry of the rift. A central horst contains theoriginally deeper central parts of the rift, including thick sections of volcanic rocks. Flanking basinscontain wedges of thinner volcanic sequences and overlying sedimentary rocks.

The great volume of basaltic rocks and related intrusive rocks in the rift, combined with limitedamounts of lithoshperic extension imply that the mantle was anomalously hot during rifling; probablyabout 200°C hotter than present day asthenosphere. Petrochemical studies indicate that much of themagma was generated by partial melting of primitive, enriched mantle. Together, these featurespoint to the existence of a mantle plume beneath the rift, and more specifically, suggest that the riflingand volcanism was a consequence of the initiation of a new plume and the arrival of a plume head atthe base of the lithosphere. Modelling and numeric simulation of plume behavior shows that when anew plume forms and rises from depth, it generates a large mushroom-shaped head of hotasthenosphere. The head grows as it rises slowly and is fed by its own narrower but faster rising stem.The arrival of a plume head at the base of the lithosphere and at depths where adiabatic partialmelting occurs creates a short period of intense magma generation. Under suitable stress conditionsin the lithosphere, the plume can also initiate extension and rifting. The arrival of a new mantleplume at the base of the lithosphere at about 1108 Ma provides the most coherent explanation for theorigin of the rift and its great volume of igneous rocks.

Rift sedimentation occurred in basins somewhat broader than the volcanic basin. The sedimentarybasins do not appear to be strongly controlled by faulting. Rather, they appear as smooth flexuraldepressions on seismic reflection sections. Subsidence was most likely a result of cooling of thelithosphere as the initial plume head spread and cooled conductively. Sediments were initiallyderived dominantly from volcanic rocks in the rift. As sedimentation progressed, however, more olderterranes outside of the rift were tapped as a sediment source along with continued erosion of riftvolcanic rocks.

Inversion of the central portion of the rift occurred during the later stages of sedimentation. A set ofhigh angle reverse faults and thrusts, in part reactivated normal faults, produced displacements of asmuch as several tens of kilometers. The time of faulting is imprecisely known, but most movementmust have been in the interval 1080-1040 Ma.

Viewed on a more regional scale, the Midcontinent rift has been historically somewhat of an enigmain that it is a major extensional feature which formed immediately west of the seemingly coevalGrenville orogen. In recent years, however, precise radiometric dating shows that the short interval(1108-1094 Ma) of extension in the Midcontinent rift appears to coincide with an interval of tectonicquiescence, or possibly extension, in the multi-phase Grenvillian orogeny. Thus, a mantle plumearriving at 1108 Ma could have begun a period of extension during this Grenvillian quiescent period.At about 1090 Ma compression in the Grenville Province was renewed and a period of northwest-directed thrusting, the Ottawan orogeny, continued for tens of millions of years. The onset ofthrusting in the Grenville Province coincides remarkably closely with the end of extension in theMideontinent rift and the onset of compressive deformation.

It appears then that development of the Midcontinent rift marks the arrival of a new plume head at thebase in the lithosphere of the Lake Superior region at about 1108 Ma. Melting of the plume head and

20

emplaced both within the crust and as crustal underplating during this same period. Basalt eruption ceased rather abruptly at about 1094 Ma and subsidence rates declined correspondingly. Further filling of the rift was with continental sedimentary rocks, mostly red sandstone, and minor reduced lacustrine rocks. The duration of sedimentation is not well constrained but most likely continued, along with subsidence, at declining rates for at least several tens of millions of years. During sedimentation, inversion of the central portions of the rift took place along a set of major reverse faults. These events combined to form the present geometry of the rift. A central horst contains the originally deeper central parts of the rift, including thick sections of volcanic rocks. Flanking basins contain wedges of thinner volcanic sequences and overlying sedimentary rocks.

The great volume of basaltic rocks and related intrusive rocks in the rift, combined with limited amounts of lithoshperic extension imply that the mantle was anomalously hot during rifting; probably about 200° hotter than present day asthenosphere. Petrochemical studies indicate that much of the magma was generated by partial melting of primitive, enriched mantle . Together, these features point to the existence of a mantle plume beneath the rift, and more specifically, suggest that the rifting and volcanism was a consequence of the initiation of a new plume and the arrival of a plume head at the base of the lithosphere. Modelling and numeric simulation of plume behavior shows that when a new plume forms and rises from depth, it generates a large mushroom-shaped head of hot asthenosphere. The head grows as it rises slowly and is fed by its own narrower but faster rising stem. The arrival of a plume head at the base of the lithosphere and at depths where adiabatic partial melting occurs creates a short period of intense magma generation. Under suitable stress conditions in the lithosphere, the plume can also initiate extension and rifting. The arrival of a new mantle plume at the base of the lithosphere at about 1 108 Ma provides the most coherent explanation for the origin of the rift and its great volume of igneous rocks.

Rift sedimentation occurred in basins somewhat broader than the volcanic basin. The sedimentary basins do not appear to be strongly controlled by faulting. Rather, they appear as smooth flexural depressions on seismic reflection sections. Subsidence was most likely a result of cooling of the lithosphere as the initial plume head spread and cooled conductively. Sediments were initially derived dominantly from volcanic rocks in the rift. As sedimentation progressed, however, more older terranes outside of the rift were tapped as a sediment source along with continued erosion of rift volcanic rocks.

Inversion of the central portion of the rift occurred during the later stages of sedimentation. A set of high angle reverse faults and thrusts, in part reactivated normal faults, produced displacements of as much as several tens of kilometers. The time of faulting is imprecisely known, but most movement must have been in the interval 1080- 1040 Ma.

Viewed on a more regional scale, the Midcontinent rift has been historically somewhat of an enigma in that it is a major extensional feature which formed immediately west of the seemingly coeval Grenville orogen. In recent years, however, precise radiometric dating shows that the short interval (1 108-1 094 Ma) of extension in the Midcontinent rift appears to coincide with an interval of tectonic quiescence, or possibly extension, in the multi-phase Grenvillian orogeny. Thus, a mantle plume arriving at 1108 Ma could have begun a period of extension during this Grenvillian quiescent period. At about 1090 Ma compression in the Grenville Province was renewed and a period of northwest- directed thrusting, the Ottawan orogeny, continued for tens of millions of years. The onset of thrusting in the Grenville Province coincides remarkably closely with the end of extension in the Midcontinent rift and the onset of compressive deformation.

It appears then that development of the Midcontinent rift marks the arrival of a new plume head at the base in the lithosphere of the Lake Superior region at about 1108 Ma. Melting of the plume head and

stresses generated by lateral spread of the plume caused the intensely volcanic rift. Under somecircumstances this event might have led to continental breakup. In this case, however, initiation ofthe Ottawan orogeny in the adjacent Grenville Province, transmitted northwest-southeast directedstresses into the Lake Superior region. This newly applied stress field not only terminated riftextension, but initiated a period of compressive deformation and rift inversion.

The processes that formed the rift were also important in forming the major mineral deposits forwhich the rift is well known. The vast new igneous rocks were fertile starting material for metal-concentrating processes. Some concentration occurred early, during fractional crystallization ofmagma. The very large Cu-Ni sulfide deposits of the Duluth complex and other intrusions formed atthis stage, as did concentrations of platinum group elements. The rifting and related processes in themantle also provided a large heat engine that drove hydrothermal circulation. Those hydrothermalsystems formed the Keweenaw native copper district, the White Pine and related sediment-hostedcopper deposits, and polymetallic vein deposits. The heat generated by the rift also caused regionalmetamorphism that converted Early Proterozoic iron-formations from non-magnetic to magnetictaconite in places such as the Mesabi Range and Gogebic Range.

Much progress has been made in the past 15 years in understanding fundamental causative factors ofthe Midcontinent rift. The data gathered in that period lay the foundation for continued refinementsof knowledge of this important structure. Research continues on refining knowledge of the age ofrifting events, internal stratigraphy of rift units, the nature of plume-lithosphere interaction, andpatterns of regional metamorphism and alteration. Advances in the detail and precision ofinformation on the geometry, age, and composition of the rift from recent and continuing research,combined with modem computational techniques, allows a new phase of quantitative numericalresearch on rift evolution to be undertaken. Examples of this new phase of research are currentstudies of the transient thermal history of the rift, and the paleohydrology of the rift as it related toformation of mineral deposits.

21

stresses generated by lateral spread of the plume caused the intensely volcanic rift. Under some circumstances this event might have led to continental breakup. In this case, however, initiation of the Ottawan orogeny in the adjacent Grenville Province, transmitted northwest-southeast directed stresses into the Lake Superior region. This newly applied stress field not only terminated rift extension, but initiated a period of compressive deformation and rift inversion.

The processes that formed the rift were also important in forming the major mineral deposits for which the rift is well known. The vast new igneous rocks were fertile starting material for metal- concentrating processes. Some concentration occurred early, during fractional crystallization of magma. The very large Cu-Ni sulfide deposits of the Duluth complex and other intrusions formed at this stage, as did concentrations of platinum group elements. The rifting and related processes in the mantle also provided a large heat engine that drove hydrothermal circulation. Those hydrothermal systems formed the Keweenaw native copper district, the White Pine and related sediment-hosted copper deposits, and polyrnetallic vein deposits. The heat generated by the rift also caused regional metamorphism that converted Early Proterozoic iron-formations from non-magnetic to magnetic taconite in places such as the Mesabi Range and Gogebic Range.

Much progress has been made in the past 15 years in understanding fundamental causative factors of the Midcontinent rift. The data gathered in that period lay the foundation for continued refinements of knowledge of this important structure. Research continues on refining knowledge of the age of rifting events, internal stratigraphy of rift units, the nature of plume-lithosphere interaction, and patterns of regional metamorphism and alteration. Advances in the detail and precision of information on the geometry, age, and composition of the rift from recent and continuing research, combined with modem computational techniques, allows a new phase of quantitative numerical research on rift evolution to be undertaken. Examples of this new phase of research are current studies of the transient thermal history of the rift, and the paleohydrology of the rift as it related to formation of mineral deposits.

Selected References on the Midcontinent Rift (post 1982)

Allen, D.J., Hinze, Wi., Dickas, A.B., and Mudrey, M.G., Jr.,1997, Integrated geophysical modeling of the NorthAmerican Midcontinent Rift System: newinterpretations for western Lake Superior,northwestern Wisconsin, and eastern Minnesota, inOjakangas, R.W., and others, eds., MiddleProterozoic to Cambrian rifting, central NorthAmerica: Geological Society of America SpecialPaper 312, p. 47-72.

Allen, D.J., Braile. L.W., Hinze, W.J., and Mariano, J., 1995,The Midcontinent rift system, U.S.A.-a majorProterozoic continental rift: in Olsen, K.H., (ed.),Continental rifts: evolution, structure, tectonics,International Lithosphere Program Publication no.264, p. 373-407.

Behrendt, J.C., and seven others, 1988, Crustal structure of theMidcontinent rift system: results from GLIMPCEdeep seismic reflection profiles: Geology, v. 16, p.81-85.

Behrendt, J.C., and seven others, 1990, Seismic reflection(GLIMPCE) evidence of deep crustal and uppermantle intrusions and magmatic underplatingassociated with the Midcontinent rift system ofNorth America: Tectonophysics, v. 173, p.617-626.

Berg, J.H., and Klewin, K.W., 1988, High-MgO lavas from theKeweenawan Midcontinent rift near MamainsePoint, Ontario: Geology, v. 16, p. 1003-1006.

Bomhorst, T.J., 1997, Tectonic context of native copperdeposits of the North American Midcontinent Riftsystem, in Ojakangas, R.W., and others, eds., MiddleProterozoic to Cambrian rifting, central NorthAmerica: Geological Society of America SpecialPaper 312, p. 127-136.

Campbell, I.H., and Griffiths, R.W., 1990, Implications ofmantle plume structure for the evolution of floodbasalts: Earth and Planetaiy Science Letters, v. 99, p.79-93.

Campbell McCuaig, T., and Kissin, S.A., 1997, The PortCoidwell veins, northern Ontario: Pb-Zn-Ag depositsin a rift setting, in Ojakangas. R.W., and others, eds.,Middle Proterozoic to Cambrian rifting, centralNorth America: Geological Society of AmericaSpecial Paper 312, p. 187-196.

Cannon, W.F., 1992, The Midcontinent rift in the Lake Superiorregion with emphasis on its geodynamic evolution:Tectonophysics, v. 213, p. 41-48.

Cannon, W.F., 1994, Closing of the Midcontinent rift — afar-field effect of Grenvillian compression: Geology, v.22,p. 155-158.

Cannon, W.F., and Nicholson, S.W., 1996, Middle ProterozoicMidcontinent Rift System: in Sims, P.K., and Carter,L.H.M., eds. Archean and Proterozoic geology of theLake Superior region, U.S.A. U.S. GeologicalSurvey Professional Paper 1556, p.60-67.

22

Cannon, W.F., and Hinze, W.J., 1992, Speculations on theorigin of the North American Midcontinent rift:Tectonophysics, v. 213, p. 49-55.

Cannon, W.F., and McGervy, T.A., 1991, Map showing mineraldeposits of the Midcontinent rift, Lake Superiorregion, United States and Canada: U.S. GeologicalSurvey Miscellaneous Field Studies Map MF-2153,scale 1:500,000.

Cannon, W.F., and ten others, 1990, The Midcontinent riftbeneath Lake Superior from GLIMPCE seismicreflection profiling: Tectonics, v. 8, p.305-332.

Cannon, W.F., Peterman, Z.E., and Sims, P.K., 1993, Crustal-scale thrusting and origin of the Montreal Rivermonocline—a 35-km-thick cross section of theMidcontinent rift in northern Michigan andWisconsin: Tectonics, v. 12, p. 728-744.

Chandler, V.W., McSwiggen, P.L., Morey, G.B., Hinze, W.J.,and Anderson, R.L., 1989, Interpretation of seismicreflection, gravity and magnetic data across theMiddle Proterozoic Midcontinent Rift System inwestern Wisconsin, eastern Minnesota, and centralIowa: American Association of PetroleumGeologists Bulletin, v. 73, p. 261-275.

Davis, D.W., and Green, J.C., 1997, Geochronology of theNorth American Midcontinent rift in western LakeSuperior and implications for its geodynamicevolution: Canadian Journal of Earth Sciences, v. 34,p. 476-488.

Davis, D.W., and Sutdliffe, R.H., 1985, U-Pb ages from theNipigon plate and northern Lake Superior:Geological Society of America Bulletin, v. 96, p.1572-1579.

Davis, D.W., and Paces, J.B., 1990, Time resolution of geologicevents on the Keweenaw Peninsula and implicationsfor the development of the Midcontinent rift system:Earth and Planetary Science Letters, v. 97, p. 54-64.

Dickas, A.B., 1986, Comparative Precambrian stratigraphy andstructure along the Mid-Continent Rift: AmericanAssociation of Petroleum Geologists, v. 70, p. 225-238.

Dickas, A.B., and Mudrey, M.G., Jr., 1997, Segmented structureof the Middle Proterozoic Midcontinent Rift System,North America, in Ojakangas, R.W., and others, eds.,Middle Proterozoic to Cambrian rifting, centralNorth America: Geological Society of AmericaSpecial Paper 312, p.37-46.

Green, A.G., and nine others, 1989, A GLIMPCE of the deepcrust beneath the Great Lakes: AmericanGeophysical Union Monograph 51, p. 65-80.

Green, J.C., 1983, Geologic and geochemical evidence for thenature and development of the middle Proterozoic(Keweenawan) Midcontinent Rift of North America:Tectonophysics, v. 91, p. 413-437.

Selected References on the Midcontinent Rift (post 1982)

&en, D.J., Hinze, W.J., Dickas, A.B., and Mudrey, M.G., Jr., 1997, Integrated geophysical modeling of the North American Midcontinent Rift System: new interpretations for western Lake Superior, northwestern Wisconsin, and eastern Minnesota, in Ojakangas, R.W., and others, eds., Middle Proterozoic to Cambrian rifting, central North America: Geological Society of America Special Paper 312, p. 47-72.

Allen, D.J., Braile. L.W., Hinze, W.J., and Mariano, J., 1995, The Midcontinent rift system, U.S.A.-a major Proterozoic continental rift: in Olsen, K.H., (ed.), Continental rifts: evolution, structure, tectonics, International Lithosphere Program Publication no. 264, p. 373-407.

Behrendt, J.C., and seven others, 1988, Crustal structure of the Midcontinent rift system: results from GLIMPCE deep seismic reflection profiles: Geology, v. 16, p. 81-85.

Behrendt, J.C., and seven others, 1990, Seismic reflection (GLIMPCE) evidence of deep crustal and upper mantle intrusions and magmatic underplating associated with the Midcontinent rift system of North America: Tectonophysics, v. 173, p.617-626.

Berg, J.H., and Klewin, K.W., 1988, High-MgO lavas from the Keweenawan Midcontinent rift near Mamainse Point, Ontario: Geology, v. 16, p. 1003-1006.

Bornhorst, T.J., 1997, Tectonic context of native copper deposits of the North American Midcontinent Rift system, in Ojakangas, R.W., and others, eds., Middle Proterozoic to Cambrian rifting, central North America: Geological Society of America Special Paper 312, p. 127-136.

Campbell, I.H., and Griffiths, R.W., 1990, Implications of mantle plume structure for the evolution of flood basalts: Earth and Planetary Science Letters, v. 99, p. 79-93.

Campbell McCuaig, T., and Kissin, S.A., 1997, The Port Coldwell veins, northern Ontario: Pb-Zn-Ag deposits in a rift setting, in Ojakangas. R.W., and others, eds., Middle Proterozoic to Cambrian rifting, central North America: Geological Society of America Special Paper 312, p. 187-196.

Cannon, W.F., 1992, The Midcontinent rift in the Lake Superior region with emphasis on its geodynamic evolution: Tectonophysics, v. 213, p. 41-48.

Cannon, W.F., 1994, Closing of the Midcontinent rift - a far- field effect of Grenvillian compression: Geology, v. 22, p. 155-158.

Cannon, W.F., and Nicholson, S.W., 1996, Middle Proterozoic Midcontinent Rift System: in Sims, P.K., and Carter, L.H.M., eds. Archean and Proterozoic geology of the Lake Superior region, U.S.A. U.S. Geological Survey Professional Paper 1556, p.60-67.

Cannon, W.F., and Hinze, W.J., 1992, Speculations on the origin of the North American Midcontinent rift:

Cannon, W.F., and McGervy, T.A., 1991, Map showing mineral deposits of the Midcontinent rift. Lake Superior region, United States and Canada: U.S. Geological Survey Miscellaneous Field Studies Map MF-2153, scale 1:500,000.

Cannon, W.F., and ten others, 1990, The Midcontinent rift beneath Lake Superior from GLIMPCE seismic reflection profiling: Tectonics, v. 8, p.305-332.

Cannon, W.F., Peterman, Z.E., and Sirns, P.K., 1993, Crustal- scale thrusting and origin of the Montreal River monocline-a 35-km-thick cross section of the Midcontinent rift in northern Michigan and Wisconsin: Tectonics, v. 12, p. 728-744.

Chandler, V.W., McSwiggen, P.L., Morey, G.B., Hinze, W.J., and Anderson, R.L., 1989, Interpretation of seismic '

reflection, gravity and magnetic data across the Middle Proterozoic Midcontinent Rift System in western Wisconsin, eastern Minnesota, and central Iowa: American Association of Petroleum Geologists Bulletin, v. 73, p. 261-275.

Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Sciences, v. 34, p. 476-488.

Davis, D.W., and Sutcliffe, R.H., 1985, U-Pb ages from the Nipigon plate and northern Lake Superior: Geological Society of America Bulletin, v. 96, p. 1572-1579.

Davis, D.W., and Paces, J.B., 1990, Time resolution of geologic events on the Keweenaw Peninsula and implications for the development of the Midcontinent rift system: Earth and Planetary Science Letters, v. 97, p. 54-64.

Dickas, A.B., 1986, Comparative Precambrian stratigraphy and structure along the Mid-Continent Rift: American Association of petroleum Geologists, v. 70, p. 225- 238.

Dickas, A.B., and Mudrey, M.G., Jr., 1997, Segmented structure of the Middle Proterozoic Midcontinent Rift System, North America, in Ojakangas, R.W., and others, eds., Middle Proterozoic to Cambrian rifting, central North America: Geological Society of America Special Paper 312, p. 37-46.

Green, A.G., and nine others, 1989, A GLIMPCE of the deep crust beneath the Great Lakes: American Geophysical Union Monograph 51, p. 65-80.

Green, J.C., 1983, Geologic and geochemical evidence for the nature and development of the middle Proterozoic (Keweenawan) Midcontinent Rift of North America: Tectonophysics, v. 91, p. 413-437.

Green, IC., 1989, Physical volcanology of mid-Proterozoicplateau lavas: the Keweenawan North ShoreVolcanic Group, Minnesota: Geological Society ofAmerica Bulletin, v 101, p. 486-500.

Green, J.C., and six others, 1987, Keweenawan dykes of theLake Superior region: evidence for the evolution ofthe Middle Proterozoic Midcontinent Rift of NorthAmerica, in Halls, H.C. and Fahrig, W.F., eds.,Mafic dyke swarms, Geological Association ofCanada Special Paper 34, p. 289-302.

Griffiths, R.W., and Campbell, I.H., 1991, Interaction of mantleplume heads with the Earth's surface and the onsetof small-scale convection: Journal of GeophysicalResearch, v.96, p. 18295-183 10.

Hauk, S.A., and seven others, 1997, An overview of the geologyand oxide, sulfide, and platinum-group elementmineralization along the western and northerncontacts of the Duluth complex, in Ojakangas, R.W.,and others, eds., Middle Proterozoic to Cambrianrifting, central North America: Geological Society ofAmerica Special Paper 312, p. 137-186.

Heamon, L.M., and Machado, N., 1992, Timing and origin ofthe Midcontinent rift alkaline magmatism, NorthAmerica: evidence from the CoIdwell Complex:Contributions to Mineralogy and Petrology, v. 110,p. 289-303.

Hieshima, G.B., and Pratt, L.M., 1991, Sulfur/carbon ratios andextractable organic matter of the Middle ProterozoicNonesuch formation, North American MidcontinentRift: Precambrian Research, v. 54, p. 65-79.

Hinze, W.J., Allen, D.J., Braile, L.W., and Mariano, J, 1997,The Midcontinent Rift System: a major Proterozoiccontinental rift, in Ojakangas, R.W., and others, eds.,Middle Proterozoic to Cambrian rifting, centralNorth America: Geological Society of AmericaSpecial Paper 312, p. 7-36.

Hinze, W.J., Braile, L.W., and Chandler, V.W., 1990, Ageophysical profile of the southern margin of theMidcontinent Rift System in western Lake Superior:Tectonics, v.9, p. 303-3 10.

Hutchinson, D.R., White, R.S., Cannon, W.F., and Schulz, K.J.,1990, Keweenaw hot spot: Geophysical evidence fora 1.1 Ga mantle plume beneath the Midcontinent RiftSystem: Journal of Geophysical Research, v. 95, p.10869-10884.

Kiewin, K.W., 1989, Polybaric fractionation in an evolvingcontinental rift: Evidence from the KeweenawanMid-continental Rift: Journal of Geology, v, 97, p.65-76.

Kiewin, K.W., and Berg, J.H., 1991, Petrology of theKeweenawan Mamainse Point lavas, Ontario:Journal of Geophysical Research, v. 96, p. 457-474.

Kiewin, K.W., and Shirey, S.B., 1992, The igneous petrologyand magmatic evolution of the Midcontinent RiftSystem: Tectonophysics, v. 213, p. 33-40.

23

Manson, M.L., and Halls, H.C., 1997, Proterozoic reactivationof the southern Superior Province and its role in theevolution of the Midcontinent rift: Canadian Journalof Earth Sciences, v. 34, p. 562-5 75.

Mariano, J., and Hinze, W.J., 1994, Structural interpretation ofthe Midcontinent Rift in eastern Lake Superior fromseismic reflection and potential field studies:Canadian Journal of Earth Sciences, v.31, p. 619-628.

Mariano, J., and Hinze, W.J., 1994, Gravity and magneticmodels of the Midcontinent Rift in eastern LakeSuperior: Canadian Journal of Earth Sciences, v. 31,p. 661-674.

McSwiggen, P.L., Morey, G.B., and Chandler, V.W., 1987,New model of Midcontinent rift in easternMinnesota and western Wisconsin: Tectonics, v. 6,p. 677-685.

Miller, J.D., Jr., and Chandler, V.W., 1997, Geology, petrology,and tectonic significance of the Beaver BayComplex, northeastern Minnesota, in Ojakangas,R.W., and others, eds., Middle Proterozoic toCambrian rifting, central North America: GeologicalSociety of America Special Paper 312, p. 73-96.

Nicholson, SW., Shirey, S.B., Schulz, K.J., and Green, J.C.,1997, Rift-wide correlation of 1.1 Ga Midcontinentrift system basalts: implications for multiple mantlesources during rift development: Canadian Journal ofEarth Sciences, v. 34, p. 504-520.

Nicholson, SW., Cannon, W.F., and Schulz, K.J., 1992,Metallogeny of the Midcontinent Rift System ofNorth America: Precambrian Research, v., 58, p.355-386.

Nicholson, SW., and Shirey, S.B., 1990, Evidence for aPrecambrian mantle plume: a Sr, Nd, and Pb isotopicstudy of the Midcontinent Rift System in the LakeSuperior region: Journal of Geophysical Research, v.95, p. 10851-10868.

Paces, J.B., and Bell, K., 1989, Non-depleted subeontinentalmantle beneath the Superior Province of theCanadian Shield: Nd-Sr isotopic and trace elementevidence from Midcontinent rift basalts: Geochimicaet Cosmochimica Acta, v. 53, p. 2023-2035.

Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages ofDuluth Complex and related mafic intrusions,northeastern Minnesota: Geochronological insightsto physical, petrogenetic, paleomagnetic and tectono-magmatic processes associated with the 1.1 GaMidcontinent Rift system: Journal of GeophysicalResearch, v. 98, p. 13997-140 13.

Palmer, H.C., and Davis, D.W., 1987, Paleomagnetism and U-Pb geochronology of volcanic rocks fromMichipicoten Island, Lake Superior, Canada: Precisecalibration of the Keweenawan polar wander track:Precambrian Research, v. 37, p. 157-171.

Green, J.C., 1989, Physical volcanology of mid-Proterozoic plateau lavas: the Keweenawan North Shore Volcanic Group, Minnesota: Geological Society of America Bulletin, v 101, p. 486-500,

Green, J.C., and six others, 1987, Keweenawan dykes of the Lake Superior region: evidence for the evolution of the Middle Proterozoic Midcontinent Rift of North America, in Halls, H.C. and Fahrig, W.F., eds., Mafic dyke swarms, Geological Association of Canada Special Paper 34, p. 289-302.

Griffiths, R.W., and Campbell, I.H., 1991, Interaction of mantle plume heads with the Earth's surface and the onset of small-scale convection: Journal of Geophysical Research, v. 96, p. 18295-18310.

Hauk, S.A., and seven others, 1997, An overview of the geology and oxide, sulfide, and platinum-group element mineralization along the western and northern contacts of the Duluth complex, in Ojakangas, R.W., and others, eds., Middle Proterozoic to Cambrian rifting, central North America: Geological Society of America Special Paper 312, p. 137-186.

Heamon, L.M., and Machado, N., 1992, Timing and origin of the Midcontinent rift alkaline magmatism, North America: evidence from the Coldwell Complex: Contributions to Mineralogy and Petrology, v. 110, p. 289-303.

Hieshima, G.B., and Pratt, L.M., 1991, Sulfurlcarbon ratios and extractable organic matter of the Middle Proterozoic Nonesuch formation, North American Midcontinent Rift: Precambrian Research, v. 54, p. 65-79.

Hinze, W.J., Allen, D.J., Braile, L.W., and Mariano, J, 1997, The Midcontinent Rift System: a major Proterozoic continental rift, in Ojakangas, R.W., and others, eds., Middle Proterozoic to Cambrian rifting, central North America: Geological Society of America Special Paper 312, p. 7-36.

Hinze, W.J., Braile, L.W., and Chandler, V.W., 1990, A geophysical profile of the southern margin of the Midcontinent Rift System in western Lake Superior: Tectonics, v. 9, p. 303-310.

Hutchinson, D.R., White, R.S., Cannon, W.F., and Schulz, K.J., 1990, Keweenaw hot spot: Geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent Rift System: Journal of Geophysical Research, v. 95, p. 10869-1 0884.

Klewin, K.W., 1989, Polybaric fractionation in an evolving continental rift: Evidence from the Keweenawan Mid-continental Rift: Journal of Geology, v, 97, p. 65-76.

Klewin, K.W., and Berg, J.H., 1991, Petrology of the Keweenawan Mamainse Point lavas, Ontario: Journal of Geophysical Research, v. 96, p. 457-474.

Klewin, K.W., and Shirey, S.B., 1992, The igneous petrology and magmatic evolution of the Midcontinent Rift System: Tectonophysics, v. 213, p. 33-40.

Manson, M.L., and Halls, H.C., 1997, Proterozoic reactivation of the southern Superior Province and its role in the evolution of the Midcontinent rift: Canadian Journal of Earth Sciences, v. 34, p. 562-575.

Mariano, J., and Hinze, W.J., 1994, Structural interpretation of the Midcontinent Rift in eastern Lake Suverior from seismic reflection and potential field studies: Canadian Journal of Earth Sciences, v. 3 1, p. 619- 628.

Mariano, J., and Hinze, W.J., 1994, Gravity and magnetic models of the Midcontinent Rift in eastern Lake Superior: Canadian Journal of Earth Sciences, v. 31, p. 661-674.

McSwiggen, P.L., Morey, G.B., and Chandler, V.W., 1987, New model of Midcontinent rift in eastern Minnesota and western Wisconsin: Tectonics, v. 6, p. 677-685.

Miller, J.D., Jr., and Chandler, V.W., 1997, Geology, petrology, and tectonic significance of the Beaver Bay Complex, northeastern Minnesota, in Ojakangas, R.W., and others, eds., Middle Proterozoic to Cambrian rifting, central North America: Geological Society of America Special Paper 312, p. 73-96.

Nicholson, S.W., Shirey, S.B., Schulz, K.J., and Green, J.C., 1997, Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development: Canadian Journal of Earth Sciences, v. 34, p. 504-520.

Nicholson, S.W., Cannon, W.F., and Schulz, K.J., 1992, Metallogeny of the Midcontinent Rift System of North America: Precambrian Research, v., 58, p. 355-386.

Nicholson, S.W., and Shirey, S.B., 1990, Evidence for a Precambrian mantle plume: a Sr, Nd, and Pb isotopic study of the Midcontinent Rift System in the Lake Superior region: Journal of Geophysical Research, v. 95, p. 10851-10868.

Paces, J.B., and Bell, K., 1989, Non-depleted subcontinental mantle beneath the Superior Province of the Canadian Shield: Nd-Sr isotopic and trace element evidence from Midcontinent rift basalts: Geochimica et Cosmochimica Acta, v. 53, p. 2023-2035.

Paces, J.B., and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic and tectono- magmatic processes associated with the 1.1 Ga Midcontinent Rift system: Journal of Geophysical Research, v. 98, p. 13997-14013.

Palmer, H.C., and Davis, D.W., 1987, Paleomagnetism and U- Pb geochronology of volcanic rocks from Michipicoten Island, Lake Superior, Canada: Precise calibration of the Keweenawan polar wander track: Precambrian Research, v. 37, p. 157-171.

Peterman, Z.E., and Sims, P.K., 1988, The Goodman Swell: alithospheric flexure caused by crustal loading alongthe Midcontinent Rift System: Tectonics, v. 7, p.1077-1090.

Samson, C., and West, G.F., 1992, Crustal structure of theMidcontinent Rift system in eastern Lake Superiorfrom controlled-amplitude analysis of GLIMPCEdeep reflection seismic data: Canadian Journal ofEarth Sciences, v. 29, p. 636-649.

Shirey, SB., 1997, Re-Os isotopic compositions ofMidcontinent rift system picrites: implications forplume-lithosphere interaction and enriched mantlesources: Canadian Journal of Earth Sciences, v. 34,p. 489-503.

Suszek, T., 1997, Petrography and sedimentation of the MiddleProterozoic (Keweenawan) Nonesuch Formation,western Lake Superior region, Midcontinent Riftsystem, in Ojakangas, R.W., and others, eds., MiddleProterozoic to Cambrian rifting, central NorthAmerica: Geological Society of America SpecialPaper 312, p. 195-210.

Van Schmus, W.R., 1992, Tectonic setting of the MidcontinentRift System: Tectonophysics, v. 213, p. 1-15.

Van Schmus, W.R., and Hinze, W.J., 1985, The MidcontinentRift System: Annual Reviews of Earth AndPlanetary Science, v. 13, p. 345-383.

Vervoort, J.D., and Green, J.C., 1997, Origin of evolvedmagmas in the Midcontinent rift system, northeastMinnesota: Nd-isotopic evidence for melting ofArchean crust: Canadian Journal of Earth Sciences,v. 34, p. 521-535.

Weiblin, P.W., 1993, Midcontinent rift system, the LakeSuperior region and Trans-Hudson orogen, in Reed,J.C., Jr., and others, eds., Precambrian-ConterminousUnited States: Boulder, Cob., Geological Society ofAmerica, The Geology of North America, v. C2, p.72-8 1.

White, R.S., 1997, Mantle temperature and lithospheric thinningbeneath the Midcontinent rift system: evidence frommagmatism and subsidence: Canadian Journal ofEarth Sciences, v. 34, p. 464-475.

White, R., and McKenzie, D., 1989, Magmatism at rift zones:The generation of volcanic continental margins andflood basalts: Journal of Geophysical Research, v.94, p. 7685-7729.

Wirth, K.R., Vervoort, J.D., and Naiman, Z.J., 1997, TheChengwatana Volcanics, Wisconsin and Minnesota:petrogenesis of the southernmost volcanic rocksexposed in the Midcontinent rift: Canadian Journalof Earth Sciences, v. 34, p. 536-548.

Zartman, R.E., Nisholson, SW., Cannon, W.F., and Morey,G.B., 1997, U-Th-Pb zircon ages of someKeweenawan Supergroup rocks from the southernshore of Lake Superior: Canadian Journal of EarthSciences, v. 34, p. 549-561.

Zhu, T., and Brown, L., 1986, Consortium for ContinentalReflection Profiling Michigan surveys: reprocessingand results: Journal of Geophysical Research, v. 91,p. 11477-11495.

Selected references on anorogenic plutons

Anderson, J.L., 1993, The Wolf River batholith, the LakeSuperior region and Trans-Hudson orogen, in Reed,J.C., Jr., and others, eds., Precambrian-ConterminousUnited States: Boulder, Cob., Geological Society ofAmerica, The Geology of North America, v. C2, p.69-7 1.

Anderson, J.L., 1983, Proterozoic anorogenic granite plutonismof North America, in Medaris, L.G., Jr., and others,eds., Proterozoic geology: selected papers from anInternational Proterozoic Symposium: GeologicalSociety of America Memoir, 161, p. 133-154.

Anderson, .LL., 1980, Mineral equilibria and crystallizationconditions in the late Precambrian Wolf Riverrapakivi massif, Wisconsin: American Journal ofScience, v. 280, p. 289-332.

Anderson, J.L., and Cullers, R.L., 1978, Geochemistry andevolution of the Wolf River batholith, a latePrecambrian rapakivi massif in northern Wisconsin,USA: Precambrian Research, v. 7, p. 287-324.

24

Anderson, J.L., Cullers, R.L., and Van Schmus, W.R., 1980,Anorogenic metaluminous and peraluminous graniteplutonism in the mid-Proterozoic of Wisconsin,USA: Contributions to Mineralogy and Petrology, v.74, p. 311-328.

Nelson, B.K., and DePaobo, D.J., 1985, Rapid production ofcontinental crust 1.7-1.9 b.y. ago- Nd and Sr isotopicevidence from the basement of the North Americanmidcontinent: Geological Society of AmericaBulletin, v. 96, p. 746-754.

Sims, P.K., 1996, Early and Middle Proterozoic intracratonicrocks, in Sims, P.K. and Carter, L.M.H., eds.,Archean and Proterozoic geology of the LakeSuperior region, U.S.A.: U.S. Geological SurveyProfessional Paper 1556, p. 57-60.

Van Schmus, W.R., , Medaris, L.G., and Banks, P.O., 1975,Chronology of Precambrian rocks in Wisconsin, I:the Wolf River batholith, a rapikivi massifapproximately 1500 m.y. old: Geological Society ofAmerica Bulletin, v. 86, p. 907-9 14.

Peterman, Z.E., and Sims, P.K., 1988, The Goodman Swell: a lithospheric flexure caused by crustal loading along the Midcontinent Rift System: Tectonics, v. 7, p. 1077-1090.

Samson, C., and West, G.F., 1992, Crustal structure of the Midcontinent Rift system in eastern Lake Superior from controlled-amplitude analysis of GLIMPCE deep reflection seismic data: Canadian Journal of Earth Sciences, v. 29, p. 636-649.

Shirey, S.B., 1997, Re-0s isotopic compositions of Midcontinent rift system picrites: implications for plume-lithosphere interaction and enriched mantle sources: Canadian Journal of Earth Sciences, v. 34, p. 489-503.

Suszek, T., 1997, Petrography and sedimentation of the Middle Proterozoic (Keweenawan) Nonesuch Formation, western Lake Superior region, Midcontinent Rift system, in Ojakangas, R.W., and others, eds., Middle Proterozoic to Cambrian rifting, central North America: Geological Society of America Special Paper 312, p. 195-210.

Van Schmus, W.R., 1992, Tectonic setting of the Midcontinent Rift System: Tectonophysics, v. 213, p. 1-15.

Van Schmus, W.R., and Hinze, W.J., 1985, The Midcontinent Rift System: Annual Reviews of Earth And Planetary Science, v. 13, p. 345-383.

Vervoort, J.D., and Green, J.C., 1997, Origin of evolved magmas in the Midcontinent rift system, northeast Minnesota: Nd-isotopic evidence for melting of Archean crust: Canadian Journal of Earth Sciences, V. 34, p. 521-535.

Weiblin, P.W., 1993, Midcontinent rift system, the Lake Superior region and Trans-Hudson orogen, in Reed, J.C., Jr., and others, eds., Precambrian-Conterminous United States: Boulder, Colo., Geological Society of America, The Geology of North America, v. C2, p. 72-81.

White, R.S., 1997, Mantle temperature and lithospheric thinning beneath the Midcontinent rift system: evidence from magmatism and subsidence: ~ h a d i a n Journal of Earth Sciences, v. 34, p. 464-475.

White, R., and McKenzie, D., 1989, Magmatism at rift zones: The generation of volcanic continental margins and flood basalts: Journal of Geophysical Research, v. 94, p. 7685-7729.

Wirth, K.R., Vervoort, J.D., and Naiman, Z.J., 1997, The Chengwatana Volcanics, Wisconsin and Minnesota: petrogenesis of the southernmost volcanic rocks exposed in the Midcontinent rift: Canadian Journal of Earth Sciences, v. 34, p. 536-548.

Zartman, R.E., Nisholson, S.W., Cannon, W.F., and Morey, G.B., 1997, U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the southern shore of Lake Superior: Canadian Journal of Earth Sciences, v. 34, p. 549-561.

Zhu, T., and Brown, L., 1986, Consortium for Continental Reflection Profiling Michigan surveys: reprocessing and results: Journal of Geophysical Research, v. 91, p. 11477-1 1495.

Selected references on anorogenic plutons

Anderson, J.L., 1993, The Wolf River batholith, the Lake Superior region and Trans-Hudson orogen, in Reed, J.C., Jr., and others, eds., Precambrian-Conterminous United States: Boulder, Colo., Geological Society of America, The Geology of North America, v. C2, p. 69-7 1.

Anderson, J.L., 1983, Proterozoic anorogenic granite plutonism of North America, in Medaris, L.G., Jr., and others, eds., Proterozoic geology: selected papers from an International Proterozoic Symposium: Geological Society of America Memoir, 161, p. 133-154.

Anderson, J.L., 1980, Mineral equilibria and crystallization conditions in the late Precambrian Wolf River rapakivi massif, Wisconsin: American Journal of Science, v. 280, p. 289-332.

Anderson, J.L., Cullers, R.L., and Van Schmus, W.R., 1980, Anorogenic metaluminous and peraluminous granite plutonism in the mid-Proterozoic of Wisconsin, USA: Contributions to Mineralogy and Petrology, v. 74, p. 3 11-328.

Nelson, B.K., and DePaolo, D.J., 1985, Rapid production of continental crust 1.7-1.9 b.y. ago- Nd and Sr isotopic evidence from the basement of the North American midcontinent: Geological Society of America Bulletin, v. 96, p. 746-754.

Sims, P.K., 1996, Early and Middle Proterozoic intracratonic rocks, in Sims, P.K. and Carter, L.M.H., eds., Archean and Proterozoic geology of the Lake Superior region, U.S.A.: U.S. Geological Survey Professional Paper 1556, p. 57-60.

Anderson, J.L., and Cullers, R.L., 1978, Geochemistry and Van Schmus, W.R., , Medaris, L.G., andBanks, P.O., 1975, evolution of the Wolf River batholith, a late Chronology of Precambrian rocks in Wisconsin, I: Precambrian rapakivi massif in northern Wisconsin, the Wolf River batholith, a rapikivi massif USA: Precambrian Research, v. 7, p. 287-324. approximately 1500 my. old: Geological Society of

America Bulletin, v. 86, p. 907-914.

PALEOZOIC ROCKS IN THE NORTHERN PART OF THE CENTRALMIDCONTINENT OF NORTH AMERICA

RUNKEL, Anthony C., Minnesota Geological Survey, 2642 University Ave SE, StPaul, MN, 55114-1057

INTRODUCTION

Paleozoic strata in the central midcontinent region of North American consist of thinunits of carbonate, sandstone, and shale distributed across tens of thousands of squarekilometers. This overview focuses on the northernmost extent of lower Paleozoic stratawhich were deposited on the stable cratonic shelf northwest of the Illinois and Michiganbasins (Figs. 1 and 2). They are exposed in a sinuous belt of outcrops in southernMinnesota, Wisconsin and northern Michigan, on the southern flanks of theTranscontinental arch and the Wisconsin dome and arch.

Deposition of Paleozoic sediments began in Middle to Late Cambrian time. Coarsesiliciclastic sediments of the Mt Simon Sandstone covered the eroded Precambrian surface.Overlying strata were deposited in two broad, laterally equivalent facies belts across thecentral midcontinent region (Fig. 1 inset) (Palmer, 1960). An inner detrital belt wascomposed of shallow marine siiciclastics derived from subaerially exposed Precambrianshield areas on and to the north of the Wisconsin Dome. Thin, laterally extensive unitsdominated by either fme- to coarse-grained quartzose sandstone, very fme sandstone, orshale were deposited in the inner detrital belt. The middle carbonate belt consisted of subtidalto intertidal carbonate and shale deposits that accumulated to the south. The boundarybetween belts shifted, with inner detrital belt sedimentation dominant during Cambrian andpart of Middle Ordovician time, and middle carbonate belt sedimentation dominant duringthe Early and Late Ordovician, as well as in Silurian and Devonian time (Fig. 2).

Early Paleozoic deposition occurred on a virtually horizontal, cratonic shelf with a verylow subsidence rate. Late Cambrian subsidence averaged less than lOrn/m.y. (Sloss,1988), about one-fifth to one-tenth the rate in the contemporaneous Illinois Basin (Sargent,1991) and orders of magnitude slower than that of better known, younger basins in NorthAmerica. Maximum paleobathymetry was typically less than 100 m (e.g. Byers and Dott,1995; Ludvigson and others, 1996) and the shelf had a low gradient slope of about0.1 rn/km.

The Wisconsin arch and dome and Transcontinental arch were positive structuralfeatures that influenced early Paleozoic sedimentation patterns, and eventually controlled thedistribution and configuration of gently folded and faulted Paleozoic strata today. Innorthern Iowa, southern Minnesota, and southwestern Wisconsin Paleozoic strata arepreserved in what is known as the Hollandale Embayment, which is a broad syncline thatlies between these two positive features. South and east of the Wisconsin arch and domePaleozoic rocks dip less than 1 degree into the Illinois and Michigan basins.

DEPOSITIONAL MODELS

The depositional history of lower Paleozoic siliciclastic strata of this region remainspoorly understood despite over 100 years of study. Many workers have lamented anapparent absence of modern or ancient depositional analogues. In addition, dozens of localstudies have been conducted (e.g. Nelson, 1954; Haddox and Dott, 1990) but there are fewregional-scale investigations that incoiporate several lithofacies into a temporally constrainedstratigraphic framework. Present-day understanding of early Paleozoic siliciclasticdeposition is based mostly on work conducted since 1950, beginning with thepredominantly stratigraphic investigations of Berg (1954), Nelson (1956), Berg and others(1956), Bell and others (1956), and Ostrom (1964, 1970). Subsequent work has beenchiefly sedimentologic, focusing on the interpretations of near-shore marine facies (Fraser,1976; Driese and others, 1981; Dott and others, 1986; Haddox and Dott, 1990; Barnes and

25

PALEOZOIC ROCKS IN THE NORTHERN PART OF THE CENTRAL MIDCONTINENT OF NORTH AMERICA

RUNKEL, Anthony C., Minnesota Geological Survey, 2642 University Ave SE, St Paul, MN, 55 114-1057

INTRODUCTION

Paleozoic strata in the central midcontinent region of North American consist of thin units of carbonate, sandstone, and shale distributed across tens of thousands of square kilometers. This overview focuses on the northernmost extent of lower Paleozoic strata which were deposited on the stable cratonic shelf northwest of the Illinois and Michigan basins (Figs. 1 and 2). They are exposed in a sinuous belt of outcrops in southern Minnesota, Wisconsin and northern Michigan, on the southern flanks of the Transcontinental arch and the Wisconsin dome and arch.

Deposition of Paleozoic sediments began in Middle to Late Cambrian time. Coarse siliciclastic sediments of the Mt Simon Sandstone covered the eroded Precambrian surface. Overlying strata were deposited in two broad, laterally equivalent facies belts across the central midcontinent region (Fig. 1 inset) (Palmer, 1960). An inner detrital belt was composed of shallow marine siliciclastics derived from subaerially exposed Precambrian shield areas on and to the north of the Wisconsin Dome. Thin, laterally extensive units dominated by either fine- to coarse-grained quartzose sandstone, very fine sandstone, or shale were deposited in the inner detrital belt. The middle carbonate belt consisted of subtidal to intertidal carbonate and shale deposits that accumulated to the south. The boundary between belts shifted, with inner detrital belt sedimentation dominant during Cambrian and part of Middle Ordovician time, and middle carbonate belt sedimentation dominant during the Early and Late Ordovician, as well as in Silurian and Devonian time (Fig. 2).

Early Paleozoic deposition occurred on a virtually horizontal, cratonic shelf with a very low subsidence rate. Late Cambrian subsidence averaged less than lOrn1m.y. (Sloss, 1988), about one-fifth to one-tenth the rate in the contemporaneous Illinois Basin (Sargent, 1991) and orders of magnitude slower than that of better known, younger basins in North America. Maximum paleobathymetry was typically less than 100 m (e.g. Byers and Dott, 1995; Ludvigson and others, 1996) and the shelf had a low gradient slope of about O.lm/km.

The Wisconsin arch and dome and Transcontinental arch were positive structural features that influenced early Paleozoic sedimentation patterns, and eventually controlled the distribution and configuration of gently folded and faulted Paleozoic strata today. In northern Iowa, southern Minnesota, and southwestern Wisconsin Paleozoic strata are preserved in what is known as the Hollandale Embayment, which is a broad syncline that lies between these two positive features. South and east of the Wisconsin arch and dome Paleozoic rocks dip less than 1 degree into the Illinois and Michigan basins.

DEPOSITIONAL MODELS

The depositional history of lower Paleozoic siliciclastic strata of this region remains poorly understood despite over 100 years of study. Many workers have lamented an apparent absence of modem or ancient depositional analogues. In addition, dozens of local studies have been conducted (e.g. Nelson, 1954; Haddox and Dott, 1990) but there are few regional-scale investigations that incorporate several lithofacies into a temporally constrained stratigraphic framework. Present-day understanding of early Paleozoic siliciclastic deposition is based mostly on work conducted since 1950, beginning with the predominantly stratigraphic investigations of Berg (1954), Nelson (1956), Berg and others (1956), Bell and others (1956), and Ostrom (1964,1970). Subsequent work has been chiefly sedimentologic, focusing on the interpretations of near-shore marine facies (Fraser, 1976; Driese and others, 1981; Dott and others, 1986; Haddox and Dott, 1990; Barnes and

;

LITHOLOGY

i.L— I — —

GROUP!

FORMATION

Cedar VaHey

J

EngaduneManustique

—I

—

- — I — — I

I—I-J

— ——

!:!.

Burnt BluffCataract

MaquoketaDubuqueGalenaDecorahPlattevilleood

JdeGroup

Jordan.••

St Lawrence

Franconia(LoneRock)

Ga(Wonewoc)

E •.

-: —Eau Claire andBonneterre

• . Mt. Simon

Figure 1. Location map showingearly Paleozoic tectonic features,major paleotopographic highs ofPrecambrian rocks (stippled) and theapproximate distribution ofCambrian (C), Ordovician (0),Silurian (S), and Devonian (D)strata in the northern part of thecentral midcontinent region. Theinset shows the study area relative tothe Late Cambrian inner (stipple)and outer (dashes) detrital belts andthe middle carbonate belt (blockpattern) of Palmer (1960). Modifiedfrom Runkel and others (1998)

Figure 2. Generalized stratigraphic column (noscale) for lower Paleozoic rocks in the northern partof the central midcontinent region. The Cambrianand Ordovician nomenclature is that used insoutheastern Minnesota and Wisconsin, theSilurian from eastern Wisconsin and northernMichigan, and the Devonian from southernmostMinnesota and northern Iowa.

(uatzose sandstone

(] Very fine s.s., siltstone, shale

— Carbonate (dashed where shaly)

No record

Unconformity

Figure 1. Location map showing early Paleozoic tectonic features, major paleotopographic highs of Precambrian rocks (stippled) and the approximate distribution of Cambrian (C) , Ordovician (0), Silurian (S), and Devonian (D) strata in the northern part of the central midcontinent region. The inset shows the study area relative to the Late Cambrian inner (stipple) and outer (dashes) detrital belts and the middle carbonate belt (block pattern) of Palmer (1960). Modified from Runkel and others (1998)

Figure 2. Generalized stratigraphic column (no scale) for lower Paleozoic rocks in the northern part of the central midcontinent region. The Cambrian and Ordovician nomenclature is that used in southeastern Minnesota and Wisconsin, the Silurian from eastern Wisconsin and northern Michigan, and the Devonian from southernmost Minnesota and northern Iowa.

E l -sandstone Very fine s.s., sdtstone, shale

Carbonate (dashed where shaly)

No record - Unconfonnity

others, 1992; Mossier, 1992; Runkel, 1994). In a very important and widely cited studyDott and others (1986) examined the transition from nonmarine to marine environmentsrecorded in quartzose sandstones, and provided the first set of criteria for identification ofnonmarine facies.They described an early Paleozoic terrestrial setting with broad braidedstreams and eolian sheet and erg systems that delivered sand to a shallow marineenvironment dominated by small three dimensional dunes. Dott and others (1986) stressedthe differences, such as the lack of vegetation, between early Paleozoic depositionalenvironments and those inferred for both younger rocks and modem settings. Adepositional model of a simple, storm-dominated, texturally graded shelf was proposed byRunkel and others (1998), and is noteworthy in its similarity to models based on bothyounger rocks and modem systems. Their study was essentially a regional-scale outgrowthof the excellent outcrop work of Charlie Bell and his students from the University ofMinnesota in the 1950's. Runkel and others (1998) suggested that early Paleozoicdepositional processes and facies distribution are analogous to those depicted in well-knownmodels of the Cretaceous interior seaway and the modem Bering Shelf. The thin widespreadsiliciclastic units characteristic of lower Paleozoic strata result from large lateral faciesmigrations which occurred in response to continental-scale changes in relative sea level.

Taylors Falls Precambrianlava flow rrdges, (J,/

Mayor sand

——

source from

CARBONATEI —

— SILIcICLASTIC OFFSHORE SHELF —

— 0

ORE — — — — — - =—

— SHOREFACE 0• Dome

SHELF(M0StIV — — —

— o FLUVIAL&

— — — —

— 0 . EOLIAN

— — - — — —

o sand0 SANDPLAIN

— — — —

— transport

— —— Storm t,anspolt of 0 by storms

— — — — !° sand. suit. shale 0 01 0

source

— —

— !rom shoretacesandstone

- — — — — — —

—s.. .._- on Arch

— —

— 00Rudge : — — — —

-0 Tudal

- Tempestltes- - 'mmodks0 •.:

— Depth 5Pm — — — — —

— 00 0 00 00 00 :

— = -o 4OUI0 0 0 0

—— I —

— 0000-o 2O4O= — 0000

— — — 0

(East) 0

Figure 3. Conceptualdepositional model of LateCambrian storm-dominated,texturally graded shelf.Quartzose fme- to coarse-grained sand accumulated innearshore terresirial andshoreface environments. Finergrained , feldspathicsiiciclastic sediments andsubtidal carbonatesaccumulated in deeper water ofthe offshoreshelf.

Carbonate rocks within the lower Paleozoic sequence have historically been regarded asmore amenable to comparison with both modem and ancient facies models. Depositionoccurred in environments that range from evaporitic, supratidal conditions for parts of thePrairie du Chien Group (Smith and others, 1993) to subtidal settings in depths of 100 m ormore for parts of units such as the Decorah Shale (Ludvigson and others, 1996). Mostinvestigations have been local in scope relative to the great lateral extent of these units andthere are too many to cite them individually. Ordovician carbonate strata of the Hollandaleembayment are the focus of a monograph edited by Sloan (1986). Silurian carbonates on thewestem flank of the Michigan Basin have recently been studied by Harris and Waldhuetter(1996). There are many excellent studies of Ordovician through Devonian age strata innorthern Iowa, largely conducted by the Iowa Geological Survey; the results can often beextrapolated to Minnesota and Wisconsin (e.g. several papers in Witzke and others, 1996)Some regional scale studies have recently been completed; a stratigraphic framework hasbeen developed for the Prairie du Chien Group from the Michigan basin to the Hollandaleembayment (Smith and others, 1993, 1996; Barnes and others, 1996). Simo and others(1997) are currently constructing a large scale stratigraphic framework for Ordovician rocksof the central midcontinent in order to address long-standing problems such as identificationof the controls responsible for the apparently synchronous regional-scale changes from"tropical" to "temperate" styles of carbonate deposition.

27

others, 1992; Mossler, 1992; Runkel, 1994). In a very important and widely cited study Dott and others (1986) examined the transition from nonmarine to marine environments recorded in quartzose sandstones, and provided the fmt set of criteria for identification of nonmarine facies.They described an early Paleozoic terrestrial setting with broad braided streams and eolian sheet and erg systems that delivered sand to a shallow marine environment dominated by small three dimensional dunes. Dott and others (1986) stressed the dzfferences, such as the lack of vegetation, between early Paleozoic depositional environments and those inferred for both younger rocks and modem settings. A depositional model of a simple, storm-dominated, texturally graded shelf was proposed by Runkel and others (1998), and is noteworthy in its similari~ to models based on both younger rocks and modem systems. Their study was essentially a regional-scale outgrowth of the excellent outcrop work of Charlie Bell and his students from the University of Minnesota in the 1950's. Runkel and others (1998) suggested that early Paleozoic depositional processes and facies distribution are analogous to those depicted in well-known models of the Cretaceous interior seaway and the modem Bering Shelf. The thin widespread siliciclastic units chamcteristic of lower Paleozoic strata result from large lateral facies migrations which o c c m d in response to continental-scale changes in relative sea level.

Figure 3. Conceptual depositional model of Late Cambrian storm-dominated, textxraily graded shelf. Quartzose fme- to coarse- gained sand accumulated in nearshore terrestrial and shoreface environments. Finer grained , feldspathic siliciclastic sediments and subtidal carbonates accumulated in deeper water of the offshoreshelf.

Carbonate rocks within the lower Paleozoic sequence have historically been regarded as more amenable to comparison with both modem and ancient facies models. Deposition occurred in environments that m g e from evaporitic, supratidal conditions for parts of the Prairie du Chien Group (Smith and others, 1993) to subtidal settings in depths of 100 m or more for parts of units such as the Decorah Shale (Ludvigson and others, 1996). Most investigations have been local in scope relative to the g m t lateral extent of these units and there are too many to cite them individually. Ordovician carbonate strata of the Hollandale embayment are the focus of a monograph edited by Sloan (1986). Silurian carbonates on the westem flank of the Michigan Basin have recently been studied by Harris and Waldhuetter (1996). There are many excellent studies of Ordovician through Devonian age strata in northem Iowa, largely conducted by the Iowa Geological Survey; the results can often be extrapolated to Minnesota and Wisconsin (e.g. several papers in Witzke and others, 1996) Some regional scale studies have recently been completed; a stmtigxaphic framework has been developed for the Fhkie du Chien Group from the Michigan basin to the Hollandale embayment (Smith and others, 1993,1996; Barnes and others, 1996). Simo and others (1997) are currently constructing a large scale stratigraphic framework for Ordovician rocks of the central midcontinent in order to address long-standing problems such as identification of the controls responsible for the apparently synchronous regional-scale changes from %opicalV1 to "temperate1' styles of carbonate deposition. .

NOTABLE FEATURES AND LONGSTANDING PROBLEMS

Lower Paleozoic strata in the central midcontinent region are among the longest studiedsedimentary rocks in North America. They are well known to geologists outside of the areafor several enigmatic features (Dott and Byers, 1980). Most notable is the extreme texturaland mineralogical maturity of the fme to coarse-grained sandstones, and the sheet-likegeometry of these and other siliciclastic units. The overall dearth of shale has puzzledsedimentologists, and the fundamental controls on the episodic change from siliciclastic-dominated to carbonate-dominated sedimentation remain poorly understood. Lastly, thepresence, position and magnitude of unconformities has been debated for decades. Theremainder of this abstract summarizes the progress made in understanding these enigmaticfeatures since the overview publication by Dott and Byers (1980) on lower Paleozoic strata.

Sheet geometry A long-standing question has been how sheet-like siliciclastic layers wereinitially deposited and then preserved across tens of thousands of square kilometers of thecratonic shelf. Most workers have attributed the formation of such sheets to depositionalprocesses and environments that are markedly different from those described for mostyounger rocks and modern settings. Lochman-Balk (1970) inferred the presence ofenormous tidal flats extending well over two hundred kilometers perpendicular to theshoreline. Dott and Byers (1980) suggested that quartzose sand aggraded more or lessvertically into a sheet under high-energy conditions across a vast shallow sea. The mostwidely cited hypothesis is one which attributes the origin of quartzose sandstone sheets tofluvial and eolian processes that dispersed sand across a vegetation free landscape prior tomarine reworking during a transgression (Dott and others 1986). Recent studies, however,have demonstrated that some quartzose sheets were deposited entirely within the marinerealm under regressive conditions (e.g. Runkel, 1994, Hughes and Hesselbo, 1997, Runkeland others 1998), and that the depositional processes responsible for the dispersal ofsiliciclastic sediments were similar to those operating during the deposition of ancientsandstones that are not sheet-like.

The lateral persistence and sheet-like geometry of lower Paleozoic siliciclastic unitsprobably reflect the roles of basin physiography and tectonics in controlling thelithostratigraphic architecture, rather than the existence of atypical sedimentaryenvironments. Deposition was characterized by a continuous and abundant supply ofsediment to a relatively stable, nearly fiat basin with a slow, uniform rate of subsidence.Individual sheets of siliciclastic sediment were deposited when discrete facies (e.g. Fig. 3)migrated great distances during changes in sea level. Deep incision of the individual sheetsduring episodes of subaerial exposure did not occur, resulting in more or less uniformpreservation.

Textural maturity of sandstones The extreme textural and mineralogical maturity of the fine-to coarse- grained sandstone units, such as the St Peter Sandstone, may be the best knownand longest studied feature in the lower Paleozoic strata of the central midcontinent. Thesandstones contain more than 98% quartz and most grains are moderately to well rounded.Such textural and compositional maturity could not have been achieved solely by fluvial andmarine abrasion even over transport distances of hundreds of kilometers if the grains werederived directly from crystalline source rocks. Rare grains with abraded overgrowths arereworked from older sedimentary rocks, but the volumetric significance of such recycling isuncertain and a source of suitable sedimentary rock has never been identified. Odom (1975,1978) conducted the most comprehensive mineralogical study of lower Paleozoicsandstones; he noted that the very fme grained fraction is feldspathic, whereas the fine tocoarse-grained fraction contains more than 98% quartz (with the exception of some lower MtSimon Sandstone beds). He suggested that a long history of abrasion in a marine setting

28

NOTABLE FEATURES AND LONGSTANDING PROBLEMS

Lower Paleozoic strata in the central midcontinent region are among the longest studied sedimentary rocks in North America. They are well known to geologists outside of the area for several enigmatic features (Dott and Byers, 1980). Most notable is the extreme textural and mineralogical maturity of the fine to coarse-grained sandstones, and the sheet-like geometry of these and other siliciclastic units. The overall dearth of shale has p u l e d sedimentologists, and the fundamental controls on the episodic change from siliciclastic- dominated to carbonate-dominated sedimentation remain poorly understood. Lastly, the presence, position and magnitude of unconformities has been debated for decades. The remainder of this abstract summarizes the progress made in understanding these enigmatic features since the overview publication by Dott and Byers (1980) on lower Paleozoic strata.

Sheet geometry A long-standing question has been how sheet-like siliciclastic layers were initially deposited and then preserved across tens of thousands of square kilometers of the cratonic shelf. Most workers have attributed the formation of such sheets to depositional processes and environments that are markedly different from those described for most younger rocks and modern settings. Lochman-Balk (1970) inferred the presence of enormous tidal flats extending well over two hundred kilometers perpendicular to the shoreline. Dott and Byers (1980) suggested that quartzose sand aggraded more or less vertically into a sheet under high-energy conditions across a vast shallow sea. The most widely cited hypothesis is one which attributes the origin of quartzose sandstone sheets to fluvial and eolian processes that dispersed sand across a vegetation free landscape prior to marine reworking during a bansgression (Dott and others 1986). Recent studies, however, have demonstrated that some quartzose sheets were deposited entirely within the marine realm under regressive conditions (e.g. Runkel, 1994, Hughes and Hesselbo, 1997, Runkel and others 1998), and that the depositional processes responsible for the dispersal of siliciclastic sediments were similar to those operating during the deposition of ancient sandstones that are not sheet-like.

The lateral persistence and sheet-like geometry of lower Paleozoic siliciclastic units probably reflect the roles of basin physiography and tectonics in controlling the lithostratigmphic architecture, rather than the existence of atypical sedimentary environments. Deposition was characterized by a continuous and abundant supply of sediment to a relatively stable, nearly flat basin with a slow, uniform rate of subsidence. Individual sheets of siliciclastic sediment were deposited when discrete facies (e.g. Fig. 3) migrated great distances during changes in sea level. Deep incision of the individual sheets during episodes of subaerial exposure did not occur, resulting in more or less uniform preservation.

Textural maturitv of sandstones The extreme textural and mineralogical maturity of the fine- to coarse- grained sandstone units, such as the St Peter Sandstone, may be the best known and longest studied feature in the lower Paleozoic strata of the central midcontinent. The sandstones contain more than 98% quartz and most grains are moderately to well rounded. Such textural and compositional maturity could not have been achieved solely by fluvial and marine abrasion even over transport distances of hundreds of kilometers if the grains were derived directly from crystalline source rocks. Rare grains with abraded overgrowths are reworked from older sedimentary rocks, but the volumetric significance of such recycling is uncertain and a source of suitable sedimentary rock has never been identified. Odom (1975, 1978) conducted the most comprehensive mineralogical study of lower Paleozoic sandstones; he noted that the very fine grained fraction is feldspathic, whereas the fine to coarse-grained fraction contains more than 98% quartz (with the exception of some lower Mt Simon Sandstone beds). He suggested that a long history of abrasion in a marine setting

could account for both the maturity of the quartzose grains and the reduction in size offeldspar grains. Dott and others (1986) were the first to clearly identify the presence ofsubstantial eolian deposits within some quartzose sandstones, and they suggested thateolian abrasion could contribute to the textural and mineralogic features described by Odom(1975, 1978). Lastly, chemical weathering in the source area must be taken intoconsideration (Morey, 1972). Odom (1975, 1978) noted that the very fine grained,feldspathic sandstones are rich in K-feldspar but have only a "trace" amount of plagioclasegrains, even though plagioclase was presumably dominant in the source area of thePrecambrian shield. Selective diagenetic leaching of plagioclase grains from the sandstonewas discounted by Odom. Thus, chemical weathering probably preferentially dissolvedplagioclase crystals and reduced the size and amount of other relatively unstable mineralsprior to mechanical abrasion during transport to the shoreline.

While each of the processes described above could have been responsible in part for themineralogic and textural attributes of lower Paleozoic sandstones, they do not entirelyaccount for the compositional record that appears to indicate that virtually all fine- tocoarse-grained sand was exceptionally mature when it arrived to the early Paleozoicshoreline. Fine- to coarse-grained immature sand would have almost certainly beendeposited locally if the marine abrasion model of Odom (1975) were accurate. It is difficultto reconstruct a terrestrial setting in which all sand is subjected to prolonged eolian abrasion(Dott and others 1986) prior to deposition in a marine environment.

Dearth of Shale Lower Paleozoic rocks of the central midcontinent region are noted for adearth of shale compared to other shallow marine deposits. Two very different hypotheseshave been proposed to account for this dearth of shale. In a "marine bypassing" model,Pettijohn and others (1973) suggested that clay- and silt-sized particles were delivered to theshoreline, but subsequently carried in suspension by repeated storms across the shallowshelf before final burial in basinward areas. In contrast, Dairymple and others (1985)suggested that strong winds blew clay and silt particles hundreds to thousands of kilometersoffshore, to be ultimately deposited in the outer detrital belt (Fig 1).

Recent investigations of Upper Cambrian siliciclastic strata support the marine bypassingmodel of Pettijohn and others (1973), demonstrating that the well known dearth of shale ischaracteristic only of the outcrop belt on the flanks of the Wisconsin dome and arch(McKay, 1988, Runkel and others, 1998). Basinward of the Wisconsin arch, subsurfacerecords from Minnesota and Iowa show that facies dominated by shale and siltstone arecommon. The shale and siltstone dominated facies are offshore equivalents to the shale-poorsandstone facies of the outcrop belt, and their geographic position changed through time inresponse to changes in the position of the storm-fairweather wave base. The outcrop belt hasa dearth of shale simply because the depositional record is dominated by highstand shorefacefacies deposited above fairweather wave base. This stratigraphic and sedimentologicattribute is not unique to the lower Paleozoic rocks of the central midcontinent region-it iscommon in many younger wave-dominated sequences along the landward margins ofindividual basins. The notion of a shale-free early Paleozoic epeiric sea is a deeplyentrenched idea that simply reflects the bias of the outcrop belt. The dearth of shale inlower Paleozoic rocks should be removed from the list of enigmatic features unless thescarcity can be demonstrated to exist on a scale larger than that of the outcrop belt.

Siliciclastic-Carbonate cycles The fundamental controls on the transition from nearshoresiliciclastic- to carbonate-dominated deposition have never been satisfactorily understood.For example, in Early Ordovician time carbonate deposition apparently dominated across theentire region even in the shallowest water conditions (Smith and others 1993). Both older(Jordan Sandstone) and younger (St Peter Sandstone) shallow marine environments weredominated by siliciclastics. Only speculative and vague hypotheses have been suggested toaccount for such fundamental changes. For example, Adams (1978) suggested that the EarlyOrdovician change in depositional styles may have been in response to drowning of the

29

could account for both the maturity of the quartzose grains and the reduction in size of feldspar grains. Dott and others (1986) were the fmt to clearly identi@ the presence of substantial eolian deposits within some quarkose sandstones, and they suggested that eolian abrasion could contribute to the textural and mineralogic features described by Odom (1975, 1978). Lastly, chemical weathering in the source area must be taken into consideration (Morey, 1972). Odom (1975, 1978) noted that the very fine grained, feldspathic sandstones are rich in K-feldspar but have only a "trace" amount of plagioclase grains, even though plagioclase was presumably dominant in the source area of the Precambrian shield. Selective diagenetic leaching of plagioclase grains from the sandstone was discounted by Odom. Thus, chemical weathering probably preferentially dissolved plagioclase crystals and reduced the size and amount of other relatively unstable minerals prior to mechanical abrasion during transport to the shoreline.

While each of the processes described above could have been responsible in part for the mineralogic and textural attributes of lower Paleozoic sandstones, they do not entirely account for the compositional record that appears to indicate that virtually all fme- to coarse-grained sand was exceptionally mature when it arrived to the early Paleozoic shoreline. Fine- to coarse-grained immature sand would have almost certainly been deposited locally if the marine abmion model of Odom (1975) were accurate. It is difficult to reconstruct a terrestrial setting in which all sand is subjected to prolonged eolian abrasion (Dott and others 1986) prior to deposition in a marine environment.

Dearth of Shale Lower Paleozoic rocks of the centml midcontinent region are noted for a dearth of shale compared to other shallow marine deposits. Two very d i f fe~nt hypotheses have been proposed to account for this dearth of shale. In a "marine bypassing" model, Pettijohn and others (1973) suggested that clay- and silt-sized particles were delivered to the shoreline, but subsequently carried in suspension by repeated storms across the shallow shelf before final burial in basinward areas. In contrast, Dalrymple and others (1985) suggested that strong winds blew clay and silt particles hundreds to thousands of kilometers offshore, to be ultimately deposited in the outer detrital belt (Fig 1).

Recent investigations of Upper Cambrian siliciclastic strata support the marine bypassing model of Pettijohn and others (1973), demonstrating that the well known dearth of shale is characteristic only of the outcrop belt on the flanks of the Wisconsin dome and arch (McKay, 1988, Runkel and others, 1998). Basinward of the Wisconsin arch, subsurface records from Minnesota and Iowa show that facies dominated by shale and siltstone are common. The shale and siltstone dominated facies are offshore equivalents to the shale-poor sandstone facies of the outcrop belt, and their geographic position changed through time in response to changes in the position of the storm-fairweather wave base. The outcrop belt has a dearth of shale simply because the depositional record is dominated by highstand shoreface facies deposited above fairweather wave base. This stratigraphic and sedimentologic attribute is not unique to the lower Paleozoic rocks of the central midcontinent region-it is common in many younger wave-dominated sequences along the landward margins of individual basins. The notion of a shale-free early Paleozoic epeiric sea is a deeply entrenched idea that simply reflects the bias of the outcrop belt. The dearth of shale in lower Paleozoic rocks should be removed from the list of enigmatic features unless the scarcity can be demonstrated to exist on a scale larger than that of the outcrop belt.

Siliciclastic-Carbonate cycles The fundamental controls on the transition from nearshore siliciclastic- to carbonate-dominated deposition have never been satisfactorily understood. For example, in Early Ordovician time carbonate deposition apparently dominated across the entire region even in the shallowest water conditions (Smith and others 1993). Both older (Jordan Sandstone) and younger (St Peter Sandstone) shallow marine environments were dominated by siliciclastics. Only speculative and vague hypotheses have been suggested to account for such fundamental changes. For example, Adams (1978) suggested that the Early Ordovician change in depositional styles may have been in response to drowning of the

siliciclastic source area by a shallow sea or the result of siliciclastics being "deflected" toanother region. Early Paleozoic carbonate-dominated systems developed following extendedperiods of high sea level (Smith and others, 1993) which suggests that the siliciclasticsource area may have been covered with a blanket of carbonate rocks. The spread of certainforms of terrestrial life such as bacterial encrustation, and later the development of vascularplants, may have also reduced siliciclastic input.

Crvntic unconformities Previous investigations of lower Paleozoic strata of the centralmidcontinent region have also been hindered by an inability to recognize unconformities.The unconformities bounding the major sequences of Sloss (1963) are fairly wellestablished within the strata of the outcrop belt. The presence and position of "lesser"unconformities, especially within siliciclastic units, have been a matter of considerabledebate. Stratigraphic relations and interpreted depositional histories indicate that suchunconformities "should" be present. However, little or no evidence for regionally extensivesubaerial erosion has been documented. Such sequence-bounding unconformities aredifficult to recognize where they are contained within coarse siiciclastics because theyseparate quartzose sandstones that are texturally and mineralogically similar, and becausethey are relatively flat-reflecting erosion that occurred on a loose, sandy substrate along alow, uniform gradient, and in a nonvegetated terrestrial environment. Furthermore, the ultramature mineral composition of the exposed substrate inhibits development of a distinctiveweathering profile.

Recent work has shown some promise for identifying subtle subaerial surfaces oferosion. Recognition of cryptic unconformities requires interpretation of regionalstratigraphic relations in conjunction with physical evidence collected at individual outcropsand high resolution biostratigraphic data. Smith and others (1993) interpreted local depositsof silica cement in uppermost Cambrian and Lower Ordovician strata as subaerially formedsilcrete. Runkel and others (1998) suggested that cryptic unconformities within the frontonand Galesville Sandstones and at the top of the Jordan Sandstone and can be identified onlyby overlying lag deposits. These lags deposits are the coarsest bed within nearshoresandstone successions; they separate a regionally traceable, decameter-scale, coarsening-upward interval below from a decameter-scale, fming-upward sequence above. Preliminaryresults of high resolution biostratigraphic dating using conodonts has verified the presenceof an erosion surface on top of the Jordan Sandstone, and demonstrates promise formeasuring the magnitude of such cryptic unconformities with detailed paleontologic work.

REFERENCES

Adams, R. L.,1978, Stratigraphy and petrology of the lower Oneota Dolomite (Ordovician),south-central Wisconsin: Wisconsin Geological and Natural History Survey Field TripGuidebook 3, p. 82-90.

Barnes D. A., Lundgren C. E., and Longman, M. W., 1992, Sedimentology and diagenesisof the St. Peter Sandstone, Central Michigan Basin, United States: American Associationof Petroleum Geologists Bulletin, v. 76, no. 10, p. 1507-1532.

Barnes, D. A., Harrison Ill, W. B., and Shaw, T. H., 1996, Lower-Middle OrdovicianLithofacies and Interregional Correlation, Michigan Basin, USA, in, Witzke, B.J.,Ludvigson, G. A., and Day, J., eds., Paleozoic Sequence Stratigraphy:Views from theNorth American Craton: Geological Society of America Special Paper 306, p. 35-54.

Bell, W. C., Berg, R. R., and Nelson, C. A., 1956, Croixan type area—Upper MississippiValley, in Rodgers, J., ed., El Sistema Cambrico, su Paleogeografia y el problema de suBase, Tomo II, Parte fl:Australia, America: XX Congreso Geological Intemacional,Mexico, p. 415-446.

Berg, R. R., 1954, Franconia Formation of Minnesota and Wisconsin: Geological Society ofAmerica Bulletin, v. 65, p. 857-882.

30

siliciclastic source area by a shallow sea or the result of siliciclastics being "deflected" to another region. b l y Paleozoic carbonate-dominated systems developed following extended periods of high sea level (Smith and others, 1993) which suggests that the siliciclastic source area may have been covered with a blanket of carbonate rocks. The spread of certain forms of terrestrial life such as bacterial encrustation, and later the development of vascular plants, may have also reduced siliciclastic input.

C w t i c unconformities Previous investigations of lower Paleozoic strata of the central midcontinent region have also been hindered by an inability to recognize unconformities. The unconformities bounding the major sequences of Sloss (1963) are fairly well established within the strata of the outcrop belt. The presence and position of "lesser" unconformities, especially within siliciclastic units, have been a matter of considerable debate. Stratigraphic relations and interpreted depositional histories indicate that such unconformities "should" be present. However, little or no evidence for regionally extensive subaerial erosion has been documented. Such sequence-bounding unconformities are difficult to recognize where they are contained within coarse siliciclastics because they separate quartzose sandstones that are texturally and mineralogically similar, and because they are relatively flat-reflecting erosion that occurred on a loose, sandy substrate along a low, uniform gradient, and in a nonvegetated terrestrial environment. Furthermore, the ultra mature mineral composition of the exposed substrate inhibits development of a distinctive weathering profile.

Recent work has shown some promise for identifying subtle subaerial surfaces of erosion. Recognition of cryptic unconformities requires interpretation of regional stratigraphic relations in conjunction with physical evidence collected at individual outcrops and high resolution biostratigraphic data. Smith and others (1993) interpreted local deposits of silica cement in uppermost Cambrian and Lower Ordovician strata as subaerially formed silcrete. Runkel and others (1998) suggested that cryptic unconformities within the Ironton and Galesville Sandstones and at the top of the Jordan Sandstone and can be identified only by overlying lag deposits. These lags deposits are the coarsest bed within nearshore sandstone successions; they separate a regionally traceable, decameter-scale, coarsening- upward interval below from a decameter-scale, fining-upward sequence above. Preliminary results of high resolution biostratigraphic dating using conodonts has verified the presence of an erosion surface on top of the Jordan Sandstone, and demonstrates promise for measuring the magnitude of such cryptic unconformities with detailed paleontologic work.

REFERENCES

Adams, R. L., 1978, Stratigraphy and petrology of the lower Oneota Dolomite (Ordovician), south-central Wisconsin: Wisconsin Geological and Natural History Survey Field Trip Guidebook 3, P- 82-90.

Barnes D. A., Lundgren C. E., and Longman, M. W., 1992, Sedimentology and diagenesis of the St. Peter Sandstone, Central Michigan Basin, United States: American Association of Petroleum Geologists Bulletin, v. 76, no. 10, p. 1507-1532.

Barnes, D. A., Harrison 111, W. B., and Shaw, T. H., 1996, Lower-Middle Ordovician Lithofacies and Interregional Correlation, Michigan Basin, USA, in, Witzke, B.J., Ludvigson, G. A., and Day, J., eds., Paleozoic Sequence Stratigraphy:Views from the North American Craton: Geological Society of America Special Paper 306, p. 35-54.

Bell, W. C., Berg, R. R., and Nelson, C. A., 1956, Croixan type area-Upper Mississippi Valley, in Rodgers, J., ed., El Sistema Cambrico, su Paleogeografia y el problema de su Base, Tom0 11, Parte IkAustralia, America: XX Congreso Geological Internacional, Mexico, p. 415-446.

Berg, R. R., 1954, Franconia Formation of Minnesota and Wisconsin: Geological Society of America Bulletin, v. 65, p. 857-882.

Berg, R. R., Nelson, C. A., and Bell, W. C., 1956, Upper Cambrian rocks in southeastMinnesota, in Sloan, R., and Schwartz, G. M., eds., Lower Paleozoic geology of theUpper Mississippi Valley: Geological Society of America Guidebook Series, Field Trip 2,p. 1-23.

Byers, C. W., and Doll, R. H., Jr., 1995, Sedimentology and depositional sequences of theJordan Formation (Upper Cambrian), Northern Mississippi Valley: Journal ofSedimentary Petrology, v. B65, no. 3, p. 289-305.

Dairymple, R. W., Narbonne, G. M., and Smith, L., 1985, Eolian action and the distributionof Cambrian shales in North America: Geology, v. 13, p. 607-610.

Dott, R. H., Jr., 1978, Sedimentology of Upper Cambrian cross-bedded sandstone facies asexemplified by the Van Oser Sandstone: Wisconsin Geological and Natural HistorySurvey Field Trip Guidebook 3, p. 52-66.

Dott, R. H., Jr, and Byers, C. W., 1980, SEPM research conference on modern shelf andancient cratonic sedimentation—the orthoquartzite-carbonate suite revisited: Society ofEconomic Paleontologists and Mineralogists Research Conference Field Trip Guidebookno. 1, 61 p.

Dolt, R. H., Jr., Byers, C. W., Fielder, G. W., Stenzel, S. R., and Winfree, K. E., 1986,Aeolian to marine transition in Cambro-Ordovician cratonic sheet sandstones of thenorthern Mississippi Valley, USA: Sedimentology, v. 33, p. 345-367.

Driese, S. G., Byers, C. W., and Dolt, R. H., Jr., 1981, Tidal deposits in the basal UpperCambrian Mt. Simon Sandstone in Wisconsin: Journal of Sedimentary Petrology, v. 51,p. 367-381.

Fraser, G. S., 1976, Sedimentology of a middle Ordovician quartz arenite-carbonate transitionin the Upper Mississippi Valley: Geological Society of America Bulletin , v. 86, p. 833-845.

Haddox, C. A., and Dolt, R. H., Jr., 1990, Cambrian shoreline deposits in northernMichigan: Journal of Sedimentary Petrology, v. 60., no. 5., p. 697-7 16.

Harris, M. T., and Waldhuetter, K. R., 1996, Silurian of the Great Lakes region, Part 3:Landoveiy strata of the Don Peninsula: Milwaukee Museum Contributions in Biology andGeology, n. 90, 162 p.

Hughes, N. C., and Hesselbo, S. P., 1997, Stratigraphy and sedimentology of the St.Lawrence Formation, Upper Cambrian of the northern Mississippi Valley: MilwaukeePublic Museum Contributions in Biology and Geology, no.91, SOp..

Lochman-Balk, C., 1970, Upper Cambrian faunal patterns on the craton: Geological Societyof America Bulletin, v. 81, p. 3197-3224.

Ludvigson, G. A., Jacobson, S. R., Witzke, B. J., and Gonzalez, L. A., 1996, Carbonatecomponent chemostratigraphy and depositional history of the Ordovician DecorahFormation, Upper Mississippi Valley, in Witzke, B. J., Ludvigson, G. A., and Day, J.,eds., Paleozoic Sequence Stratigraphy: Views from the North American Craton:GeologicalSociety of America Special Paper 306, p.67-86.

McKay, R. M., 1988, Stratigraphy and lithofacies of the Dresbachian (Upper Cambrian) EauClaire Formation in the subsurface of eastern Iowa: in Ludvigson, G. A., and Bunker B.A., eds., New perspectives on the Paleozoic history of the Upper Mississippi Valley,Guidebook for the 18th Field Conference of the Great Lakes section, Society of EconomicPaleontologists and Mineralogists, p. 33-53.

Morey, G. B., 1972, Pre-Mt Simon Regolith, in Sims, P. K., and Morey, G. B., eds.,Geology of Minnesota: A Centennial Volume: Minnesota Geological Survey, p. 506-508.

Mossler, J. H., 1992, Sedimentary rocks of Dresbachian age (Late Cambrian), HollandaleEmbayment, southeastern Minnesota: Minnesota Geological Survey Report ofInvestigations 40, 71 p.

Nelson, C. A., 1956, Upper Croixan stratigraphy, Upper Mississippi Valley: GeologicalSociety of America Bulletin, v. 67, p. 165-183.

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Berg, R. R., Nelson, C. A., and Bell, W. C., 1956, Upper Cambrian rocks in southeast Minnesota, in Sloan, R., and Schwartz, G. M., eds., Lower Paleozoic geology of the Upper Mississippi Valley: Geological Society of America Guidebook Series, Field Trip 2, p. 1-23.

Byers, C. W., and Dott, R. H., Jr., 1995, Sedimentology and depositional sequences of the Jordan Formation (Upper Cambrian), Northern Mississippi Valley: Journal of Sedimentary Petrology, v. B65, no. 3, p. 289-305.

Dalrymple, R. W., Narbonne, G. M., and Smith, L., 1985, Eolian action and the distribution of Cambrian shales in North America: Geology, v. 13, p. 607-610.

Dott, R. H., Jr., 1978, Sedimentology of Upper Cambrian cross-bedded sandstone facies as exemplified by the Van Oser Sandstone: Wisconsin Geological and Natural History Survey Field Trip Guidebook 3, p. 52-66.

Dott, R. H., Jr, and Byers, C. W., 1980, SEPM research conference on modem shelf and ancient cratonic sedimentation-the orthoquartzite-carbonate suite revisited: Society of Economic Paleontologists and Mineralogists Research Conference Field Trip Guidebook no. 1, 61 p.

Dott, R. H., Jr., Byers, C. W., Fielder, G. W., Stenzel, S. R., and Winfree, K. E., 1986, Aeolian to marine transition in Cambro-Ordovician cratonic sheet sandstones of the northern Mississippi Valley, USA: Sedimentology, v. 33, p. 345-367.

Driese, S. G., Byers, C. W., and Dott, R. H., Jr., 1981, Tidal deposits in the basal Upper Cambrian Mt. Simon Sandstone in Wisconsin: Journal of Sedimentary Petrology, v. 5 1, p. 367-381.

Fraser, G. S., 1976, Sedimentology of a middle Ordovician quartz arenite-carbonate transition in the Upper Mississippi Valley: Geological Society of America Bulletin , v. 86, p. 833- 845.

Haddox, C. A., and Dott, R. H., Jr., 1990, Cambrian shoreline deposits in northern Michigan: Journal of Sedimentary Petrology, v. 60., no. 5., p. 697-716.

Harris, M. T., and Waldhuetter, K. R., 1996, Silurian of the Great Lakes region, Part 3: Landovery strata of the Dorr Peninsula: Milwaukee Museum Contributions in Biology and Geology, n. 90, 162 p.

Hughes, N . C., and Hesselbo, S. P., 1997, Stratigraphy and sedimentology of the St. Lawrence Formation, Upper Cambrian of the northern Mississippi Valley: Milwaukee Public Museum Contributions in Biology and Geology, no.91,50 p. .

Lochman-Balk, C., 1970, Upper Cambrian faunal patterns on the craton: Geological Society of America Bulletin, v. 81, p. 3197-3224.

Ludvigson, G. A., Jacobson, S. R., Witzke, B. J., and Gonzalez, L. A., 1996, Carbonate component chemostratigmphy and depositional history of the Ordovician Decorah Formation, Upper Mississippi Valley, in Witzke, B. J., Ludvigson, G. A., and Day, J., eds., Paleozoic Sequence Stratigraphy: Views from the North American Craton:Geological Society of America Special Paper 306, p.67-86.

McKay, R. M., 1988, Stratigraphy and lithofacies of the Dresbachian (Upper Cambrian) Eau Claire Formation in the subsurface of eastern Iowa: in Ludvigson, G. A., and Bunker B. A., eds., New perspectives on the Paleozoic history of the Upper Mississippi Valley, Guidebook for the 18th Field Conference of the Great Lakes section, Society of Economic Paleontologists and Mineralogists, p. 33-5 3.

Morey, G. B., 1972, Pre-Mt Simon Regolith, in Sims, P. K., and Morey, G. B., eds., Geology of Minnesota: A Centennial Volume: Minnesota Geological Survey, p. 506-508.

Mossier, J. H., 1992, Sedimentary rocks of Dresbachian age (Late Cambrian), Hollandale Embayment, southeastern Minnesota: Minnesota Geological Survey Report of Investigations 40,71 p.

Nelson, C. A., 1956, Upper Croixan stratigraphy, Upper Mississippi Valley: Geological Society of America Bulletin, v. 67, p. 165-1 83.

Odom, I. E., 1975, Feldspar-grain size relations in Cambrian arenites, Upper MississippiValley: Journal of Sedimentary Petrology, v. 45, no. 3, p. 636-650.

Odom I. E. 1978, Mineralogy of Cambrian sandstones, Upper Mississippi Valley:Wisconsin Geological and Natural History Survey Field Trip Guidebook 3, p. 23-45.

Ostrom, M. E., 1964, Pre-Cincinnatian Paleozoic cyclic sediments in the Upper MississippiValley: A discussion: Kansas Geological Survey Bulletin 169, p. 38 1-398.

Ostrom, M. E., 1970, Sedimentation cycles in Lower Paleozoic rocks of western Wisconsin:Wisconsin Geological and Natural History Survey Information Circular 11, p. 10-34.

Palmer, A. R., 1960, Some aspects of the early Upper Cambrian stratigraphy of White PineCounty , Nevada, and Vicinity, in Geology of east Central Nevada: IntermountainAssociation of Petroleum Geologists Guidebook 11th Annual Field Conference, p. 53-58.

Pettijohn, F. J., Potter, P. E., and Siever, R., 1973, Sand and Sandstone:New York,Springer-Verlag, 618p.

Runkel, A. C., 1994, Deposition of the uppermost Cambrian (Croixan) Jordan Sandstone,and the nature of the Cambrian-Ordovician boundary in the Upper Mississippi Valley:Geological Society of America Bulletin, v. 106, p. 492-506.

Runkel, A. C., McKay R. M., and Palmer, A. R.,1998, Origin of a classic cratonic sheetsandstone:Stratigraphy across the Sauk Il-Sauk III boundary in the Upper MississippiValley: Geological Society of America Bulletin v. 110, no. 2, p.188-210

Sargent, M. L., 1991, Sauk Sequence: Cambrian System through Lower Ordovician Series,in Leighton, M. W., Kolata, D. R., Oltz, D. F., and Eidel, J. J., eds., Interior cratonicbasins: American Association of Petroleum Geologists Memoir 51, p.75-86.

Simo, J. A., Choi, L, Freiberg, P., Byers, C. W., Doft, R. H., Jr., and Saylor, B., 1997,Sedimentology, Sequence Stratigraphy, and Paleoceanography of the Middle Ordovicianof eastern Wisconsin, in, Mudrey, M. G., ed, Guide to Field trips in Wisconsin andadjacent areas of Minnesota: Wisconsin Geological and Natural History Survey, p. 95-114.

Sloan, R. E., ed, 1986, Middle and Late Ordovician lithostratigraphy of the UpperMississippi Valley: Minnesota Geological Survey report of Investigations 35, 232p.

Sloss, L. L., 1963, Sequences in the cratonic interior of North America: Geological Society ofAmerica Bulletin, v. 74, p. 93-1 14.

Sloss, L. L., 1988, Tectonic evolution of the craton in Phanerozoic time, in Sloss, L.L., ed.,Sedimentary cover—North American craton, United States: The Geology of NorthAmerica, DNAG Volume D-2, p. 25-51.

Smith, G. L., Byers, C. W., and Dott, R. H., Jr., 1993, Sequence stratigraphy of the LowerOrdovician Prairie du Chien Group on the Wisconsin Arch and in the Michigan Basin:American Association of Petroleum Geologists Bulletin, v. 77, p. 49-67.

Smith, G. L., Byers, C. W., and Dott, R. H., Jr., 1996, Sequence stratigraphy of the Prairiedu Chien Group, Lower Ordovician, Midcontinent, USA, in, Witzke, B.J., Ludvigson,0. A., and Day, J., eds., Paleozoic Sequence Stratigraphy:Views from the NorthAmerican Craton: Geological Society of America Special Paper 306, p.23-34.

Witzke, B. J., Ludvigson, G. A., and Day, J., 1991, eds., Paleozoic SequenceStratigraphy:Views from the North American Craton: Geological Society of AmericaSpecial Paper 306, 446p.

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Odom, I. E., 1975, Feldspar-grain size relations in Cambrian arenites, Upper Mississippi Valley: Journal of Sedimentary Petrology, v. 45, no. 3, p. 636-650.

Odom I. E. 1978, Mineralogy of Cambrian sandstones, Upper Mississippi Valley: Wisconsin Geological and Natural History Survey Field Trip Guidebook 3, p. 23-45.

Ostrom, M. E., 1964, Pre-Cincinnatian Paleozoic cyclic sediments in the Upper Mississippi Valley: A discussion: Kansas Geological Survey Bulletin 169, p. 381-398.

Ostrom, M. E., 1970, Sedimentation cycles in Lower Paleozoic rocks of western Wisconsin: Wisconsin Geological and Natural History Survey Information Circular 11, p. 10-34.

Palmer, A. R., 1960, Some aspects of the early Upper Cambrian stratigraphy of White Pine County , Nevada, and Vicinity, in Geology of east Central Nevada: Intermountain Association of Petroleum Geologists Guidebook 1 lth Annual Field Conference, p. 53-58.

Pettijohn, F. J., Potter, P. E., and Siever, R., 1973, Sand and Sandstone:New York, Springer-Verlag, 6 18p.

Runkel, A. C., 1994, Deposition of the uppermost Cambrian (Croixan) Jordan Sandstone, and the nature of the Cambrian-Ordovician boundary in the Upper Mississippi Valley: Geological Society of America Bulletin, v. 106, p. 492-506.

Runkel, A. C., McKay R. M., and Palmer, A. R.,1998, Origin of a classic cratonic sheet sandstone:Stratigraphy across the Sauk 11-Sauk I11 boundary in the Upper Mississippi Valley: Geological Society of America Bulletin v. 110, no. 2, p.188-210

Sargent, M. L., 1991, Sauk Sequence: Cambrian System through Lower Ordovician Series, in Leighton, M. W., Kolata, D. R., Oltz, D. F., and Eidel, J. J., eds., Interior cratonic basins: American Association of Petroleum Geologists Memoir 51, p.75-86.

Simo, J. A., Choi, L, Freiberg, P., Byers, C. W., Dott, R. H., Jr., and Saylor, B., 1997, Sedimentology, Sequence Stratigraphy, and Paleoceanography of the Middle Ordovician of eastern Wisconsin, in, Mudrey, M. G., ed, Guide to Field trips in Wisconsin and adjacent areas of Minnesota: Wisconsin Geological and Natural History Survey, p. 95- 114.

Sloan, R. E., ed, 1986, Middle and Late Ordovician lithostratigraphy of the Upper Mississippi Valley: Minnesota Geological Survey report of Investigations 35,232~.

Sloss, L. L., 1963, Sequences in the cratonic interior of North America: Geological Society of America Bulletin, v. 74, p. 93-1 14.

Sloss, L. L., 1988, Tectonic evolution of the craton in Phanerozoic time, in Sloss, L.L., ed., Sedimentary cover-North American craton, United States: The Geology of North America, DNAG Volume D-2, p. 25-5 1.

Smith, G. L., Byers, C. W., and Dott, R. H., Jr., 1993, Sequence stratigraphy of the Lower Ordovician Prairie du Chien Group on the Wisconsin Arch and in the Michigan Basin: American Association of Petroleum Geologists Bulletin, v. 77, p. 49-67.

Smith, G. L., Byers, C. W., and Dott, R. H., Jr., 1996, Sequence stratigraphy of the Prairie du Chien Group, Lower Ordovician, Midcontinent, USA, in, Witzke, B.J., Ludvigson, G. A., and Day, J., eds., Paleozoic Sequence Stratigraphy:Views from the North American Craton: Geological Society of America Special Paper 306, p.23-34.

Witzke, B. J., Ludvigson, G. A., and Day, J., 1991, eds., Paleozoic Sequence Stratigraphy:Views from the North American Craton: Geological Society of America Special Paper 306,446~.

MODELS FOR INTERPRETING THE QUATERNARY HISTORYOF THE LAKE SUPERIOR REGION

PATTERSON, Carrie J., Minnesota Geological Survey

The interpretation of the Quaternaiy glacial and interglacial history for the Lake Superiorregion is in the midst of change as a result of two new models. Firstly, oxygen isotopedata have provided evidence leading to the development of a model that indicates a largenumber of glaciations occurred during the last two million years. Secondly, a newmechanical model for the dynamics of ice flow has been developed. In order to integratethe new models into our interpretations, a broad geographic perspective is required.Rather than provide a summary of the history of the investigations in the Lake Superiorregion, I will present relevant observations in the context of these new models. Even ifthe models are eventually replaced, they will have forced us to look beyond the confinesof the Upper Midwest and the traditional glacial interpretations.

Number of glaciationsEvidence indicates that the number of Quaternary glaciations is significantly greater thanthe four-fold glacial chronology devised for North America around the turn of the centurywhich included the Wisconsinan, Illinoian, Kansan and Nebraskan glaciations (Flint,1957). The new precision is based on nonterrestrial records of global ice volume,including the oxygen-isotope record of ocean carbonates. During glaciation, 160 ispreferentially evaporated from the oceans and is stored in the ice sheets, enriching theglacial oceans in 180. Fluctuations in the amount of 160 and 180 can be measured inocean sediments and ice cores (Shakleton and others, 1984) and used to determine globalice volume. Based on these calculations for global ice volumes, a series of oxygen-isotope stages has been identified. Even numbered oxygen-isotopes stages representglaciations, and odd numbers represent interglacial periods. There were 40glaciallinterglacial oscillations involving moderate global ice volumes between 2.4 Maand .9 Ma, and there were 22 oscillations involving greater ice volumes between .9 Maand the present (Shackleton et al., 1984). For the most recent 11 glaciations, oxygen-isotope stages 2, 6, 12 and 16 show the greatest ice volumes, possibly corresponding totraditional four-fold record of terrestrial glaciations in North America.

Another feature of the traditional four-fold record of glaciations is that the most extensiveglaciation formerly recognized, (the Nebraskan) was also interpreted to be the oldest.Successively less extensive glaciations, (the Kansan, Illinoian and Wisconsinan) wereinterpreted to be successively younger (Flint, 1957). This chronology is probably anartifact of the preservation of terrestrial glacial sediments. While there were clearly morethan four glacial periods during the past 0.9 my, each glacial advance obscures, either byerosion or burial, the record of previous, less extensive advances. If, for example, therewere ten successive glacial advances, each advancing some random distance, probabilityanaylsis suggests that only three glacial limits will survive at the surface and the oldestsurviving advance will be the most extensive (Gibbons and others, 1984). It is thereforenot surprising that of the 11 most recent glaciations we have historically recognized onlyfour glacial limits at the surface.

The oxygen isotope record shows that glaciations in the first half of the Pleistocene didnot have as great a global ice volume as those in the latter half (Shaldeton and others,1984). However, these early advances of ice were apparently extensive enough in North

33

MODELS FOR INTERPRETING THE QUATERNARY HISTORY OF THE LAKE SUPERIOR REGION

PATTERSON, Carrie J., Minnesota Geological Survey

The interpretation of the Quaternary glacial and interglacial history for the Lake Superior region is in the midst of change as a result of two new models. Firstly, oxygen isotope data have provided evidence leading to the development of a model that indicates a large number of glaciations occurred during the last two million years. Secondly, a new mechanical model for the dynamics of ice flow has been developed. In order to integrate the new models into our interpretations, a broad geographic perspective is required. Rather than provide a summary of the history of the investigations in the Lake Superior region, I will present relevant observations in the context of these new models. Even if the models are eventually replaced, they will have forced us to look beyond the confines of the Upper Midwest and the traditional glacial interpretations.

Number of glaciations Evidence indicates that the number of Quaternary glaciations is significantly greater than the four-fold glacial chronology devised for North America around the turn of the century which included the Wisconsinan, Illinoian, Kansan and Nebraskan glaciations (Flint, 1957). The new precision is based on nonterrestrial records of global ice volume, including the oxygen-isotope record of ocean carbonates. During glaciation, 160 is preferentially evaporated from the oceans and is stored in the ice sheets, enriching the glacial oceans in 180. Fluctuations in the amount of 160 and 180 can be measured in ocean sediments and ice cores (Shakleton and others, 1984) and used to determine global ice volume. Based on these calculations for global ice volumes, a series of oxygen- isotope stages has been identified. Even numbered oxygen-isotopes stages represent glaciations, and odd numbers represent interglacial periods. There were 40 glacialhterglacial oscillations involving moderate global ice volumes between 2.4 Ma and .9 Ma, and there were 22 oscillations involving greater ice volumes between .9 Ma and the present (Shackleton et al., 1984). For the most recent 11 glaciations, oxygen- isotope stages 2,6, 12 and 16 show the greatest ice volumes, possibly corresponding to traditional four-fold record of terrestrial glaciations in North America.

Another feature of the traditional four-fold record of glaciations is that the most extensive glaciation formerly recognized, (the Nebraskan) was also interpreted to be the oldest. Successively less extensive glaciations, (the Kansan, Illinoian and Wisconsinan) were interpreted to be successively younger (Flint, 1957). This chronology is probably an artifact of the preservation of terrestrial glacial sediments. While there were clearly more than four glacial periods during the past 0.9 my, each glacial advance obscures, either by erosion or burial, the record of previous, less extensive advances. If, for example, there were ten successive glacial advances, each advancing some random distance, probability anaylsis suggests that only three glacial limits will survive at the surface and the oldest surviving advance will be the most extensive (Gibbons and others, 1984). It is therefore not surprising that of the 11 most recent glaciations we have historically recognized only four glacial limits at the surface.

The oxygen isotope record shows that glaciations in the f i r t half of the Pleistocene did not have as great a global ice volume as those in the latter half (Shakleton and others, 1984). However, these early advances of ice were apparently extensive enough in North

America to reach Iowa and Nebraska (Hallberg, 1986) and the foothills of the CanadianRockies (Kiassen, 1989). The sediment of these early advances may be preserved inMinnesota (e.g. Meyer, 1997; Patterson, 1997) and in Canada in the subsurface (e.g.Kiassen, 1989) in areas of net glacial deposition, but the record is discontinuous andlocalized in deep, preglacial valleys.

Discontinuous, subsurface glacial sediment is difficult to place in a stratigraphicframework because: (1) absolute dating of the units is difficult; (2) till of different glacialperiods is lithologically and texturally similar if the source terrane traversed by theglacier is the same; (3) although till may change composition gradually with distance inthe ice-flow direction, isolated exposures of till may be dissimilar enough to preventcorrelation. Radiocarbon dating of organic remains is only useful for Wisconsinan-agedeposits because of the limitations of the half life of 14C. A few dates from scattered pre-Wisconsinan samples have been secured using vertebrate paleontology (e.g. Klassen,1989), dating of volcanic ash present in the glacial sediment (summarized in Richmondand Fullerton, 1986), studies of remnant magnetization in till and lake sediment (e.g.Baker and others, 1983), and cosmosgenic isotope age estimates of striated rock surfaces(Bierman and others, 1998).

Owing to the different substrates over which the ice flowed, it is possible to distinguishtills using matrix color, texture and mineralogy, and clast lithology. During the LateWisconsinan, Minnesota was nearly equidistant from the two major ice accumulationcenters of the Laurentide ice sheet and received ice from both (Dyke and Prest, 1986).Minnesota's unique position has facilitated provenance studies to determine ice-flowpaths and source areas. The glacial stratigraphy of Canada and Minnesota (Fulton, 1989;Meyer, 1997) indicate that ice sheets had similar geometries throughout the Quaternary.

At least five ice lobes advanced into Minnesota during oxygen-isotope stage 2 (LateWisconsinan). Many of these lobes had multiple readvances. During advance each lobehad the potential to erode older sediment and rock, and to deposit its own glacialsediment. Therefore, a single glacial period may be represented by ten's ofcompositionally and texturally different, incompletely preserved glacial units — eachhaving several depositional facies.

Interpretation of the most recent advances is most straightforward because the depositsare continuous, and the associated landforms are well preserved. For this reason,research in Minnesota and neighboring states has historically focused of the activity ofice lobes during the Late Wisconsinan (for regional summaries see Wright, 1972; Claytonand Moran, 1982).

Dynamics of ice flowDuring oxygen-isotope stage 2 (Late Wisconsinan), and probably during stages 6, 12 and16 (all pre-Late Wisconsinan), Minnesota was marginal to an ice sheet that extendedacross most of Canada. Minnesota and the Great Lakes region were affected mainly bythin, dynamic ice lobes that advanced beyond the main body of the ice sheet. Mucheffort has been made to correlate the advances of these lobes (Clayton and Moran, 1982;Mickelson and others, 1983) but it has become apparent that they were not synchronouson a time scale of hundreds to thousands of years. Movement of the ice lobes isultimately driven by the overall mass balance of the ice sheet, which is controlled byclimate. Ice lobes, however, respond quickly to changes in mass balance and bed

34

America to reach Iowa and Nebraska (Hallberg, 1986) and the foothills of the Canadian Rockies (Klassen, 1989). The sediment of these early advances may be preserved in Minnesota (e.g. Meyer, 1997; Patterson, 1997) and in Canada in the subsurface (e.g. Klassen, 1989) in areas of net glacial deposition, but the record is discontinuous and localized in deep, preglacial valleys.

Discontinuous, subsurface glacial sediment is difficult to place in a stratigraphic framework because: (1) absolute dating of the units is difficult; (2) till of different glacial periods is lithologically and texturally similar if the source terrane traversed by the glacier is the same; (3) although till may change composition gradually with distance in the ice-flow direction, isolated exposures of till may be dissimilar enough to prevent correlation. Radiocarbon dating of organic remains is only useful for Wisconsinan-age deposits because of the limitations of the half life of 14C. A few dates from scattered pre- Wisconsinan samples have been secured using vertebrate paleontology (e.g. Klassen, 1989), dating of volcanic ash present in the glacial sediment (summarized in Richmond and Fullerton, 1986), studies of remnant magnetization in till and lake sediment (e.g. Baker and others, 1983), and cosmosgenic isotope age estimates of striated rock surfaces (Bierman and others, 1998).

Owing to the different substrates over which the ice flowed, it is possible to distinguish tills using matrix color, texture and mineralogy, and clast lithology. During the Late Wisconsinan, Minnesota was nearly equidistant from the two major ice accumulation centers of the Laurentide ice sheet and received ice from both (Dyke and Prest, 1986). Minnesota's unique position has facilitated provenance studies to determine ice-flow paths and source areas. The glacial stratigraphy of Canada and Minnesota (Fulton, 1989; Meyer, 1997) indicate that ice sheets had similar geometries throughout the Quaternary.

At least five ice lobes advanced into Minnesota during oxygen-isotope stage 2 (Late Wisconsinan). Many of these lobes had multiple readvances. During advance each lobe had the potential to erode older sediment and rock, and to deposit its own glacial sediment. Therefore, a single glacial period may be represented by ten's of compositionally and texturally different, incompletely preserved glacial units - each having several depositional facies.

Interpretation of the most recent advances is most straightforward because the deposits are continuous, and the associated landforms are well preserved. For this reason, research in Minnesota and neighboring states has historically focused of the activity of ice lobes during the Late Wisconsinan (for regional summaries see Wright, 1972; Clayton and Moran, 1982).

Dynamics of ice flow During oxygen-isotope stage 2 (Late Wisconsinan), and probably during stages 6,12 and 16 (all pre-Late Wisconsinan), Minnesota was marginal to an ice sheet that extended across most of Canada. Minnesota and the Great Lakes region were affected mainly by thin, dynamic ice lobes that advanced beyond the main body of the ice sheet. Much effort has been made to correlate the advances of these lobes (Clayton and Moran, 1982; Mickelson and others, 1983) but it has become apparent that they were not synchronous on a time scale of hundreds to thousands of years. Movement of the ice lobes is ultimately driven by the overall mass balance of the ice sheet, which is controlled by climate. Ice lobes, however, respond quickly to changes in mass balance and bed

conditions of their respective icesheds and may therefore appear to be out of phase withnearby lobes and regional climate.

Because the ice lobes were comparatively thin—decreasing from approximately 1 kmwhere they left the ice sheet— exisiting structural and alluvial lowlands controlled thedirection of ice flow. These lowlands were deepened even more owing to the enhancedglacial erosion resulting from fast, focused flow of the ice. The Great Lakes are wellknown examples of basins that controlled the flow of lobes; others include Great BearLake in the Northwest Territories, Great Slave Lake in the District of MacKenzie, LakeAthabasca, on the Alberta-Saskatchewan border, Reindeer Lake in Saskatchewan, andLakes Winnipeg and Winnipegosis in Manitoba.

Evidence indicates that the development of ice lobes (or 'outlet lobes') may be the resultof ice streams (Patterson, 1997). Ice flow hundreds of km up-ice from the outlet lobesmay begin to converge into a narrow, fast-moving streams of ice (Dyke and Prest, 1986;Fulton, 1989; Thorliefsson and Kristjansson 1993; Fulton, 1995). The nature anddistribution of glacial sediment and landforms implies that ice flow was much different inthese narrow convergent zones. Till associated with ice streams is more distally derived,homogenous, traceable for hundreds of km, and typically has a level-to-streamlinedsurface expression. Tills deposited outside these narrow convergent zones are usuallythin and locally derived.

Models for the mechanics of ice movement that are based on the study of modem icesheets (Fowler and Johnson, 1995) predict that slow, uniform flow of ice is unstable andtherefore unlikely. What is predicted and observed, by contrast, is a bimodal style of iceflow, with fast ice streams (km/yr) moving within slower ice (cm/yr). The location of icestreams appears to change over time, but is influenced by bed topography andhydrogeology. Ice streams and their outlet lobes are the major discharge areas for an icesheet, and their development is largely a function of subglacial water. Ice streams drawdown the ice mass in the ice accumulation centers. Ice-lobe advances therefore representa redistribution of ice, and may actually signal the overall decay of the ice sheet ratherthan an increase in volume of the ice sheet.

The deglacial record of the Late Wisconsinan Laurentide ice sheet indicates that in theareas of continuous ice cover in Canada, ice streams developed within the ice sheet andfed the ice lobes that moved through the Great Lakes basins into the upper Midwest(Dyke and Prest, 1986; Hicock and Dreimanis, 1992; Thorleifson and Kristjansson,1993). The ice streams were active throughout the retreat of the ice across Canada asdocumented in the pattern of moraines (Dyke and Prest, 1986; Fulton, 1995). It isunclear if ice streams were also important during ice-sheet growth because the effects ofsubglacial water are difficult to ascertain. This model of ice flow can be cautiouslyextended to earlier glaciations, bearing in mind that bed conditions could have been verydifferent from those of more recent events.

The advantages of studying the geological record of Quaternary glaciations include: (1)the relatively excellent preservation, continuity and accessibility of the deposits; (2) thelocation of the deposits within a well established paleogeographic, paleomagnetic, andtectonic setting; (4) records for floral and faunal succession and pluvial lake-levelhistories that allow the construction of predictive global circulation models; (4) directevidence of global ice volume from the ocean oxygen istope record as well as the recordof crustal rebound; and (5) fairly easy access to modem analogs (for example, the study

35

conditions of their respective icesheds and may therefore appear to be out of phase with nearby lobes and regional climate.

Because the ice lobes were comparatively thin-decreasing from approximately 1 km where they left the ice sheet- exisiting structural and alluvial lowlands controlled the direction of ice flow. These lowlands were deepened even more owing to the enhanced glacial erosion resulting from fast, focused flow of the ice. The Great Lakes are well known examples of basins that controlled the flow of lobes; others include Great Bear Lake in the Northwest Territories, Great Slave Lake in the District of MacKenzie, Lake Athabasca, on the Alberta-Saskatchewan border, Reindeer Lake in Saskatchewan, and Lakes Winnipeg and Winnipegosis in Manitoba.

Evidence indicates that the development of ice lobes (or 'outlet lobes') may be the result of ice streams (Patterson, 1997). Ice flow hundreds of km up-ice from the outlet lobes may begin to converge into a narrow, fast-moving streams of ice (Dyke and Prest, 1986; Fulton, 1989; Thorliefsson and Kristjansson 1993; Fulton, 1995). The nature and distribution of glacial sediment and landforms implies that ice flow was much different in these narrow convergent zones. Till associated with ice streams is more distally derived, homogenous, traceable for hundreds of km, and typically has a level-to-streamlined surface expression. Tills deposited outside these narrow convergent zones are usually thin and locally derived.

Models for the mechanics of ice movement that are based on the study of modem ice sheets (Fowler and Johnson, 1995) predict that slow, uniform flow of ice is unstable and therefore unlikely. What is predicted and observed, by contrast, is a bimodal style of ice flow, with fast ice streams (km/yr) moving within slower ice (cdyr). The location of ice streams appears to change over time, but is influenced by bed topography and hydrogeology. Ice streams and their outlet lobes are the major discharge areas for an ice sheet, and their development is largely a function of subglacial water. Ice streams draw down the ice mass in the ice accumulation centers. Ice-lobe advances therefore represent a redistribution of ice, and may actually signal the overall decay of the ice sheet rather than an increase in volume of the ice sheet.

The deglacial record of the Late Wisconsinan Laurentide ice sheet indicates that in the areas of continuous ice cover in Canada, ice streams developed within the ice sheet and fed the ice lobes that moved through the Great Lakes basins into the upper Midwest (Dyke and Prest, 1986; Hicock and Dreimanis, 1992; Thorleifson and Kristjansson, 1993). The ice streams were active throughout the retreat of the ice across Canada as documented in the pattern of moraines (Dyke and Prest, 1986; Fulton, 1995). It is unclear if ice streams were also important during ice-sheet growth because the effects of subglacial water are difficult to ascertain. This model of ice flow can be cautiously extended to earlier glaciations, bearing in mind that bed conditions could have been very different from those of more recent events.

The advantages of studying the geological record of Quaternary glaciations include: (1) the relatively excellent preservation, continuity and accessibility of the deposits; (2) the location of the deposits within a well established paleogeographic, paleomagnetic, and tectonic setting; (4) records for floral and faunal succession and pluvial lake-level histories that allow the construction of predictive global circulation models; (4) direct evidence of global ice volume from the ocean oxygen istope record as well as the record of crustal rebound; and (5) fairly easy access to modem analogs (for example, the study

of Antarctic ice is relatively easy compared to the study of a deep magma chamber).These advantages also provide the greatest challenge to Quaternary researchers, who arerequired to integrate data across a wide range of disciplines and geographic areas. Theultimate goal is to provide a integrated picture of the geosphere, hydrosphere andbiosphere for a specific period of earth's history, and to extend this understanding to moreobscure glacial periods in the geologic record.

REFERENCES

Baker, R. W., Diehi, J. F., Simpson, T. W., Zelazny, L. W., and Beske-Diehl, S., 1983, Pre-Wisconsinanglacial stratigraphy, chronology, paleomagnetics of west-central Wisconsin: Geological Societyof America Bulletin, v. 94, p. 1442-1449.

Bierman, P. R., Marsella, K. A., Patterson, C., Davis, P. T. Caffee, M., 1998, Mid-Pleistocene cosmogenicminimum-age limits for pre-Wisconsinan glacial surfaces in southwestern Minnesota and southernBaffin Island -- a multiple nuclide approach. Accepted for publication in Geomorphology.

Clayton, L., and Moran, S. R., 1982, Chronology of late Wisconsin glaciation in middle North America.Quaternary Science Reviews, v. 1 p. 55-82.

Dyke A. S. and Prest, V. K., 1986, Paleogeography of northern North America, 18,000-5000 years ago.Geological Survey of Canada, Map 1703A, scale 1: 12,500,000.

Flint, R. F., 1957. Glacial and Pleistocene Geology. New York, John Wiley and Sons, 553 p.Fowler, A. C., and Johnson, C., 1995, Hydraulic run-away: A mechanism for thermally regulated surges of

ice sheets: Journal of Glaciology, v. 41, p. 554-56 1.Fulton, R. J., 1989, Foreward in Fulton, R. J., ed., Quaternary geology of Canada and Greenland: Ottawa,

Geological Survey of Canada, Geology of Canada series, v. 1;(Geological Society of America,Geology of North America, v. K-i), p. 1-11.

Fulton, R. J., 1995, Surficial map of Canada: Geological Survey of Canada Map 1880A, scale 1:5,000,000.Gibbons, A. B., Megeath, J. D., and Pierce, K. L., 1984, Probability of moraine survival in a succession of

glacial advances. Geology, v. 12, p. 327-330.Hallberg, G. R., 1986, Pre-Wisconsin glacial stratigraphy of the central plains region in Iowa, Nebraksa,

Kansas, and Missouri. Quaternary Science Reviews, v. 5, p. 11-15.Hicock, S. T., and Dreimanis, A., 1992, Deformation till in the Great Lakes region: implications for rapid

flow along the south-central margin of the Laurentide Ice Sheet. Canadian Journal of EarthSciences, v. 29, p. 1565-1579.

Kiassen, R. W., 1989, Quaternary geology of the southern Canadian interior plains, in Fulton, R. J., ed.,Quaternary geology of Canada and Greenland: Ottawa, Geological Survey of Canada, Geology ofCanada series, v. 1; (Geological Society of America, Geology of North America), v. K-i, p. 138-174.

Mickelson, D. M., Clayton, L., Fullerton, D. S., and Borns, H. W. Jr., 1983. The late Wisconsin glacialrecord of the Laurentide ice sheet in the United States, in Wright, H. E., Jr., and Porter, S. C., Late-Quaternary Environments of the United States, Vol. 1, The Late Pleistocene. Univeristy ofMinnesota Press, Minneapolis, p. 3-37.

Meyer, G. N., 1997, Pre-late Wisconsinan till stratigraphy of north-central Minnesota. MinnesotaGeological Survey Report of Investigations no. 48, 67 p.

Patterson, C. J., 1997, Quaternary geology of southwestern Minnesota, in Patterson, C. J., Contributions tothe Geology of Southwestern Minnesota, Minnesota Geological Survey Report of Investigations 47,p. 1-45.

Richmond, G. M., and Fullerton, D. 5., 1986, An introduction to Quaternary glaciations in the United Statesof America, Quaternary Science Reviews, v. 5, p. 3-10.

Shackelton, N. J. and others, 1984, Oxygen isotope calibration of the onset of ice-rafting and history ofglaciation in the North Atlantic region: Nature, v. 307, p. 216-219.

Thorleifson, L. H., and Kristjansson, F. J., 1993, Quaternary geology and drift prospecting, Beardmore-Geraldton area, Ontario: Geological Survey of Canada Memoir 435, 146 p.

Wright, H. E. Jr, 1972, Quaternary History of Minnesota, in Sims, P. K. and Morey, G. B, eds., Geology ofMinnesota: A centennial volume. Minnesota Geological Survey, p. 5 15-547.

36

of Antarctic ice is relatively easy compared to the study of a deep magma chamber). These advantages also provide the greatest challenge to Quaternary researchers, who are required to integrate data across a wide range of disciplines and geographic areas. The ultimate goal is to provide a integrated picture of the geosphere, hydrosphere and biosphere for a specific period of earth's history, and to extend this understanding to more obscure glacial periods in the geologic record.

REFERENCES

Baker, R. W., Diehl, J. F., Simpson, T. W., Zelazny, L. W., and Beske-Diehl, S., 1983, Re-Wisconsinan glacial stratigraphy, chronology, palmmagnetics of west-central Wisconsin: Geological Society of America Bulletin, v. 94, p. 1442-1449.

Bierman, P. R., Marsella, K. A., Patterson, C., Davis, P. T. Caffee, M., 1998, Mid-Pleistocene cosmogenic minimum-age limits for pre-Wisconsinan glacial surfaces in southwestern Minnesota and southern Baffin Island -- a multiple nuclide approach. Accepted for publication in Geomorphology.

Clayton, L., and Moran, S. R., 1982, Chronology of late Wisconsin glaciation in middle North America. Quaternary Science Reviews, v. 1 p. 55-82.

Dyke A. S. and Prest, V. K., 1986, Paleogeography of northern North America, 18,000-5000 years ago. Geological Survey of Canada, Map 1703A, scale 1 : 12,500,000.

Hint, R. F., 1957. Glacial and Pleistocene Geology. New York, John Wiley and Sons, 553 p. Fowler, A. C., and Johnson, C., 1995, Hydraulic run-away: A mechanism for thermally regulated surges of

ice sheets: Journal of Glaciology, v. 41, p. 554-561. Fulton, R. J., 1989, Foreward in Fulton, R. J., ed., Quaternary geology of Canada and Greenland: Ottawa,

Geological Survey of Canada, Geology of Canada series, v. l;(Geological Society of America, Geology of North America, v. K-1), p. 1-1 1.

Fulton, R. J., 1995, Surficial map of Canada: Geological Survey of Canada Map 1880A, scale 1:5,000,000. Gibbons, A. B., Megeath, J. D., and Pierce, K. L., 1984, Probability of moraine survival in a succession of

glacial advances. Geology, v. 12, p. 327-330. Hallberg, G. R., 1986, Pre-Wisconsin glacial stratigraphy of the central plains region in Iowa, Nebraksa,

Kansas, and Missouri. Quaternary Science Reviews, v. 5, p. 11-15. Hicock, S. T., and Dreimanis, A., 1992, Deformation till in the Great Lakes region: implications for rapid

flow along the south-central margin of the Laurentide Ice Sheet. Canadian Journal of Earth Sciences, v. 29, p. 1565-1579.

Klassen, R. W., 1989, Quaternary geology of the southern Canadian interior plains, in Fulton, R. J., ed., Quaternary geology of Canada and Greenland: Ottawa, Geological Survey of Canada, Geology of Canada series, v. 1; (Geological Society of America, Geology of North America), v. K-1, p. 138- 174.

Mickelson, D. M., Clayton, L., Fullerton, D. S., and Boms, H. W. Jr., 1983. The late Wisconsin glacial record of the Laurentide ice sheet in the United States, in Wright, H. E., Jr., and Porter, S, C., Late- Quaternary Environments of the United States, Vol. 1, The Late Pleistocene. Univeristy of Minnesota Press, Minneapolis, p. 3-37.

Meyer, G. N., 1997, Pre-late Wisconsinan till stratigraphy of north-central Minnesota. Minnesota Geological Survey Report of Investigations no. 48,67 p.

Patterson, C. J., 1997, Quaternary geology of southwestern Minnesota, in Patterson, C. J., Contributions to the Geology of Southwestern Minnesota, Minnesota Geological Survey Report of Investigations 47, p. 1-45.

Richmond, G. M., and Fullerton, D. S., 1986, An introduction to Quaternary glaciations in the United States of America, Quaternary Science Reviews, v. 5, p. 3-10.

Shackelton, N. J. and others, 1984, Oxygen isotope calibration of the onset of ice-rafting and history of glaciation in the North Atlantic region: Nature, v. 307, p. 216-219.

Thorleifson, L. H., and Kristjansson, F. J., 1993, Quaternary geology and drift prospecting, Beardmore- Geraldton area, Ontario: Geological Survey of Canada Memoir 435, 146 p.

Wright, H. E. Jr, 1972, Quaternary History of Minnesota, in Sims, P. K. and Morey, G. B, eds., Geology of Minnesota: A centennial volume. Minnesota Geological Survey, p. 5 15-547.

WHAT'S NEXT FOR GEOLOGY IN THE LAKE SUPERIOR AREA?

SOUTHWICK, David L., Minnesota Geological Survey, 2642 University Ave. W., St. Paul, MN55114-1057

Marvelous progress has been made in deciphering the complex, diverse geological framework of theLake Superior region over the past twenty years. In particular, our comprehension of the Precambrianrock record has progressed remarkably through excellent programs of geologic mapping, regionalgeophysics, geochronology, and related topical research supported largely by government and justifiedon the premise that geologic understanding of Precambrian terranes will lead to discovery of economicmineral deposits. One hopes that this momentum can be sustained and that intellectual andtechnological progress toward the goal of finding and developing viable ore deposits will continue.Sustaining it will not be easy in the prevailing political climate, however.

Ultimately, albeit indirectly, the policies of democratic governments are determined by publicopinion that is transmitted to elected representatives. At the present time the public is not particularlyconcerned about mineral commodities and therefore minerals issues do not have a high politicalprofile. There are no pressing shortages, prices are moderate and stable, unemployment is generallylow, and concerns for present-day "quality of life" issues greatly outweigh concerns for the long-termavailability of mineral and energy resources in the minds of most people. As a consequence, electedrepresentatives are being urged by their constituents to preserve and promote environmental quality,provide recreational opportunities in wilderness areas, improve urban infrastructure, combat crime,improve health care, extend educational opportunites, and reduce taxes. They are not being urged toprovide for the well-being of a mining industry that we geologists know is vital to the long-termsustainability of the quality of life, because there is a powerful human tendency not to worry aboutfuture difficulties until circumstances absolutely demand it. Instead of mining, the plexus ofenvironmental issues that immediately affect human health and happiness will be the primary driverof public resource policy and derivative geological investigations in the Lake Superior region over thenext twenty years. Although metal mining and related industries surely wifi remain significant to localeconomies and will receive passive political support, they wifi not in themselves provide the rationalefor increasing public expenditures on geological investigations.

The following will be growth areas for applied geologic thinking and research in the Lake Superiorregion. (1) Detailed characterization of unconsolidated materials within the uppermost 100 m of thegeologic section. This work will involve collaboration among glacial geologists, stratigraphers,geophysicists, geochemists, hydrogeologists, and soil scientists and will have direct application to anumber of societal concerns. It will also benefit mineral exploration, but it wifi not be justified on thatbasis. (2) The development and deployment of improved geophysical tools for obtaining informationabout the shallow subsurface. (3) Detailed geologic mapping of surficial deposits, especially in urban,urbanizing, and agricultural areas. Regional mapping to provide the stratigraphic framework of theQuaternary materials will be necessary as well, but the more pressing need will be for detail. (4)Hydrogeologic studies on a variety of scales, with particular emphasis on the geological controls ofground-water flow. Fractured-rock hydrogeology will be a developing area of research in whichsedimentologists, petrologists, and structural geologists will become increasingly involved. (5) Low-temperature rock-water geochemistry, with applications to problems of surface-water and ground-water composition and the transport of dissolved chemical species. (6) The mapping andcharacterization of aggregate resources. (7) Characterization and control of mined land and mine waste,in which the full spectrum of hydrogeological and geochemical methodologies are brought to bear. (8)Geological and geophysical studies within Lake Superior itself that will be predicated on calibrationof the paleoclimate record but will extend to investigations of lake-basin evolution.

As the geological profession becomes ever more dependent on government policies, our near-termresearch opportunities inevitably will reflect public priorities. This need not and will not mean thetotal demise of economic geology and the hard-rock disciplines that support it. However, traditionalhard-rock work will diminish in favor of non-traditional investigations in which hard-rock thinkingand skills can be applied. Flexibility of training and outlook wifi be critical to professional success.

37

WHAT'S NEXT FOR GEOLOGY IN THE LAKE SUPERIOR AREA?

SOUTHWICK, David L., Minnesota Geological Survey, 2642 University Ave. W., St. Paul, MN 55114-1057

Marvelous progress has been made in deciphering the complex, diverse geological framework of the Lake Superior region over the past twenty years. In particular, our comprehension of the Precambrian rock record has progressed remarkably through excellent programs of geologic mapping, regional geophysics, geochronology, and related topical research supported largely by government and justified on the premise that geologic understanding of Precambrian terranes will lead to discovery of economic mineral deposits. One hopes that this momentum can be sustained and that intellectual and technological progress toward the goal of finding and developing viable ore deposits will continue. Sustaining it will not be easy in the prevailing political climate, however.

Ultimately, albeit indirectly, the policies of democratic governments are determined by public opinion that is transmitted to elected representatives. At the present time the public is not p&ticularly concerned about mineral commodities and therefore minerals issues do not have a high political profile. There are no pressing shortages, prices are moderate and stable, unemployment is generally low, and concerns for present-day "quality of life" issues greatly outweigh concerns for the long-term availability of mineral and energy resources in the minds of most people. As a consequence, elected representatives are being urged by their constituents to preserve and promote environmental quality, provide recreational opportunities in wilderness areas, improve urban infrastructure, combat crime, improve health care, extend educational opportunites, and reduce taxes. They are not being urged to provide for the well-being of a mining industry that we geologists know is vital to the long-term sustainability of the quality of life, because there is a powerful human tendency not to worry about future difficulties until circumstances absolutely demand it. Instead of mining, the plexus of environmental issues that immediately affect human health and happiness will be the primary driver of public resource policy and derivative geological investigations in the Lake Superior region over the next twenty years. Although metal mining and related industries surely will remain significant to local economies and will receive passive political support, they will not in themselves provide the rationale for increasing public expenditures on geological investigations.

The following will be growth areas for applied geologic thinking and research in the Lake Superior region. (1) Detailed characterization of unconsolidated materials within the uppermost 100 m of the geologic section. This work will involve collaboration among glacial geologists, stratigraphers, geophysicists, geochemists, hydrogeologists, and soil scientists and will have direct application to a number of societal concerns. It will also benefit mineral exploration, but it will not be justified on that basis. (2) The development and deployment of improved geophysical tools for obtaining information about the shallow subsurface. (3) Detailed geologic mapping of surficial deposits, especially in urban, urbanizing, and agricultural areas. Regional mapping to provide the stratigraphic framework of the Quaternary materials will be necessary as well, but the more pressing need will be for detail. (4) Hydrogeologic studies on a variety of scales, with particular emphasis on the geological controls of ground-water flow. Fractured-rock hydrogeology will be a developing area of research in which sedimentologists, petrologists, and structural geologists will become increasingly involved. (5) Low- temperature rock-water geochemistry, with applications to problems of surface-water and ground- water composition and the transport of dissolved chemical species. (6) The mapping and characterization of aggregate resources. (7) Characterization and control of mined land and mine waste, in which the full spectrum of hydrogeological and geochemical methodologies are brought to bear. (8) Geological and geophysical studies within Lake Superior itself that will be predicated on calibration of the paleoclimate record but will extend to investigations of lake-basin evolution.

As the geological profession becomes ever more dependent on government policies, our near-term research opportunities inevitably will reflect public priorities. This need not and will not mean the total demise of economic geology and the hard-rock disciplines that support it. However, traditional hard-rock work will diminish in favor of non-traditional investigations in which hard-rock thinking and skills can be applied. Flexibility of training and outlook will be critical to professional success.

GENERAL TECHNICAL SESSIONS

39

ABSTRACTSACTS

PETROGRAPHY AND GEOCHEMISTRY OF MIDCONTINENT RIFT RHYO-LITE (CHENGWATANA VOLCANICS) NEAR CLAM FALLS, WISCONSIN

ABBOTT, Kathleen M., THOLE, Jeffrey T., WIRTH, Karl R., Geology Department,Macalester College, St. Paul MN, 55105; kabbott@macalester.edu;thole@macalester.edu; wirth@macalester.edu

The southwest limb of the Midcontinent Rift (MCR) includes the poorly-exposed ChengwatanaVolcanics (CV) which are comprised of predominantly tholeiitic basalts (Wirth et al., 1997,Naiman et al., this volume), with minor rhyolites which are believed to be the southernmostexposed rocks of this type within the MCR. Recent analysis of these rhyolites found near ClamFalls, Wisconsin, provides physical, chemical, and isotopic data which has helped constrain thetiming (Wirth and Gehrels, this volume), and magmatic processes (Naiman and Wirth, thisvolume) involved in the generation of the CV and the southern MCR. These data are also usedto compare the CV rhyolites with the Portage Lake rhyolites of northern Michigan (Nicholson,1992, Nicholson and Shirey, 1990), and the felsic rocks of the North Shore Volcanic Group(Green and Fitz, 1993, Vervoort and Green, 1997).

Two exposures of rhyolite are found just west of Clam Falls including about 5 meters ofrhyolite (KC-302) which is approximately 640 meters above the base of the exposed volcanicsection. The only other outcrop of rhyolite (KC-310) is exposed near the base of the sectionapproximately 3 kilometers south of KC-302 and is less than 10 meters thick. No flow contactsor foliations are present but the outcrops appear to be concordant with surrounding basaltswhich trend northeast and dip approximately 15 degrees to the northwest in this area. The baseof the outcrop at KC-302 appears to be an intrusive contact with the underlying basalt andlocally, the rhyolite contains subangular inclusions of ophitic basalt. The nature of emplace-ment of these rhyolite bodies is unclear.

The rhyolites are generally weakly (<5%) porphyritic containing small phenocrysts (—.1mm)of euhedral to subhedral plagioclase (avg. Ab94 An2 Or4) and trace amounts of subhedral Fe-Tioxides. Quartz phenocrysts and albite glomerocrysts are present only locally. Amphiboles,partly replaced by chlorite/epidote/carbonate, are rare. The groundmass is commonly spheru-litic (up to 2- 3 mm) with phenocryst nuclei, containing tabular quartz (tridymite paramorphs),dusty, anhedral alkali feldspar (avg. Ab4 An1 Or95), very fine-grained disseminated Fe-oxides,and local poikilitic anhedral quartz. Ac-cessory zircon is present as minute

I I I

euhedral prisms in most samples. Thepresence of minor epidote and carbonate C1fl Falls (CV)

within the groundmass is ubiquitous. D BSa1t

Some samples contain minor diktytaxitic 3 . . Rhyolite -

cavities. The presence of tridymite a

paramorphs indicate these rocks are simi-lar to some rhyolites of the NSVG which 2 -

Do -

are believed to have erupted at high tern- Cperatures (Green and Fitz, 1993). No ap- f-4 00°parent flow foliation or pyroclastic tex-

1

° -

tures were observed in the exposures nearClam Falls.

Only basalt and rhyolite are present I

in the Clam Falls' section; no rocks of 050 60 70 80

intermediate composition have been iden-S.tifled. Rocks of intermediate composi- 1 2 wt /0

41

PETROGRAPHY AND GEOCHEMISTRY O F MIDCONTINENT RIFT RHYO- LITE (CHENGWATANA VOLCANICS) NEAR CLAM FALLS, WISCONSIN

ABBOTT, Kathleen M., THOLE, Jeffrey T., WIRTH, Karl R., Geology Department, Macalester College, St. Paul MN, 55105; kabbott@macalester.edu;

The southwest limb of the Midcontinent Rift (MCR) includes the poorly-exposed Chengwatana Volcanics (CV) which are comprised of predominantly tholeiitic basalts (Wirth et al., 1997, Naiman et al., this volume), with minor rhyolites which are believed to be the southernmost exposed rocks of this type within the MCR. Recent analysis of these rhyolites found near Clam Falls, Wisconsin, provides physical, chemical, and isotopic data which has helped constrain the timing (Wirth and Gehrels, this volume), and magmatic processes (Naiman and Wirth, this volume) involved in the generation of the CV and the southern MCR. These data are also used to compare the CV rhyolites with the Portage Lake rhyolites of northern Michigan (Nicholson, .

1992, Nicholson and Shirey, 1990), and the felsic rocks of the North Shore Volcanic Group (Green and Fitz, 1993, Vervoort and Green, 1997).

Two exposures of rhyolite are found just west of Clam Falls including about 5 meters of rhyolite (KC-302) which is approximately 640 meters above the base of the exposed volcanic section. The only other outcrop of rhyolite (KC-310) is exposed near the base of the section approximately 3 kilometers south of KC-302 and is less than 10 meters thick. No flow contacts or foliations are present but the outcrops appear to be concordant with surrounding basalts which trend northeast and dip approximately 15 degrees to the northwest in this area. The base of the outcrop at KC-302 appears to be an intrusive contact with the underlying basalt and locally, the rhyolite contains subangular inclusions of ophitic basalt. The nature of emplace- ment of these rhyolite bodies is unclear.

The rhyolites are generally weakly (4%) porphyritic containing small phenocrysts (- lmm) of euhedral to subhedral plagioclase (avg. Abo4 An2 Or4) and trace amounts of subhedral Fe-Ti oxides. Quartz phenocrysts and albite glomerocrysts are present only locally. Amphiboles, partly replaced by chlorite/epidote/carbonate, are rare. The groundmass is commonly spheru- litic (up to 2 - 3 mm) with phenocryst nuclei, containing tabular quartz (tridymite paramorphs), dusty, anhedral alkali feldspar (avg. Ab4 Ani Org5), very fine-grained disseminated Fe-oxides, and local poikilitic anhedral quartz. Ac- cessory zircon is present as minute euhedral prisms in most samples. The presence of minor epidote and carbonate within the groundmass is ubiquitous. Some samples contain minor diktytaxitic cavities. The presence of tridymite g pararnorphs indicate these rocks are simi- lar to some rhyolites of the NSVG which 2 are believed to have erupted at high tern- 0 peratures (Green and Fitz, 1993). No ap- parent flow foliation or pyroclastic tex- 1 tures were observed in the exposures near Clam Falls.

Only basalt and rhyolite are present in the Clam Falls' section; no rocks of

I I I a

Clam Falls (CV) a Basalt

- Rhyolite - a

- ^ - a & I L7

o n a

- n o - 00 -

I I I

40 50 60 70 intermediate composition have been iden-

80

tified. Rocks of intermediate composi- SiO, wt %

DI

0

-4

— -8

-12

-16

-20

_______________________________

40 50 60 70 80Si02 wt %

tions are found in both the NSVG and the Portage Lake Volcanics. Chengwatana basalt geochem-istry is discussed by Naiman et a!., this volume. The felsic Chengwatana rocks can be classifiedas tholeiitic rhyolites. The rocks are metaluminous and only one sample has normative corun-dum (0.16%). In general, the rhyolite chemistry is similar to both NSVG and Type I PortageLake rhyolites. Notable variations from these two groups include slightly less Si02 (71-72%),higher total Fe (4.87 to 5.78 Fe203%), higher Th (25-27 ppm), and significantly higher Zr (7 13-986 ppm).

Initial ENd (1100 Ma) values on two Chengwatana rhyolites are 0.95 and -0.06. Thesevalues are similar to Chengwatana basalts of the Clam Falls region (-2 to +3.4). The Chengwatanarhyolite ENd values are most similar to Portage Lake (Type I) rhyolites which range from -0.3 to-4.7 (Nicholson and Shirey, 1990) and are distinct from rhyolites of the NSVG which rangefrom -2.3 to -14.8 (Vervoort and Green, 1997). NSVG granophyres and icelandites have ENdvalues with a smaller range than NSVG rhyolites and these values are also lower thanChengwatana rhyolites. Large degrees of crustal melting are invoked as a major process informing the NSVG rhyolites as indicated by the low ENd values (Vervoort and Green, 1997).The Chengwatana rhyolites at Clam Falls lack the pronounced crustal Nd isotopic signatureevident in the NSVG rhyolites and are believed to be fractionates of a primary basaltic magmaderived from an enriched mantle source.

References CitedGreen, J.C. and Fitz, T.J., 1993, Journal of Volcanology and Geothermal Research, v. 54, p. 177-196.Nicholson, S.W., and Shirey, S.B., 1990, Journal of Geophysical Research, v. 95, p. 10,851-10868.Nicholson, S.W., 1992, U.S.G.S. Bulletin 1970, 57 pp.Wirth, K.R., Vervoort, J.D., Naiman, Z.J., 1997, Canadian Journal of Earth Sciences, v. 34, no. 4, p. 536-548.Vervoort, J.D., and Green, J.C., 1997, Canadian Journal of Earth Sciences, v. 34, p. 521-535.

42

NSVGPortage Lake0

ocJJ 00

0

NSVG Icelandites

Clam Falls (CV)

0 Basalt

I Rhyolite

NSVG Rhyolites

Portage LakeType II

I I Portage Lak

NSVG Granophyres \ Â

om a

a

NSVG Icelandi

Clam Falls (CV)

Basalt

Rhyolite

tions are found in both the NSVG and the Portage Lake Volcanics. Chengwatana basalt geochem- istry is discussed by Naiman et al., this volume. The felsic Chengwatana rocks can be classified as tholeiitic rhyolites. The rocks are metaluminous and only one sample has normative corun- dum (0.16%). In general, the rhyolite chemistry is similar to both NSVG and Type I Portage Lake rhyolites. Notable variations from these two groups include slightly less Si02 (7 1-72%), higher total Fe (4.87 to 5.78 Fe203%), higher Th (25-27 ppm), and significantly higher Zr (713- 986 ppm).

Initial ENd (1100 Ma) values on two Chengwatana rhyolites are 0.95 and -0.06. These values are similar to Chengwatana basalts of the Clam Falls region (-2 to +3.4). The Chengwatana rhyolite ENd values are most similar to Portage Lake (Type I) rhyolites which range from -0.3 to -4.7 (Nicholson and Shirey, 1990) and are distinct from rhyolites of the NSVG which range from -2.3 to -14.8 (Vervoort and Green, 1997). NSVG granophyres and icelandites have ENd values with a smaller range than NSVG rhyolites and these values are also lower than Chengwatana rhyolites. Large degrees of crustal melting are invoked as a major process in forming the NSVG rhyolites as indicated by the low ENd values (Vervoort and Green, 1997). The Chengwatana rhyolites at Clam Falls lack the pronounced crustal Nd isotopic signature evident in the NSVG rhyolites and are believed to be fractionates of a primary basaltic magma derived from an enriched mantle source.

References Cited Green, J.C. and Fitz, T.J., 1993, Journal of Volcanology and Geothermal Research, v. 54, p. 177-196. Nicholson, S.W., and Shirey, S.B., 1990, Journal of Geophysical Research, v. 95, p. 10,851-10868. Nicholson, S.W., 1992, U.S.G.S. Bulletin 1970,57 pp. Wirth, K.R., Vervoort, J.D., Naiman, Z.J., 1997, Canadian Journal of Earth Sciences, v. 34, no. 4, p. 536-548. Vervoort, J.D., and Green, J.C., 1997, Canadian Journal of Earth Sciences, v. 34, p. 521-535.

A PRELIMINARY DETAILED GEOLOGICAL DESCRIPTION OF THE NEWHIGH GRADE SILVER-BASE METAL DISCOVERY IN THE LUMBY LAKEMETAVOLCANIC BELT NORTHEAST OF ATIKOKAN, ONTARIO,CANADA

BERNATCHEZ, Raymond A., President & Consulting Geologist to Atikokan ResourcesInc., P.O. Box 1376, 126 Willow Rd., Atikokan, Ontario, Canada, POT 1CO. Tel.807-597-4526, Fax 807-597-4636 (rbernatc@atikokan.lakeheadu.ca)

A high grade silver—zinc—lead mineral occurrence has been discovered within an east-weststratabound felsic volcaniclastic horizon at the southern edge of the Lumby Lake MetavolcarncBelt. Located north of the creek connecting Lumby and Herontrack Lakes, the discoveryoccurs at the southern margin of a 1.5 km thick east-west trending felsic volcanic package nearthe southern contact of the Lumby Lake Metavolcanic Belt with the Marmion Lake tonalitebatholith to the south. The Lumby Lake volcanic belt is located 23 air miles northeast ofAtikokan, Ontario, Canada and forms the northeasterly extension of the Steep Rock andFinlayson Lake volcanic stratigraphy. This discovery is contained within highly(hydrothermally) altered felsic volcaniclastic rocks having geological and geochemical featuressimilar to other Archean massive sulphide deposits. However, the felsic volcanic rocks hostingthis high grade silver—base metal occurrence have been dated at 3.0 Ga (Mesoarchean; Davisand Jackson, 1988), whereas all other base metal deposits found in the Archean in Ontariohave been dated at 2.7 Ga. The occurrence contains unusually high silver content with somerandom grab samples assaying as high as 416 ounces/ton (14,265 gm/tonne) or 270 ozftonover 1.1 meter (9292 gm/tonne).

The Lumby Lake greenstone belt is located within the Wabigoon subprovince, 30 km northof the Wabigoon—Quetico subprovince boundary and 37 air km northeast of Atikokan, Ontario.The Lumby Lake greenstone belt consists of a 60 km x 20 km synclinal supracrustal sequenceof rocks composed of mafic (pillowed and massive), ultramafic (komatiitic), and felsic (tuff,lapili tuff, tuff breccia, massive), metavolcanic rocks with lesser amounts of metasedimentaryrocks (chert, argillite, arkose, conglomerate and iron formations). The metavolcanic—metasedimentary assemblage is in contact with the Mannion Lake Batholith to the south and theWhite Otter Batholith to the northwest. The belt has been intruded by two larger oval andheterogeneous monzodiorite-granite plutons, the Norway Lake and van Nostrand Lake Stockand smaller ones (the Bar Lake and Viking Lake stocks).

The Belt has been subjected to four major structural events: at least two folding events andtwo faulting-shearing events. The Belt has had a major synclinal folding event with its axistrending east-west through the central portion of the belt through Seahorse-Garnet Bay-Pinecone-Hematite Lakes system. The van Nostrand Lake stock intrudes this axis in thecentral part of the belt. A second folding event occurs within the interfiow sedimentary units.The Redpaint Lake Fault truncates the Lumby Lake belt to the west. Two east-westdeformation zones have been noted: the Bufo Lake - Spoon Lake Deformation Zone and theGarnet Bay-Viking Lake-Hematite Lake Deformation Zone.

The silver—base metal discovery is located in the western block of claims known as theHerontrack Lake Block and within the southwest portion of the Lumby Lake greenstone belt.A 130 km grid has been established in the central portion of this block from Lumby Lake toHutt Lake. A detailed exploration program over this grid consisting of geological mapping,soil geochemical sampling, magnetic and partial VLF EM and IP surveys, lithogeochemical

43

A PRELIMINARY DETAILED GEOLOGICAL DESCRIPTION OF THE NEW HIGH GRADE SILVER-BASE METAL DISCOVERY IN THE LUMBY LAKE METAVOLCANIC BELT NORTHEAST OF ATIKOKAN, ONTARIO, CANADA

BERNATCHEZ, Raymond A., President & Consulting Geologist to Atikokan Resources Inc., P.O. Box 1376, 126 Willow Rd., Atikokan, Ontario, Canada, POT 1CO. Tel. 807-597-4526, Fax 807-597-4636 (rbernatc @ atikokan.1akeheadu.ca)

A high grade silver-zinc-lead mineral occurrence has been discovered within an east-west stratabound felsic volcaniclastic horizon at the southern edge of the Lumby Lake Metavolcanic Belt. Located north of the creek connecting Lumby and Herontrack Lakes, the discovery occurs at the southern margin of a 1.5 km thick east-west trending felsic volcanic package near the southern contact of the Lumby Lake Metavolcanic Belt with the Marmion Lake tonalite batholith to the south. The Lumby Lake volcanic belt is located 23 air miles northeast of Atikokan, Ontario, Canada and forms the northeasterly extension of the Steep Rock and Finlayson Lake volcanic stratigraphy. This discovery is contained within highly (hydrothermally) altered felsic volcaniclastic rocks having geological and geochemical features similar to other Archean massive sulphide deposits. However, the felsic volcanic rocks hosting this high grade silver-base metal occurrence have been dated at 3.0 Ga (Mesoarchean; Davis and Jackson, 1988), whereas all other base metal deposits found in the Archean in Ontario have been dated at 2.7 Ga. The occurrence contains unusually high silver content with some random grab samples assaying as high as 416 ounces/ton (14,265 gdtonne) or 270 ozlton over 1.1 meter (9292 gdtonne).

The Lumby Lake greenstone belt is located within the Wabigoon subprovince, 30 km north of the Wabigoon-Quetico subprovince boundary and 37 air km northeast of Atikokan, Ontario. The Lumby Lake greenstone belt consists of a 60 km x 20 km synclinal supracrustal sequence of rocks composed of mafic (pillowed and massive), ultramafic (komatiitic), and felsic (tuff, lapilli tuff, tuff breccia, massive), metavolcanic rocks with lesser amounts of metasedimentary rocks (chert, argillite, arkose, conglomerate and iron formations). The metavolcanic- metasedimentary assemblage is in contact with the Marmion Lake Batholith to the south and the White Otter Batholith to the northwest. The belt has been intruded by two larger oval and heterogeneous monzodiorite-granite plutons, the Norway Lake and van Nostrand Lake Stock and smaller ones (the Bar Lake and Viking Lake stocks).

The Belt has been subjected to four major structural events: at least two folding events and two faulting-shearing events. The Belt has had a major synclinal folding event with its axis trending east-west through the central portion of the belt through Seahorse-Garnet Bay- Pinecone-Hematite Lakes system. The van Nostrand Lake stock intrudes this axis in the central part of the belt. A second folding event occurs within the interflow sedimentary units. The Redpaint Lake Fault truncates the Lumby Lake belt to the west. Two east-west deformation zones have been noted: the Bufo Lake - Spoon Lake Deformation Zone and the Garnet Bay-Viking Lake-Hematite Lake Deformation Zone.

The silver-base metal discovery is located in the western block of claims known as the Herontrack Lake Block and within the southwest portion of the Lumby Lake greenstone belt. A 130 krn grid has been established in the central portion of this block from Lumby Lake to Hutt Lake. A detailed exploration program over this grid consisting of geological mapping, soil geochemical sampling, magnetic and partial VLF EM and IP surveys, lithogeochemical

sampling and overburden mechanical stripping has identified and exposed numerous suiphidebearing felsic volcaniclastic horizons containing economic to subeconomic values in zinc,copper, silver, lead and gold.

The detail geological mapping has identified five rock types: mafic volcanic, massive andpilowed; felsic volcanic, tuff, lapili tuff, tuff breccia and coarse breccia, massive rhyolite asfeldspar and/or quartz porphyries and cherty rhyolite; mafic to intermediate intrusive rocks(gabbro and diorite) as dykes and sills intruding the above; Marmion Lake batholith (tonalite)and finally felsic dikes intruding the felsic volcanic rocks.

The bulk of the mineralization is contained within stratabound interflow sediments. Pyrite,and in some interfiow units, pyrrhotite make up the bulk of the sulfides. The high grade silver—zinc—lead discovery zone contains significant amounts of pyrite, sphalerite, galena, acanthiteand native silver with minor chalcopyrite. This mineralization is contained within highlyaltered felsic volcanic rocks. The felsic rocks have been altered to sericite and chlorite in somelocations. The sericite was previously interpreted as an altered and sheared quartz diorite; itnow appears that this alteration was caused as the result of the footwall alteration associatedwith massive sulphide deposits. This has been partially confirmed by some limitedlithogeochemical rock sampling.

The exploration program carried out to date has identified three highly altered base metalmineralized horizons. The most economically significant horizon identified to date is theLumby Lake—Spoon Lake Horizon which hosts the high grade silver discovery. Several othersignificant silver and base metal-bearing occurrences have been discovered along this 250 to300 meter wide sulphide horizon that has now been traced for a strike length of about 6 km bymechanical stripping and geophysical surveys. Two additional altered suiphide base metalbearing felsic volcanic horizons (Delos Lake and Pond Lake Horizon) have been located a fewhundred meters north of the silver discovery horizon.

The Questor Airborne Geophysical Survey carried out by the Ontario Geological Survey in1980 did not identify any input anomalies along the Lumby Lake—Spoon Lake mineralizedhorizon. However, a broad magnetic low covers the area over the newly identified felsicstrata. This magnetic low can be traced east—west along the southern volcanic—granite contactof the Lumby Lake greenstone belt for over 12 km from Bufo Lake to Hutt Lake. Other similarmagnetic lows can be traced eastward to Old Man Lake a distance of over 13 km. Similar felsicvolcanic rocks have been identified in these areas. A new copper—zinc—silver occurrence hasbeen found along this magnetic low response near a series of airborne input anomalies at thewest end of Old Man Lake. The OGS airborne geophysical survey has detected numerousother anomalies in the Lumby Lake greenstone belt, of which most have never been examinedor tested. What other base metal discoveries will the Belt reveal?

References cited:Davis, D.W., and Jackson, M.C., 1988, Geochronology of the Lumby Lake greenstone belt: a

3 Ga complex within the Wabigoon Subprovince, northwest Ontario: Bull. G.S.A., v.100, p. 818-824.

44

sampling and overburden mechanical stripping has identified and exposed numerous sulphide bearing felsic volcaniclastic horizons containing economic to subeconomic values in zinc, copper, silver, lead and gold.

The detail geological mapping has identified five rock types: mafic volcanic, massive and pillowed; felsic volcanic, tuff, lapilli tuff, tuff breccia and coarse breccia, massive rhyolite as feldspar andfor quartz porphyries and cherty rhyolite; mafic to intermediate intrusive rocks (gabbro and diorite) as dykes and sills intruding the above; Marmion Lake batholith (tonalite) and finally felsic dikes intruding the felsic volcanic rocks.

The bulk of the mineralization is contained within stratabound interflow sediments. Pyrite, and in some interflow units, pyrrhotite make up the bulk of the sulfides. The high grade silver- zinc-lead discovery zone contains significant amounts of pyrite, sphalerite, galena, acanthite and native silver with minor chalcopyrite. This mineralization is contained within highly altered felsic volcanic rocks. The felsic rocks have been altered to sericite and chlorite in some locations. The sericite was previously interpreted as an altered and sheared quartz diorite; it now appears that this alteration was caused as the result of the footwall alteration associated with massive sulphide deposits. This has been partially confirmed by some limited lithogeochemical rock sampling.

The exploration program carried out to date has identified three highly altered base metal mineralized horizons. The most economically significant horizon identified to date is the Lumby Lake-Spoon Lake Horizon which hosts the high grade silver discovery. Several other significant silver and base metal-bearing occurrences have been discovered along this 250 to 300 meter wide sulphide horizon that has now been traced for a strike length of about 6 km by mechanical stripping and geophysical surveys. Two additional altered sulphide base metal bearing felsic volcanic horizons (Delos Lake and Pond Lake Horizon) have been located a few hundred meters north of the silver discovery horizon.

The Questor Airborne Geophysical Survey carried out by the Ontario Geological Survey in 1980 did not identify any input anomalies along the Lumby Lake-Spoon Lake mineralized horizon. However, a broad magnetic low covers the area over the newly identified felsic strata. This magnetic low can be traced east-west along the southern volcanic-granite contact of the Lumby Lake greenstone belt for over 12 km from Bufo Lake to Hutt Lake. Other similar magnetic lows can be traced eastward to Old Man Lake a distance of over 13 km. Similar felsic volcanic rocks have been identified in these areas. A new copper-zinc-silver occurrence has been found along this magnetic low response near a series of airborne input anomalies at the west end of Old Man Lake. The OGS airborne geophysical survey has detected numerous other anomalies in the Lumby Lake greenstone belt, of which most have never been examined or tested. What other base metal discoveries will the Belt reveal?

References cited: Davis, D.W., and Jackson, M.C., 1988, Geochronology of the Lumby Lake greenstone belt: a

3 Ga complex within the Wabigoon Subprovince, northwest Ontario: Bull. G.S.A., v. 100, p. 818-824.

45

L5

r

I

Ii

0 -'z0

Ic

ii I

DIMENSION STONE PRODUCTS OF MINNESOTA

BOERBOOM, Terrence J., Minnesota Geological Survey, 2642 University Avenue, St.Paul, MN 55114; boerbO01@tc.umn.edu; and OBERHELMAN, Matt, MinnesotaDepartment of Natural Resources - Division of Minerals, 1525 E. 3rd Street, Hibbing,MN 55746-1461; matt.oberhelman@dnr.state.mn.us

The first dimension stone quarry was opened in 1868, to supply gray granite from the St.Cloud District for construction of the St. Paul Customs House and Post Office. Since then,the dimension stone industry in Minnesota has grown considerably. At present there arefourteen active dimension stone quarries in the state, and several others that are active on anintermittent basis. Dimension stone products from Minnesota are marketed worldwide, and areused for a variety of purposes that include interior tiling, counter tops, monuments andmemorials, "surface plates" for precision machine mounts, acid-resistant industrialapplications, decorative and structural exterior panels for major buildings, and grinding millliners and balls.

The results of a recent Minnesota Department of Natural Resources' dimension stoneinventory show that northern Minnesota offers excellent potential for further development ofhigh quality dimension stone products. Field investigations have identified twenty-twoprospect sites that exhibit potential for quarry development. Prospects have been identified inMiddle Proterozoic rocks (ca. 1100 m.y.) of the Duluth Complex and Archean rocks (ca. 2700m.y.) of the Vermilion Granitic Complex and other granitoid rock units. Prospects includeblack, green, pink, beige, and multi-colored stone with a variety of textures. Prospectevaluation included outcrop observations that considered joint spacing, color, texture, anddeleterious materials. Results of the inventory are described in the following three MDNRreports: Dimension Stone Inventory of Northern Minnesota 1991, Report 289; DimensionStone Inventory of Northern Minnesota 1993, Report 298; and Dimension Stone Inventory ofNorthern Minnesota 1995, Report 298-2.

Cold Spring Granite Co. is currently leasing three of the prospect sites identified by thedimension stone inventory study, and have opened quarries on two of these leases. Bothutilize gabbroic anorthosite of the Duluth Complex; the stone from these sites is marketedunder the names Mesabi Black and Lake Superior Green.

This poster display shows products from all of the currently active quarries in the state ofMinnesota, along with samples of dimension stone prospects that were collected and preparedby the Minnesota DNR-Minerals Division (see below). The numbers on the samples refer tolocations on the index map. The table on the following page summarizes the geologic aspectsof the various dimension stone products of Minnesota.

ACKNOWLEDGMENTSAll of the quarry companies listed on the following table kindly contributed samples of theirproducts to the Minnesota Geological Survey for this display.

46

DIMENSION STONE PRODUCTS OF MINNESOTA

BOERBOOM, Terrence J., Minnesota Geological Survey, 2642 University Avenue, St. Paul, MN 551 14; boerbOOl@tc.umn.edu; and OBERHELMAN, Matt, Minnesota Department of Natural Resources - Division of Minerals, 1525 E. 3rd Street, Hibbing, MN 55746-1461 ; matt.oberhelman@dm.state.mn.us

The fmt dimension stone quarry was opened in 1868, to supply gray granite from the St. Cloud District for construction of the St. Paul Customs House and Post Office. Since then, the dimension stone industry in Minnesota has grown considerably. At present there are fourteen active dimension stone quarries in the state, and several others that are active on an intermittent basis. Dimension stone products from Minnesota are marketed worldwide, and are used for a variety of purposes that include interior tiling, counter tops, monuments and memorials, "surface platest' for precision machine mounts, acid-resistant industrial applications, decorative and structural exterior panels for major buildings, and grinding mill liners and balls.

The results of a recent Minnesota Department of Natural Resources' dimension stone inventory show that northern Minnesota offers excellent potential for krther development of high quality dimension stone products. Field investigations have identified twenty-two prospect sites that exhibit potential for quarry development. Prospects have been identified in Middle Proterozoic rocks (ca. 1100 m.y.) of the Duluth Complex and Archean rocks (ca. 2700 may.) of the Vermilion Granitic Complex and other granitoid rock units. Prospects include black, green, pink, beige, and multi-colored stone with a variety of textures. Prospect evaluation included outcrop observations that considered joint spacing, color, texture, and deleterious materials. Results of the inventory are described in the following three MDNR reports: Dimension Stone Inventory of Northern Minnesota 1991, Report 289; Dimension Stone Inventory of Northern Minnesota 1993, Report 298; and Dimension Stone Inventory of Northern Minnesota 1995, Report 298-2.

Cold Spring Granite Co. is currently leasing three of the prospect sites identified by the dimension stone inventory study, and have opened quarries on two of these leases. Both utilize gabbroic anorthosite of the Duluth Complex; the stone h m these sites is marketed under the names Mesabi Black and Lake Superior Green.

This poster display shows products from all of the currently active quarries in the state of Minnesota, along with samples of dimension stone prospects that were collected and prepared by the Minnesota DNR-Minerals Division (see below). The numbers on the samples refer to locations on the index map. The table on the following page summarizes the geologic aspects of the various dimension stone products of Minnesota.

AcmowLEDGMENTs All of the quarry companies listed on the following table kindly contributed samples of their products to the Minnesota Geological Survey for this display.

1 CSG

2 CSG

3 CSG

4 CSG

5 CSG

6 CSG

7 CSG

8 CSG

9 DGC

10 JSC

11 CSG

12 MKS

13 VSC

14 BSC

Trade Name

Mesabi Black

Lake Superior Green

Indian

Charcoal Black

Diamond Pink

Rockville Beige

Rockville White

Agate

Bellingham Granite

Rainbow

Varieties of pink-gray

Varieties of pink-gray

Winona Dolomite

Geologic Unit

Duluth Complex

Duluth Complex

Isle Granite

Reformatory granodiorite

Rockville Granite

Rockville Granite

Rockville Granite

Unnamed

Unnamed

Sioux Quartzite

Morton Gneiss

Oneota Dolomite

Oneota Dolomite

Oneota Dolomite

Age

—1090 Ma

—1090 Ma

1812- 1770 Ma

1812±9 Ma 1

1812±9 Ma 1

1812±9 Ma 1

1812±9 Ma 1

2618±1 Ma 2

2618±lMa?

1700- 1750 Ma

2600 - 3600 Ma

Earliest Ordovician

Earliest Ordovician

Earliest Ordovician

DominantMineralogyP, cpx, urn

P, cpx, gp, ilm

K, P, Q, bi

P, K, Q, bi, hbl, sph

K, P, Q, bi, hbl, sph

K, P, Q, bi, hbl, sph

K, P, Q, bi, hbl, sph

K, P, Q, bi

K, Q, P, bi

Q

K, P, Q, bi, hbl

dolomite

dolomite

dolomite

Texture

mgr, lam, un, oph

very cgr, porph

m-cgr, porph

m-cgr, si. porph.

m-cgr, porph, rkv,

cgr, porph, rkv,

cgr, porph, rkv,

cgr

cgr

recrystallized quartzite

gneissic banded, gbl

bioturbated micro-xln.

bioturbated micro-xln.

bioturbated micro-xln.

Company abbreviations: CSG - Cold Spring Granite, JSC - Jasper Stone Company, DGC - Dakota Granite Company, MKS - Mankato-Kasota Stone, Inc.,VSC - Vetter Stone Co., BSC - Biesanz Stone Co.

Mineral abbreviations: P - plagioclase, K - potassic feldspars, Q - quartz, cpx - clinopyroxene, hbl - hornblende, bi - biotite, jim - ilmenite, sph - sphene,gp - granophynic quartz/feldspar, xln - crystalline

Texture abbreviations: mgr - medium-grained, cgr - coarse-grained, lam - igneous lamination (cumulus or trachytoid), tin - lineated, rkv - rapakivi, gbl -granoblastic, oph - ophitic, porph - porphyritic

1U-Pb zircon date (Goldich pers. commun., as reported in Horan and others, 1987)2U.Pb zircon date (Mark Schmitz, Massachussettes Institite of Technology, pers. comm., 1998)

Sample CompanySample Company Trade Name Geologic Unit Age Dominant Texture -

Mineralogy 1 CSG Mesabi Black Duluth Complex -1090 Ma P, cpx, ilm mgr, lam, lin, oph

CSG

CSG

CSG

CSG

CSG

CSG

CSG

DGC

JSC

CSG

MKS

vsc BSC

Lake Superior Green

Iridian

Charcoal Black

Diamond Pink

Rockville Beige

Rockville White

Agate

Bellingham Granite

Rainbow

Varieties of pink-gray

Varieties of pink-gray

Winona Dolomite

Duluth Complex -1090 Ma

Isle Granite 1812 - 1770 Ma

Reformatory granodiorite 18 1 Z?H Ma 1

Rockville Granite 1 8 1 2 s Ma 1

Rockville Granite 1812&9 Ma 1

Rockville Granite 1 8 1 2 s Ma 1

Unnamed 261833 Ma 2

Unnamed 2618klMa ?

Sioux Quartzite 1700 - 1750 Ma

Morton Gneiss 2600 - 3600 Ma

Oneota Dolomite Earliest Ordovician

Oneota Dolomite Earliest Ordovician

Oneota Dolomite Earliest Ordovician

p, cpx, gp, ilm

K, P, Q, bi

P, K, Q, bi, hbl, sph

K, P, Q, bi, hbl, sph

K, P, Q, bi, hbl, sph

K, P, Q, bi, hbl, sph

K, P, Q, bi

K, Q, P, bi

Q K, P, Q, bi, hbl

dolomite

dolomite

dolomite

very cgr, porph

m-cgr, porph

m-cgr, sl. porph.

m-cgr, porph, rkv,

cgr, porph, rkv,

cgr, porph, rkv,

ClY

ClY

recrystallized quartzite

gneissic banded, gbl

bioturbated micro-xln.

bioturbated micro-xln.

bioturbated micro-xln.

Company abbreviations: CSG - Cold Spring Granite, JSC - Jasper Stone Company, DGC - Dakota Granite Company, MKS - Mankato-Kasota Stone, Inc., VSC - Vetter Stone Co., BSC - Biesanz Stone Co.

Mineral abbreviations: P - plagioclase, K - potassic feldspars, Q - quartz, cpx - clinopyroxene, hbl - hornblende, bi - biotite, ilm - ilmenite, sph - sphene, gp - granophyric quartdfeldspar, xln - crystalline

Texture abbreviations: mgr - medium-grained, cgr - coarse-grained, lam - igneous lamination (cumulus or trachytoid), lin - lineated, rkv - rapakivi, gbl- granoblastic, oph - ophitic, porph - porphyritic

I u - P ~ zircon date (Goldich pers. commun., as reported in Horan and others, 1987) 2 ~ - ~ b zircon date (Mark Schmitz, Massachussettes Institite of Technology, pers. comm., 1998)

GROUND WATER RECHARGE, DISCHARGE AND RESIDENCE TIME IN RICECOUNTY, MINNESOTA -- IMPLICATIONS FOR LAND USE PLANNING

CAMPION, Moira -- Minnesota Department of Natural Resourcese-mail -- moira.campion@dnr.state.mn.us

Rice county is located 40 miles south of the Minneapolis-St. Paul metropolitan area. Thecounty contains urban, suburban, and rural areas and faces increasing development pressurefor housing as well as for recreational and high intensity agricultural purposes. Water supplyin the county is primarily from ground water and most of the 1600 wells in the county database are completed in the St. Peter-Prairie du Chien-Jordan aquifer. This bedrock aquifer isconfined throughout the western and southeastern portions of the county and is unconfined inthe east-central and northeast. The potentiometric surface shows a regional high in southwestRice County and lows in the central and northeast parts of the county where the aquiferdischarges into local streams.

Throughout most of the county the aquifer is protected from direct surface recharge byvarious consolidated and unconsolidated low permeability materials, except in the northeastwhere the aquifer is exposed at or near the surface. The various low permeability depositsand their distribution result in different recharge conditions to the bedrock aquifer. In thenortheast, where the aquifer is not protected by overlying material, recharge to the aquiferoccurs directly through percolation from the land surface. In western Rice County, there areabundant lakes and wetlands in a hummocky terrain of thick supraglacial tills. Recharge tothe bedrock aquifer occurs in complicated flowpaths through the lakes and tills in this part ofthe county. In the southeast, the Decorah-Platteville-Glenwood confining unit and someolder tills cover the aquifer. Where this relatively thin package of shales and limestones iscontinuous, the St. Peter-Prairie du Chien-Jordan aquifer receives very little vertical rechargefrom the surface. Conversely, near the edge of this confining unit, recharge may be as muchas 30 times greater than underneath this unit (Delin, 1991). Discharge from the aquiferoccurs where ground water drains into the Cannon River, the Straight River, and PrairieCreek. Residence time of ground water was estimated using tritium and 14C age-dating.These isotopic age estimates confirm the conceptual model of recharge in the county.Ground water is youngest in the northeast where low permeability cover is absent and oldestin the southeast under the Decorah-Platteville-Glenwood confining unit. Isotopic results inwestern Rice County show ages greater than tritium dating limits but less than a fewcenturies old.

Knowledge of these differing recharge conditions, coupled with geologic and hydrologicinformation, provides county planners with an important water resource management tool.This information allows county staff to decide where modification to current land usepractices would minimize potential negative impacts on ground water quality. Basing landuse planning decisions on resource management concepts rather than on politically basedsituations will increase the chance of resource protection.

48

GROUND WATER RECHARGE, DISCHARGE AND RESIDENCE TIME IN RICE COUNTY, MINNESOTA -- IMPLICATIONS FOR LAND USE PLANNING

CAMPION? Moira -- Minnesota Department of Natural Resources e-mail-- moira.campion@d~.state.mn.us

Rice county is located 40 miles south of the Minneapolis-St. Paul metropolitan area. The county contains urban, suburban, and rural areas and faces increasing development pressure for housing as well as for recreational and high intensity agricultural purposes. Water supply in the county is primarily from ground water and most of the 1600 wells in the county data base are completed in the St. Peter-Prairie du Chien-Jordan aquifer. This bedrock aquifer is confined throughout the western and southeastern portions of the county and is unconfined in the east-central and northeast. The potentiometric surface shows a regional high in southwest Rice County and lows in the central and northeast parts of the county where the aquifer discharges into local streams.

Throughout most of the county the aquifer is protected from direct surface recharge by various consolidated and unconsolidated low permeability materials, except in the northeast where the aquifer is exposed at or near the surface. The various low permeability deposits and their distribution result in different recharge conditions to the bedrock aquifer. In the northeast, where the aquifer is not protected by overlying material, recharge to the aquifer occurs directly through percolation from the land surface. In western Rice County, there are abundant lakes and wetlands in a hummocky terrain of thick supraglacial tills. Recharge to the bedrock aquifer occurs in complicated flowpaths through the lakes and tills in this part of the county. In the southeast, the Decorah-Platteville-Glenwood confining unit and some older tills cover the aquifer. Where this relatively thin package of shales and limestones is continuous7 the St. Peter-Prairie du Chien-Jordan aquifer receives very little vertical recharge from the surface. Conversely, near the edge of this confining unit, recharge may be as much as 30 times greater than underneath this unit (Delin, 1991). Discharge fiom the aquifer occurs where ground water drains into the Cannon River, the Straight River, and Prairie Creek. Residence time of ground water was estimated using tritium and 1% age-dating. These isotopic age estimates confirm the conceptual model of recharge in the county. Ground water is youngest in the northeast where low permeability cover is absent and oldest in the southeast under the Decorah-Platteville-Glenwood confining unit. Isotopic results in western Rice County show ages greater than tritium dating limits but less than a few centuries old.

Knowledge of these differing recharge conditions, coupled with geologic and hydrologic information, provides county planners with an important water resource management tool. This information allows county staff to decide where modification to current land use practices would minimize potential negative impacts on ground water quality. Basing land use planning decisions on resource management concepts rather than on politically based situations will increase the chance of resource protection.

Potentiometric

Cross section

A Well sampledfor chemistry

+ Well measured for waterlevel, but not sampled

Saturated thickness in feet

LIII 0-100100-200200-300300-400400-500500-600

Aquifer absent

'1 Aquifer unconfined;aquifer confined to west

49

RICE COUNTY, MINNESOTAHydrogeology of the

St. Peter-Prairie du Chien-Jordan Aquifer

5)5

(0

0

Map Legend Cross Section Legend

R UnsaturatedRecent waters; tritium >10 TUMixed waters 1< tntium <10 TUVintage waters; tritium <1 TU andbetween 50 to 5,000 years oldVintage waters; tritium <1 TU andmore than 5,000 to 10,000 years oldDecorah-Platteville-GlennwoodConfining unitSt. Peter-Prairie du Chien-Jordan aquifer

Original Scale 1:100,000. From: Rice County Geologic Atlas. Part 6,1997Copyright: Minnesota Dept. of Natural Resources

— Direction of groundwater flow

RICE COUNTY, MINNESOTA Hydrogeology of the

St. Peter-Prairie du Chien-Jordan Aquifer

m - 5 ln -

* -

6 4 -

0 -

0 2 4 6 Miles I 1 I I

Map Legend Cross Section Legend

/V Potentiometric contour

Cross section

A Well sampled for chemistry

Saturated thickness in feet

+ Well measured for water Aquifer absent level, but not sampled ix Aquifer unconfined;

aquifer confined to west

E Unsaturated Recent waters; tritium sf0 TU 0 Mixed waters; 7 c tritium c 7 0 TU

Vintage waters; tritium c7 TU and between 50 to 5,000 years old Vintage waters; tritium el TU and more than 5,000 to 70,000 years old Decorah-Plaffeville-Glennwmd Confining unit St. Peter-Prairie du ChienJordan aquifer

+ Direction of groundwater flow

Original Scale l : l C J O , O ~ . Fmm: Rice Counly Geologic Atlas, Part 6.1997 Cop*ght: Minnesota Dept. of Natural Resources

THE GEOLOGICAL SOURCE OF ARSENIC IN GROUND WATER IN SOUTHEASTERNMICHIGAN

W.F. Cannon and Alan Kolker, U.S. Geological Survey, Reston, VAD.B. Westjohn, U.S. Geological Survey, Lansing, MI

The ground water in parts of nine counties in southeastern Michigan contains anomalous quantities ofdissolved arsenic, presumably derived from a natural source. Many wells tested by the MichiganDepartment of Community Health and by the USGS exceed the U.S.Environmental ProtectionAgency's maximum contaminant level (MCL) of 50 .tg/L for drinking water. Extreme values are ashigh as 350 jtg/L. In one county (Huron) about 30% of the tested wells exceeded the MCL for arsenic.Most wells with elevated arsenic were completed in the Mississippian Marshall Sandstone, but otherunits, particularly glacial aquifers, can also yield arsenic-contaminated water.

The nine affected counties have a combined population of more than 2 million people. The MarshallSandstone is the principal bedrock aquifer in most of the affected counties and in some areas is thesole-source aquifer. These conditions combine to make knowledge of the natural source for arsenic acritical aspect in designing a strategy to provide a continued supply of safe drinking water for thisregion.

As part of a larger study of this problem, we have been examining the lithology, mineralogy, andchemistry of the Marshall Sandstone to better define the geographic, stratigraphic, and mineralogicdistribution of arsenic in bedrock. In October 1997, a well was drilled and cored to collect a completestratigraphic section of the Marshall Sandstone and immediately overlying and underlying units. Thehole was drilled in Huron County, near one of the most contaminated wells known in the study area.Core from that hole and cuttings and core from other producing wells have formed the basis for thisstudy.

The Marshall Sandstone is a fluvial to marginal marine sequence that is present at subcrop in a beltencircling the central part of the Michigan Basin. Arsenic-contaminated ground water is known onlyalong the eastern flank of the basin. As seen in the Huron County core, the Marshall is a veryheterogeneous unit of medium- to coarse-grained, gray to brown sandstone interbedded with massiveto laminated gray and red siltstone and thin units of black to gray shale. Fossil plant debris is presentand pyrite is common in accessory amounts in many units.

Arsenic is very unevenly distributed in the Marshall. Highest values in the Huron County core, up to255 mg/kg, are in black shale units. Most sandstone and siltstone have an arsenic content below theanalytically detectable limit (5 mg/kg), but some sandstone beds contain as much as 25 mg/kg arsenic.Average sandstones contain about 2 mg/kg arsenic, and so even these relatively low values areanomalous. Cuttings from wells in adjacent Lapeer County contain as much as 350 mg/kg arsenic. Thestrongly anomalous arsenic content of the Marshall Sandstone in the area of arsenic contaminatedground water lends strong support to the earlier belief that the source of arsenic is natural and withinthe Marshall aquifer.

Electron microprobe studies of well cuttings from Lapeer and Tuscola Counties show that essentiallyall arsenic is in pyrite. Pyrite is ubiquitous in the Marshall Sandstone, typically in trace concentrations,but locally constitutes from a few percent to as much as 20 percent of the rock, mostly as pore-occluding cement. Pyrite formed during several stages of diagenesis. Early diagenetic pyrite formedcoatings on detrital grains. Framboidal pyrite formed in intermediate stages of diagenesis andprecipitated on authigenic carbonate and chlorite or on authigenic quartz overgrowths. Late stagepyrite encapsulated framboids and in places formed displacive pyrite masses.

50

THE GEOLOGICAL SOURCE OF ARSENIC IN GROUND WATER IN SOUTHEASTERN MICHIGAN

W.F. Cannon and Alan Kolker, U.S. Geological Surveyy Reston? VA D.B. Westjohn? U.S. Geological Surveyy Lansing? MI

The ground water in parts of nine counties in southeastern Michigan contains anomalous quantities of dissolved arsenic? presumably derived from a natural source. Many wells tested by the Michigan Department of Community Health and by the USGS exceed the U.S.Environmenta1 Protection Agency's maximum contaminant level (MCL) of 50 pg/L for drinking water. Extreme values are as high as 350 pgL. In one county (Huron) about 30% of the tested wells exceeded the MCL for arsenic. Most wells with elevated arsenic were completed in the Mississippian Marshall Sandstone? but other units? particularly glacial aquifers? can also yield arsenic-contaminated water.

The nine affected counties have a combined population of more than 2 million people. The Marshall Sandstone is the principal bedrock aquifer in most of the affected counties and in some areas is the sole-source aquifer. These conditions combine to make knowledge of the natural source for arsenic a critical aspect in designing a strategy to provide a continued supply of safe drinking water for this region.

As part of a larger study of this problem, we have been examining the lithology, mineralogy, and chemistry of the Marshall Sandstone to better define the geographic? stratigraphic? and mineralogic distribution of arsenic in bedrock. In October 1997? a well was drilled and cored to collect a complete stratigraphic section of the Marshall Sandstone and immediately overlying and underlying units. The hole was drilled in Huron County? near one of the most contaminated wells known in the study area. Core from that hole and cuttings and core from other producing wells have formed the basis for this study.

The Marshall Sandstone is a .fluvial to marginal marine sequence that is present at subcrop in a belt encircling the central part of the Michigan Basin. Arsenic-contaminated ground water is known only along the eastern flank of the basin. As seen in the Huron County core, the Marshall is a very heterogeneous unit of medium- to coarse-grained, gray to brown sandstone interbedded with massive to laminated gray and red siltstone and thin units of black to gray shale. Fossil plant debris is present and pyrite is common in accessory amounts in many units.

Arsenic is very unevenly distributed in the Marshall. Highest values in the Huron County core, up to 255 m a g y are in black shale units. Most sandstone and siltstone have an arsenic content below the analytically detectable limit (5 mgkg) but some sandstone beds contain as much as 25 mglkg arsenic. Average sandstones contain about 2 mgkg arsenic? and so even these relatively low values are anomalous. Cuttings from wells in adjacent Lapeer County contain as much as 350 mgkg arsenic. The strongly anomalous arsenic content of the Marshall Sandstone in the area of arsenic contaminated ground water lends strong support to the earlier belief that the source of arsenic is natural and within the Marshall aquifer.

Electron microprobe studies of well cuttings from Lapeer and Tuscola Counties show that essentially all arsenic is in pyrite. Pyrite is ubiquitous in the Marshall Sandstone? typically in trace concentrationsy but locally constitutes from a few percent to as much as 20 percent of the rock? mostly as pore- occluding cement. Pyrite formed during several stages of diagenesis. Early diagenetic pyrite formed coatings on detrital grains. Framboidal pyrite formed in intermediate stages of diagenesis and precipitated on authigenic carbonate and chlorite or on authigenic quartz overgrowths. Late stage pyrite encapsulated framboids and in places formed displacive pyrite masses.

Arsenic is very unevenly distributed in pyrite at virtually all scales, even to the limit of resolution ofthe microprobe. Many pyrite grains have little or no arsenic at the limit of detection (about 0.01 masspercent), whereas nearby grains (within the same rock chip) may have extreme arsenic enrichment.Individual analyzed points contain as much as 6.5 mass percent arsenic. Arsenic shows a variety ofmodes of occurrence, but the most common is as strong enrichments in rims on individual framboids(see figure). It appears that a majority of arsenic was introduced during an intermediate stage of pyritediagenetic growth, at the end of framboidal pyrite formation. Both framboid cores and later overgrownpyrite are not enriched in arsenic. Some other trace elements also show enrichment. Some arsenic-enriched pyrite contains as much as 0.7 mass % Ni and 0.5 mass % Co.

A critical remaining question is where and how arsenic is released from pyrite to contaminate groundwater. In general, the oxidation of arsenic-bearing pyrite must be the fundamental control of arsenicrelease. Most arsenic seems to be contained in black shale units, which are not aquifers and have notbeen observed to be undergoing oxidation. It seems likely, therefore, that arsenic is introduced toground water from smaller concentrations in the coarser sandstones that form the aquifers within theMarshall Sandstone. Observed arsenic content of coarse sandstone is as high as 25 mg/kg over severalfeet, or tens of feet, of section. The sandstones, therefore, have concentrations several hundred timesgreater than the MCL for drinking water and are able in quantitative terms to provide enough arsenic tocontaminate a large volume of ground water. Results to date indicate that the potential for naturalarsenic contamination of ground water is controlled by a variety of factors including total arseniccontent of the rock and hydrologic characteristics of the rock, and by poorly know factors such asbiological mediation of pyrite solution and variable redox conditions of ground water in both time andplace.

Electron microprobe map showing framboids of arsenic-poor pyrite encased in arsenic-rich pyrite, inturn overgrown by arsenic-poor pyrite. Map of iron distribution (left) outlines large pyrite grains(lightest shade) that have grown in pore space between quartz clasts (black). Authigenic clay(intermediate gray) also fills pore space. Arsenic distribution is shown on the right, the same field ofview as the iron map. Arsenic is concentrated on rims of pyrite framboids now completely enclosedwithin late stage pyrite.

51

Arsenic is very unevenly distributed in pyrite at virtually all scales, even to the limit of resolution of the microprobe. Many pyrite grains have little or no arsenic at the limit of detection (about 0.01 mass percent), whereas nearby grains (within the same rock chip) may have extreme arsenic enrichment. Individual analyzed points contain as much as 6.5 mass percent arsenic. Arsenic shows a variety of modes of occurrence, but the most common is as strong enrichments in rims on individual framboids (see figure). It appears that a majority of arsenic was introduced during an intermediate stage of pyrite diagenetic growth, at the end of framboidal pyrite formation. Both framboid cores and later overgrown pyrite are not enriched in arsenic. Some other trace elements also show enrichment. Some arsenic- enriched pyrite contains as much as 0.7 mass % Ni and 0.5 mass % Co.

A critical remaining question is where and how arsenic is released from pyrite to contaminate ground water. In general, the oxidation of arsenic-bearing pyrite must be the fundamental control of arsenic release. Most arsenic seems to be contained in black shale units, which are not aquifers and have not been observed to be undergoing oxidation. It seems likely, therefore, that arsenic is introduced to ground water from smaller concentrations in the coarser sandstones that form the aquifers within the Marshall Sandstone. Observed arsenic content of coarse sandstone is as high as 25 mglkg over several feet, or tens of feet, of section. The sandstones, therefore, have concentrations several hundred times greater than the MCL for drinking water and are able in quantitative terms to provide enough arsenic to contaminate a large volume of ground water. Results to date indicate that the potential for natural arsenic contamination of ground water is controlled by a variety of factors including total arsenic content of the rock and hydrologic characteristics of the rock, and by poorly know factors such as biological mediation of pyrite solution and variable redox conditions of ground water in both time and place.

Electron microprobe map showing framboids of arsenic-poor pyrite encased in arsenic-rich pyrite, in turn overgrown by arsenic-poor pyrite. Map of iron distribution (left) outlines large pyrite grains (lightest shade) that have grown in pore space between quartz clasts (black). Authigenic clay (intermediate gray) also fills pore space. Arsenic distribution is shown on the right, the same field of view as the iron map. Arsenic is concentrated on rims of pyrite frarnboids now completely enclosed within late stage pyrite.

REiNTERPRETATION OF THE PENOKEAN CONTINENTAL MARGIN INPART OF NORTHERN WISCONSIN AND MICHIGAN

W.F. Cannon, U.S. Geological Survey, Reston, VAG.L. Laberge, UW-Oshkosh, Oshkosh, WIJ. S. Kiasner, Western Illinois University, Macomb, ILK.J. Schulz, U.S. Geological Survey, Reston, VA

The depositional and tectonic history of part of the Penokean continental margin has beenclarified by field studies, and examination of previously proprietary' magnetic, electro-magnetic, and drill core data. A continental margin terrane, informally called the Marquetteterrane, inasmuch as it is composed largely of rocks of the Marquette Range Supergroup andits Archean basement, consists of three subterranes, each with its own characteristic depo-sitional and tectonic history (see figure). The subterranes are separated by major Penokeanfaults, each probably a northward-directed thrust.

PINE LAKE SUBTERRANEMarquette Range Supergroup

metasedimentary & metavolcanic rocks

Archean granitic rocks

Archean greenstones

PARK FALLS SUBTERRANE

Graywacke, schist, metavolcanic rocks

gi Archeangneiss

[i] Graywacke. metavolcanic rocks, schistBasal ferrugenous unit

Archean gneiss

WISCONSIN MAGMATIC TERRANES

±Xg GraniteMetavolcanic rocks

— A.. — Keweenwan thrust faults

y Penokean thrust faults

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POWELL SUBTERRANE

'We thank Cominco American Incorporated and Kerr-McGee Corporation for penuission to publish detailedaeromagnetic and electromagnetic data for parts of our study area

REINTERPRETATION OF THE PENOKEAN CONTINENTAL MARGIN IN PART OF NORTHERN WISCONSIN AND MICHIGAN

W.F. Cannon, U.S. Geological Survey, Reston, VA G.L. Laberge, UW-Oshkosh, Oshkosh, WI J. S. Klasner, Western Illinois University, Macomb, IL K J. Schulz, U. S. Geological Survey, Reston, VA

The depositional and tectonic history of part of the Penokean continental margin has been clarified by field studies, and examination of previously proprietary1 magnetic, electro- magnetic, and drill core data. A continental margin terrane, informally called the Marquette terrene, inasmuch as it is composed largely of rocks of the Marquette Range Supergroup and its Archean basement, consists of three subterranes, each with its own characteristic depo- sitional and tectonic history (see figure). The subterranes are separated by major Penokean faults, each probably a northward-directed thrust.

PINE LAKE SUBTERRANE POWELL SUBTERRANE Vflyj Marquette Range Supergroup

metasedirnentary & metavolcanic rocks

Archean granitic rocks Archean greenstones

Graywacke, metavolcanic rocks, schist

Basal ferrugenous unit

Archean gneiss

PARK FALLS SUBTERRANE WISCONSIN MAGMATICTERRANES

Graywacke, schist, metavolcanic rocks

- - Keweenwan thrust faults

7 Penokean thrust faults

1 We thank Cominco American Incorporated and Kerr-McGee Corporation for permission to publish detailed aeromagnetic and electromagnetic data for parts of our study area

52

The Marquette terrane includes those areas where the Marquette Range Supergroup wasdeposited in a tectonically unstable environment for at least part of its depositional historyand was subsequently significantly deformed and metamorphosed during the Penokeanorogeny. The terrane is bounded on the north by areas where the Marquette RangeSupergroup was deposited in a stable cratonic or foreland setting and was not appreciablydeformed during the Penokean orogeny (central Gogebic Iron Range for instance), and onthe south by the volcanic arcs of the Pembine-Wausau terrane. The essential characteristicsof each subterrane are outlined below.

Pine Lake subterrane• Basement is Late Archean granite, greenstone, and metagraywacke.• Early Proterozoic has quartzite and dolomite of Chocolay Group at base.• Menominee Group consists of Palms Formation, iron-formation including shallow-water oolitic jasper, and

abundant volcanic rocks. Group was deposited in tectonically active region, probably in syndepositionalgrabens.

• Early Proterozoic rocks were strongly deformed, generally with increasing intensity of deformation to thesouth.

• Archean basement rocks were not deformed penetratively during the Penokean orogeny.• Penokean granitic rocks are rare.• Metamorphic grade ranges from chlorite to garnet (locally staurolite).

Powell subterrane• Basement is Early and Late Archean gneiss.• Early Proterozoic rocks include a widespread basal unit of ferruginous slate and lean iron-formation,

quartzite, and black suffidic slate and an overling succession of metapelitic and lesser metavolcanic rocks.The rocks are probably broadly correlative with the Marquette Range Supergroup, but details of correlationare not known.

• Archean basement was penetratively deformed along with Early Proterozoic cover during the Penokeanorogeny.

• Dikes and segregations of Penokean granitic rocks are widespread.• High temperature and high pressure metamorphism occurs throughout Pelitic assemblages are biotite-

gamet-staurolite-kyarnte.

Park Falls subterrane• Basement is Archean gneiss.• Early Proterozoic rocks are peitic schist, carbonaceous sulliclic slate, and subordinate metavolcanic rocks.

The rocks are probably broadly equivalent to the Marquette Range Supergroup but details of correlation areunknown.

• Rocks are intensely folded in multiple folding events.• Dikes and segregations of Penokean granitic rocks are abundant.• Metamorphism is moderate temperature and pressure. Pelitic assemblages are biotite-garnet-sillimanite.

The assemblage of subterranes hereby defined in Wisconsin and Michigan is similar to theassemblage known in analogous parts of the Penokean orogen in Minnesota. In particular,we suggest that the Flambeau Flowage Fault in Wisconsin and Michigan is the eastwardextension of the Malmo discontinuity in Minnesota. Both structures thrust Archean gneissand highly metamorphosed Early Proterozoic strata northward over less deformed andmetamorphosed Early Proterozoic strata.

53

The Marquette terrane includes those areas where the Marquette Range Supergroup was deposited in a tectonically unstable environment for at least part of its depositional history and was subsequently significantly deformed and metamorphosed during the Penokean orogeny. The terrane is bounded on the north by areas where the Marquette Range Supergroup was deposited in a stable cratonic or foreland setting and was not appreciably deformed during the Penokean orogeny (central Gogebic Iron Range for instance), and on the south by the volcanic arcs of the Pembine-Wausau terrane. The essential characteristics of each subterrane are outlined below.

Pine Lake subterrane 0 Basement is Late Archean granite, greenstone, and metagraywacke. 0 Early Proterozoic has quartzite and dolomite of Chocolay Group at base.

Menominee Group consists of Palms Formation, iron-formation including shallow-water oolitic jasper, and abundant volcanic rocks. Group was deposited in tectonically active region, probably in syndepositional grabens. Early Proterozoic rocks were strongly deformed, generally with increasing intensity of deformation to the south. Archean basement rocks were not deformed penetratively during the Penokean orogeny. Penokean granitic rocks are rare. Metamorphic grade ranges from chlorite to garnet (locally staurolite).

Powell subterrane Basement is Early and Late Archean gneiss.

0 Early Proterozoic rocks include a widespread basal unit of ferruginous slate and lean iron-formation, quartzite, and black sulfidic slate and an overling succession of metapelitic and lesser metavolcanic rocks. The rocks are probably broadly correlative with the Marquette Range Supergroup, but details of correlation are not known. Archean basement was penetratively deformed along with Early Proterozoic cover during the Penokean orogeny. Dikes and segregations of Penokean granitic rocks are widespread. High temperature and high pressure metamorphism occurs throughout. Pelitic assemblages are biotite- garnet-staurolite-kyanite.

Park Falls subterrane Basement is Archean gneiss.

0 Early Proterozoic rocks are politic schist, carbonaceous sulfidic slate, and subordinate metavolcanic rocks. The rocks are probably broadly equivalent to the Marquette Range Supergroup but details of correlation are unknown.

0 Rocks are intensely folded in multiple folding events. 0 Dikes and segregations of Penokean granitic rocks are abundant.

Metamorphism is moderate temperature and pressure. Pelitic assemblages are biotite-garnet-sillimanite.

The assemblage of subterranes hereby defined in Wisconsin and Michigan is similar to the assemblage known in analogous parts of the Penokean orogen in Minnesota. In particular, we suggest that the Flambeau Flowage Fault in Wisconsin and Michigan is the eastward extension of the Malmo discontinuity in Minnesota. Both structures thrust Archean gneiss and highly metamorphosed Early Proterozoic strata northward over less deformed and metamorphosed Early Proterozoic strata.

GEOGRAPHIC INFORMATION SYSTEM ON THE GEOLOGY AND COPPERDEPOSITS OF THE KEWEENAW PEMNSULA

W.F. Cannon, U.S. Geological Survey, Reston, VAMichele E. McRae, Oak Ridge Associated Universities, Reston, VASuzanne W. Nicholson, U.S. Geological Survey, Reston, VA

Two new products on the geology and mineral deposits of the Keweenaw Peninsula andsurrounding area have recently been developed: a traditional 1:100,000 scale U.S. GeologicalSurvey geologic map (now in press) and a geographic information system (GIS) database.These products are based largely on detailed geologic maps, many at a scale of 1:24,000,published during the past 50 years. They present new data and interpretations in parts of thearea. The paper product was compiled digitally and digital production techniques are beingused to streamline the printing process.

54

/1 \

FormationsPaleozoic undivided

J Jacobsville SandstoneFreda SandstoneNonesuch FormationCopper Harbor ConglomerateIndiana FelsitePortage Lake VolcanicsDiabase dikesSiemens Creek VolcanicsMichigamme FormationGranitic gneiss

Geology of the Keweenaw Peninsula and vicinity generalized by formation.

GEOGRAPHIC INFORMATION SYSTEM ON THE GEOLOGY AND COPPER DEPOSITS OF THE KEWEENAW PENINSULA

W.F. Cannon, U.S. Geological Survey, Reston, VA Michele E. McRae, Oak Ridge Associated Universities, Reston, VA Suzanne W. Nicholson, U.S. Geological Survey, Reston, VA

Two new products on the geology and mineral deposits of the Keweenaw Peninsula and surrounding area have recently been developed: a traditional 1 : 100,000 scale U. S. Geological Survey geologic map (now in press) and a geographic information system (GIs) database. These products are based largely on detailed geologic maps, many at a scale of 1 :24,000, published during the past 50 years. They present new data and interpretations in parts of the area. The paper product was compiled digitally and digital production techniques are being used to streamline the printing process.

Geology of die Keweenaw Peninsula and vicinity generalized by formation.

The data were developed using the Environmental System Resource Institute's ARC/INFOsoftware and exported to ArcView. ArcView is a desktop mapping software package thatprovides a variety of tools for the display, query, analysis, and output of geographicallyreferenced data sets. This particular GIS consists of coverages and tabular data on thegeology, structure, mines, mineral deposits, hydrography, and transportation networks of thearea. Most of the detail from the source maps has been incorporated into the database. Mapviews showing full detail can be constructed; however, the data have been structured to allowgeneralization by age, rock type, tectonic setting, or stratigraphic rank. The figure above wasgenerated in ArcView, by generalizing the geologic units based on stratigraphic formation.ArcView also allows the user to access information either by interactively selecting featureson a map view, or by performing logical selection on the data tables. For example, clickingon a mineral deposit with the 'identify' tool will open a window that provides tabularinformation on mines that worked the deposit, amount of production, years of production,and an estimate of the amount and grade of remaining identified resources. A logical searchcould be used to identify native copper lodes with greater than 1% Cu. Finally, this productwill include geologic cross sections, a correlation chart, a description of map units, andinterpretative text. The cross sections are dynamically linked to the cross sections lines onthe map view.

55

The data were developed using the Environmental System Resource Institute's ARCIINFO software and exported to ArcView. ArcView is a desktop mapping software package that provides a variety of tools for the display, query, analysis, and output of geographically referenced data sets. This particular GIs consists of coverages and tabular data on the geology, structure, mines, mineral deposits, hydrography, and transportation networks of the area. Most of the detail from the source maps has been incorporated into the database. Map views showing fall detail can be constructed; however, the data have been structured to allow generalization by age, rock type, tectonic setting, or stratigraphic rank. The figure above was generated in ArcView, by generalizing the geologic units based on stratigraphic formation. ArcView also allows the user to access information either by interactively selecting features on a map view, or by performing logical selection on the data tables. For example, clicking on a mineral deposit with the 'identify' tool will open a window that provides tabular information on mines that worked the deposit, amount of production, years of production, and an estimate of the amount and grade of remaining identified resources. A logical search could be used to identify native copper lodes with greater than 1% Cu. Finally, this product will include geologic cross sections, a correlation chart, a description of map units, and interpretative text. The cross sections are dynamically linked to the cross sections lines on the map view.

GRAVITY AND MAGNETIC STUDIES IN THE VIRGINIA HORN AREA,NORTHEASTERN MINNESOTA

CHANDLER, Va! W., JIRSA, Mark A., and LIVELY, Richard L. MinnesotaGeological Survey, 2642 University Ave., St. Paul, MN 55114(chand004@maroon.tc.umn.edu)

Gravity and magnetic data were used to supplement geologic field mapping of Archean rocksof the Virginia Horn area of northeastern Minnesota. Although outcrop control is locallyexcellent, gravity and magnetic data assist to a minor degree in mapping areas covered byglacial deposits and mine dumps. More importantly, they provide additional information ongeologic structure at depth. Preexisting subdivisions of the Archean and Paleoproterozoicrocks were used to guide mapping and geophysical work. Although the gross lithologicdivisions have remained essentially unchanged, many details of their stratigraphic andstructural relations are now much better known. Gravity data used in this study consist ofpreexisting stations in the state-wide database together with 203 new stations. The newgravity stations fill gaps in the previous coverage, and allow development of a 50 kmnortheast-southwest profile across the Archean rocks of the Virginia Horn area, together withfive shorter (5-15 km) northwest-southeast profiles. The magnetic data are from the highresolution aeromagnetic survey of Minnesota; line spacing in the Virginia Horn area is 400m. Subsurface structure was investigated using gravity and magnetic modeling on the longprofile.

The gravity and magnetic data were interpreted using grid images of processed data.Unfiltered and derivative-enhanced versions of the gravity and aeromagnetic grids revealmany details of the bedrock geology. The northern part of the study area is underlain byArchean granitic rocks of the Giants Range Batholith. It is characterized by a stronglynegative Bouguer gravity anomaly and a busy, moderate- to high-amplitude magneticsignature. The presence of intermediate to mafic phases within the batholith is indicated bymore magnetic areas that are associated with highs in the derivative-enhanced gravity data.Derivative-enhanced magnetic data also delineate several previously unidentified northwest-strildng faults that extend across the batholith. A discontinuous sliver of high-grademetavolcanic and metasedimentary rocks, the Minntac sequence, occurs along the southernmargin of the batholith. A negative Bouguer gravity anomaly implies that the supracrustalrocks of the Minntac sequence are under-plated by granitic rocks at shallow depths. TheMinntac sequence is separated from Archean rocks to the south by the east-west strikingLaurentian fault. South of the Laurentian fault sub- to low-greenschist grade metavolcanicand metasedimentary rocks of the Archean Mud Lake and Midway sequences arecharacterized by a positive gravity anomaly and an extremely subdued magnetic signature.Rocks of the Mud Lake sequence form a broad southwest-plunging syncline. To the southand west, the supracrustal rocks of the Minntac, Midway and Mud Lake sequences arecovered by the Paleoproterozoic Animikie Group. Regional gravity data indicate that theArchean rocks extend southwest as a major belt beneath the Animikie basin. The BiwabikIron Formation of the lower Animilde Group is associated with complex, high-amplitudemagnetic signatures; magnetic highs delineate oxide-rich taconites and magnetic lowsdelineate natural ores and faults.

Gravity and magnetic modeling was constrained by surface geologic mapping and rockproperties determined from surface samples. The models indicate that most structures in theArchean rocks have near-vertical dips. The Giants Range Batholith and the low-gradesupracrustal rocks of the Midway and Mud Lake sequences extend to about 5 km depth. Thecomposition of the crust below 5 km remains uncertain, although the Minntac sequence(which we infer to have been uplifted along the Laurentian fault) may yield clues because ithas been extensively invaded by tonalitic and other early phases of the Giants RangeBatholith. Models clearly show that the low-grade sedimentary rocks of the Mud Lakesequence lie within a structural trough; low-density sediments as thick as 1 km are underlain

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GRAVITY AND MAGNETIC STUDIES IN THE VIRGINIA HORN AREA, NORTHEASTERN MINNESOTA

CHANDLER, Val W ., JIRSA, Mark A., and LIVELY, Richard L. Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 551 14 (chandOO4@maroon.tc.umn.edu)

Gravity and magnetic data were used to supplement geologic field mapping of Archean rocks of the Virginia Horn area of northeastern Mimesota. Although outcrop control is locally excellent, gravity and magnetic data assist to a minor degree in mapping areas covered by glacial deposits and mine dumps. More importantly, they provide additional information on geologic structure at depth. Preexisting subdivisions of the Archean and Paleoproterozoic rocks were used to guide mapping and geophysical work. Although the gross lithologic divisions have remained essentially unchanged, many details of their stratigraphic and structural relations are now much better known. Gravity data used in this study consist of preexisting stations in the state-wide database together with 203 new stations. The new gravity stations fill gaps in the previous coverage, and allow development of a 50 km northeast-southwest profile across the Archean rocks of the Virginia Horn area, together with five shorter (5-15 km) northwest-southeast profiles. The magnetic data are from the high resolution aeromagnetic survey of Minnesota; line spacing in the Virginia Horn area is 400 m. Subsurface structure was investigated using gravity and magnetic modeling on the long profile.

The gravity and magnetic data were interpreted using grid images of processed data. Unfiltered and derivative-enhanced versions of the gravity and aeromagnetic grids reveal many details of the bedrock geology. The northern part of the study area is underlain by Archean granitic rocks of the Giants Range Batholith. It is characterized by a strongly negative Bouguer gravity anomaly and a busy, moderate- to high-amplitude magnetic signature. The presence of intermediate to mafic phases within the batholith is indicated by more magnetic areas that are associated with highs in the derivative-enhanced gravity data. Derivative-enhanced magnetic data also delineate several previously unidentified northwest- striking faults that extend across the batholith. A discontinuous sliver of high-grade metavolcanic and metasedimentary rocks, the Mimtac sequence, occurs along the southern margin of the batholith. A negative Bouguer gravity anomaly implies that the supracrustal rocks of the Mimtac sequence are under-plated by granitic rocks at shallow depths. The Minntac sequence is separated from Archean rocks to the south by the east-west striking Laurentian fault. South of the Laurentian fault sub- to low-greenschist grade metavolcanic and metasedimentary rocks of the Archean Mud Lake and Midway sequences are characterized by a positive gravity anomaly and an extremely subdued magnetic signature. Rocks of the Mud Lake sequence form a broad southwest-plunging syncline. To the south and west, the supracrustal rocks of the Minntac, Midway and Mud Lake sequences are covered by the Paleoproterozoic Animikie Group. Regional gravity data indicate that the Archean rocks extend southwest as a major belt beneath the Animikie basin. The Biwabik Iron Formation of the lower Animikie Group is associated with complex, high-amplitude magnetic signatures; magnetic highs delineate oxide-rich taconites and magnetic lows delineate natural ores and faults.

Gravity and magnetic modeling was constmined by surface geologic mapping and rock properties determined from surface samples. The models indicate that most structures in the Archean rocks have near-vertical dips. The Giants Range Batholith and the low-grade supracrustal rocks of the Midway and Mud Lake sequences extend to about 5 km depth. The composition of the crust below 5 km remains uncertain, although the Mimtac sequence (which we infer to have been uplifted along the Laurentian fault) may yield clues because it has been extensively invaded by tonalitic and other early phases of the Giants Range Batholith. Models clearly show that the low-grade sedimentary rocks of the Mud Lake sequence lie within a structural trough; low-density sediments as thick as 1 km are underlain

by moderately high-density rocks inferred to be largely basaltic. Thickness of thesedimentary rocks increases in stepwise fashion northwestward towards the center of thebasin against northeast-striking faults. Sediment thickness is also interpreted to increaseappreciably to the northeast. These results demonstrate that gravity and magnetic studies arevaluable to a geologic mapping program, even in areas of abundant outcrop control.

ACKNOWLEDGMENTSThis study was supported by the Minnesota Legislature through the State SpecialAppropriation and an appropriation recommended by the Minnesota Minerals CoordinatingCommittee. The authors thank U. S. Steel Corporation, Inland Steel Mining Company,Cliffs Mining Services Company, LTV Mining Company, American Shield Company, theDuluth Mesabi and Iron Range Railway Company, and the Minnesota Power Company foraccess to their properties and for providing elevation data.

57

by moderately high-density rocks inferred to be largely basaltic. Thickness of the sedimentary rocks increases in stepwise fashion northwestward towards the center of the basin against northeast-striking faults. Sediment thickness is also interpreted to increase appreciably to the northeast. These results demonstrate that gravity and magnetic studies are valuable to a geologic mapping program, even in areas of abundant outcrop control.

ACKNOWLEDGMENTS This study was supported by the Minnesota Legislature through the State Special Appropriation and an appropriation recommended by the Minnesota Minerals Coordinating Committee. The authors thank U. S. Steel Corporation, Inland Steel Mining Company, Cliffs Mining Services Company, LTV Mining Company, American Shield Company, the Duluth Mesabi and Iron Range Railway Company, and the Minnesota Power Company for access to their properties and for providing elevation data.

MINERAL POTENTIAL ASSESSMENT OF NORTHERN ST.LOUIS COUNTY,SOUTHEASTERN KOOCHICHING COUNTY, AND NORTHEASTERN ITASCA COUNTY,

MINNESOTA

Va! W. Chandler, M.A. Jirsa, and G.B. Morey, Minnesota Geological SurveyPresented by Tom Lawler, Department of Natural Resources, Division of Minerals

The Minnesota Geological Survey in a contract with the Department of Natural Resources,Division of Minerals produced a geologic map and mineral-potential assessment of a contiguoustwenty-six township area in northern St. Louis, southeastern Koochiching, and northeasternItasca Counties, Minnesota. Six tholeiitic to caic-alkaline volcanic sequences of the ArcheanWawa subprovince are resolved that are usually separated by faults or metasedimentary belts,and are intruded by a variety of syn- to late tectonic granitoid plutons. Eight criteria are identifiedwhich indicate potential for twenty-two lode gold deposits; Six criteria identify potential for twoiron-formation hosted replacement gold deposits; Seven criteria identify potential for fourvolcanic associated massive sulfide deposits; Seven criteria identify potential for mafic-ultramafic intrusion hosted Cu-Ni-PGE deposits; Six criteria identify potential for komatiiteassociated Ni-Cu-PGE deposits (although the criteria were developed only one area with PGEpotential was identified); and Two criteria identify potential for two kimberlite hosted diamonddeposits. All of these areas are to be regarded with appropriate caution and further evaluationwould require detailed exploration including drilling.

The compilation of the bedrock geologic map (Plate 6), the magnetic and gravity model cross-sections (Plate 7) and the mineral potential assessment map (Plate 9) used available geologicdata combined with gravity and airborne magnetic data. The interpretation used gridded forms ofgeophysical data that have been enhanced to emphasize near-surface geologic phenomena. Usingthe UTM based grid the aeromagnetic data were enhanced by reduction to vertical polarizationand calculation of the second vertical derivative. These procedures shift anomalies more directlyover their sources and eliminate interference from regional scale anomalies to help clarify theshort wavelength signatures of shallow sources that lie at or near the Precambrian surface. With asimilar procedure the gravity data were enhanced by the calculation of the second verticalderivative after smoothing by continuation to a level of two kilometers above surface to eliminate"noise" caused by variations in overburden thickness. Much of the quantitative analysis of thisstudy are based on the Werner deconvolution method of inverse modeling using the approachand proprietary software developed by R.J. Ferderer (1988).

The contract resulted in a twenty-seven page open-file report 97-5: Chandler, V.W., Jirsa, M.A.and Morey, G.B., (1997) Mineral potential assessment of northern St. Louis County,southeastern Koochiching County, and northeastern Itasca County, Minnesota. The reportincludes a detailed account of analytical procedures and results, rock property data, six crosssectional studies using gravity and magnetic modeling, five tables and nine plates displayingresults. This report and the plates are available in hard copy and digital format at the MinnesotaGeological Survey, 2642 University Avenue, St. Paul, Minnesota, 55114-1057, Phone (612) 627-4780 also the Minnesota Department of Natural Resources, 1525 Third Avenue East, Hibbing,Minnesota 55746-1461, Phone (218) 262-6767.

58

MINERAL POTENTIAL ASSESSMENT OF NORTHERN ST.LOU1S COUNTY, SOUTHEASTERN KOOCHICHING COUNTY, AND NORTHEASTERN ITASCA COUNTY,

MINNESOTA

Val W. Chandler, M.A. Jirsa, and G.B. Morey, Minnesota Geological Survey Presented by Tom Lawler, Department of Natural Resources, Division of Minerals

The Minnesota Geological Survey in a contract with the Department of Natural Resources, Division of Minerals produced a geologic map and mineral-potential assessment of a contiguous twenty-six township area in northern St. Louis, southeastern Koochiching, and northeastern Itasca Counties, Minnesota. Six tholeiitic to calc-alkaline volcanic sequences of the Archean Wawa subprovince are resolved that are usually separated by faults or metasedimentary belts, and are intruded by a variety of syn- to late tectonic granitoid plutons. Eight criteria are identified which indicate potential for twenty-two lode gold deposits; Six criteria identifj potential for two iron-formation hosted replacement gold deposits; Seven criteria identifj potential for four volcanic associated massive sulfide deposits; Seven criteria identi@ potential for mafic- ultramafic intrusion hosted Cu-Ni-PGE deposits; Six criteria identifj potential for komatiite associated Ni-Cu-PGE deposits (although the criteria were developed only one area with PGE potential was identified); and Two criteria identifj potential for two kimberlite hosted diamond deposits. All of these areas are to be regarded with appropriate caution and W h e r evaluation would require detailed exploration including drilling.

The compilation of the bedrock geologic map (Plate 61, the magnetic and gravity model cross- sections (Plate 7) and the mineral potential assessment map (Plate 9) used available geologic data combined with gravity and airborne magnetic data. The interpretation used gridded forms of geophysical data that have been enhanced to emphasize near-surface geologic phenomena. Using the UTM based grid the aeromagnetic data were enhanced by reduction to vertical polarization and calculation of the second vertical derivative. These procedures shift anomalies more directly over their sources and eliminate interference fiom regional scale anomalies to help clarifj the short wavelength signatures of shallow sources that lie at or near the Precambrian surface. With a similar procedure the gravity data were enhanced by the calculation of the second vertical derivative after smoothing by continuation to a level of two kilometers above surface to eliminate "noise" caused by variations in overburden thickness. Much of the quantitative analysis of this study are based on the Werner deconvolution method of inverse modeling using the approach and proprietary software developed by R.J. Ferderer (1 988).

The contract resulted in a twenty-seven page open-file report 97-5: Chandler, V.W., Jirsa, M.A. and Morey, G.B., (1997) Mineral potential assessment of northern St. Louis County, southeastern Koochiching County, and northeastern Itasca County, Minnesota. The report includes a detailed account of analytical procedures and results, rock property data, six cross sectional studies using gravity and magnetic modeling, five tables and nine plates displaying results. This report and the plates are available in hard copy and digital format at the Minnesota Geological Survey, 2642 University Avenue, St. Paul, Minnesota, 551 14-1057, Phone (612) 627- 4780 also the Minnesota Department of Natural Resources, 1525 Third Avenue East, Hibbing, Minnesota 55746-1461, Phone (21 8) 262-6767.

EXTENSION OF THE HURONIAN MAGMATIC SUITE INSIDE THEGRENVILLE PROVINCE: NEW ZIRCON U-Pb EVIDENCE FROM THEGRENVILLE FRONT TECTONIC ZONE IN STREET TOWNSHIP, SUDBURYREGION, ONTARIO.

F. Corfu, Royal Ontario Museum, 100 Queen's Park, Toronto, ON M5S 2C6 andR.M. Easton, Precambrian Geoscience Section, Ontario Geological Survey, 933Ramsey Lake Road, Sudbury, ON P3E 6B5

The Sudbury region is characterized by very complex geological relationships due to the

confluence and overlapping of multiple geological domains ranging in age from Archean to

Neoproterozic. Early Paleoproterozoic rifling of the Archean crust led to the deposition of

extensive clastic sedimentary assemblages of the Huron ian Supergroup and was initially

accompanied by the emplacement of bimodal mafic and felsic, intrusive and extrusive rocks.

Although the distribution and petrogenetic features of these magmatic rocks is well understood in

the region west of Sudbury, the fate of the Huronian remains a matter of speculation farther to the

east and south of the Grenville Front, the only exception being the previously dated River Valley

gabbro-anorthosite. In conjunction with an Ontario Geological Survey mapping program along the

Southern-Grenville Province boundary, we have examined two examples of metamorphosed

granitic bodies and a metapyroxenite plug straddling the Grenville Front tectonic zone in Street

Township, roughly 15 km east of Sudbury.

The metapyroxenite is part of a suite of small bodies, generally less than 500m in size, that occur

within the Grenville Front tectonic zone between Coniston and River Valley. Mineralogically,

these bodies consist of roughly equal amounts of orthopyroxene phenocrysts (0.5-5 cm in size) in

an amphibole matrix; locally olivine phenocrysts are preserved. Metamorphic crystallization in

these bodies increases with increasing distance from the Grenville Front, consequently, sampling

was conducted on a body located only 250m southeast of the Grenville Front. The metapyroxenite

contains highly resorbed, prismatic zircon yielding a U-Pb data array that points toward an upper

intercept age of about 2490 Ma. The age corresponds approximately to that of the East-Bull Lake

and Shakespeare-Dunlop (a.k.a. Agnew Lake) intrusions, emplaced in Archean crust at the margin

of the Huronian basin east of Sudbury. On the basis of SEM examination, most of the zircon found

in this sample is located within the orthopyroxene phenocrysts, in conjunction with Cr-spinel and

chromite. In addition, the matrix of the metaproxenite is chemically differentiated. Thus, we are

confident that the zircon fraction yielding the upper intercept age of 2490 Ma dates emplacement

of these rocks. The metapyroxenite was metamorphosed and developed metamorphic zircon during

an event at about 1700-1600 Ma, and was subsequently overprinted by Grenvillian-age

metamorphism.

Two granitic bodies were sampled. The first was a foliated monzogranite located in an area south

of the Ess Creek fault and north of the Grenville Front boundary fault as mapped by Lumbers

59

EXTENSION OF THE HURONIAN MAGMATIC SUITE INSIDE THE GRENVILLE PROVINCE: NEW ZIRCON U-Pb EVIDENCE FROM THE GRENVILLE FRONT TECTONIC ZONE IN STREET TOWNSHIP, SUDBURY REGION, ONTARIO.

F. C o f i , Royal Ontario Museum, 100 Queen's Park, Toronto, ON M5S 2C6 and R.M. Easton, Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, Sudbury, ON P3E 6B5

The Sudbury region is characterized by very complex geological relationships due to the confluence and overlapping of multiple geological domains ranging in age from Archean to Neoproterozic. Early Paleoproterozoic rifting of the Archean crust led to the deposition of extensive clastic sedimentary assemblages of the Huronian Supergroup and was initially accompanied by the emplacement of bimodal mafic and felsic, intrusive and extrusive rocks. Although the distribution and petrogenetic features of these magmatic rocks is well understood in the region west of Sudburyy the fate of the Huronian remains a matter of speculation farther to the east and south of the Grenville Fronty the only exception being the previously dated River Valley gabbro-anorthosite. In conjunction with an Ontario Geological S u ~ e y mapping program along the Southern-Grenville Province boundary, we have examined two examples of metamorphosed granitic bodies and a metapyroxenite plug straddling the Grenville Front tectonic zone in Street Township, roughly 15 km east of Sudbury.

The metapyroxenite is part of a suite of small bodiesy generally less than 500m in size, that occur within the Grenville Front tectonic zone between Coniston and River Valley. Mineralogically, these bodies consist of roughly equal amounts of orthopyroxene phenocrysts (0.5-5 cm in size) in an amphibole matrix; locally olivine phenocrysts are preserved. Metamorphic crystallization in these bodies increases with increasing distance from the Grenville Front, consequently, sampling was conducted on a body located only 250m southeast of the Grenville Front. The metapyroxenite contains highly resorbed, prismatic zircon yielding a U-Pb data array that points toward an upper intercept age of about 2490 Ma. The age corresponds approximately to that of the East-Bull Lake

and Shakespeare-Dunlop (a.k.a. Agnew Lake) intrusionsy emplaced in Archean crust at the margin

of the Huronian basin east of Sudbury. On the basis of SEM examinationy most of the zircon found

in this sample is located within the orthopyroxene phenocrysts, in conjunction with Cr-spinel and

chromite. In addition, the matrix of the metaproxenite is chemically differentiated. Thus, we are confident that the zircon fraction yielding the upper intercept age of 2490 Ma dates emplacement

of these rocks. The metapyroxenite was metamorphosed and developed metamorphic zircon during

an event at about 1700- 1600 Ma, and was subsequently overprinted by Grenvillian-age

metamorphism.

Two granitic bodies were sampled. The first was a foliated monzogranite located in an area south of the Ess Creek fault and north of the Grenville Front boundary fault as mapped by Lumbers

(1973). This fault-bounded region has been variously assigned to the Southern or Grenville

provinces, and has been subjected to middle to upper amphibolite facies metamorphism. The

second body is located about 15 km south of the first, and is one of several similar bodies located

within the Grenville Province. Both sampled granites, as well as other bodies south of the Grenville

Front, are chemically similar, suggesting they form part of a larger plutonic body or a suite of

intrusions. In particular, they are characterized by Si02 66-72%, Al203 11.9-12.6%, FeOt0 >35%CaO >1.5%, EufEu* .55-.70, La/Yb 5-8, and Gd/Yb 1.2-1.65, and are identical in major and REE

geochemistry with Stobie Formation dacites from both the Sudbury and Street Township areas. On

various geochemical discrimination diagrams, they plot in the Within-Plate granite field. In

contrast, felsic rocks of the Murray and Creighton granites and the Copper Cliff rhyolite have

lower FeOt0 contents and higher Si02 and alkali contents. In addition, REE data, only available

from the Copper Cliff rhyolites, have EufEu* <.5, La/Yb <5.5 and Gd/Yb <.75. The foliated

monzogranite defines a U-Pb zircon age of about 2460-2450 Ma, which overlaps the age of some

of the youngest magmatic expressions of the Huronian rifting event such as the Copper Cliff

rhyolite and the Hearst dyke swarm. The granite was overprinted by Grenvillian metamorphism

that strongly, but not totally reset the titanite ages. The zircon data for the second granitic body

display more pronounced effects of both a 1700-1600 Ma event and the Grenvillian orogeny at

about 990-980 Ma, but are nevertheless consistent with a Huronian age and with the chemical

similarities between these granitic intrusions.

The ages on the bodies reported here are consistent with field relations which show a close spatial

relationship between the metapyroxenites, the granites, and mesocratic to anorthositic gabbros

likely correlative with the ca. 2475 Ma River Valley gabbro-anorthosite. In addition, the foliated

monzogranite is spatially associated with a thin sliver of mafic and felsic metavolcanic rocks that

have been correlated with the Huronian Stobie Formation. The latter are the only Huronian

volcanic rocks so far identified east of Sudbury. These field relationships, in conjunction with the

geochronological results reported herein, suggest that the Huronian magmatic province is more

extensive east of Sudbury than previously recognized. In addition, felsic magmatism of Huronian

age is no longer confined to the vicinity of Copper Cliff, and indeed, could be more extensive than

previously thought, particularly in the area of the little studied eastern Cobalt plate.

Reference:Lumbers, S.B. 1973. River Valley area; Ontario Division of Mines, Preliminary Map, P.844.

60

(1973). This fault-bounded region has been variously assigned to the Southern or Grenville provinces, and has been subjected to middle to upper amphibolite facies metamorphism. The second body is located about 15 km south of the first, and is one of several similar bodies located within the Grenville Province. Both sampled granites, as well as other bodies south of the Grenville Front, are chemically similar, suggesting they form part of a larger plutonic body or a suite of intrusions. In particular, they are characterized by Si02 66-72%, A1203 1 1.9-12.6%, FeotO&' >3.5%, CaO >IS%, Eu/Eu* -55--70, L a b 5-8, and GdNb 1.2-1 -65, and are identical in major and REE geochemistry with Stobie Formation dacites from both the Sudbury and Street Township areas. On various geochemical discrimination diagrams, they plot in the Within-Plate granite field. In contrast, felsic rocks of the Murray and Creighton granites and the Copper Cliff rhyolite have lower Feotoh' contents and higher Si02 and alkali contents. In addition, REE data, only available from the Copper Cliff rhyolites, have Eu/Eu* c.5, L a b -3.5 and Gdffb C.75. The foliated monzogranite defines a U-Pb zircon age of about 2460-2450 Ma, which overlaps the age of some of the youngest magmatic expressions of the Huronian rifting event such as the Copper Cliff

rhyolite and the Hearst dyke swarm. The granite was overprinted by Grenvillian metamorphism that strongly, but not totally reset the titanite ages. The zircon data for the second granitic body display more pronounced effects of both a 1700- 1600 Ma event and the Grenvillian orogeny at about 990-980 Ma, but are nevertheless consistent with a Huronian age and with the chemical similarities between these granitic intrusions.

The ages on the bodies reported here are consistent with field relations which show a close spatial relationship between the metapyroxenites, the granites, and mesocratic to anorthositic gabbros likely correlative with the ca. 2475 Ma River Valley gabbro-anorthosite. In addition, the foliated monzogranite is spatially associated with a thin sliver of mafic and felsic metavolcanic rocks that have been correlated with the Huronian Stobie Formation. The latter are the only Huronian volcanic rocks so far identified east of Sudbury. These field relationships, in conjunction with the geochronological results reported herein, suggest that the Huronian magmatic province is more extensive east of Sudbury than previously recognized. In addition, felsic magmatism of Huronian age is no longer confined to the vicinity of Copper Cliff, and indeed, could be more extensive than

previously thought, particularly in the area of the little studied eastern Cobalt plate.

Reference: Lumbers, S.B. 1973. River Valley area; Ontario Division of Mines, Preliminary Map, P.844.

KINEMATIC FABRICS NEAR MINE CENTRE, ONTARIO: EVIDENCE FOR AMODIFIED TRANSPRESSION MODEL.

Czeck, D. M. and Hudleston, P. J., Department of Geology andGeophysics, University of Minnesota, 310 Pillsbury Dr. SE,Minneapolis, MN 55455

The boundary between the Quetico and Wabigoon subprovinces of the Archean SuperiorProvince is characterized by ductily deformed Greenschist- Amphibolite facies rocks. Thetwo subprovinces have been amalgamated through continental accretion processes. Theworking kinematic model to describe this area is transpression, which explains thedominant flattening fabric in the vertical plane and the evidence for noncoaxial strain notedin the horizontal plane. Theoretical work on classical transpression has shown that the longaxis of the strain ellipsoid,' and thus the mineral lineation, are necessarily either vertical orhorizontal. Structural field work in this study has shown that the lineations along theWabigoon-Quetico boundary plunge between 0-90° within the foliation plane. Therefore, amodification to the three-dimensional transpression model is necessary to adequatelydescribe the deformation process. One possible modification to transpression whichexplains the oblique lineations is heterogeneous extrusion due to anastomosing shearzones.

61

KINEMATIC FABRICS NEAR MINE CENTRE, ONTARIO: EVIDENCE FOR A MODIFIED TRANSPRESSION MODEL.

Czeck, D. M. and Hudleston, P. J., Department of Geology and Geophysics, University of Minnesota, 3 10 Pillsbury Dr. SE, Minneapolis, MN 55455

The boundary between the Quetico and Wabigoon subprovinces of the Archean Superior Province is characterized by ductily deformed Greenschist- Arnphibolite facies rocks. The two subprovinces have been amalgamated through continental accretion processes. The working kinematic model to describe this area is transpression, which explains the dominant flattening fabric in the vertical plane and the evidence for noncoaxial strain noted in the horizontal plane. Theoretical work on classical transpression has shown that the long axis of the strain ellipsoid,and thus the mineral lineation, are necessarily either vertical or horizontal. Structural field work in this study has shown that the lineations along the Wabigoon-Quetico boundary plunge between 0-90' within the foliation plane. Therefore, a modification to the three-dimensional transpression model is necessary to adequately describe the deformation process. One possible modification to transpression which explains the oblique lineations is heterogeneous extrusion due to anastomosing shear zones.

New Aeromagnetic Surveys in Wisconsin by the U.S. Geological Survey

David L. Daniels, Stephen L. Snyder, Suzanne W. Nicholson, William F. Cannon, U.S. GeologicalSurvey, MS 954 National Center, Reston, VA 20192

Aeromagnetic surveying in Wisconsin over the past 10 years by the U.S. GeologicalSurvey (USGS) has added considerably to the coverage in the state and to the digitalaeromagnetic database of the USGS Mineral Resources Program. Initial efforts were directedtoward completing the coverage in the northern part of the state in which Precambrian bedrock isat the surface or covered by glacial deposits. The flight-lines were flown in a N-S directionbecause of the predominant easterly or northeasterly grain of the geology, were spaced '/2-mileapart, and flown at 500 or 1000 ft above terrain. The surveys were designed to extend the largesurvey conducted by John Karl of the University of Wisconsin, Oshkosh (Karl, 1986; King, 1990),prior to 1977.

In 1988, the USGS flew a 3900 line-mile aeromagnetic survey in the northwestern cornerof Wisconsin south of Lake Superior. In 1996, USGS surveying was continued in two areasadjacent to the 1988 survey (see index map) for an additional 6700 line-miles. These two surveyscompleted the coverage of the volcanic and sedimentary rocks in the Midcontinent rift system.These new data have led to new interpretations of structures in the highly magnetic Keweenawanbasalt flows in the St. Croix horst (Cannon and others, 1997). In the most recent (10/97-3/98)survey, aeromagnetic data have been acquired in three blocks in a broad swath through thecentral part of the state, in the Marinette-Green Bay area, the Wisconsin Rapids area, and theMississippi River area (about 16,000 line-miles). In addition, two small areas along theWisconsin-Michigan border were filled in.

In much of the area covered by the most recent survey, a thin (<1 50m) veneer ofmagnetically transparent Cambrian sandstone overlies the more highly magnetic Early and MiddleProterozoic rocks; in the western part non-magnetic Middle Proterozoic sedimentary rocks formpart of the basement. Thus, although this is an area of very little outcrop, the aeromagnetic dataeffectively delineate the structure and character of the buried basement rocks. The enhancedunderstanding of the basement geology provided by the aeromagnetic survey will allow betterevaluation of the mineral potential of this region and aid in regional correlations of basementterranes in the region. Maps of the new aeromagnetic survey and the current aeromagneticcompilation of Wisconsin will be shown.

References Cited

Cannon, W.F., Daniels, D.L., Snyder, S.L., 1997, New aeromagnetic map of the MidcontinentRift in Northwestern Wisconsin and adjacent Minnesota; Geological Society of America 1997abstracts with programs, v. 29, no. 4, p. 9.

Karl, J.H., 1986, Total magnetic intensity map of northern Wisconsin; Wisconsin Geological andNatural History Survey, Map 86-7, scale 1:250,000.

King, E.R., 1990, Precambrian terrane of north-central Wisconsin: an aeromagnetic perspective;Canadian Journal of Earth Science, v. 27, pp. 1472-1477.

62

New Aeromagnetic Surveys in Wisconsin by the U.S. Geological Survey

David L. Daniels, Stephen L. Snyder, Suzanne W. Nicholson, William F. Cannon, U.S. Geological Survey, MS 954 National Center, Reston, VA 20192

Aeromagnetic surveying in Wisconsin over the past 10 years by the U. S . Geological Survey (USGS) has added considerably to the coverage in the state and to the digital aeromagnetic database of the USGS Mineral Resources Program. Initial efforts were directed toward completing the coverage in the northern part of the state in which Precambrian bedrock is at the surface or covered by glacial deposits. The flight-lines were flown in a N-S direction because of the predominant easterly or northeasterly grain of the geology, were spaced %-mile apart, and flown at 500 or 1000 ft above terrain. The surveys were designed to extend the large survey conducted by John Karl of the University of Wisconsin, Oshkosh (Karl, 1986; King, 1990), prior to 1977.

In 1988, the USGS flew a 3900 line-mile aeromagnetic survey in the northwestern comer of Wisconsin south of Lake Superior. In 1996, USGS surveying was continued in two areas adjacent to the 1988 survey (see index map) for an additional 6700 line-miles. These two surveys completed the coverage of the volcanic and sedimentary rocks in the Midcontinent rift system. These new data have led to new interpretations of structures in the highly magnetic Keweenawan basalt flows in the St. Croix horst (Cannon and others, 1997). In the most recent (10197-3198) survey, aeromagnetic data have been acquired in three blocks in a broad swath through the central part of the state, in the Marinette-Green Bay area, the Wisconsin Rapids area, and the Mississippi River area (about 16,000 line-miles). In addition, two small areas along the Wisconsin-Michigan border were filled in.

In much of the area covered by the most recent survey, a thin (<150m) veneer of magnetically transparent Cambrian sandstone overlies the more highly magnetic Early and Middle Proterozoic rocks; in the western part non-magnetic Middle Proterozoic sedimentary rocks form part of the basement. Thus, although this is an area of very little outcrop, the aeromagnetic data effectively delineate the structure and character of the buried basement rocks. The enhanced understanding of the basement geology provided by the aeromagnetic survey will allow better evaluation of the mineral potential of this region and aid in regional correlations of basement terranes in the region. Maps of the new aeromagnetic survey and the current aeromagnetic compilation of Wisconsin will be shown.

References Cited

Cannon, W.F., Daniels, D.L., Snyder, S.L., 1997, New aeromagnetic map of the Midcontinent Rift in Northwestern Wisconsin and adjacent Minnesota; Geological Society of America 1997 abstracts with programs, v. 29, no. 4, p. 9.

Karl, J.H., 1986, Total magnetic intensity map of northern Wisconsin; Wisconsin Geological and Natural History Survey, Map 86-7, scale 1:250,000.

King, E.R., 1990, Precambrian terrane of north-central Wisconsin: an aeromagnetic perspective; Canadian Journal of Earth Science, v. 27, pp. 1472-1477.

Index of Recent Aeromagnetic Surveys in Wisconsin

9T 91 9O 8993 88 8T

46

45

44

43

92 91° 90° 89° 88°

50 1000 150 200 KM

63

Index of Recent Aeromagnetic Surveys in Wisconsin

93- 92' 91' 90' 89' 88- 87'

POLYMETAMORPHISM OF SKARNS RELATED TO THE MORIN ANORTHOSITECOMPLEX, GRENVILLE PROVINCE, QUEBEC.

DEANGELIS1, M. T., PECK, W. H., and VALLEY, J. W., Department of Geology and Geophysics,University of Wisconsin, Madison, Madison, WI, 53706, william@geology.wisc.edu. 1studentauthor

Skarn caic-silicate assemblages associated with the Mona Anorthosite Complex are exposed —2 km west ofthe town of St. Jovite, Quebec, and display field relations and reaction textures which indicate apolymetamorphic origin. A set of —25 small dioritic and monzonitic intrusive bodies, which are locatedbetween two mapped bodies of mangerite (Martignole and Coniveau, 1993) outcrop in a —300 meter longseries of four roadcuts in calcite marble along Highway 117. These bodies are rimmed by 10-20 cm thickskarns containing metasomatic banding of garnet, diopside, and wollastonite. This locality occurs adjacentto the western margin of the Morin Anorthosite Massif which was emplaced at 1155 ± 3 Ma (Doig, 1991).The anorthosite is associated with mangerite bodies that were emplaced 1135 ±3 Ma (Doig, 1991). Thearea was metamorphosed under granulite facies conditions following emplacement of the anorthosite massifto pressures of 6-8 kb and temperatures of 650-775 °C (Indares and Martignole, 1989). Thismetamorphism has been dated at 1070-1100 Ma by Rb-Sr whole rock isochrons (see Doig, 1991).

Descriptions of skarnsWell developed skarns appear on both diorite and monzonite bodies. We interpret these bodies as

intrusive because of cross-cutting relationships between different intrusions and relict igneous textures. Onexposed surfaces, diorite bodies vary in both size and shape, but are typically circular to oval with diametersranging from 1 to 10 meters. The diorite intrusives are composed primarily of plagioclase with minoramounts of clinopyroxene, amphibole, biotite, magnetite, pyrite, and sphene. Metamorphic mineralsinclude calcite, garnet and clinozoisite. Surrounding the diorite intrusives are skarns which are made up ofconcentric garnet-, cinopyroxene-, and wollastonite-rich bands. Next to the diorite, garnet-rich bands are—10 cm thick. Wollastonite-rich bands (in the middle of the skarn) are thinner (�1 cm) and sometimes arenot present. Clinopyroxene-rich bands are —15 cm thick and gradually grade into the marble. The garnet-rich bands contain garnet, clinopyroxene, plagioclase, quartz, calcite, sphene, and small amounts of pyrite.The wollastonite-nich bands are composed of wollastonite, clinopyroxene, calcite, plagioclase, garnet,quartz, sphene, and small amounts of graphite and pyrite. The clinopyroxene-rich bands are made up ofclinopyroxene, calcite, garnet, plagioclase, quartz, sphene, with minor amounts of graphite and pyrite.

The monzomte intrusives are composed ofpotassium feldspar and plagioclase with minor amounts ofquartz, clinopyroxene, and sphene. Metamorphic mineralsinclude calcite, garnet, and Fe-vesuvianite(Fe/Fe+Mg=0.65 determined by electron microprobe). Fe-vesuvianite is visibly zoned in hand sample, with Fe-richrims (FeIFe+Mg0.67) and less Fe-rich cores(Fe/Fe+Mg=0.61). Fe-vesuviamte crystals can reachlengths of —6 cm and are found within the monzonite andthe inner most skarn band. Within one monzonite body,an unusual texture consisting of a cylinder of matrixminerals (potassium feldspar, plagioclase, garnet, andquartz) encased by a single crystal of elongate (—2-6 cmlong, width�50 mm) clinopyroxene (En26Fs23Wo50Ac1)occurs. This gives these crystals a "hollow" appearance inthin section and on the outcrop. Monzonite occurs bothas semi-circular bodies (1-10 m in diameter) as well asdikes which cross cut diorite. Skarns around themonzonite consist of small (3-5 cm) garnet- andclinopyroxene-rich bands. Garnet-rich bands containgarnet, clinopyroxene, calcite, potassium feldspar,plagioclase, Fe-vesuvianite, and sphene. Clinopyroxene-rich bands contain clinopyroxene, calcite, garnet,potassium feldspar, plagioclase, and sphene. Away from the igneous bodies, marble is composed of primarilyequigranular calcite with small amounts of clinopyroxene, garnet, sphene, and feldspar. Trace amounts ofgraphite and pyrite are also observed. Bogoch et al (1997) report similar mineralogies of calcite, potassiumfeldspar, vesuvianite, quartz, cinopyroxene, plagioclase, wollastonite, graphite, and spbene in a body 2.5 mlong and 50 cm wide in calcite marble approximately 12 km west of the field area.

64

Figure 1. Fragmented diorite intrusion.Skam is developed where hot dioritecame into contact with marble. Brittledeformation of diorite and skarn post-dates intrusion.

POLYMETAMORPHISM OF SKARNS RELATED TO THE MORIN ANORTHOSITE COMPLEX, GRENVILLE PROVINCE, QUEBEC.

DEANGELIS l , M. T., PECK, W. H., and VALLEY, J. W., Department of Geology and Geophysics, University of Wisconsin, Madison, Madison, WI, 53706, william@geology.wisc.edu. ^student author

Skarn calc-silicate assemblages associated with the Morin Anorthosite Complex are exposed -2 km west of the town of St. Jovite, Quebec, and display field relations and reaction textures which indicate a polymetamorphic origin. A set of -25 small dioritic and monzonitic intrusive bodies, which are located between two mapped bodies of mangerite (Martignole and Corriveau, 1993) outcrop in a -300 meter long series of four roadcuts in calcite marble along Highway 117. These bodies are rimmed by 10-20 cm thick skarns containing metasomatic banding of garnet, diopside, and wollastonite. This locality occurs adjacent to the western margin of the Morin Anorthosite Massif which was emplaced at 1155 Â 3 Ma (Doig, 1991). The anorthosite is associated with mangerite bodies that were emplaced 1135 Â 3 Ma (Doig, 1991). The area was metamorphosed under granulite facies conditions following emplacement of the anorthosite massif to pressures of 6-8 kb and temperatures of 650-775 OC (Indares and Martignole, 1989). This metamorphism has been dated at 1070-1 100 Ma by Rb-Sr whole rock isochrons (see Doig, 1991).

Descriptions of skarns Well developed skarns appear on both diorite and monzonite bodies. We interpret these bodies as

intrusive because of cross-cutting relationships between different intrusions and relict igneous textures. On exposed surfaces, diorite bodies vary in both size and shape, but are typically circular to oval with diameters ranging from 1 to 10 meters. The diorite intrusives are composed primarily of plagioclase with minor amounts of clinopyroxene, amphibole, biotite, magnetite, pyrite, and sphene. Metamorphic minerals include calcite, garnet and clinozoisite. Surrounding the diorite intrusives are skarns which are made up of concentric garnet-, clinopyroxene-, and wollastonite-rich bands. Next to the diorite, garnet-rich bands are -10 cm thick. Wollastonite-rich bands (in the middle of the skarn) are thinner (51 cm) and sometimes are not present. Clinopyroxene-rich bands are -15 cm thick and gradually grade into the marble. The garnet- rich bands contain garnet, clinopyroxene, plagioclase, quartz, calcite, sphene, and small amounts of pyrite. The wollastonite-rich bands are composed of wollastonite, clinopyroxene, calcite, plagioclase, garnet, quartz, sphene, and small amounts of graphite and pyrite. The clinopyroxene-rich bands are made up of clinopyroxene, calcite, garnet, plagioclase, quartz, sphene, with minor amounts of graphite and pyrite.

The monzonite intrusives are composed of potassium feldspar and plagioclase with minor amounts of quartz, clinopyroxene, and sphene. Metamorphic minerals include calcite, garnet, and Fe-vesuvianite (Fe/Fe+Mg=0.65 determined by electron microprobe). Fe- vesuvianite is visibly zoned in hand sample, with Fe-rich rims (Fe/Fe+Mg=0.67) and less Fe-rich cores (Fe/Fe+Mg=0.61). Fe-vesuvianite crystals can reach lengths of -6 cm and are found within the monzonite and the inner most skarn band. Within one monzonite body, an unusual texture consisting of a cylinder of matrix minerals (potassium feldspar, plagioclase, garnet, and quartz) encased by a single crystal of elongate (-2-6 cm long, width90 mm) clinopyroxene (En2(;Fs23W050Acl) occurs. This gives these crystals a "hollow" appearance in thin section and on the outcrop. Monzonite occurs both Figure 1. Fragmented diorite intrusion. as semi-circular bodies (1-10 m in diameter) as well as Skam is developed where hot diorite dikes which cross cut diorite. Skarns around the came into contact with marble. Brittle monzonite consist of small (3-5 cm) garnet- and deformation of diorite and skarn post- clinopyroxene-rich bands. Garnet-rich bands contain dates intrusion.

garnet, clinopyroxene, calcite, potassium feldspar, plagioclase, Fe-vesuvianite, and sphene. Clinopyroxene-rich bands contain clinopyroxene, calcite, garnet, potassium feldspar, plagioclase, and sphene. Away from the igneous bodies, marble is composed of primarily equigranular calcite with small amounts of clinopyroxene, garnet, sphene, and feldspar. Trace amounts of graphite and pyrite are also observed. Bogoch et a1 (1997) report similar mineralogies of calcite, potassium feldspar, vesuvianite, quartz, clinopyroxene, plagioclase, wollastonite, graphite, and sphene in a body 2.5 m long and 50 cm wide in calcite marble approximately 12 krn west of the field area.

Deformation and mineral reactionsThe majority of the exposed skarns parallel the

edge of the igneous bodies and maintain uniformthickness. However, some skarns and igneous rock arelocally deformed. Some igneous bodies and their skarnsare folded and boudined, 2-25 cm size skarn fragments arecommon away from igneous rock; some of which arevariably folded. Skarn fragments and skarn minerals areobserved being transported by ductile flow of the marble(e.g. into boudin necks and out of fold hinges). Someigneous bodies and their skarns show brittle deformation,breaking both skarn and intrusion into fragments (see Fig.1). The lack of skarn development on some intrusion-marble contacts of dismembered igneous bodies indicatesthat brittle deformation (of igneous rock and skarn)occurred after igneous crystallization and skam formation.

Mineral textures show that original skarnmineralogies have been modified by a later granulite-faciesmetamorphic event (see also Martignole and Schriver1970). Garnet (Gr92) rims surrounding calcite,wollastonite, and plagioclase indicate the reaction calcite + quartz + anorthite + wollastonite -> grossular +CO2 (see Fig. 2). Garnet-quartz intergrowths indicate the reaction anorthite + wollastomte ->grossular +quartz. The univariant assemblage of garnet (Gr95), plagioclase (An19), quartz, wollastomte is observed asfine-grained intergrowths. Dilution of the anorthite component of plagioclase by albite moves theunivariant assemblage to lower pressures and temperatures (Windom and Boettcher 1976), consistent withthe estimates of regional metamorphism (Indares and Martignole 1990). The presence of wollastomte+vesuvianite, garnet+ plagioclase+ quartz+ wollastomte, clinozoisite+ plagioclase+ quartz, and spheneindicate conditions of high XH2O if metamorphism was fluid saturated. These assemblages also consistentwith estimates of low H20 activity from granulite facies terrains if conditions are fluid absent or if non C-0-H fluid species are important (Valley et al., 1990).

Comparison to the AdirondacksEconomic wollastonite deposits in the Adirondack Highlands (—250 km to the south) are skarns

associated with the Marcy Anorthosite Massif and related granitic rocks. Skarn formation adjacent to theMarcy anorthosite was due to the infiltration of oxidizing meteoric water into the high temperature contactzone of a shallow (<10km) anorthosite body. The Adirondacks underwent regional granulite faciesmetamorphism with high pressure, fluid-absent conditions —100 Ma later (McLelland and Chiarenzelli,1990), but wollastonite and other skarn minerals remained stable through this later metamorphism due tolow fco2 "fluid-absent" conditions (Valley et al. 1990). Post-intrusion deformation and mineral textures inMonn Complex skarns suggest polymetamorphism, but published geochronology (i.e. dating of hightemperature metamorphic minerals) at present does not allow a distinction between two separatemetamorphic events, or a continuum of metamorphism under changing conditions following anorthositeintrusion or depth of intrusion.

ReferencesBogoch, R., Kumarapeli, S., and Matthews, A. (1997) Can Mm 35:1269-1275.Doig R. (1991) J Geology 99: 729-738.Martignole J., Corriveau L. (1993) Geol Surv Can Open File 2640.Martignole J., Schrijver K. (1970) Geol Soc Finland Bull 42: 165-209.McLelland, J., Chiarenzelli, J. (1990) J Geology 98:19-41.Indares A., Martignole J. (1989) Can J Earth Sci 27: 37 1-386.Valley, J., Bohlen, S. R., Essene, E. J., and Lamb, W. (1990) J Petrology 3 1:555-596.Windom, K. E., Boettcher, A. L. (1976) Am Mm 61:889-896.

65

Figure 2. Calcite rimmed with garnet(Gç) This texture and garnet rimmingwolliastonite is evidence for thereaction calcite+ quartz+ anorthite+wollastonite-> grossular+ CO2.

Deformation and mineral reactions The majority of the exposed skarns parallel the

edge of the igneous bodies and maintain uniform thickness. However, some skarns and igneous rock are locally deformed. Some igneous bodies and their skarns are folded and boudined, 2-25 cm size skarn fragments are common away from igneous rock; some of which are variably folded. Skarn fragments and skarn minerals are observed being transported by ductile flow of the marble (e.g. into boudin necks and out of fold hinges). Some igneous bodies and their skarns show brittle deformation, breaking both skarn and intrusion into fragments (see Fig. 1). The lack of skarn development on some intrusion- marble contacts of dismembered igneous bodies indicates that brittle deformation (of igneous rock and skarn) occurred after igneous crystallization and skarn formation. (G ). This texture and garnet nmnung

Mineral textures show that original skarn wopastonite is evidence for the mineralogies have been modified by a later granulite-facies reaction calcite+ quartz+ anorthite+ metamorphic event (see also Martignole and Schriver wollastonite-> grossular+ COT

1970). Garnet (Gr92) rims surrounding calcite, wollastonite, and plagioclase indicate the reaction calcite + quartz + anorthite + wollastonite -> grossular + C02 (see Fig. 2). Garnet-quartz intergrowths indicate the reaction anorthite + wollastonite ->grossular + quartz. The univariant assemblage of garnet (Gr95), plagioclase (An19), quartz, wollastonite is observed as fine-grained intergrowths. Dilution of the anorthite component of plagioclase by albite moves the univariant assemblage to lower pressures and temperatures (Windom and Boettcher 1976), consistent with the estimates of regional metamorphism (Indares and Martignole 1990). The presence of wollastonite+ vesuvianite, garnet+ plagioclase+ quartz+ wollastonite, clinozoisite+ plagioclase+ quartz, and sphene indicate conditions of high Xu20 if metamorphism was fluid saturated. These assemblages also consistent with estimates of low H2O activity from granulite facies terrains if conditions are fluid absent or if non C- 0-H fluid species are important (Valley et al., 1990).

Comparison to the Adirondacks Economic wollastonite deposits in the Adirondack Highlands (-250 km to the south) are skarns

associated with the Marcy Anorthosite Massif and related granitic rocks . Skarn formation adjacent to the Marcy anorthosite was due to the infiltration of oxidizing meteoric water into the high temperature contact zone of a shallow (<10km) anorthosite body. The Adirondacks underwent regional granulite facies metamorphism with high pressure, fluid-absent conditions -100 Ma later (McLelland and Chiarenzelli, 1990), but wollastonite and other skarn minerals remained stable through this later metamorphism due to low fc02 "fluid-absent" conditions (Valley et al. 1990). Post-intrusion deformation and mineral textures in Morin Complex skarns suggest polymetamorphism, but published geochronology (i.e. dating of high temperature metamorphic minerals) at present does not allow a distinction between two separate metamorphic events, or a continuum of metamorphism under changing conditions following anorthosite intrusion or depth of intrusion.

References Bogoch, R., Kumarapeli, S., and Matthews, A. (1997) Can Min 351269-1275. Doig R. (1991) J Geology 99: 729-738. Martignole J., Corriveau L. (1993) Geol Surv Can Open File 2640. Martignole J., Schrijver K. (1970) Geol Soc Finland Bull 42: 165-209. McLelland, J., Chiarenzelli, J. (1990) J Geology 98:19-41. Indares A., Martignole J. (1989) Can J Earth Sci 27: 371-386. Valley, J., Bohlen, S. R., Essene, E. J., and Lamb, W. (1990) J Petrology 31555-596. Windom, K. E., Boettcher, A. L. (1976) Am Min 61:889-896.

THE AGE AND PROVENANCE OF THE GUNFLINT LAPILLI TUFF

FRALICK, P.W. and KISSIN, S.A., Department of Geology, LakeheadUniversity, Thunder Bay, Ontario, P7B 5E1, and DAVIS, D.W., RoyalOntario Museum, 100 Queen's Park, Toronto, Ontario, M5S 2C6.

The Gunflint Formation forms the middle unit of the Animikie Group in northwesternOntario, outcropping proximally to the western end of Lake Superior. It consists of anassemblage of chemical and fine-grained elastic sediments deposited in the strand-proximalzone of a south facing shelf as recently interpreted by Pufahl (1996).

Previous attempts at assigning an age to the Gunflint Formation (or the conformablyoverlying Rove Formation) and equivalents can be classified into three groups: (1) ages from1.556 to 1.63 Ga based on whole-rock Rb/Sr or K/Ar techniques (Hurley et a!., 1962; Peterman1966; Faure & Kovach, 1969; and Franklin, 1978); (2) ages from 2.08 to 2.111 Ga based onwhole-rock NdJSm techniques (Stille & Plauer, 1985; Gerlach et a!, 1988); and (3) 1.86 and1.99 Ga based on whole-rock Pb/Pb from the Virginia Formation (Hemmings et al; 1995).

Petrographic examination of a lapilli tuff unit present in the upper Gunflint at KakabekaFalls identified euhedral zircons forming a small portion of the silt population together withlarge, monomineralic clasts of quartz and sanidine likely from an explosive, volcanic source.The stratigraphic section present here has a large algal bioherm complex at its lowest level(Figure 1). This is overlain by a thick sequence of parallel-laminated black shale with sporadicdevelopment of layer rip-ups caused by current activity. A series of what has been describedas lapilli tuff beds occurs midway through this sequence (Shegelski, 1982). These are massiveto cross-stratified and graded to disorganized bedded. Mudstone rip-ups are common. Thelapilli consist of Fe-rich chlorite and are internally massive. The above strongly suggests thatthe lapilli are not accretionary volcanic-rainout debris, but rounded volcaniclastic mud rip-ups,which were eroded, abraded and transported by storm-induced currents. The black shale unitis correlated with a fmd-grained, resedimented, volcaniclastic shale, which extends throughoutupper Gunflint equivalents in the U.S.A. It may also be correlated with a small regioncontaining basaltic flow rocks.

Approximately 100 zircons were recovered from the reworked tuffs, including altered,abraided and euhedral brown populations. Although the majority of the population analysedgave reset Archaean ages, a euhedral zircon gave a concordant age of 1878± 2Ma BP (Figure2). This most likely represents the age of volcanism that was penecontemporaneous withsedimentation.

Preliminary geochemical data for Gunflint basalts and volcaniclastic sediment indicatesthe possibility of a deep mantle source for the melts and rules out any involvement ofsubducting lithosphere. Similarities with the Emperor and Hemlock volcanics suggest anorthward time-transgressive, extensional, intraplate eruptive event, which may in part overlapchronologically with the commencement of arc volcanism to the south.

66

THE AGE AND PROVENANCE OF THE GUNFLINT LAPILLI TUFF

FRALICK, P.W. and KISSIN, S.A., Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B 5E1, and DAVIS, D.W., Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario, M5S 2C6.

The Gunflint Formation forms the middle unit of the Animikie Group in northwestern Ontario, outcropping proximally to the western end of Lake Superior. It consists of an assemblage of chemical and fine-grained elastic sediments deposited in the strand-proximal zone of a south facing shelf as recently interpreted by Pufahl(1996).

Previous attempts at assigning an age to the Gunflint Formation (or the conformably overlying Rove Formation) and equivalents can be classified into three groups: (1) ages from 1.556 to 1.63 Ga based on whole-rock Rb/Sr or KIAr techniques (Hurley et al., 1962; Peterman 1966; Faure & Kovach, 1969; and Franklin, 1978); (2) ages from 2.08 to 2.1 11 Ga based on whole-rock NdlSm techniques (Stille & Plauer, 1985; Gerlach et al, 1988); and (3) 1.86 and 1.99 Ga based on whole-rock PbIPb from the Virginia Formation ( Hemmings et al; 1995).

Petrographic examination of a lapilli tuff unit present in the upper Gunflint at Kakabeka Falls identified euhedral zircons forming a small portion of the silt population together with large, monomineralic clasts of quartz and sanidine likely from an explosive, volcanic source. The stratigraphic section present here has a large algal bioherm complex at its lowest level (Figure 1). This is overlain by a thick sequence of parallel-laminated black shale with sporadic development of layer rip-ups caused by current activity. A series of what has been described as lapilli tuff beds occurs midway through this sequence (Shegelski, 1982). These are massive to cross-stratified and graded to disorganized bedded. Mudstone rip-ups are common. The lapilli consist of Fe-rich chlorite and are internally massive. The above strongly suggests that the lapilli are not accretionary volcanic-rainout debris, but rounded volcaniclastic mud rip-ups, which were eroded, abraded and transported by storm-induced currents. The black shale unit is correlated with a find-grained, resedimented, volcaniclastic shale, which extends throughout upper Gunflint equivalents in the U.S.A. It may also be correlated with a small region containing basaltic flow rocks.

Approximately 100 zircons were recovered from the reworked tuffs, including altered, abraided and euhedral brown populations. Although the majority of the population analysed gave reset Archaean ages, a euhedral zircon gave a concordant age of 1878Â 2Ma BP (Figure 2). This most likely represents the age of volcanism that was penecontemporaneous with sedimentation.

Preliminary geochemical data for Gunflint basalts and volcaniclastic sediment indicates the possibility of a deep mantle source for the melts and rules out any involvement of subducting lithosphere. Similarities with the Emperor and Hemlock volcanics suggest a northward time-transgressive, extensional, intraplate eruptive event, which may in part overlap chronologically with the commencement of arc volcanism to the south.

.34

.338

.336

.334

.332

References

Faure, G., & Kovach, J. (1969). Geol. Soc. Amer. Bull. , 1725-1736.Franklin, J.M. (1978). Geol. Surv. Can. Pap. 77-14, 35-39.Gerlach, D.C., Shirey, S.B. & Carlson, R.W. (1988). EOS , 1515.Hemming, S.R., McLennan, S.M. & Hanson, G.N. (1995). Jour. Geol. jQ., 147-168.Hurley, P.M., Fairbaim, H.W., Pinson, W.H., & Hower, J. (1962). Jour. Geol. , 489-492.Peterman, Z.E. (1966). Geol. Soc. Amer. Bull. fl, 103 1-1044.Pufahl, P.K. (1996). M.Sc. Thesis, Lakehead Univ., 167 p.Shegeiski, R.J. (1982). Geol. Assoc. Can./Mineral. Assoc. Can. Field Trip Guidebook 4, 15-31.

5.2 5.25 5.3 5.35 5.4 5.4510-01-1997

Fig. 2. Pb/U plot for euhedral, brownzircons from the lapilli tuff unit.

67

Fig. 1. Stratigraphic section of the upperGunflint Formation containing lapillituff units in the Kakabeka Gorge.

BrecciaStromatojites

'' Wavy Beddinc'..-.. Cross -

Stratification

Granules tosmall Pebbles

Sandstone

Siltstone

References

Faure? G.? & Kovach? J. (1969). Geol. SOC. Amer. Bull. SOy 1725-1736. Franklin, J.M. (1978). Geol. Sum. Can. Pap. 77-14,35-39. Gerlachy D.C.? Shirey, S.B. & Carlson, R.W. (1988). EOS @, 15 15. Hemming, S.R.? McLennan, S.M. & Hanson, G.N. (1 995). Jour. Geol. my 147-1 68. Hurley, P.M., Fairbairn, H.W., Pinson? W.HSy & Hower, J. (1962). Jour. Geol. a, 489-492, Petermany Z.E. (1966). Geol. SOC. Amer. Bull. a, lo3 1-1044. Pufahl, P.K. (1996). M.Sc. Thesis, Lakehead Univ., 167 p. Shegelski, R.J. (1982). Geol. Assoc. CanJMineral. Assoc. Can. Field Trip Guidebook 4? 15-3 1.

. Brown euheclral

1878 +I- 2 Ma

20'Pb / 236U

Fig. 2. Pb/U plot for euhedraly brown zircons fiom the lapilli tuff unit.

Granules to / a .small Pebbles .-. .::. .:.- . . . . . . . Sandstone . . . . . . . . .............. . . . . . . siltstone . --.

Shale

Chert

Breccia Stromatolites Wavy Beddin( Cross - Stratification

Fig. 1. Stratigraphic section of the upper Gunflint Formation containing lapilli tuff units in the Kakabeka Gorge.

PRE-WISCONSINAN GRAY TILL IN THE MANKATO AREA OF THE MINNESOTA RIVERVALLEY

GRAMSTAD, Sally D., student, Department of Geological and Atmospheric Sciences, IowaState University, Ames, IA 50011, sgramstaiastate.edu

Exposures of pre-Wisconsinan gray till in the Mankato area of the Minnesota River Valley show thatthis unit is up to 30 m thick and is somewhat continuous. Low shale content, moderate carbonatecontent, and NW-SE trending fabrics indicate a Winnipeg provenance. The texture and lithology ofthis unit are similar to those of other pre-Wisconsinan tills in the Midwest, including the Browervilleand Elmdale tills in north-central and central Minnesota, and unit 1 at Salisbury Hill, which is twentymiles northeast of the study area.

The topography of the bedrock valley that is present in the Mankato area may have affectedice flow of the glacier that deposited the pre-Wisconsinan gray till unit. Influence of this bedrockvalley may explain several anomalous fabrics, as well as the fact that the thick till unit is interruptedby sand and gravel layers at some sites and is completely undisturbed at others.

68

PRE-WISCONSINAN GRAY TILL IN THE MANKATO AIZEA OF THE MINNESOTA RIVER VALLEY

GRAMSTAD, Sally D., student, Department of Geological and Atmospheric Sciences, Iowa State University, Ames, IA 5001 1, sgramsta@iastate.edu

Exposures of pre-Wisconsinan gray till in the Mankato area of the Minnesota River Valley show that this unit is up to 30 m thick and is somewhat continuous. Low shale content, moderate carbonate content, and NW-SE trending fabrics indicate a Winnipeg provenance. The texture and lithology of this unit are similar to those of other pre-Wisconsinan tills in the Midwest, including the Browewille and Elmdale tills in north-central and central Minnesota, and unit 1 at Salisbury Hill, which is twenty miles northeast of the study area.

The topography of the bedrock valley that is present in the Mankato area may have affected ice flow of the glacier that deposited the pre-Wisconsinan gray till unit. Influence of this bedrock valley may explain several anomalous fabrics, as well as the fact that the thick till unit is interrupted by sand and gravel layers at some sites and is completely undisturbed at others.

A MINERALOGRAPHIC STUDY OF MAGNETITE IN THE BIWABIK IRON-FORMATION, MESABIRANGE, M[NNESOTA

T. M. Han, Senior Research Scientist (Retired), Cleveland-Cliffs' Research Laboratory

A substantial number of polished sections prepared from specimens collected from the Biwabik Iron-Formation were microscopically examined before and after an induced oxidation procedure. The Biwabik Iron-Formation is simply classified here as cherty and slaty members. It is considered a regionally metamorphosedsediment before the the intrusion of the Duluth complex. Based on the nature and magnitude of the changes,the existing iron-formation may be divided into three metamorphosed sectors progressing from west to east, i.e.regionally, thermally, and pyrometasomatically. During regional metamorphism, hematite functioned as a"starting point" for the development of much of the magnetite through Fe diffusion. Thermal metamorphisminvolves mineralogical transformations of an isochemical nature, which may be subdivided into transitional,amphibole, and pyroxene zones. Pyrometasomatism process involved advanced recrystallization andreplacement by gabbro adjacent to, and within, the intrusion.

This poster focuses on the genesis of magnetite in the regionally metamorphosed iron-formation andthe textural and mineralogical changes caused by thermal metamorphism and pyrometasomatism. Most of thefeatures observed during the course of study are photographically and photomicrographically shown. It covers:

A. Lithological characteristics from different metamorphic zones.

The irregularly and evenly banded macrostructures of the regionally metamorphosed iron-formationshow little change, whereas the mineralogical composition and microstructures have experienced significantprogressive elimination and transformation due to the thermal metamorphism and pyrometasomatism.

B. Hematite-Magnetite Relations in the Regional Metamorphosed Iron-Formation

1. Before Induced Oxidation. The hematite and magnetite occur either in separate layers or inintimate relations in the same layer. Hematite is almost always much fmer and less abundant than magnetite.The replacement of hematite by magnetite is readily evident. The magnetite differs in external morphology, i.e.octahedral and pseudomorphs after other minerals.

2. After Induced Oxidation. The morphologies and arrangement of precursor hematite in existing magnetitevary significantly. In the evenly banded slaty lithology, the precursor hematite is almost exclusively lath-shaped and exhibits a decussate microstructure. In the irregularly banded lithology of both the slaty and chertymembers, the precursor hematite exhibits a wide variety of morphologies. It is either randomly arranged ordisplays vuggy or microgeodic structures within magnetite that is present as irregular, coalesced crystals,granules, or pinch-and-swell layers. The external morphology of the present magnetite is governed by theprecursor hematite. Some of the precursor hematite might have been enlarged by authigenesis beforemagnetite development. Magnetite with precursor hematite inclusions is present in nearly all the mineralassemblages of the iron-formation. The size and morphologies of precursor hematite in magnetite differs fromthe existing hematite, either coexisting with or playing host to the magnetite.

C. Effect of Thermal Metamorphism on Hematite and Magnetite

In the transitional zone, more than one stage of overgrowth on euhedral magnetite (with or without theprecursor hematite inclusions) is frequently observed. Two to three external morphologies of magnetite werealso seen in coexistence. The induced oxidation pattern progressively changes from grain boundary oxidationalong the precursor hematite to cleavage oxidation along octahedral planes of the existing magnetite as themetamorphic grade increases. The hematite of the regionally metamorphosed iron-formation is progressivelyreplaced by magnetite. The euhedral magnetite is progressively changed to subangular to nearly round grains.However, the outlines of the magnetite pseudomorphs and magnetite granules are still preserved. Magnetiteporphyroblasts appear in the fine-grained coalesced subangular magnetite. Some siliceous magnetite blades or

69

A MINERALOGRAPHIC STUDY OF MAGNETITE IN THE BIWABIK IRON-FORMATION, MESABI RANGE. MrNNESOTA

T. M. Han, Senior Research Scientist (Retired), Cleveland-Cliffs' Research Laboratory

A substantial number of polished sections prepared from specimens collected from the Biwabik Iron- Formation were microscopically examined before and after an induced oxidation procedure. The Biwabik Iron- Formation is simply classified here as cherty and slaty members. It is considered a regionally metamorphosed sediment before the the intrusion of the Duluth complex. Based on the nature and magnitude of the changes, the existing iron-formation may be divided into three metamorphosed sectors progressing from west to east, i.e. regionally, thermally, and pyrometasomatically. During regional metamorphism, hematite functioned as a "starting point" for the development of much of the magnetite through ~ e + + difhsion. Thermal metamorphism involves mineralogical transformations of an isochemical nature, which may be subdivided into transitional, amphibole, and pyroxene zones. Pyrometasomatism process involved advanced recrystallization and replacement by gabbro adjacent to, and within, the intrusion.

This poster focuses on the genesis of magnetite in the regionally metamorphosed iron-formation and the textural and mineralogical changes caused by thermal metamorphism and pyrometasomatism. Most of the features observed during the course of study are photographically and photomicrographically shown. It covers:

A. Lithological characteristics fiom different metamorphic zones.

The irregularly and evenly banded macrostructures of the regionally metamorphosed iron-formation show little change, whereas the mineralogical composition and microstructures have experienced significant progressive elimination and transformation due to the thermal metamorphism and pyrometasomatism.

B. Hematite-Magnetite Relations in the Regional Metamorphosed Iron-Formation

1. Before Induced Oxidation. The hematite and magnetite occur either in separate layers or in intimate relations in the same layer. Hematite is almost always much fmer and less abundant than magnetite. The replacement of hematite by magnetite is readily evident. The magnetite differs in external morphology, i.e. octahedral and pseudbmorphs after other minerals.

2. After Induced Oxidation. The morphologies and arrangement of precursor hematite in existing magnetite vary significantly. In the evenly banded slaty lithology, the precursor hematite is almost exclusively lath- shaped and exhibits a decussate microstructure. In the irregularly banded lithology of both the slaty and cherty members, the precursor hematite exhibits a wide variety of morphologies. It is either randomly arranged or displays vuggy or microgeodic structures within magnetite that is present as irregular, coalesced crystals, granules, or pinch-and-swell layers. The external morphology of the present magnetite is governed by the precursor hematite. Some of the precursor hematite might have been enlarged by authigenesis before magnetite development. Magnetite with precursor hematite inclusions is present in nearly all the mineral assemblages of the iron-formation. The size and morphologies of precursor hematite in magnetite differs fiom the existing hematitey either coexisting with or playing host to the magnetite.

C. Effect of Thermal Metamorphism on Hematite and Magnetite

In the transitional zoney more than one stage of overgrowth on euhedral magnetite (with or without the precursor hematite inclusions) is frequently observed. Two to three external morphologies of magnetite were also seen in coexistence. The induced oxidation pattern progressively changes from grain boundary oxidation along the precursor hematite to cleavage oxidation along octahedral planes of the existing magnetite as the metamorphic grade increases. The hematite of the regionally metamorphosed iron-formation is progressively replaced by magnetite. The euhedral magnetite is progressively changed to subangular to nearly round grains. However, the outlines of the magnetite pseudomorphs and magnetite granules are still preserved. Magnetite porphyroblasts appear in the fine-grained coalesced subangular magnetite. Some siliceous magnetite blades or

cubic cavities are occasionally observed in magnetite distributed in the pyroxene zone. However, somehematite coexisting with magnetite has also been found in the pyroxene zone.

D. Effect of Pyrometasomatism on Hematite and Magnetite

1. Advanced Recrystallized Zone. The magnetite here is substantially coarser than in the regionallyand thermally metamorphosed zones. The hematite, magnetite pseudomorphs, and magnetite granules nolonger exist. Two generations of magnetite and the introduction of hercyanite along magnetite cleavage planesare evident. The magnetite grains commonly have fme parallel to subparallel fractures, whereas the host doesnot. The fractured magnetite sometimes resembles specular hematite. Minute euhedral gangue inclusions arecommonly present. Cleavage oxidation is typical. Magnetite inclusions within magnetite are either outlined bythe effect of differential oxidation or by their grain boundaries.

2. Gabbroized Zone. The advanced recrystallized magnetite gradationally transforms to atitanomagnetite approaching the gabbroized zone. The latter contains minute inclusions of hercyanite, ilmenite,and ulvospinel, and an unidentified mineral. Most of these mineral phases are products of exsolution. Bothmagnetite and titanomagnetite may be observed in adjacent layers of the same specimen. Small octahedralinclusions of titanomagnetite are occasionally seen and exhibit essentially the same composition and texture astheir titanomagnetite host. Some titanomagnetite contains apatite, pyroxene, and other gangue grains; someexhibits a myrmekitic intergrowth texture in pyroxene; and some appears to be replaced by ilmenite andsulfides.

Economically, regional metamorphism is the key contributor in generating magnetite for the presentmining industry. Thermal metamorphism has a negative impact on magnetite size liberation but a positiveeffect on magnetite weight recovery. Advanced recrystallization has a positive effect on both size liberationand weight recovery, whereas the gabbroized iron-formation can no longer be considered iron ore.

The details relative to this study were reported in the following papers and abstracts.

T. M. Han (1978) "Microstructures of Magnetite as Guides to Its Origin in Some Precambrian Iron-Formations." Fortschr. Mineral. 56, 105-142

(1982) "Iron-Formation of Precambrian Age: Hematite-Magnetite Relationships in Some ProterozoicIron Deposits - A Microscopic Observation." Ore Genesis -The State of the Art, Springer-Verlag, Berlin,Heidelberg, New York.

(1988) "Origin of Magnetite in Precambrian Iron-Formations of Low Metamorphic Grade."Proceedings of the Seventh Quadrennial IAGOD Symposium, E Schweizerbart'sche Verlagsbuchhandlung, D-7000 Stuttgart I

(1996) "Mineralogical Evolution of Precambrian Iron-Formation of Low Metamorphic Grade and ItsContribution to the U.S. Steel Industry" (Abstract), 30th IGC, Beijing, China

T. M. Han, R. G. Graber, and Ruth Kramer (1992) "Effect of Duluth Gabbro Intrusion on Ore Mineralogy ofthe Biwabik Iron-Formation, Lake Superior District," USA International Symposium on Mineralization Relatedto Mafic and Ultramafic Rocks, 8th Quadrennial IAGOD Symposium, Orleans, France (Abstract)

70

cubic cavities are occasionally observed in magnetite distributed in the pyroxene zone. However, some hematite coexisting with magnetite has also been found in the pyroxene zone.

D. Effect of Pyrometasomatism on Hematite and Magnetite

1. Advanced Recrystallized Zone. The magnetite here is substantially coarser than in the regionally and thermally metamorphosed zones. The hematite, magnetite pseudomorphs, and magnetite granules no longer exist. Two generations of magnetite and the introduction of hercyanite along magnetite cleavage planes are evident. The magnetite grains commonly have fme parallel to subparallel fractures, whereas the host does not. The fractured magnetite sometimes resembles specular hematite. Minute euhedral gangue inclusions are commonly present. Cleavage oxidation is typical. Magnetite inclusions within magnetite are either outlined by the effect of differential oxidation or by their grain boundaries.

2. Gabbroized Zone. The advanced recrystallized magnetite gradationally transforms to a titanomagnetite approaching the gabbroized zone. The latter contains minute inclusions of hercyanite, ilmenite, and ulvospinel, and an unidentified mineral. Most of these mineral phases are products of exsolution. Both magnetite and titanomagnetite may be observed in adjacent layers of the same specimen. Small octahedral inclusions of titanomagnetite are occasionally seen and exhibit essentially the same composition and texture as their titanomagnetite host. Some titanomagnetite contains apatite, pyroxene, and other gangue grains; some exhibits a myrmekitic intergrowth texture in pyroxene; and some appears to be replaced by ilmenite and sulfides.

Economically, regional metamorphism is the key contributor in generating magnetite for the present mining industry. Thermal metamorphism has a negative impact on magnetite size liberation but a positive effect on magnetite weight recovery. Advanced recrystallization has a positive effect on both size liberation and weight recovery, whereas the gabbroized iron-formation can no longer be considered iron ore,

The details relative to this study were reported in the following papers and abstracts.

T. M. Han (1978) "Microstructures of Magnetite as Guides to Its Origin in Some Precambrian Iron- Formations." Fortschr. Mineral. 56, 105-142

(1 982) "Iron-Formation of Precambrian Age: Hematite-Magnetite Relationships in Some Proterozoic Iron Deposits - A Microscopic Observation." Ore Genesis - The State of the Art, Springer-Verlag, Berlin, Heidelberg, New York.

(1988) "Origin of Magnetite in Precambrian Iron-Formations of Low Metamorphic Grade." Proceedings of the Seventh Quadrennial IAGOD Symposium, E Schweizerbart'sche Verlagsbuchhandlung, D- 7000 Stuttgart I

(1996) "Mineralogical Evolution of Precambrian Iron-Formation of Low Metamorphic Grade and Its Contribution to the U.S. Steel Industry" (Abstract), 30th IGC, Beijing, China

T. M. Han, R. G . Graber, and Ruth Kramer (1992) "Effect of Duluth Gabbro Intrusion on Ore Mineralogy of the Biwabik Iron-Formation, Lake Superior District," USA International Symposium on Mineralization Related to Mafic and Ultramafic Rocks, 8th Quadrennial IAGOD Symposium, Orleans, France (Abstract)

FACIES AND DEPOSITIONAL ENVIRONMENTS OF THE EARLY PROTEROZOICIRONWOOD IRON FORMATION, MT. WHITTLESEY WISCONSIN.

*EJ Hensel, *ck Joslin, and Dan Lehrmann; Dept. of Geology, University of Wisconsin-

Oshkosh, Oshkosh, Wi .54901

There are numerous models of depositional environments of Early Proterozoic iron fonnation. Thepuipose of the study was to evaluate which of these models best fit the sedimentaiy facies and stratigraphicpatterns in the Ironwood iron formation exposed at the Berkeshire Mine Ruins, at Mt. Whitfiesey, in theGogebic District, near Mellen Wisconsin. Soil and vegetation were stripped from this area by the abandonedmining effort. Approximately 100 meters of stratigraphic section was described bed by bed at a decimeterscale. An additional 150 meters was descr bed in reconnaissance sections. Samples were collected, cut anddescribed in detail to help understand depositional environments.

Five different fades were recognized from outcrop observation and sample descriptions. These aresummarized as follows. 1) The horizontal laminated facies consists of continuous and discontinuous laminaeof alternating inagnetite and chert. Sedimentary structures include thin, graded laminae, scour marks, v-shaped cracks that probably resulted from differential compaction (j)ossibly desiccation ?), and discontinuouslenses of granular facies. The lenses range from a few to 15 cm thick and commonly pinch out within a fewmeters laterally. 2) The wavy laminated facies consists of wavy laminae of magnetite and chert with fewintraclasts of cheit Sedimentary structures include crinidey laminations that probably are horizontalstromatolites, convolute laminae, and lenticular layers. 3) The intraclasticfacies consists of intraclasts ofmagnetite and chert derived from the wavy and granular facies. The clasts were cemented early by silica.Sedimentary structures include, planar cross beds, imbricated clasts, differential syn-sedimentaiy compaction,autoclastic breccia, and abundant v-shaped cracks penetrating the margins of intraclasts. These may bedesiccation cracks. Some of the intraclasts are rounded and are concentrically coated with cheit 4) Thestromatolitefacies contains domal stromatolites composed of chert and magnetite. Intraclastic material,containing stromatolite fragments and clasts of granular fades, is interbedded with and occurs between thestromatolites. 5) The granular facies consists of coarse to fine sand and minor silt sized chert and magnetitegrains. Most of these grains are peloids (featureless rounded grains), some are ooids. The granular faciesappears to be winnowed with little fine grained matrix and locally contains ripple cross-lamination.

The most obvious stratigraphic pattern at Mt. Whittlesey is a meter- to decimeter-scale intercalationof the wavy, granular, and intraclastic facies (fig. 1). Careful observation reveals, however, that that there isalso a larger-scale change in facies associations in the outcrop that defines 3 lithologic cycles. These cycles areapproximately 50 m thick. Within the cycles there is a preferential association in which the following faciesmore frequently occur together: 1) the granular and wavy laminated facies, 2) the intraclastic and stromatoliticfacies, and 3) the horizontal laminated facies. The cycles consist of a repetition of the three facies associationsabove. Interestingly, the cycles are asymmetric in the sense that the intraclastic and stromatolitic faciesassociation always occurs beneath the horizontally laminated facies and not above it (fig. 2).

Our data supports the interpretation that the iron formation was deposited in a shallow-marine, intertidalto subtidal shelf environment. This is similar to the environments that have been most widely interpreted foriron formations (CL Simonson, 1985; Lougheed, 1983; LaBerge, 1987; Simonson and Hassler, 1996).Sedimentary features that support this interpretation include the stromatolites, ooids, ripple cross lamination,and cross bedding. We interpret that the iron formation formed as a biochemical precipitate of iron and silicaminerals from sea-water. The similarity to Proterozoic or Early Paleozoic marine carbonate facies supports theidea that these are biochemical rocks. We found no evidence to support the interpretation that these wereoriginally carbonate sediments that were diagenetically replaced by silica or for freshwater deposition that hasbeen postulated by other authors (Hough, 1958; Eugster and Chou; and Trendall, 1973).

In our preferred depositional model, the wavy laminated facies formed on the tidal flats, the granular,intraclastic, stromatolite, facies formed progressively further seaward in shallow, wave agitated subtidalenvironments and the horizontally laminated facies formed in deeper environments between nonnal wave baseand storm wave base (fig. 3). The horizontal laminations are interpreted to have formed in quiet waters belownormal wave base. The abundant scour structures and discontinuous lenses of granular facies found in thehorizontally laminated facies are interpreted to represent agitation and "spill over lobes" of granular material

71

FACIES AND DEPOSITIONAL ENVIRONMENTS OF THE E A m Y PROTEROZOIC IRONWOOD IRON FORMATION, MT. WHITTLESEY WISCONSIN.

*EM Hensely *Rick J o s h , and Dan Lehrmaq Dept. of Geoloa, University of Wisconsin-

Oshkosh7 Oshkosh, Kl54901

There are numerous models of depositional environments of Early Proterozoic i k n formation. The purpose of the study was to evaluate which of these models best fit the sedimentary facies and stratigraphic patterns in the Ironwood iron formation e x p o d at the Berkeshire Mine Ruins, at Mt. Whittlesey, in the Gogebic District, near Mellen Wisconsin. Soil and vegetation were stripped from this area by the abandoned mining &ort. Approximately 100 meters of stratigraphic section was described bed by bed at a decimeter d e . An additional 150 meters was described in reconnaissance sections. Samples were collected, cut and described in detail to help understand depositional environments.

Five different facies were recognized from outcrop observation and sample descriptions. These are s u m a r i d as follows. 1) The horizontal hmin'atedfiia consists of continuous and discontinuous laminae of alternating magnetite and chert. Sedimentary structures include thin, graded laminae, scour marks, v- shaped cracks that probably resulted from Merentid compaction (possibly desiccation ?), and discontinuous lenses of granular facies. The lenses range fiom a few to 15 cm thick and commonly pinch out within a few meters laterally. 2) The wavy hminatedfacies consists of wavy laminae of magnetite and chert with few intraclasts of chert. Sedimentary structures include crinkley laminations that probably are horizontal stromatolites, convolute laminae, and lenticular layers. 3) The infracladcfacies consists of intraclasts of magnetite and chert derived from the wavy and granular facies. The clasts were cemented early by silica. Sedimentary structures include7 planar cross beds, imbricated clasts, differential syn-sedimentary compaction, autoclastic breccia, and abundant v-shaped cracks penetrating the margins of intraclasts. These may be desiccation cracks. Some of the intraclasts are rounded and are concentrically coated with chert. 4) The ~omatolifefacies contains domal stromatolites composed of chert and magnetite. Intraclastic material, containing stromatolite hgments and clasts of granular facies* is interbedded with and occurs between the stromatolites. 5) The granuhrfacies consists of coarse to fine sand and minor silt sized chert and magnetite grains. Most of these grains are peloids (featureless rounded grains), some are mids. The granular facies appears to be winnowed with little fine grained matrix and locally contains ripple cross-lamination.

The most obvious stratigraphic pattern at Mt. Whittlesey is a meter- to decimeter-scale intercalation of the wavy, granular, and intraclastic facies (fig. 1). Carefid obsemtion reveals, however, that that there is also a larger-scale change in facies associations in the outcrop that defines 3 lithologic cycles. These cycles are approximately 50 m thick Within the cycles there is a preferential association in which the following facies more frequently occur together: 1) the granular and wavy laminated facies, 2) the intraclastic and stromatolitic facies, and 3) the horizontal laminated facies. The cycles consist of a repetition of the three facies associations above. Interestingly, the cycles are asymmetric in the sense that the intraclastic and stmmatolitic facies association always occurs beneath the horizontally laminated facies and not above it (fig. 2).

Our data supports the interpretation that the iron formation was deposited in a shallow-marine, intertidal to subtidal shelf environment. This is similar to the environments that have been most widely interpreted for iron formations (cf. Simonson, 1985; hugheed, 1983; Merge, 1987; Simonson and Hassler, 1996). Sedimentary fatures that support this interpretation include the stromatolites, mi&, ripple cross lamination, and cross bedding. We interpret that the iron formation formed as a biochemical precipitate of iron and silica minerals from sea-water. The similarity to Proterozoic or Early Paleozoic marine carbonate facies supports the idea that these are biochemical rocks. We found no evidence to support the interpretation that these were originally carbonate sediments that were diagenetically replaced by silica or for h h w a t e r deposition that has been postulated by other authors (Hough, 1958; Eugster and Chou; and Trendall, 1973).

In our p r e f e d depositional model, the wavy laminated facies formed on the tidal flats, the granular, intraclastic, stromatolite7 facies formed progressively fiuther seaward in shallow, wave agitated subtidal environments and the horizontally laminated facies formed in deeper environments between normal wave base and storm wave base (fig. 3). The horizontal laminations are interpreted to have formed in quiet waters below normal wave base. The abundant scour structures and discontinuous lenses of granular facies found in the horizontaIly laminated facies are interpreted to represent agitation and "spill over lobesn of granular material

washed seaward during storms (fig. 3). Originally we entertained the idea that the horizontally laminatedfacies formed landward of the oolite shoals in tidal flat ponds. If the v-shaped cracks were desiccation features,that would support such an interpretation. The v-shaped cracks are more commonly associated with syn-depositional deformation structures however, and the horizontally laminated facies is rarely associated withthe wavy laminated facies. This, along with the asymmetrical pattern of the large-scale depositional cyclessupports the deeper offshore environment. In this model, the cycles in facies associations would represent longterm (—million year) asynunetric transgressions and regressions of the shelf. The stromatolites and intraclasticfacies formed preferentially during initial transgressions, due to increased wave energy and increasing waterdepths. Maximum deepening is represented by the horizontally laminated facies and regression is representedby the shift back to the wavy laminated - granular facies association.

Previous studies have commonly interpreted the horizontally laminated facies as being a deep waterdeposit. In some cases it has been interpreted to be a vezy deep, pelagic and turbiditic facies. Our data suggeststhat, at Mt. Whittlesey at least, this is not the case. The granular units interbedded in the horizontal facies arefar too discontinuous to represent distal turbidites and they lack Bouma sequences.

Eugster, H. P., Chou, I.M., 1973, The depositional environments Precambrian banded iron formations:Economic Geology and the Bulletin of the Society for Economic Geology, v. 68, n. 7, p. 1144-1168.

Hough, J. L., 1958, Fresh water environment of deposition of Precambrian banded iron formation: Journal ofSedimentary Petrology, v. 28, n. 4, p. 414-430.

LaBerge, G. L., Robbins, E. I. Schmidt, K 0., 1987, A model for the biological precipitation of Precambrianiron formations: Precambrian Iron Formations, Theophrastus Pubi., Athens, p. 69-96.

Lougheed, M. S., 1983, Origin of Precambrian iron formations in the lake superior region: Geological Societyof America Bulletin, v. 94, ii. 3, p. 325-340.

Simonson, B. M., 1985, Sedimentological constraints on the origins of Precambrian iron formations:Geological Society of America Bulletin, v. 96, n. 2, p. 244-252.

Simonson, B. M. and Hassler, S. W., 1996, Was the deposition of large Precambrian iron formations linked tomajor marine transgressions?: Journal of Geology, v. 104, n. 6, p. 665-676.

Trendall, A. F., 1973, Iron formation of the Hamersley Group of Western Australia; type examples of varvedevaporites: Earth Science (Paris), v. 9, p. 257-270.

Fades

___

nfradasc

urnFigure 1

Fades Associations

EHorizontally-LaminatedIntraclastic IStromatoliticWavy Laminated /Granular

FadesWavy Iamânated Intradastic

Granu

Figure 3: Depositional Model

72

washed seaward during storms (fig. 3). Originally we entertained the idea that the horizontally laminated facies formed landward of the oolite shoals in tidal flat ponds. If the v-shaped cracks were desiccation features, that would support such an interpretation. The v-shaped cracks are more commonly associated with syn- depositional deformation structures however, and the horizontally laminated facies is rarely associated with the wavy laminated facies. This, along with the asymmetrical pattern of the large-scale depositional cycles supports the deeper offshore environment. In this model, the cycles in facies associations would represent long term (--million year) asymmetric transgressions and regressions of the shelf. The stromatolites and intraclastic facies formed preferentially during initial transgressions, due to increased wave energy and increasing water depths. Maximum deepening is represented by the horizontally laminated facies and regression is represented by the shift back to the wavy laminated - granular facies association.

Previous studies have commonly interpreted the horizontally laminated facies as being a deep water deposit. In some cases it has been interpreted to be a very deep, pelagic and turbiditic facies. Our data suggests that, at ML Whittlesey at least, this is not the case. The granular units interbedded in the horizontal facies are far too discontinuous to represent distal turbidites and they lack Bouma sequences.

Eugster, H. P., Chou, I.M., 1973, The depositional environments Precambrian banded iron formations: Economic Geology and the Bulletin of the Society for Economic Geology, v. 68, n. 7, p. 1144-1 168.

Hough, J. L., 1958, Fresh water environment of deposition of Precambrian banded iron formation: Journal of Sedimentary Petrology, v. 28, n. 4, p. 414-430.

LaBerge, G. L., Robbins, E. I. Schmidt, R G., 1987, A model for the biological precipitation of Precambrian iron formations: Precambrian Iron Formations, Theophrastus Publ., Athens, p. 69-96.

Lougheed, M. S., 1983, Origin of Precambrian iron formations in the lake superior region: Geological Society of America Bulletin, v. 94, n. 3, p. 325-340.

Simonson, B. M, 1985, Sedimentological constraints on the origins of Precambrian iron formations: Geological Society of America Bulletin, v. 96, n. 2, p. 244-252.

Simonson, B. M and Hassler, S. W., 1996, Was the deposition of large Precambrian iron formations linked to major marine transgressions?: Journal of Geology, v. 104, n. 6, p. 665-676.

Trendall, A. F., 1973, Iron formation of the Hamersley Group of Western Australia; type examples of varved evaporites: Earth Science (Paris), v. 9, p. 257-270.-

Fades

TO- ,"-

I1 Figure 1

. "-- Wavy laminated [S t.nbad&c

a Horizontally- Laminated

1 Horizontally- Laminated Intraclastic I Strornatolitic

3 Wavy Laminated I Granular

FIom Figure 2

Figure 3: Depositional Model

AGE AND DEFORMATION OF EARLY PROTEROZOIC QUARTZITES IN THESOUTHERN LAKE SUPERIOR REGION: IMPLICATIONS FOR EXTENT OFFORELAND DEFORMATION DURING FINAL ASSEMBLY OF LAURENTIA

Daniel Hoim, Department of Geology, Kent State University, Kent, OH 44242David Schneider, Dept. of Earth and Environmental Sciences, Lehigh University, Bethlehem, PAChristopher D. Coath, Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095

Post-accretion stabilization in the Lake Superior region at 1770-1760 Ma resulted in deposition oflocally thick successions of Early Proterozoic mature quartzites in Wisconsin and southern Minnesota.Their age of deposition and the age of the deformation which caused widespread folding of many of thequartzite units has long been a matter of considerable importance and controversy. We present newevidence for both the maximum and minimum age of these quartzites. Also, we document the spatial co-existence of a thermal front in Precambrian crystalline basement with a deformational front in theoverlying quartzite units. The age of the front suggests that post-Penokean shortening of the Penokeanprovince is likely related to the final stages of formation of the Laurentian supercontinent at 1650 Ma.

The bulk of Laurentia formed by rapid aggregation of Archean continents at 1900-1800 Ma during theTrans-Hudson and Penokean orogenies (Hoffman, 1989). Subsequent accretion of juvenile crust alongthe southern margin of pre-1800 Ma Laurentia formed the Transcontinental Proterozoic provinces whichconsist broadly of a northern 1800-1700 Ma inner accretionary belt and a southern 1700-1600 Ma outertectonic belt. The transition zone between the two tectonic belts of the TPP represents the region ofknown pre-1700 Ma rocks metamorphosed and deformed at —1650 Ma during formation of the outertectonic belt. The eastward continuation of this transition zone and of the outer tectonic belt from thecentral plains is problematic as no 1800-1600 Ma juvenile crust exists in the entire southern Great Lakesregion (Van Schmus et al., 1993). However, on the basis of 1680-1640 Ma orogenic deformation and theexistence of 1650 Ma batholithic intrusive rocks in Labrador, Van Schmus et al. (1993) have proposedthat the outer tectonic belt was a single coherent continental arc extending from California to Labrador.

The Penokean orogeny involved island arc/microcontinent collision along the southern passive marginof the Superior Province at 1870-1830 Ma. In northern Wisconsin the south-dipping Niagara fault zonerepresents the main suture which separates uniformly metamorphosed (upper greenschistlloweramphibolite facies) island arc rocks of the Pembine-Wausau terrane from deformed continental marginrocks which exhibit a nodal metamorphic pattern imposed during collapse of the orogen (Schneider et aL,1996; Marshak et al., 1997). Abundant mica Ar/Ar cooling dates from central Minnesota andnorthernmost Wisconsin and Michigan indicate that collapse and orogenic unroofing occurred at 1750-1700 Ma shortly after an episode of widespread magmatism at 1770-1760 Ma (Holm and Lux, 1996;Schneider et al., 1996). In contrast, Rb-Sr whole rock isocbron and biotite mineral dates in northern andcentral Wisconsin (Peterman and Sims, 1988) are mostly Middle Proterozoic (1600-1100 Ma) and reflectvariable thermal resetting associated with a low-grade —1630 Ma metamorphic event (Van Scbmus andWoolsey, 1975; Van Schmus et al., 1975), intrusion of the 1470 Ma Wolf River batholith, andKeweenawan activity. Post-accretion rapid stabilization resulted in the accumulation of post-tectonicquartz arenites in the southern Lake Superior region (Van Schmus et al., 1993). Deformed quartzite unitsin central and southern Wisconsin yield post-Penokean detrital zircon Pb-Pb ages (Van Wyck, 1995). Thefolded Baraboo quartzite is depositional on a 1752 Ma granite (Medaris et a!., 1996) and contains detritalzircons as young as 1712 Ma (Dott et a!., 1997). The minimum age of these quartzite units is constrainedonly by the fact that some are intruded by the 1470 Ma Wolf River batholith, although Dott (1983) andVan Schmus et al. (1993) speculated that they may have been deformed during the 1630 Ma event.

The predominantly Middle Proterozoic mica mineral dates of central Wisconsin contrast sharply withthe well-grouped 1750-1700 Ma mica dates obtained from central Minnesota, northernmost Wisconsin,and northern Michigan. The 1750-1700 Ma dates are the oldest mica dates obtained from the internalportions of the Penokean orogen and thus almost certainly reflect primary cooling through mica closuretemperatures following Penokean metamorphism. Considering that primary cooling at 1750-1700 Mawas orogen wide, it is likely that the younger mica dates represent thermal resetting. An —1630 Ma agecontour thus separates basement with typical post-Penokean cooling ages to the north (and west, inMinnesota) from basement with thermally reset ages to the south. The eastward extent of the chrontour isnot precisely located, however, north of the Flambeau quartzite in northwest Wisconsin the chrontour issharply defined by Rb-Sr, K-Ar, and Ar-Ar mica dates.

Importantly, the Flambeau thermal front in northwest Wisconsin coincides spatially with an apparentdeformational front in overlying post-Penokean quartzites. In Minnesota, the subhorizontal Sioux

73

AGE AND DEFORMATION OF EARLY PROTEROZOIC QUARTZITES IN THE SOUTHERN LAKE SUPERIOR REGION: IMPLICATIONS FOR EXTENT OF FORELAND DEFORMATION DURING FINAL ASSEMBLY OF LAURENTIA

Daniel Holm, Department of Geology, Kent State University, Kent, OH 44242 David Schneider, Dept. of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA Christopher D. Coath, Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095

Post-accretion stabilization in the Lake Superior region at 1770-1760 Ma resulted in deposition of locally thick successions of Early Proterozoic mature quartzites in Wisconsin and southern Minnesota. Their age of deposition and the age of the deformation which caused widespread folding of many of the quartzite units has long been a matter of considerable importance and controversy. We present new evidence for both the maximum and minimum age of these quartzites. Also, we document the spatial co- existence of a thermal front in Precambrian crystalline basement with a deformational front in the overlying quartzite units. The age of the front suggests that post-Penokean shortening of the Penokean province is likely related to the final stages of formation of the Laurentian supercontinent at -1650 Ma.

The bulk of Laurentia formed by rapid aggregation of Archean continents at 1900-1 800 Ma during the Trans-Hudson and Penokean orogenies (Hoffman, 1989). Subsequent accretion of juvenile crust along the southern margin of pre-1800 Ma Laurentia formed the Transcontinental Proterozoic provinces which consist broadly of a northern 1800-1700 Ma inner accretionary belt and a southern 1700-1600 Ma outer tectonic belt. The transition zone between the two tectonic belts of the TPP represents the region of known pre-1700 Ma rocks metamorphosed and deformed at -1650 Ma during formation of the outer tectonic belt. The eastward continuation of this transition zone and of the outer tectonic belt from the central plains is problematic as no 1800-1600 Ma juvenile crust exists in the entire southern Great Lakes region (Van Schmus et al., 1993). However, on the basis of 1680-1640 Ma orogenic deformation and the existence of 1650 Ma batholithic intrusive rocks in Labrador, Van Schmus et al. (1993) have proposed that the outer tectonic belt was a single coherent continental arc extending from California to Labrador.

The Penokean orogeny involved island arc/microcontinent collision along the southern passive margin of the Superior Province at 1870-1830 Ma. In northern Wisconsin the south-dipping Niagara fault zone represents the main suture which separates uniformly metamorphosed (upper greenschist/lower amphibolite facies) island arc rocks of the Pembine-Wausau terrane from deformed continental margin rocks which exhibit a nodal metamorphic pattern imposed during collapse of the orogen (Schneider et al., 1996; Marshak et al., 1997). Abundant mica ArIAr cooling dates from central Minnesota and northernmost Wisconsin and Michigan indicate that collapse and orogenic unroofing occurred at 1750- 1700 Ma shortly after an episode of widespread magmatism at 1770-1760 Ma (Holm and Lux, 1996; Schneider et al., 1996). In contrast, Rb-Sr whole rock isochron and biotite mineral dates in northern and central Wisconsin (Peterman and Sims, 1988) are mostly Middle Proterozoic (1600-1 100 Ma) and reflect variable thermal resetting associated with a low-grade -1630 Ma metamorphic event (Van Schmus and Woolsey, 1975; Van Schmus et al., 1975), intrusion of the 1470 Ma Wolf River batholith, and Keweenawan activity. Post-accretion rapid stabilization resulted in the accumulation of post-tectonic quartz arenites in the southern Lake Superior region (Van Schmus et al., 1993). Deformed quartzite units in central and southern Wisconsin yield post-Penokean detrital zircon Pb-Pb ages (Van Wyck, 1995). The folded Baraboo quartzite is depositional on a 1752 Ma granite (Medaris et al., 1996) and contains detrital zircons as young as 1712 Ma (Dott et al., 1997). The minimum age of these quartzite units is constrained only by the fact that some are intruded by the 1470 Ma Wolf River batholith, although Dott (1983) and Van Schmus et al. (1993) speculated that they may have been deformed during the 1630 Ma event.

The predominantly Middle Proterozoic mica mineral dates of central Wisconsin contrast sharply with the well-grouped 1750-1700 Ma mica dates obtained from central Minnesota, northernmost Wisconsin, and northern Michigan. The 1750-1700 Ma dates are the oldest mica dates obtained from the internal portions of the Penokean orogen and thus almost certainly reflect primary cooling through mica closure temperatures following Penokean metamorphism. Considering that primary cooling at 1750-1700 Ma was orogen wide, it is likely that the younger mica dates represent thermal resetting. An -1630 Ma age contour thus separates basement with typical post-Penokean cooling ages to the north (and west, in Minnesota) from basement with thermally reset ages to the south. The eastward extent of the chrontour is not precisely located, however, north of the Flambeau quartzite in northwest Wisconsin the chrontour is sharply defined by Rb-Sr, K-Ar, and Ar-Ar mica dates.

Importantly, the Flambeau thermal front in northwest Wisconsin coincides spatially with an apparent deformational front in overlying post-Penokean quartzites. In Minnesota, the subhorizontal Sioux

quartzite lies just southwest of Penokean internal zone rocks which cooled rapidly through mica closuretemperatures at —1750 Ma. In northwest Wisconsin, the Barron quartzite is essentially flat-lying and isdepositional on Precambrian basement with mica ages between 1730-1700 Ma. South of the thermalfront, and only 25 km south of the Barron quartzite, are exposures of steeply-dipping Flambeau quartzite.Here the quartzite is folded into a moderately west-plunging synform (Myers, 1974) similar to the style ofdeformation exhibited by the Baraboo and McCaslin quartzites. The relatively undeformed Barron andSioux Proterozoic quartzites must either lie outside the region of significant post-Penokean deformation(as seems suggested by the spatial coincidence of a thermal front with a deformational front) or beyounger quartzite packages deposited after significant post-Penokean deformation. To test whether thesubhorizontal quartzites are correlatable with the deformed quartzites we obtained single-grain, single-spot 207Pb/206Pb ages on detrital zircons separated from the Sioux, Barron, and Flambeau quartzites.Because we are mostly interested in constraining the maximum age of each quartzite we concentrated ourefforts on dating euhedral or subhedral crystals wherever possible. All three quartzite bodies yieldedEarly Proterozoic and Late Archean detrital zircon dates comparable to dates obtained by Van Wyck(1995) for deformed quartzite bodies in Wisconsin. Many of the late Early Proterozoic (<2000 Ma) datesfall along a discordia with a lower intercept at or near the origin indicating recent lead loss attributed toupper crustal fluid circulation. Reliable 207Pb/206Pb Proterozoic ages are between 1730 and 1850 Ma forthe Sioux (9 grains), between 1714 and 1880 Ma for the Flambeau (9 grains), and between 1750 and 1880Ma for the Barron (6 grains). These data attest to the fact that all three quartzite bodies post-date the 1760Ma magmatic event in the Lake Superior region. Thus far there is no evidence to suggest that the Barronand Sioux quartzites are younger than the deformed quartzites found throughout most of Wisconsin.

Abundant new thermocbronologic data in the Lake Superior region allow us to make a simple butimportant first-order observation that provides the first direct structural evidence that the quartzites weredeformed during the low-grade —1630 Ma event in the Lake Superior region. We note that subhorizontalpost-Penokean quartzites consistently overlie crystalline basement with primary post-Penokean coolingages, whereas highly deformed quartzites everywhere overlie crystalline basement with secondary(thermally reset) cooling ages. We are fortunate that the proximity of the deformed Flambeau quartzite tothe subhorizontal Barron quartzite in northwest Wisconsin allows us to precisely locate thisdeformational/thermal front. An age of 1650-1630 Ma for the deformation seems reasonable given thatcooling ages south of the front probably post-date the deformation somewhat.

The thermal front extends eastward into northern Michigan, the geology of which is dominated bygneiss domes and classic nodal metamorphic isograds. No Early Proterozoic post-Penokean quartzites areknown north of the McCaslin quartzite. However, it's interesting to note that the Republic metamorphicnode located north of the thermal front is concentric whereas the Peavy metamorphic node located southof the thermal front is elongate east-west. It is tempting to speculate that the Penokean isograds of thePeavy metamorphic node have been shortened north-south and may therefore be yet another structuralmanifestation of the 1630 Ma deformational event. If our interpretations are correct, then the similarorientation of post-Penokean folds and Penokean-age folds (north of the front) requires that caution beused when attributing basement structures south of the front to Penokean deformation.

The low-grade —1630 Ma metamorphism has been one of the most poorly understood events in theLake Superior region. Although it has long been speculated upon that the quartzites may have beendeformed during this event (Dott, 1983; Van Schmus et al., 1993), until now there has been no directevidence of any intrusive or deformational event of that age. We believe that the data summarized hereprovides the missing structural link to that event. The timing and the strong approximately north-southshortening style of post-Penokean deformation together are consistent with it being the result of forelanddeformation associated with emplacement of the outer tectonic belt onto the southernmost margin ofLaurentia during the Mazatzal orogeny (Van Schmus et al., 1993). The rocks of the TranscontinentalProterozoic provinces were subjected to a major magmatic event during the Middle Proterozoic from1500-1300 Ma. The undeformed Wolf River batholith in central Wisconsin is surrounded by and locallyintrudes the deformed quartzite bodies, leading some to suggest that quartzite deformation was caused byMiddle Proterozoic epeirogenic doming and igneous intrusion (Greenberg and Brown, 1984). However,the existence of the Flambeau deformational front (located over 100 km from exposures of the batholith)shows that the deformation does not wane away from the batholith. Rather, the abrupt nature of the frontis characteristic of tectonic, not intrusion related, deformation. This study supports Dott's (1983) modelwhich attributed post-Penokean deformation in Wisconsin to Early Proterozoic plate collision from thesouth and supports the hypothesis of Van Schmus et al. (1993) that the 1700-1600 Ma outer tectonic beltwas a single coherent belt extending from California to Labrador.

References will be made available at the meeting or upon request (dholm@kent.edu).

74

quartzite lies just southwest of Penokean internal zone rocks which cooled rapidly through mica closure temperatures at -1750 Ma. In northwest Wisconsin, the Ban-on quartzite is essentially flat-lying and is depositional on Precambrian basement with mica ages between 1730-1700 Ma. South of the thermal front, and only 25 km south of the Barron quartzite, are exposures of steeply-dipping Flambeau quartzite. Here the quartzite is folded into a moderately west-plunging synform (Myers, 1974) similar to the style of deformation exhibited by the Baraboo and McCaslin quartzites. The relatively undeformed Barron and Sioux Proterozoic quartzites must either lie outside the region of significant post-Penokean deformation (as seems suggested by the spatial coincidence of a thermal front with a deformational front) or be younger quartzite packages deposited after significant post-Penokean deformation. To test whether the subhorizontal quartzites are correlatable with the deformed quartzites we obtained single-grain, single- spot 2 0 7 ~ b / 2 0 6 ~ b ages on detrital zircons separated from the Sioux, Barron, and Flambeau quartzites. Because we are mostly interested in constraining the maximum age of each quartzite we concentrated our efforts on dating euhedral or subhedral crystals wherever possible. All three quartzite bodies yielded Early Proterozoic and Late Archean detrital zircon dates comparable to dates obtained by Van Wyck (1995) for deformed quartzite bodies in Wisconsin. Many of the late Early Proterozoic (<2000 Ma) dates fall along a discordia with a lower intercept at or near the origin indicating recent lead loss attributed to upper crustal fluid circulation. Reliable 207Pb/2w~b Proterozoic ages are between 1730 and 1850 Ma for the Sioux (9 grains), between 1714 and 1880 Ma for the Flambeau (9 grains), and between 1750 and 1880 Ma for the Barron (6 grains). These data attest to the fact that all three quartzite bodies post-date the 1760 Ma magmatic event in the Lake Superior region. Thus far there is no evidence to suggest that the Ban-on and Sioux quartzites are younger than the deformed quartzites found throughout most of Wisconsin.

Abundant new thermochronologic data in the Lake Superior region allow us to make a simple but important first-order observation that provides the first direct structural evidence that the quartzites were deformed during the low-grade -1630 Ma event in the Lake Superior region. We note that subhorizontal post-Penokean quartzites consistently overlie crystalline basement with primary post-Penokean cooling ages, whereas highly deformed quartzites everywhere overlie crystalline basement with secondary (thermally reset) cooling ages. We are fortunate that the proximity of the deformed Flambeau quartzite to the subhorizontal Barron quartzite in northwest Wisconsin allows us to precisely locate this deformationalhhermal front. An age of 1650-1630 Ma for the deformation seems reasonable given that cooling ages south of the front probably post-date the deformation somewhat.

The thermal front extends eastward into northern Michigan, the geology of which is dominated by gneiss domes and classic nodal metamorphic isograds. No Early Proterozoic post-Penokean quartzites are known north of the McCaslin quartzite. However, it's interesting to note that the Republic metamorphic node located north of the thermal front is concentric whereas the Peavy metamorphic node located south of the thermal front is elongate east-west. It is tempting to speculate that the Penokean isograds of the Peavy metamorphic node have been shortened north-south and may therefore be yet another structural manifestation of the 1630 Ma deformational event. If our interpretations are correct, then the similar orientation of post-Penokean folds and Penokean-age folds (north of the front) requires that caution be used when attributing basement structures south of the front to Penokean deformation.

The low-grade -1630 Ma metamorphism has been one of the most poorly understood events in the Lake Superior region. Although it has long been speculated upon that the quartzites may have been deformed during this event (Dott, 1983; Van Schmus et al., 1993), until now there has been no direct evidence of any intrusive or deformational event of that age. We believe that the data summarized here provides the missing structural link to that event. The timing and the strong approximately north-south shortening style of post-Penokean deformation together are consistent with it being the result of foreland deformation associated with emplacement of the outer tectonic belt onto the southernmost margin of Laurentia during the Mazatzal orogeny (Van Schmus et al., 1993). The rocks of the Transcontinental Proterozoic provinces were subjected to a major magmatic event during the Middle Proterozoic from 1500-1300 Ma. The undeformed Wolf River batholith in central Wisconsin is surrounded by and locally intrudes the deformed quartzite bodies, leading some to suggest that quartzite deformation was caused by Middle Proterozoic epeirogenic doming and igneous intrusion (Greenberg and Brown, 1984). However, the existence of the Rambeau deformational front (located over 100 km from exposures of the batholith) shows that the deformation does not wane away from the batholith. Rather, the abrupt nature of the front is characteristic of tectonic, not intrusion related, deformation. This study supports Dott's (1983) model which attributed post-Penokean deformation in Wisconsin to Early Proterozoic plate collision from the south and supports the hypothesis of Van Schmus et al. (1993) that the 1700- 1600 Ma outer tectonic belt was a single coherent belt extending from California to Labrador.

References will be made available at the meeting or upon request (dholm@kent.edu).

THE RECOGNITION OF A LAVA DOME COMPLEX AND ITS RELATIONSHIPTO THE ARCHEAN STURGEON LAKE CALDERA, NORTHWESTERN ONTARIO

GEORGE J. HUDAK AND RONALD L. MORTONEconomic Volcanology Research Lab, Geology Department, University of Minnesota — Duluth,

Duluth, Minnesota, USA 55812

JAMES M. FRANKLINGeological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada K1A 0E8

Detailed volcanic facies mapping has led to the recognition of the Archean SturgeonLake Caldera Complex in northwestern Ontario (Morton et al., 1991). This complex is up to25 kilometers in strike length, contains up to 4500 meters of caldera fill material, and hostssix known volcanogenic massive sulfide (VMS) orebodies and numerous subeconomicmassive sulfide lenses. The volcanic rocks within the complex have been divided into tendistinct stratigraphic successions based on the types of volcanic and sedimentary rockspresent (Hudak, 1996).

The Lyon Creek Succession comprises the uppermost of these successions. Historically,these rocks were called the NBU rhyolite (Harvey and Hinzer, 1981; Severin, 1982), andwere interpreted as rhyolite tuffs, lapilli tuffs, agglomerates, and graphite-rich sediments.However, our detailed field mapping, core logging, and petrographic investigations indicatethat little or none of these rocks are pyroclastic in origin. Instead, this succession has beeninterpreted to represent a subaqueous lava dome complex.

A plagioclase-phyric dacite lava dome that is 2-3 km in strike length and up to 540meters thick forms the base of the succession. This dome occurs within the caldera edificeapproximately 1-2 km west of a major caldera margin fault. An absence of flow contacts,and an increase in the size of feldspar phenocrysts from the margins toward the core of thedome, suggests that the dome's growth was endogenous. A series of predominantly dome-derived clastic sediments overlies the dome. These include: a) matrix-supported brecciadeposits which contain up to 75% dome fragments; b) arkosic greywacke deposits; c) very-fine grained tuffaceous sandstone and siltstone deposits; and d) graphite-rich shale deposits.The clastic sediments lack an abundance of pumice (rarely >3%) and cross-bedding is absent.Silica-, carbonate-, oxide-, and/or sulfide-facies iron formations and associated chert areinterlayered with the clastic sediments and, locally, directly overlie the lava dome.

Detailed facies mapping indicates that the breccia deposits, the graphite-rich shales, andthe iron formation were formed in restricted, fault-bounded basins within the top of thedome. The iron formation resulted from relatively low temperature hydrothermal activity thatoccurred proximal to the basin marginal faults. The lack of pumice, the absence of evidencesuggestive of reworking by surface currents (e.g. cross-bedding), and the presence ofgraphitic shales suggests a relatively deep water (>500 meters), anoxic environment,although no absolute water depth estimate is possible.

Detailed studies on felsic volcanic centers indicate that the final two stages of calderadevelopment involve major ring fracture volcanism and terminal hot spring and fumarolicactivity (Smith and Bailey, 1968). The strata comprising the Lyon Creek Successionrepresent these final two evolutionary stages in the development of the Sturgeon LakeCaldera Complex.

75

THE RECOGNITION OF A LAVA DOME COMPLEX AND ITS RELATIONSHIP TO THE ARCHEAN STURGEON LAKE CALDERA, NORTHWESTERN ONTARIO

GEORGE J. HUDAK AND RONALD L. MORTON Economic Volcanology Research Lab, Geology Department, University of Minnesota - Duluth,

Duluth, Minnesota, USA 55812

JAMES M. FRANKLIN Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada KIA OE8

Detailed volcanic facies mapping has led to the recognition of the Archean Sturgeon Lake Caldera Complex in northwestern Ontario (Morton et al., 1991). This complex is up to 25 kilometers in strike length, contains up to 4500 meters of caldera fill material, and hosts six known volcanogenic massive sulfide (VMS) orebodies and numerous subeconomic massive sulfide lenses. The volcanic rocks within the complex have been divided into ten distinct stratigraphic successions based on the types of volcanic and sedimentary rocks present (Hudak, 1996).

The Lyon Creek Succession comprises the uppermost of these successions. Historically, these rocks were called the NBU rhyolite (Harvey and Hinzer, 198 1; Severin, 1982), and were interpreted as rhyolite tuffs, lapilli tuffs, agglomerates, and graphite-rich sediments. However, our detailed field mapping, core logging, and petrographic investigations indicate that little or none of these rocks are pyroclastic in origin. Instead, this succession has been interpreted to represent a subaqueous lava dome complex.

A plagioclase-phyric dacite lava dome that is 2-3 krn in strike length and up to 540 meters thick forms the base of the succession. This dome occurs within the caldera edifice approximately 1-2 krn west of a major caldera margin fault. An absence of flow contacts, and an increase in the size of feldspar phenocrysts from the margins toward the core of the dome, suggests that the dome's growth was endogenous. A series of predominantly dome- derived clastic sediments overlies the dome. These include: a) matrix-supported breccia deposits which contain up to 75% dome fragments; b) arkosic greywacke deposits; c) very- fine grained tuffaceous sandstone and siltstone deposits; and d) graphite-rich shale deposits. The clastic sediments lack an abundance of pumice (rarely >3%) and cross-bedding is absent. Silica-, carbonate-, oxide-, and/or sulfide-facies iron formations and associated chert are interlayered with the clastic sediments and, locally, directly overlie the lava dome.

Detailed facies mapping indicates that the breccia deposits, the graphite-rich shales, and the iron formation were formed in restricted, fault-bounded basins within the top of the dome. The iron formation resulted from relatively low temperature hydrothermal activity that occurred proximal to the basin marginal faults. The lack of pumice, the absence of evidence suggestive of reworking by surface currents (e.g. cross-bedding), and the presence of graphitic shales suggests a relatively deep water (>500 meters), anoxic environment, although no absolute water depth estimate is possible.

Detailed studies on felsic volcanic centers indicate that the final two stages of caldera development involve major ring fracture volcanism and terminal hot spring and h a r o l i c activity (Smith and Bailey, 1968). The strata comprising the Lyon Creek Succession represent these final two evolutionary stages in the development of the Sturgeon Lake Caldera Complex.

References

Harvey, J. D., and Hinzer, J. B., 1981, Geology of the Lyon Lake ore deposits,Noranda Mines Ltd., Sturgeon Lake, Ontario: Canadian Institute of Mining andMetallurgy Bulletin, v. 74, no. 833, p. 77-84.

Hudak, G. J., 1996, The physical volcanology and hydrothermal alterationassociated with late caldera volcanic and volcaniclastic rocks and volcanogenic massivesulfide deposits in the Sturgeon Lake region of northwestern Ontario, Canada:unpublished Ph. D. dissertation, University of Minnesota, Minneapolis, MN, 463 pages.

Morton, R. L., Walker, J. S., Hudak, G. J., and Franklin, J. M., 1991, The earlydevelopment of an Archean submarine caldera complex with emphasis on the Mattabi ashflow tuff and its relationship to the Mattabi massive sulfide deposit: Economic Geology,v. 86,p. 1002-1011.

Severin, P. W. A., 1981, Geology of the Sturgeon Lake Cu-Zn-Pb-Ag-Au deposit:Canadian Institute of Mining and Metallurgy Bulletin, v. 75, no. 846, p. 107-123.

Smith, R. L., and Bailey, R. A., 1968, Resurgent Cauldrons: Geological Society ofAmerica Memoir 116, p. 6 13-662.

76

References

Harvey, J. D., and Hinzer, J. B., 1981, Geology of the Lyon Lake ore deposits, Noranda Mines Ltd., Sturgeon Lake, Ontario: Canadian Institute of Mining and Metallurgy Bulletin, v. 74, no. 833, p. 77-84.

Hudak, G. J., 1996, The physical volcanology and hydrothermal alteration associated with late caldera volcanic and volcaniclastic rocks and volcanogenic massive sulfide deposits in the Sturgeon Lake region of northwestern Ontario, Canada: unpublished Ph. D. dissertation, University of Minnesota, Minneapolis, MN, 463 pages.

Morton, R. L., Walker, J. S., Hudak, G. J., and Franklin, J. M., 1991, The early development of an Archean submarine caldera complex with emphasis on the Mattabi ash flow tuff and its relationship to the Mattabi massive sulfide deposit: Economic Geology, v. 86, p. 1002-101 1.

Severin, P. W. A., 198 1, Geology of the Sturgeon Lake Cu-Zn-Pb-Ag-Au deposit: Canadian Institute of Mining and Metallurgy Bulletin, v. 75, no. 846, p. 107-123.

Smith, R. L., and Bailey, R. A., 1968, Resurgent Cauldrons: Geological Society of America Memoir 1 16, p. 61 3-662.

GEOLOGIC SETTING OF SUBECONOMIC GOLD DEPOSITS IN ThE VIRGINIA HORN,NORTHEASTERN MINNESOTA: A HORN OF PLENTY OR A HORNSWOGGLE?

JIRSA, Mark A., BOERBOOM, Terrence J., and CHANDLER, Val W.—Minnesota GeologicalSurvey (MGS), 2642 University Avenue, St. Paul, MN, 55114-1057 (jirsa001@tc.umn.edu)

Several significant—though, to date subeconomic—gold deposits occur within Archean bedrock in thearea known as the Virginia Horn. Three prospects were worked to varying degrees by explorationcompanies in the late 1980's, and one of these, the "Viking prospect", was extensively drilled. Theexploration focused on pervasively altered felsic porphyry intrusions having variably well developeddeformation envelopes and associated carbonate-sericite alteration. Recent mapping by the MGS (Jirsaand others, 1998), together with geochemical work by the Natural Resources Research Institute(Englebert and Hauck, 1991; and work in progress) has provided further information on the lithologicaland structural framework, potential sources of Au, and relative timing of mineralization in theseprospects and the surrounding area.

Figure 1—Schematic block diagram of the Virginia Horn. Map surface dimensions approximately30X30 km.

The Archean rocks of the Virginia Horn lie within the Wawa subprovince of Superior Province.The rocks are subdivided into northern and southern panels on the basis of metamorphic grade anddeformation style. The northern panel, immediately south of the Giants Range batholith, containsintensely lineated, amphibolite-grade schists having volcanic and clastic protoliths. The southernpanel contains lithologically similar rocks that were metamorphosed to much lower grades, rangingfrom prehnite-pumpellyite to low greenschist. The two panels are separated by the east-trending,post-metamorphic, Laurentian fault (Figure 1). The metamorphic cleavage-forming event in bothpanels was the second (1)2) of 3 major deformations—the other two deformation events produced nodiscernible metamorphic affects. The first (Di) involved upright folding, soft-sediment deformation,and complex faulting; the third (D3) produced localized semi-brittle crenulation of D1 and D2structures, brittle fractures, and selective reactivation of earlier-formed faults. Amphibolite graderocks north of the Laurentian fault comprise the Minntac sequence; the low and sub-greenschist grade

77

MINNTACSEQUENCE\-

'I''''''I,,,,,,,,, f/f,,% , .. , .' .' \ \ '. 'S,,,,,,,,,,, , ,'S S S 'S

, 1S.''', / / /1/ / , /1•5%.'.'\\'SI� / � / / / / / /1'''''S.''''''''''S'S'S''''

'S 'S 'S 'S'S.' 'S 'Ss 'S 'S'''''''S'S''',,/,//,/,,, ,'S'S5'S'S'S'S'S'''''S''''S'S'S''''"'S/,

MUD LAKE SEQUENCEGraywacke (stippled) and volcanic rocks (white)

GEOLOGIC SETTING OF SUBECONOMIC GOLD DEPOSITS IN THE VIRGINIA HORN, NORTHEASTERN MINNESOTA. A HORN OF PLENTY OR A HORNSWOGGLE?

JIRSA, Mark A., BOERBOOM, Terrence J., and CHANDLER, Val W.-Minnesota Geological Survey (MGS), 2642 University Avenue, St. Paul, MN, 55114-1057 (nrsaOOl@tc.urnn.edu)

Several significant-though, to date subeconomic-gold deposits occur within Archean bedrock in the area known as the Virginia Horn. Three prospects were worked to varying degrees by exploration companies in the late 1980'~~ and one of these, the "Viking prospect", was extensively drilled. The exploration focused on pervasively altered felsic porphyry intrusions having variably well developed deformation envelopes and associated carbonate-sericite alteration. Recent mapping by the MGS (Jirsa and others, 1998), together with geochemical work by the Natural Resources Research Institute (Englebert and Hauck, 1991; and work in progress) has provided further information on the lithological and structural framework, potential sources of Au, and relative timing of mineralization in these prospects and the surrounding area.

Ã

Figure 1-Schematic block diagram of the Virginia Horn. Map surface dimensions approximately 30x30 km.

The Archean rocks of the Virginia Horn lie within the Wawa subprovince of Superior Province. The rocks are subdivided into northern and southern panels on the basis of metamorphic grade and deformation style. The northern panel, immediately south of the Giants Range batholith, contains intensely heated, amphibolite-grade schists having volcanic and clastic protoliths. The southern panel contains lithologically similar rocks that were metamorphosed to much lower grades, ranging from prehnite-pumpellyite to low greenschist. The two panels are separated by the east-trending, post-metamorphic, Laurentian fault (Figure 1). The metamorphic cleavage-forming event in both panels was the second (D2) of 3 major deformations-the other two deformation events produced no discernible metamorphic affects. The first (Dl) involved upright folding, soft-sediment deformation, and complex faulting; the third (D3) produced localized semi-brittle crenulation of Dl and D2 structures, brittle fractures, and selective reactivation of earlier-formed faults. Amphibolite grade rocks north of the Laurentian fault comprise the Minntac sequence; the low and sub-greenschist grade

strata south of the Laurentian fault are subdivided into Mud Lake and Midway sequences. The Minntacsequence contains strongly banded schists having geochemical and outcrop-scale characteristics of maficto intermediate volcanic and volcaniclastic strata, subvolcanic mafic sills, and graywacke. Althoughthe possibility of tight folding is great in rocks of the Minntac sequence, relict grading and beddingconsistently indicate southward stratigraphic facing. In contrast, the Mud Lake sequence forms a broad,southwest-plunging, D1 syncline that is cored by graywacke, slate, and minor felsic tuff; and has outerlimbs of calc-alkalic and tholeiitic strata. The Mud Lake strata are cut by several variablyporphyritic, quartzofeldspathic intrusions. The Mud Lake sequence and the intrusions areunconformably overlain by, and are locally in fault contact with, fluvial conglomerate and subaeriallydeposited trachyandesite flows and pyroclastic rocks that comprise the Midway sequence. TheMidway has many attributes of Timiskaming -like strata in the Kirkland Lake and Shebandowan goldmining districts of Ontario. Like the Timiskaming rocks of Ontario, those in the Virginia Horn areinferred to have been deposited in a fault-bounded pull-apart basin that formed before the onset of 1)2deformation and metamorphism.

The Virginia Horn has a long history of gold 'shows', dating back to the days of J.W. Gruner andF.F. Grout (Grout, 1937), and some visible gold can still be found locally in altered rocks in and adjacentto quartz veins. Sampling by exploration companies focused largely on the quartzofeldspathicintrusions, and the country rocks were rarely analyzed. From those data and the current study, thefollowing generalizations can be made about the distribution of gold in the region:

1. Gold is most abundant in quartzofeldspathic dikes.2. The greatest gold contents (as large as 50,000 ppb) occur in rocks that are pervasively altered and

cut by quartz-rich veins.3. Anomalous quantities of gold (50- 500 ppb) also exist in sedimentary and volcanic wall-rocks of

the porphyritic intrusions.4. Gold concentrations greater than a "mineable" cut-off of about 1000 ppb (0.029 OPT) are recorded

exclusively from samples of quartzofeldspathic dikes.5. In rocks uniformly affected by carbonate-sericite alteration, gold is most abundant in zones of

anomalous arsenic content.The carbonate-sericite alteration associated with gold mineralization varies from pervasive and

not obviously related to the rock fabric, to having textures that imply strong involvement in shearing.Pyrite typically is associated with carbonate and sericite; and arsenopyrite and chalcopyrite occurlocally. Alteration is inferred to have taken place during or just after D2 deformation, as thealteration products are variably affected by S2. That alteration is best developed along major faultzones, lithologic contacts, and adjacent to and within the quartzofeldspathic intrusions. Although thesurface and shallow subsurface have been evaluated in some detail, comparisons with analog depositsin Canada imply that unexplored potential exists; 1) at depth in the low-grade rocks of the Midwayand Mud Lake sequences; and 2) throughout the high-grade Mirintac sequence.

ACKNOWLEDGMENTSMapping and geochemical study was supported by the Minnesota Legislature on recommendation of

the Minerals Coordinating Committee.

REFERENCESEnglebert, J.A., and Hauck, S.A., 1991, Bedrock geochemistry of Archean rocks in northern Minnesota: Natural

Resources Research Institute Technical Report NRRI/TR-91/12, 200 p.

Grout, F.F., 1937, Petrographic study of gold prospects of Minnesota: Economic Geology, v.37, P. 56-68.

Jirsa, M.A., Boerboom, T.J., and Morey, G.B., 1998, Bedrock geologic map of the Virginia Horn, Mesabi Iron Range, St.Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-85 (digitial), scale 1:48,000.

78

strata south of the Laurentian fault are subdivided into Mud Lake and Midway sequences. The Minntac sequence contains strongly banded schists having geochemical and outcrop-scale characteristics of mafic to intermediate volcanic and volcaniclastic strata, subvolcanic mafic sills, and graywacke. Although the possibility of tight folding is great in rocks of the Minntac sequence, relict grading and bedding consistently indicate southward stratigraphic facing. In contrast, the Mud Lake sequence forms a broad, southwest-plunging, Dl syncline that is cored by graywacke, slate, and minor felsic tuff; and has outer limbs of calc-alkalic and tholeiitic strata. The Mud Lake strata are cut by several variably porphyritic, quartzofeldspathic intrusions. The Mud Lake sequence and the intrusions are unconformably overlain by, and are locally in fault contact with, fluvial conglomerate and subaerially deposited trachyandesite flows and pyroclastic rocks that comprise the Midway seauence. The Midway has many attributes of Tirniskaming -like strata in the Kirkland Lake and Shebandowan gold mining districts of Ontario. Like the Timiskaming rocks of Ontario, those in the Virginia Horn are inferred to have been deposited in a fault-bounded pull-apart basin that formed before the onset of D2 deformation and metamorphism.

The Virginia Horn has a long history of gold "shows", dating back to the days of J.W. Gruner and F.F. Grout (Grout, 1937), and some visible gold can still be found locally in altered rocks in and adjacent to quartz veins. Sampling by exploration companies focused largely on the quartzofeldspathic intrusions, and the country rocks were rarely analyzed. From those data and the current study, the following generalizations can be made about the distribution of gold in the region:

1. Gold is most abundant in quartzofeldspathic dikes. 2. The greatest gold contents (as large as 50,000 ppb) occur in rocks that are pervasively altered and

cut by quartz-rich veins. 3. Anomalous quantities of gold (50- 500 ppb) also exist in sedimentary and volcanic wall-rocks of

the porphyritic intrusions. 4. Gold concentrations greater than a "mineable" cut-off of about 1000 ppb (0.029 OPT) are recorded

exclusively from samples of quartzofeldspathic dikes. 5. In rocks uniformly affected by carbonate-sericite alteration, gold is most abundant in zones of

anomalous arsenic content. The carbonate-sericite alteration associated with gold mineralization varies from pervasive and

not obviously related to the rock fabric, to having textures that imply strong involvement in shearing. Pyrite typically is associated with carbonate and sericite; and arsenopyrite and chalcopyrite occur locally. Alteration is inferred to have taken place during or just after D2 deformation, as the alteration products are variably affected by 82. That alteration is best developed along major fault zones, lithologic contacts, and adjacent to and within the quartzofeldspathic intrusions. Although the surface and shallow subsurface have been evaluated in some detail, comparisons with analog deposits in Canada imply that unexplored potential exists; 1) at depth in the low-grade rocks of the Midway and Mud Lake sequences; and 2) throughout the high-grade Minntac sequence.

ACKNOWLEDGMENTS Mapping and geochemical study was supported by the Minnesota Legislature on recommendation of

the Minerals Coordinating Committee.

REFERENCES Englebert, J.A., and Hauck, S.A., 1991, Bedrock geochemis of Archean rocks in northern Minnesota: Natural

Resources Research Institute Technical Report NRRI 7 TR-91/12,200 p.

Grout, F.F., 1937, Petrographic study of gold prospects of Minnesota: Economic Geology, v.37, p. 56-68.

Jirsa, M.A., Boerboom, T.J., and Morey, G.B., 1998, Bedrock geolo 'c map of the Virginia Horn, Mesabi Iron Ran e, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous Map M-85 (digitial), scale 1:48,0%.

EFFECT OF ELECTRIC PULSE DISAGGREGATION ON MICROFOSSIL-BEARINGARGILLACEOUS LIMESTONE OF THE MIDDLE ORDOVICIAN DECORAH SHALE

KJERLAND, Dean W., and KJERLAND, Marc P., amateur paleontologists,8007 Xerxes Ave S, Bloomington, MN 55431 E—mail: 73563,3520@compuserv.com

The Middle Ordovician Decorah Shale is a fossiliferous shale with lenses and thin beds ofcoquinoidal limestone and calcareous shale (Rice). The calcitic macrofossils and phosphaticconodont microfossils have been studied (Sloan, Webers). Though known from acidprocessing residues, other non—calcitic microfossils have not been documented.

We collected over 250 typical hand—sized samples of limestone from Decorah Shale bedsdisturbed during the reconstruction of Hwys 110, 55 & 13 at Mendota, Minnesota in 1993.Our selection criteria was the presence of phosphatic microfossils or fragments visible witha hand lens on either or both surfaces. Our photographic documentation of 200 of thesesurface microfossils using optical microscopy reveals a wide variety of microfossils includinglithified mud internal casts of bryozoans (Cuffey), laminated plates, and gastropod steinkerns.

Separation of microfossils from the matrix is necessary for more efficient collection anddocumentation. Full viewing of specimens is useful for classification. Mechanical crushingis not an effective method for cleanly separating either macrofossils or microfossils frommatrix. Acid processing yields clean microfossil specimens but artificially selects for eithercalcitic or phosphatic material. Both Ca and P, as well as Fe, 5, Al, Si, and Mg, have beenconfinned from our S.E.M. analyses of a sample of several dozen typical microfossils.

Saini—Eidukat and Weiblen reported the use of an Electric Pulse Disaggregator (EPD) forfossil extraction (see References). In our study we evaluated the potential of the EPD forseparating visible microfossils from the surface 'fossil hash' of Decorah Shale limestone andalso for sampling for microfossils buried in the matrix of this rock. We used the specialized,upgraded laboratory—scale version EPD at the University of Minnesota which is based on theelectric—pulse facility in St. Petersburg, Russia (Saini—Eidukat and Weiblen).

A lenticular slab fragment, 16 cm long x 12.5 cm wide weighing about 1.2 kg, was quarteredwith a rock hammer and further fragmented/delaminated with a cold chisel to producewalnut—sized fragments for electric pulse disaggregation. The rock consisted of non—weathered, compact, dark blue—gray crystalline calcite with typical cemented calciticmacrofossil fragment debris and gray interstitial surface clay on both sides and phosphaticmicrofossils and fragments visible on only one surface.

The slab pieces were hand fed in two batches into the hollow cathode of the EPD over a 6mm integral sieve and subjected to approximately 200 pulses at approximately 100 kV. Allmaterial except the suspended fines was air dried and mechanically sorted through brasssieves (Table 1.) The material which passed through the 0.25 mm sieve was processed drythrough a series of teflon meshes (Table 2.).

The aggregate produced by the EPD is fragmented crystalline calcite and yellow clay andmud—sized fines. Only a small fraction consists of whole calcite fossils or fragments andnon—calcitic microfossils and fragments (either matrix—free or partly enclosed in calcite matrix).

79

EFFECI' OF ELECI'RIC PULSE DISAGGREGATION ON MICROFOSSIL-BEARING ARGILLACEOUS LIMESTOM OF THE MIDDLE ORDOVICIAN DECORAH SHALE

KJERLAND, Dean W., and KJERLAND, Marc P., amateur paleontologists, 8007 Xerxes Ave S, Bloomington, MN 55431 E-mail: 73563,352O@compuse~.com

The Middle Ordovician Decorah Shale is a fossiliferous shale with lenses and thin beds of coquinoidal limestone and calcareous shale (Rice). The calcitic macrofossils and phosphatic conodont microfossils have been studied (Sloan, Webers). Though known from acid processing residues, other non-calcitic microfossils have not been documented.

We collected over 250 typical hand-sized samples of limestone from Decorah Shale beds disturbed during the reconstruction of Hwys 110, 55 & 13 at Mendota, Minnesota in 1993. Our selection criteria was the presence of phosphatic microfossils or fragments visible with a hand lens on either or both surfaces, Our photographic documentation of 200 of these surface microfossils using optical microscopy reveals a wide variety of microfossils including lithified mud internal casts of bryozoans (Cuffey), laminated plates, and gastropod steinkerns.

Separation of microfossils from the matrix is necessary for more efficient collection and documentation. Full viewing of specimens is useful for classification. Mechanical crushing is not an effective method for cleanly separating either macrofossils or microfossils from matrix. Acid processing yields clean microfossil specimens but artificially selects for either calcitic or phosphatic material. Both Ca and P, as well as Fe, S, A, Si, and Mg, have been confirmed from our S.E.M. analyses of a sample of several dozen typical microfossils.

Saini-Eidukat and Weiblen reported the use of an Electric Pulse Disaggregator (EPD) for fossil extraction (see References). In our study we evaluated the potential of the EPD for separating visible microfossils from the surface 'fossil hash' of Decorah Shale limestone and also for sampling for microfossils buried in the matrix of this rock. We used the specialized, upgraded laboratory-scale version EPD at the University of Minnesota which is based on the electric-pulse facility in St. Petersburg, Russia (Saini-Eidukat and Weiblen).

A lenticular slab fragment, 16 cm long x 12.5 cm wide weighing about 1.2 kg, was quartered with a rock hammer and further fragmentedldelaminated with a cold chisel to produce walnut-sized fragments for electric pulse disaggregation. The rock consisted of non- weathered, compact, dark blue-gray crystalline calcite with typical cemented calcitic macrofossil fragment debris and gray interstitial surface clay on both sides and phosphatic microfossils and fragments visible on only one surface.

The slab pieces were hand fed in two batches into the hollow cathode of the EPD over a 6 mm integral sieve and subjected to approximately 200 pulses at approximately 100 kV. All material except the suspended fines was air dried and mechanically sorted through brass sieves (Table 1.) The material which passed through the 0.25 mm sieve was processed dry through a series of teflon meshes (Table 2.).

The aggregate produced by the EPD is fragmented crystalline calcite and yellow clay and mud-sized fines. Only a small fraction consists of whole calcite fossils or fragments and non-calciticmicrofossils and fragments (either matrix-free or partly enclosed in calcite matrix).

Table 1: Grading criteria and results: Table 2. Grading of Class VIII Material

Class Sieve(in mm) Weight(in g) and % Note: The material which passed through theI >6.0 31.2 2.5% 0.25 mm brass sieve was re—graded usingII <6.0—>4.0 297.8 24.0% teflon meshes. Results are from top of mesh.III <4.0—>2.0 397.3 32.1%IV <2.0—> 1.0 207.6 16.8% Class Mesh# (mm) Weight(m g) and %V <1.0—>.701 65.6 5.3% VIlla #60 (>.250) 6.4 5.0%VI <.701—>.495 46.7 3.8% VIIIb #80 (>.177) 15.9 12.3%;VII <.495—>.250 62.9 5.1% VIlic #100(>.149) 13.0 10.1%;VIII <.250 129.1 10.4% VIlId thru #100 93.8 72.6%.

Total 1238.2 100.0% Total 129.1 100.0%

The Class VIIIa—d materials from the teflon meshes were not examined at this time. TheClass I — VII materials were examined with an optical microscope and sampled for a). non—fossil crystalline calcite, b). fossil—bearing calcite, and c). non—calcitic fossils, fragments andunknowns. Based on subjective criteria (i.e. typical, interesting, unusual or trophy categories)654 fossils, fragments and unknowns were collected. Specimen counts are: 11.8% calciticfossils, 9.8% phosphatic fossils in crystalline calcite, 74.9% loose phosphatic fossils andfragments, and 3.5% unknown. Virtually all of the matrix—free non—calcitic fossils andfragments are from that portion of the aggregate (31%) recovered from the top of the sievesin the 2.0 mm — .250 mm sieve set (Class IV—VII).

Conclusion: The EPD provides a new method for disaggregating rocks for isolatingmicrofossils in the size range from 2.0 mm to 0.25 mm. EPD processing effectively separatesphosphatic, calcitic, and pyritic microfossils from their calcitic matrix and allows efficienttesting of rocks for the presence of buried specimens. Although some specimens may breakin processing or during sample preparation, many fossils preserved in shale are commonlyfractured naturally in the matrix. We believe that EPD processing simply separates alreadydamaged fragments or whole fossils along existing fractures.

References:

Cuffey, Roger J., Professor of Paleontology, Department of Geosciences, Pennsylvania StateUniversity; personal correspondence 1997 and 1998.

Rice, William F.; "The Systematics and Biostratigraphy of the Brachiopoda of the DecorahShale at St.Paul, Minnesota", 1985, Unpublished M.S. thesis, University of Minnesota, Mpls.

Saini—Eidukat, B., and Weiblen, P.W., 1996; "A New Method of Fossil Preparation, UsingHigh—Voltage Pulses", Curator 39/2, and references therein.

Sloan, Robert E, editor, Middle and Late Ordovician Lithostratigraphy and Biostratigraphyof the Upper Mississippi Valley, Report of Investigations #35, Minnesota Geological Survey,University of Minnesota, 1987.

Webers, Gerald F.; "The Middle and Upper Ordovician Conodont Faunas of Minnesota":Minnesota Geological Survey Special Publication Series SP—4, 1966

80

Table 1: Grading criteria and results:

Class I I1 I11 Iv v VI VII VIII

Sieve(in mm) Weight(in g) and % >6.0 31.2 2.5% ~6.0->4.0 297.8 24.0% ~4.0->2.0 397.3 32.1% c2.0->1.0 207.6 16.8% ~1.0->.701 65.6 5.3% <.701->.495 46.7 3.8% c.495->.250 62.9 5.1% e.250 129.1 10.4%

Total 1238.2 100.0%

Table 2. Grading of Class VIII Material

Note: The material which passed through the 0.25 mm brass sieve was re-graded using teflon meshes. Results are from top of mesh.

Class Mesh# (mm) Weight(in g) and % VIIIa #60 (>.250) 6.4 5.0% VIIIb #80 (>.I771 15.9 12.3%; VIIIc #100(>.149) 13.0 10.1%; VIIId thru #lo0 93.8 72.6%.

Total 129.1 100.0%

The Class VIIIa-d materials from the teflon meshes were not examined at this time. The Class I - VII materials were examined with an optical microscope and sampled for a). non- fossil crystalline calcite, b). fossil-bearing calcite, and c). non-calcitic fossils, fragments and unknowns. Based on subjective criteria (i.e. typical, interesting, unusual or trophy categories) 654 fossils, fragments and unknowns were collected. Specimen counts are: 11.8% calcitic fossils, 9.8% phosphatic fossils in crystalline calcite, 74.9% loose phosphatic fossils and fragments, and 3.5% unknown. Virtually all of the matrix-free non-calcitic fossils and fragments are from that portion of the aggregate (31%) recovered from the top of the sieves in the 2.0 mm - -250 mm sieve set (Class IV-VII).

Conclusion: The EPD provides a new method for disaggregating rocks for isolating microfossils in the size range from 2.0 mm to 0.25 mm. EPD processing effectively separates phosphatic, calcitic, and pyritic microfossils from their calcitic matrix and allows efficient testing of rocks for the presence of buried specimens. Although some specimens may break in processing or during sample preparation, many fossils preserved in shale are commonly fractured naturally in the matrix. We believe that EPD processing simply separates already damaged fragments or whole fossils along existing fractures.

References:

Cuffey, Roger J., Professor of Paleontology, Department of Geosciences, Pemsylvania State University; personal correspondence 1997 and 1998.

Rice, William F.; "The Systematics and Biostratigraphy of the Brachiopoda of the Decorah Shale at St.Pau1, Mimesota", 1985, Unpublished M.S. thesis, University of Mimesota, Mpls.

Saini-Eidukat, B., and Weiblen, P.W., 1996; "A New Method of Fossil Reparation, Using High-Voltage Pulses", Curator 3912, and references therein.

Sloan, Robert E, editor, Middle and Late Ordovician Lithostratigraphy and Biostratigraphy of the Upper Mississippi Valley, Report of Investigations #35, Mimesota Geological Survey, university of Mimesota, 1987.

Webers, Gerald F.; "The Middle and Upper Ordovician Conodont Faunas of Mimesotat': Mimesota Geological Survey Special Publication Series SP-4, 1966

RESULTS OF MODELLING PROTEROZOIC THERMAL HISTORIES:EVALUATING THE POSSIBLE EFFECTS OF WOLF RIVER BATHOLITHREHEATING ON THERMOCHRONOLOGIC DATA FROM NORTHERN WISCONSIN

Jeff Loofboro (student) and Daniel Hoim, Department of Geology, Kent State University,Kent, OH 44242

Introduction. Mica Rb/Sr and Ar/Ar thermochronologic results across northern Wisconsin andnorthern Michigan (Peterman and Sims, 1988, Tectonics; Schneider and others, 1996, CJES; Hoim andothers, 1997, GSAA) reveal a 1630-1600 Ma chrontour which separates basement rock with primarycooling ages of 1760-1750 Ma to the north from <1630 Ma ages to the south. The chrontour coincidesphysically with an apparent deformational front in overlying, Early Proterozoic, post-Penokean quartzitesleading Hoim and others (1998, ILSG; and in review) to interpret the chrontour to represent completethermal resetting of micas associated with foreland deformation related to accretion from the south.However, we note that the chrontour also appears to surround the known subsurface extent of the MiddleProterozoic Wolf River batholith (Fig. 1) raising the possibility that it might be an artifact of partialresetting. In this case, the 1630-1600 Ma chrontour may represent a collection of meaningless "mixed"dates resulting from partial resetting of the older primary 1760-1750 Ma dates at 1470 Ma when thebatholith intruded. Using the MacArgon program (Uster and Baldwin, 1996, Tectonophysics) we havemodelled various Proterozoic thermal histories in an attempt to evaluate the possible effects of MiddleProterozoic reheating on thermochronologic data from northern Wisconsin.

Initial conditions and model parameters. We initially consider a rock containing muscovite with aplateau Ar/Ar date of 1765 Ma and biotite with a plateau date of 1755 Ma. Precambrian basement rocknorth of the chrontour in northwest Wisconsin are near the Early Proterozoic nonconformity suggestingthese rocks were at shallow crustal depths by —1700 Ma. Considering this, we chose three differentambient temperatures (100°, 150°, and 200°) and imposed a thermal pulse at 1470 Ma to simulateintrusion of the batholith. We varied the peak temperature of the pulse between 200° and 450°C (using50°C increments) and the duration of heating from instantaneous to 2 my (using 0.5 my increments). Inall cases the duration of cooling back to ambient temperatures lasted 2 my and hence the total duration ofthe pulse varied in our models from 2-4 my. The duration of thermal effects of plutonism on country rockis normally shorter than this (Carslaw and Jaeger, 1959, Clarendon Press), but we chose such longdurations in order to maximize the effects of partial resetting by the batholith. We assume no otherthermal overprinting affects after intrusion of the Wolf River batholith, ending the thermal history withslow cooling from the chosen ambient temperature to 0°C at 600 Ma (Fig. 2). The variables in themodelling thus include the peak temperature obtained, the duration of the thermal pulse, and the initialambient temperature. The affect of each of these parameters on argon diffusion in muscovite and biotiteis described below.

Results of thermal modeling. As might be expected, the dominant factor influencing partial resettingis the peak temperature obtained by the rock during the imposed thermal pulse. Peak thermal pulses at orbelow the closure temperature of biotite (300°C) or muscovite (350°C) had little affect on the initialcooling age regardless of the duration of heating. Peak thermal pulses of 50°C above closure temperatureresulted in considerable partial resetting. In this case the duration of the heating interval did have amoderate affect on the degree of resetting with 2 my heating intervals resulting in total gas ages which are—50 my younger than in the case for instantaneous heating. Because of the difference in closuretemperature between biotite and muscovite, the modelling reveals that large differences in the degree ofpartial resetting (and hence apparent ages obtained) are expected for 1470 Ma peak thermal pulsesbetween 300 and 450°C. Temperatures 100°C above the mineral's nominal closure temperature resultedin nearly complete resetting of the isotopic systematics regardless of the duration of heating. Varying theinitial ambient temperature between 100-200°C had little affect on the apparent ages obtained.

Implications. Existing thermochronologic data from northern Wisconsin show nearly concordantmuscovite and biotite dates near the chrontour (with muscovite around 1620 Ma and biotite around 1600Ma; Romano and others, 1997, GSAA). We are unable to obtain nearly concordant partially reset ageswith any of our simulations and conclude that Middle Proterozoic intrusion of the Wolf River batholithwas probably not responsible for generating the 1630-1600 Ma chrontour by partial resetting. Thisconclusion is indirectly supported by two independent lines of evidence. First, the degree of deformationof the Early Proterozoic quartzites in Wisconsin does not wane away from the Wolf River batholith aswould be expected for intrusion related deformation. The sharp deformational front (which coincideswith the 1630-1600 Ma clirontour) is more characteristic of tectonic-related deformation. Second,

81

RESULTS OF MODELLING PROTEROZOIC THERMAL HISTORIES: EVALUATING THE POSSIBLE EFFECTS OF WOLF FUVER BATHOLITH =HEATING ON THEMOCHRONOLOGIC DATA FROM NORTHERN WISCONSIN

Jeff Loofboro (student) and Daniel Holm, Department of Geology, Kent State University, Kent, OH 44242

Introduction. Mica RblSr and ArlAr thermochronologic results across northern Wisconsin and northern Michigan (Peterman and Sims, 1988, Tectonics; Schneider and others, 1996, CJE!S; Holm and others, 1997, GSAA) reveal a 1630-1600 Ma chrontour which separates basement rock with primary cooling ages of 1760-1750 Ma to the north from 4 6 3 0 Ma ages to the south. The chrontour coincides physically with an apparent deformational front in overlying, Early Proterozoic, post-Penokean quartzites leading Holm and others (1998, ILSG; and in review) to interpret the chrontour to represent complete thermal resetting of micas associated with foreland deformation related to accretion from the south. However, we note that the chrontour also appears to surround the known subsurface extent of the Middle Proterozoic Wolf Mver batholith (Fig. 1) raising the possibility that it might be an artifact of partial resetting. In this case, the 1630-1600 Ma chrontour may represent a collection of meaningless ?nixed'' dates resulting from partial resetting of the older primary 1760-1750 Ma dates at 1470 Ma when the batholith intruded. Using the MacArgon program (Lister and Baldwin, 1996, Tectonophysics) we have modelled various Proterozoic thermal histories in an attempt to evaluate the possible effects of Middle Proterozoic reheating on thermochronologic data from northern Wisconsin.

Initial conditions and model parameters. We initially consider a rock containing muscovite with a plateau ArIAr date of 1765 Ma and biotite with a plateau date of 1755 Ma. Precambrian basement rock north of the chrontour in northwest Wisconsin are near the Early Proterozoic nonconformity suggesting these rocks were at shallow crustal depths by -1700 Ma. Considering this, we chose three different ambient temperatures (loo0, 150° and 200') and imposed a thermal pulse at 1470 Ma to simulate intrusion of the batholith. We varied the peak temperature of the pulse between 200O and 450° (using 50° increments) and the duration of heating from instantaneous to 2 my (using 0.5 my increments). In all cases the duration of cooling back to ambient temperatures lasted 2 my and hence the total duration of the pulse varied in our models from 2-4 my. The duration of thermal effects of plutonism on country rock is normally shorter than this (Carslaw and Jaeger, 1959, Clarendon Press), but we chose such long durations in order to maximize the effects of partial resetting by the batholith. We assume no other thermal overprinting affects after intrusion of the Wolf River batholith, ending the thermal history with slow cooling from the chosen ambient temperature to O° at 600 Ma (Fig. 2). The variables in the modelling thus include the peak temperature obtained, the duration of the thermal pulse, and the initial ambient temperature. The affect of each of these parameters on argon diffusion in muscovite and biotite is described below.

Results of thermal modeling. As might be expected, the dominant factor inlluencing partial resetting is the peak temperature obtained by the rock during the imposed thermal pulse. Peak thermal pulses at or below the closure temperature of biotite (3W°C or muscovite (350°C had little affect on the initial cooling age regardless of the duration of heating. Peak thermal pulses of 50° above closure temperature resulted in considerable partial resetting. In this case the duration of the heating interval did have a moderate affect on the degree of resetting with 2 my heating intervals resulting in total gas ages which are -50 my younger than in the case for instantaneous heating. Because of the difference in closure temperature between biotite and muscovite, the modelling reveals that large differences in the degree of partial resetting (and hence apparent ages obtained) are expected for 1470 Ma peak thermal pulses between 300 and 450°C Temperatures 100OC above the mineral's nominal closure temperature resulted in nearly complete resetting of the isotopic systematics regardless of the duration of heating. Varying the initial ambient temperature between 100-200OC had little affect on the apparent ages obtained.

Implications. Existing therm6chronologic data from northern Wisconsin show nearly concordant muscovite and biotite dates near the chrontour (with muscovite around 1620 Ma and biotite around 1600 Ma; Romano and others, 1997, GSAA). We are unable to obtain nearly concordant partially reset ages with any of our simulations and conclude that Middle Proterozoic intrusion of the Wolf River batholith was probably not responsible for generating the 1630-1600 Ma chrontour by partial resetting. This conclusion is indirectly supported by two independent lines of evidence. First, the degree of deformation of the Early Roterozoic quartzites in Wisconsin does not wane away from the Wolf River batholith as would be expected for intrusion related deformation. The sharp deformational front (which coincides with the 1630-1600 Ma chrontour) is more characteristic of tectonic-related deformation. Second,

thennal effects of the Duluth Complex on Archean country rock in northeastern Minnesota exist only to amap-view distance of 10 km away from the intrusion (Hanson and others, 1975, GCA). By comparison,the 1630-1600 Ma chrontour in northern Wisconsin is located more than 40 km away from the knownsubsurface extent of the Wolf River batholith (Fig. 1). Our results support the conclusion of Hoim andothers (1998, ILSG) that the 1630-1600 Ma chrontour represents complete thermal resetting of micasassociated with latest Early Proterozoic tectonism.

Lister, G.S., and Baldwin, S.L., 1996, Modelling the effect of arbitrary P-T-t histories on argon diffusionin minerals using the MacArgon program for the Apple Macintosh: Tectonophysics, v. 253, p. 83-109.

I,fit,f' a Edge of Late Prol/

Phanerozoic cover

Baraboo qtzite / I. -!

1(tightly folded) / Approximate subsurface extent ofWRB

I I I I91 90 89 88

Figure 1. Simplified tectonic map of the central Penokean orogen, northernWisconsin and Michigan, U.S.A. (after Sims, 1992; Holm and others, in review).CM = continental margin; EPSZ = Eau Pleine shear zone; NSZ = Niagara suturezone; MT = Archean Marshfield terrane; WRB = Wolf River batholith.

E PeakT

t \ t = duration of thermal pulseAmbientT

1800 1470 Age(Ma) 600Figure 2. Temperature-time graph showing modelled parameters.

82

KEWEENAWAN

subhoilzontal, , , , , , , -Ba ffonF % .. .. % % % .. '

--

% .. % % % % % %# . / t / / / &�S:S:S:::S. . . . . S. S. % S. :-xc-t-:.:-:/ F / / / / / F F a — 3:S::SS. S. S. S. S. S. S. S. S. S S. %'..—..

S S. S. S. CM%S.S.S.S.S.S.S.S.1

—46

:Penoke

ambeauightly fold

50km

thermal effects of the Duluth Complex on Archean country rock in northeastern Minnesota exist only to a map-view distance of 10 km away from the intrusion (Hanwn and others, 1975, GCA). By comparison, the 1630-1600 Ma chrontour in northern Wisconsin is located more than 40 km away from the known subsurface extent of the Wolf River batholith (l3g. 1). Our results support the conclusion of Holm and others (1998, U G ) that the 1630-1600 Ma chrontour represents complete thermal resetting of micas associated with latest Early Proterozoic tectonism.

Lister, G.S., and Baldwin, S.L., 1996, Modelling the effect of arbitrary P-T-t histories on argon diffbsion in minerals using the MacArgon program for the Apple Macintosh: Tectonophysics, v. 253, p. 83-109.

Figure 2. Temperature-time graph showing modelled parameters.

Figure I. Simplified tectonic map of the central Penokean orogen, northern Wisconsin and Michigan, U.S.A. (after Sims, 1992; Holm and others, in review). CM = continental margin; EPSZ = Eau Pleine shear zone; NSZ = Niagara suture zone; MT = Archean Marshfield terrane; WRB = Wolf River batholith.

92 3 + 2 a

E z

Peak T

t = duration of thermal pulse 1 Ambient T ,t

1800 1470 Age (Ma) 600

An Archean subaqueous heterolithic debris flow,Irwin, Pifher, and Meader Townships,

Lake Nipigon Region, Ontario

Frank ft Luther, Geology Department, UW-Whitewater, Whitewater, WI 53190lutherf@mail.uww.edu

This extensive body of volcanic breccia is located in northern Irwin,southeastern Meader, and a large part of southern and eastern Pifher Townships.It is 10-25 km east of Lake Nipigon and 17-28 km north-northeast of Beardmore.

Mapping by Mackasey (1975) and Kresz and Zayachivsky (1989) shows thisrock to be enclosed in a thick sequence of typical greenstone assemblage rocks -pillowed and massive basalt flows, intermediate to felsic pyroclastics and flows,and lesser amount of spatially-associated volcanogenic sediments. It is cut bysmall granitic intrusions through most of its exposure area and by larger graniticintrusions in Pifher Twp to the northeast. Metamorphism Increases to thenortheast, perhaps caused by these intrusions. Keweenawan intrusions which cutthe breccia include a large diabase sill which dips gently to the south and a smalldiabase dike, locally termed greenspar (Luther, 1997; Thomas, Kean, and Luther,in preparation).

The breccia contains rounded rock fragments ranging from <1 mm up to over1 m in maximum dimension. Most fragments are disk-shaped to cigar-shaped inform although irregular on smaller scale; some smaller (5-20 cm) fragments aremore competent and nearly spherical. Sorting and alignment of fragments is, insome locations, poorly-developed primary bedding while, in other locations, thesorting and alignment is a result of later strain. The smaller fragments vary incomposition from that of the matrix to mafic or ultramafic to carbonate-rich. Largerfragments tend to be similar to the matrix in composition although minor differencesin composition or consolidation cause the fragments to weather high or low.

The matrix of the breccia is composed of fine quartz grains (<0.05 mm),euhedral to broken crystals of plagioclase (now albite) ranging in size from <0.1 to1 mm, a minor pelitic fraction (now muscovite), and, locally, fragments of quartz,pyroxene and/or hornblende (now chlorite) up to 0.5 mm. The primary texture isover-printed by a metamorphic assemblages of chlorite, epidote, actinolite,muscovite, and biotite. Representative whole rock analyses of the matrix andfragments are presented in Table 1. These analyses show the average (igneousrock equivalent) composition of the matrix to be dacitic while the fragments varyfrom dacitic to mafic.

In summary (1) the heterogeneity of the fragments, (2) the relativehomogeneity of the matrix, (3) the shape, and rounding of the fragments, (4) the

83

An Archean subaqueous heterolithic debris flow, Irwinl Pifher! and Meader Townshipsl

Lake Nipigon Region, Ontario

Frank R. Luther! Geology Departmentl UW-Whitewaterl Whitewatery Wl 53190 lutherf@mail.uww.edu

This extensive body of volcanic breccia is located in northern Irwinl southeastern Meader! and a large part of southern and eastern Pifher Townships. It is 10-25 km east of Lake Nipigon and 17-28 km north-northeast of Beardmore.

Mapping by Mackasey (1975) and Kresz and Zayachivsky (1989) shows this rock to be enclosed in a thick sequence of typical greenstone assemblage rocks - pillowed and massive basalt flowsl intermediate to felsic pyroclastics and flowsl and lesser amount of spatially-associated volcanogenic sediments. It is cut by small granitic intrusions through most of its exposure area and by larger granitic intrusions in Pifher Twp to the northeast. Metamorphism Increases to the northeast! perhaps caused by these intrusions. Keweenawan intrusions which cut the breccia include a large diabase sill which dips gently to the south and a small diabase dike! locally termed greenspar (Lutherl 1997; Thomasl Keanl and Luther! in preparation).

The breccia contains rounded rock fragments ranging from cl mm up to over I m in maximum dimension. Most fragments are disk-shaped to cigar-shaped in form although irregular on smaller scale; some smaller (5-20 cm) fragments are more competent and nearly spherical. Sorting and alignment of fragments isl in some locationsl poorly-developed primary bedding whilel in other locationsl the sorting and alignment is a result of later strain. The smaller fragments vary in composition from that of the matrix to mafic or ultramafic to carbonate-rich. Larger fragments tend to be similar to the matrix in composition although minor differences in composition or consolidation cause the fragments to weather high or low.

The matrix of the breccia is composed of fine quartz grains (~0.05 mm)l euhedrai to broken crystals of plagioclase (now albite) ranging in size from 4 . 1 to I mm, a minor pelitic fraction (now muscovite)l and! locallyl fragments of quartzl pyroxene andlor hornblende (now chlorite) up to 0.5 mm. The primary texture is over-printed by a metamorphic assemblages of chloritel epidotel actinolitel muscovitel and biotite. Representative whole rock analyses of the matrix and fragments are presented in Table 1. These analyses show the average (igneous rock equivalent) composition of the matrix to be dacitic while the fragments vary from dacitic to mafic.

In summary (I) the heterogeneity of the fragments! (2) the relative homogeneity of the matrix? (3) the shape! and rounding of the fragmentsl (4) the

great variability in size of the fragments, (5) the poorly sorted character of the wholebody of rock, and (6) the spatial association with pillowed volcanics and beddedtufts lead the author to conclude that this rock is the product of a subaqueousdebris flow or flows off the side of a volcanic edifice. The slopes presumablyconsisted of poorly-consolidated volcanogenic debris including finely-crystallinequartz and clay which was transported from nearby weathered felsic-to-intermediate volcanics and an admixed quantity of pyroclastic debris.

Table 1. Whole rock analyses of the matrix and selected fragments of thevolcanic breccia. (XRAL)

matrix fraamentsLN97-6-2 LN97-7-9-2 LN97-6-1 B LN97-6-2A LN9779A*

Si02 643 60.5 63.2 57.8 45.4Ti02 0.49 0.53 0.51 0.51 0.36A1203 15.8 16.0 16.5 16.0 22.2Fe203 1.85 1.38 1.85 1.73 7.34FeO 3.2 4.9 3.1 5.4 0.8MnO 0.08 0.09 0.07 0.09 0.12MgO 2.92 4.63 2.15 4.36 0.88CaO 4.87 3.53 6.03 4.42 17.7Na20 4.54 4.18 3.53 3.47 0.88K20 1.05 1.15 1.49 2.18 0.30

0.10 0.12 0.11 0.11 0.08LOI 0.55 2.4 0.85 1.45 2.0

TOTAL 99.75 99.41 99.39 97.52 98.06

* 95% epidoteREFERENCES

Kresz, D.U. and Zayachivsky, B., 1989, Precambrian geology, Barbara, Meaderand Pifher Townships; Ontario Geological Survey, rpt 270 with maps 2536-2537,91 p.

Mackasey, W.O., 1975, Geology of Dorothea, Sandra, and Irwin Townships, Districtof Thunder Bay; Ontario Division of Mines, rpt 122 with map 2294, 83 p.

Luther, F., 1997, The Petrology of greenspar: a Proterozoic porphyritic diabasedike; Pifher and Irwin Townships, Lake Nipigon District, Ontario (extendedabstract): 43rd Annual Institute on Lake Superior Geology meeting, Sudbury,Ontario, v. 43.

84

great variability in size of the fragments, (5) the poorly sorted character of the whole body of rock, and (6) the spatial association with pillowed volcanics and bedded tuffs lead the author to conclude that this rock is the product of a subaqueous debris flow or flows off the side of a volcanic edifice. The slopes presumably consisted of poorly-consolidated volcanogenic debris including finely-crystalline quartz and clay which was transported from nearby weathered felsic-to- intermediate volcanics and an admixed quantity of pyroclastic debris.

Table 1. Whole rock analyses of the matrix and selected fragments of the volcanic breccia. (XRAL)

Si02 Ti02 A12 0 3

Fe203 Feo MnO MgO CaO N a20 K20 p205 LO1

matrix LN97-6-2 LN97-7-9-2

fraaments LN97-6-1 B LN97-6-2A ~ ~ 9 7 - 7 - 9 ~ *

* 95% epidote REFERENCES

Kresz, D.U. and Zayachivsky, B., 1989, Precambrian geology, Barbara, Meader and Pifher Townships; Ontario Geological Survey, rpt 270 with maps 2536-2537, 91 p.

Mackasey, W.O., 1975, Geology of Dorothea, Sandra, and Irwin Townships, District of Thunder Bay; Ontario Division of Mines, rpt 122 with map 2294, 83 p.

Luther, F., 1997, The Petrology of greenspar: a Proterozoic porphyritic diabase dike; Pifher and Irwin Townships, Lake Nipigon District, Ontario (extended abstract): 43rd Annual Institute on Lake Superior Geology meeting, Sudbury, Ontario, v. 43.

NEW FIELD OBSERVATIONS OF THE CLARKSBURG VOLCANICS, UPPER PENINSULA OFMICHIGAN

MAKI, John C. and Bornhorst, Theodore J., Department of Geological Engineering andSciences, Michigan Technological University, Houghton, MI 49931

The Clarksburg Volcanics are a member of the Early Proterozoic Michigamme Formation, BaragaGroup, Marquette Supergroup and crop out near US-4 I between Champion and west Ishpeming,Marquette County, Upper Peninsula, Michigan (Cannon, 1974, 1975; Cannon and Klasner, 1977;Simmons, 1974). The Clarksburg Volcanics are stratigraphically above the Greenwood Iron-formationMember and below the Lower graywacke member of the Michigamme Formation and are laterallyrestricted over a 19 km strike length and only crop out on the southern limb of the Marquette trough.These rocks dip about 60 degrees or more, hence the exposed section only provides data on stratigraphicand strike parallel variation. Despite metamorphism to the amphibolite facies in most exposures andgreenschist fades in a few, primary textures are well preserved in the outcrops.

The Clarksburg Volcanics are composed mostly of volcanic rocks with lesser amounts of clasticsedimentary rocks and iron-formation. Field observations and available chemical data suggest that thevolcanic rocks are dominantly basalt with less amounts of andesite. In general, the volcanic rocks aretuffs and agglomerates containing fragments 0.5 to 1.5 cm in diameter. Near the towns of Humboldt andClarksburg, clast size is considerably larger than elsewhere, up to 30 cm in diameter. Laterally, thetypical clast size is considerably finer in both the extreme eastern and western exposures with argillitebeing common in the west Ishpeming exposures. These observations suggest a proximal or near ventenvironment in the central exposures and a distal environment towards the east and west. Since maficpyroclastic rocks often do not travel large distances from the volcanic vent, it is likely that the volcanowas quite near the central exposures of the Clarksburg Volcanics.

Within the Clarksburg Volcanics there are several areas with interbedded banded iron-formation. The iron-formation is composed of mixed magnetite- and detrital-bearing layers (I to 4 mmthick) and poorly defined chert layers (< 1 mm thick). Metamorphism has produced quartzite texturesin the iron-fonnation. As the exposures of iron-formation are in the proximal section of the ClarksburgVolcanics, we suggest that the interbedded iron-formation has a volcanogenic origin.

Throughout the immediate region, diabase sills, dikes, and other shaped bodies are common.Cannon (1974) suggested that the diabase was approximately equivalent in age to the ClarksburgVolcanics. The major element chemical composition of the diabase and Clarksburg Volcanics aresimilar. Just south of the town of Clarksburg is a relatively large body of diabase with a surfaceexposure of approximately 1.5 by 1.5 km. This body of diabase cuts rocks stratigraphically older thanthe Clarksburg Volcanics and is spatially adjacent to the proximal section. We suggest this diabase ispart of the subvolcanic roots of the Clarksburg volcanic system.

References

Cannon, W. F., 1974, Bedrock geologic map of the Greenwood quadrangle, Marquette County, Michigan: U.S.Geological Survey Geologic Quadrangle Map GQ-l 168.

Cannon W. F., 1975, Bedrock geologic map of the Republic quadrangle, Marquette County, Michigan: U.S.Geological Survey Miscellaneous Investigation Series Map 1-862.

Cannon, W. F., and Kiasner, J. S., 1977, Bedrock geologic map of the southern part of the Diorite and Champion7-112 minute quadrangles, Marquette County, Michigan: U.S. Geological Survey Miscellaneous Investigation SeriesMap 1-1058.

Simmons, G. C., 1974, Bedrock geological map of the Ishpeming quadrangle, Marquette County, Michigan: U.S.Geological Survey Geologic Quadrangle Map GQ-1 130.

85

NEW FIELD OBSERVATIONS OF THE CLARKSBURG VOLCANICS, UPPER PENINSULA OF MICHIGAN

MAIU, John C. and Bornhorst, Theodore J., Department of Geological Engineering and Sciences, Michigan Technological University, Houghton, MI 4993 1

The Clarksburg Volcanics are a member of the Early Proterozoic Michigamme Formation, Baraga Group, Marquette Supergroup and crop out near US-41 between Champion and west Ishpeming, Marquette County, Upper Peninsula, Michigan (Cannon, 1974, 1975; Cannon and Klasner, 1977; Simmons, 1974). The Clarksburg Volcanics are stratigraphically above the Greenwood Iron-formation Member and below the Lower graywacke member of the Michigamme Formation and are laterally restricted over a 19 km strike length and only crop out on the southern limb of the Marquette trough. These rocks dip about 60 degrees or more, hence the exposed section only provides data on stratigraphic

and strike parallel variation. Despite metamorphism to the amphibolite facies in most exposures and greenschist facies in a few, primary textures are well preserved in the outcrops.

The Clarksburg Volcanics are composed mostly of volcanic rocks with lesser amounts of clastic sedimentary rocks and iron-formation. Field observations and available chemical data suggest that the volcanic rocks are dominantly basalt with less amounts of andesite. In general, the volcanic rocks are tuffs and agglomerates containing fragments 0.5 to 1.5 cm in diameter. Near the towns of Humboldt and Clarksburg, clast size is considerably larger than elsewhere, up to 30 cm in diameter. Laterally, the typical clast size is considerably finer in both the extreme eastern and western exposures with argillite being common in the west Ishpeming exposures. These observations suggest a proximal or near vent environment in the central exposures and a distal environment towards the east and west. Since mafic pyroclastic rocks often do not travel large distances from the volcanic vent, it is likely that the volcano was quite near the central exposures of the Clarksburg Volcanics.

Within the Clarksburg Volcanics there are several areas with interbedded banded iron- formation. The iron-formation is composed of mixed magnetite- and detrital-bearing layers (1 to 4 mm thick) and poorly defined chert layers (< 1 mm thick). Metamorphism has produced quartzite textures in the iron-formation. As the exposures of iron-formation are in the proximal section of the Clarksburg Volcanics, we suggest that the interbedded iron-formation has a volcanogenic origin.

Throughout the immediate region, diabase sills, dikes, and other shaped bodies are common. Cannon (1974) suggested that the diabase was approximately equivalent in age to the Clarksburg Volcanics. The major element chemical composition of the diabase and Clarksburg Volcanics are similar. Just south of the town of Clarksburg is a relatively large body of diabase with a surface exposure of approximately 1.5 by 1.5 krn. This body of diabase cuts rocks stratigraphically older than the Clarksburg Volcanics and is spatially adjacent to the proximal section. We suggest this diabase is part of the subvolcanic roots of the Clarksburg volcanic system.

References

Cannon, W. F., 1974, Bedrock geologic map of the Greenwood quadrangle, Marquette County, Michigan: U.S. Geological Survey Geologic Quadrangle Map GQ-1168.

Cannon W. F., 1975, Bedrock geologic map of the Republic quadrangle, Marquette County, Michigan: U.S. Geological Survey Miscellaneous Investigation Series Map 1-862.

Cannon, W. F., and Klasner, J. S., 1977, Bedrock geologic map of the southern part of the Diorite and Champion 7-112 minute quadrangles, Marquette County, Michigan: U.S. Geological Survey Miscellaneous Investigation Series Map 1-1058.

Simmons, G. C., 1974, Bedrock geological map of the Ishpeming quadrangle, Marquette County, Michigan: U.S. Geological Survey Geologic Quadrangle Map GQ- 1 130.

POST-GLACIAL SHORELINES OF ISLE ROYALE - WHERE ARE THEY NOW?M.E. McRae, Oak Ridge Associated Universities, Reston, VAW.F. Cannon, U.S. Geological Survey, Reston, VAL.G. Woodruff, U.S. Geological Survey, Mounds View, MN

Well-developed shoreline features at elevations higher than the present day lake level arewell documented in the Lake Superior basin. These shorelines formed approximately 10,000to 4,500 years ago as the last glacier to occupy western Lake Superior receded to thenortheast. Further, mapping of stranded shoreline features has demonstrated that these onceflat-lying lake planes are now tilted from northeast to southwest as a consequence ofdifferential isostatic rebound. Previous researchers have been able to construct isobases ofrebound for several of the better-developed lake levels. These include Lakes Washburn andBeaver Bay (—9800 - 9700 B.P.), Lake Minong (—9500 B.P.), Lake Houghton (—8000 B.P.),and Lake Nipissing (—5500 -4700 B.P.).

Beach ridges are common features on Isle Royale, particularly on the lower elevations of thesouthwest portion of the island where glacial debris is widespread. A few wave-cut featuresin bedrock have also been mapped in the northeastern half of the island. However, all ofthese features are scattered and discontinuous. Consequently, the exact positions of formershorelines are not precisely known from observable features for most of the island.

Using geographic information systems (GIS) software, we constructed a series of griddedsurfaces representing lake level planes from the isobases of glacioisostatic rebound describedabove. Subtracting each gridded surface from the present-day digital elevation model (DEM)of Isle Royale yields a sequence of new DEM's showing the island's morphology at severaldifferent lake stages.

We then displayed the modeled shorelines with the known beach and wave-cut features. Thetwo highest modeled shorelines, Lake Washburn and Lake Beaver Bay do not correlate withany of the mapped features. This may indicate that the island was not yet free of ice at thesetimes. All mapped features fall either on or below the Lake Minong shoreline. Lengthysections of mapped beach ridges lie on the modeled Minong shore. Evidence also seems tosupport the position of the modeled Nipissing shore. No features seem to correlate with theHoughton shoreline.

By overlaying the modeled shorelines on the modern digital elevation model, it is possible todetermine the approximate elevation of the post-glacial shores around the island. Thisknowledge could serve as a guide to future mapping particularly if combined withinformation such as vegetation type, surficial material, abundance of outcrop, andaccessibility to trails.

Lastly, prehistoric mining of native copper began about 5,000 years ago, and possibly earlier.We hypothesize that former lake levels influenced the prehistoric discovery and mining ofcopper. Copper would have been easily accessible and recognizable in wave-washedshoreline rock exposures, so discovery along prehistoric shorelines seems probable. Becausewave-washed exposures remain largely barren of vegetative material for prolonged periods

86

POST-GLACIAL SHORELINES OF ISLE ROYALE - WHERE ARE THEY NOW? M.E. McRae, Oak Ridge Associated Universities, Reston, VA W.F. Cannon, U.S. Geological Survey, Reston, VA L.G. Woodruff, U.S. Geological Survey, Mounds View, MN

Well-developed shoreline features at elevations higher than the present day lake level are well documented in the Lake Superior basin. These shorelines formed approximately 10,000 to 4,500 years ago as the last glacier to occupy western Lake Superior receded to the northeast. Further, mapping of stranded shoreline features has demonstrated that these once flat-lying lake planes are now tilted from northeast to southwest as a consequence of differential isostatic rebound. Previous researchers have been able to construct isobases of rebound for several of the better-developed lake levels. These include Lakes Washbum and Beaver Bay (-9800 - 9700 B.P.), Lake Minong (-9500 B.P.), Lake Houghton (-8000 B.P.), and Lake Nipissing (-5500 - 4700 B.P.).

Beach ridges are common features on Isle Royale, particularly on the lower elevations of the southwest portion of the island where glacial debris is widespread. A few wave-cut features in bedrock have also been mapped in the northeastern half of the island. However, all of these features are scattered and discontinuous. Consequently, the exact positions of former shorelines are not precisely known from observable features for most of the island.

Using geographic information systems (GIs) software, we constructed a series of gridded surfaces representing lake level planes from the isobases of glacioisostatic rebound described above. Subtracting each gridded surface from the present-day digital elevation model (DEM) of Isle Royale yields a sequence of new DEM7s showing the island's morphology at several different lake stages.

We then displayed the modeled shorelines with the known beach and wave-cut features. The two highest modeled shorelines, Lake Washburn and Lake Beaver Bay do not correlate with any of the mapped features. This may indicate that the island was not yet free of ice at these times. All mapped features fall either on or below the Lake Minong shoreline. Lengthy sections of mapped beach ridges lie on the modeled Minong shore. Evidence also seems to support the position of the modeled Nipissing shore. No features seem to correlate with the Houghton shoreline.

By overlaying the modeled shorelines on the modem digital elevation model, it is possible to determine the approximate elevation of the post-glacial shores around the island. This knowledge could serve as a guide to future mapping particularly if combined with information such as vegetation type, surficial material, abundance of outcrop, and accessibility to trails.

Lastly, prehistoric mining of native copper began about 5,000 years ago, and possibly earlier. We hypothesize that former lake levels influenced the prehistoric discovery and mining of copper. Copper would have been easily accessible and recognizable in wave-washed shoreline rock exposures, so discovery along prehistoric shorelines seems probable. Because wave-washed exposures remain largely barren of vegetative material for prolonged periods

after they are abandoned, they remained favorable points for discovery long after the lakelevels had receded. Major prehistoric workings at the Minong Mine are in extensive bedrockexposures at or just above the projected Minong shore. During a brief reconnaissance in1997 we observed prehistoric working in two other areas of extensive exposure at theMinong shore. Did early inhabitants of the Lake Superior region, such as the Plano Indianswho inhabited the Minong shore near Thunder Bay, visit the island and discover copperduring the Lake Minong stage, well before the generally accepted date of first mining? Ordid barren rock exposures from the former Minong shore facilitate recognition of nativecopper thousands of years later? In either case, we suggest that the projected location of theMinong and younger shorelines can provide a guide to future archaeological studies.

87

Projected digital elevation model of Isle Royale during the time of Lake Minong.Shoreline of the modem island is shown in white.

after they are abandoned, they remained favorable points for discovery long after the lake levels had receded. Major prehistoric workings at the Minong Mine are in extensive bedrock exposures at or just above the projected Minong shore. During a brief reconnaissance in 1997 we observed prehistoric working in two other areas of extensive exposure at the Minong shore. Did early inhabitants of the Lake Superior region, such as the Piano Indians who inhabited the Minong shore near Thunder Bay, visit the island and discover copper during the Lake Minong stage, well before the generally accepted date of first mining? Or did barren rock exposures from the former Minong shore facilitate recognition of native copper thousands of years later? In either case, we suggest that the projected location of the Minong and younger shorelines can provide a guide to future archaeological studies.

Projected digital elevation model of Isle Royale during the time of Lake Minong. Shoreline of the modem island is shown in white.

ELECTRON MICROPROBE STUDY OF THE Pt-Pd AND RELATEDMINERALIZATION IN THE MINNAMAX/BABBITT Cu-Ni DEPOSIT

McSWIGGEN, Peter L., Minnesota Geological Survey, 2642 University Avenue,St. Paul, Minnesota 55114

The Minnamax deposit is one of several copper-nickel sulfide deposits that occur along thebase of the Duluth Complex, which is a series of mafic intrusions that are part of theKeweenawan (1100 Ma) Midcontinent Rift system (Paces and Miller, 1993). The ore bodyconsists of troctolitic and ultramafic rocks, as well as homfelsic inclusions derived fromPaleoproterozoic metasedimentary rocks, and unconformably overlying Mesoproterozoicvolcanic rocks. The copper and nickel are contained predominantly in disseminatedsulfides that make up between 1 and 5 percent of the rock. The sulfides consist largely ofchalcopyrite, cubanite, pyrrhotite and pentlandite (Severson, 1991; Severson and Barnes,1991). In addition to the copper and nickel, the deposit also contains significantconcentrations of platinum (Pt), palladium (Pd), gold (Au), silver (Ag) and cobalt (Co).Sections of core ranging from 5 to 10 ft long have as much as 7.0 ppm Pd, 3.1 ppm Pt,and 13.1 ppm Au. As pointed out by Severson (1991), the high values for the platinumgroup elements (PGE's), other precious metals, and Co have been shown by others to beassociated with the high-grade copper zones (Kuhns and others, 1990).

This electron microprobe study has shown that rocks at the Minnamax site containat least two platinum group minerals (PGM's) -- froodite (PdBi2) and cabriite (Pd2SnCu).They are commonly associated with massive sulfide mineralization and are less common insamples consisting mostly of silicates. Of the 14 samples investigated in detail, 5 werefound to contain either froodite or cabriite. Although the number of PGM grains is small,just a few of the 1-2 micron size grains could account for the reported whole rock values.Therefore there is no need to invoke PGE-sulfide phase solid solution to account for thePGE whole-rock values reported from these rocks.

Numerous examples of silver mineralization were found in these rocks. The silveris typically present either as solid solution in maucherite or as discrete grains of nativesilver. Significant gold was found in a few of the native silver grains. Values as high as16 wt. percent Au were measured in 5 to 100 micron size grains of silver, but values weremore typically in the 1 percent range. Cobalt was found at significant levels only inmaucherite and pentlandite. Detailed inspections of these samples shows that they containnumerous rare phases including dienerite (Ni3As), shadlunite [(Pb,Cd)(Fe,Cu)8S8], altaite(PbTe), laurionite [PbCl(OH)} and cotunnite (PbCl2).

Kuhns, M.P., Hauck, S.A. and Barnes, R.J., 1990, Origin and occurrence of platinumgroup elements, gold and silver, in the South Filson Creek copper-nickel mineraldeposit, Lake County, Minnesota. Natural Resources Research Institute,Univ. Minn., Duluth, Tech. Report, NRRI/GMIN-TR-89-15, 6Op.

Paces, J.B. and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and relatedmafic intrusions, northeastern Minnesota: Geochronological insights to physical,petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the1.1 Midcontinent Rift System. Journal Geophysical Research, v. 98, no. B8,p. 13,997-14,013.

Severson, M., 1991, Geology, mineralization, and geostatitics of the MinnamaxlBabbittCu-Ni Deposit (Local Boy Area), Minnesota. Natural Resources Research InstituteTechnical Report NRRIITR-91/13a, 96p.

Severson, M.J. and Barnes, R.J., 1991, Geology, mineralization, and geostatistics of theMinnamax/Babbitt Cu-Ni deposit (Local Boy Area), Minnesota; Part II:Mineralization and geostatistics. Natural Resources Research Institute TechnicalReport NRRI/TR-91/13b, 216 p.

88

ELECTRON MICROPROBE STUDY OF THE Pt-Pd AND RELATED MINERALIZATION IN THE MINNAMAXIBABBITT Cu-Ni DEPOSIT

McSWIGGEN, Peter L., Minnesota Geological Survey, 2642 University Avenue, St. Paul, Minnesota 551 14

The Minnamax deposit is one of several copper-nickel sulfide deposits that occur along the base of the Duluth Complex, which is a series of mafic intrusions that are part of the Keweenawan (1 100 Ma) Midcontinent Rift system (Paces and Miller, 1993). The ore body consists of troctolitic and ultramafic rocks, as well as hornfelsic inclusions derived from Paleoproterozoic metasedimentary rocks, and unconforrnably overlying Mesoproterozoic volcanic rocks. The copper and nickel are contained predominantly in disseminated sulfides that make up between 1 and 5 percent of the rock. The sulfides consist largely of chalcopyrite, cubanite, pyrrhotite and pentlandite (Severson, 199 1 ; Severson and Barnes, 1991). In addition to the copper and nickel, the deposit also contains significant concentrations of platinum (Pt), palladium (Pd), gold (Au), silver (Ag) and cobalt (Co). Sections of core ranging from 5 to 10 ft long have as much as 7.0 ppm Pd, 3.1 ppm Pt, and 13.1 ppm Au. As pointed out by Severson (1991), the high values for the platinum group elements (PGE's), other precious metals, and Co have been shown by others to be associated with the high-grade copper zones (Kuhns and others, 1990).

This electron microprobe study has shown that rocks at the Minnamax site contain at least two platinum group minerals (PGM's) -- froodite (PdBi2) and cabriite (PdzSnCu). They are commonly associated with massive sulfide mineralization and are less common in samples consisting mostly of silicates. Of the 14 samples investigated in detail, 5 were found to contain either froodite or cabriite. Although the number of PGM grains is small, just a few of the 1-2 micron size grains could account for the reported whole rock values. Therefore there is no need to invoke PGE-sulfide phase solid solution to account for the PGE whole-rock values reported from these rocks.

Numerous examples of silver mineralization were found in these rocks. The silver is typically present either as solid solution in maucherite or as discrete grains of native silver. Significant gold was found in a few of the native silver grains. Values as high as 16 wt. percent Au were measured in 5 to 100 micron size grains of silver, but values were more typically in the 1 percent range. Cobalt was found at significant levels only in maucherite and pentlandite. Detailed inspections of these samples shows that they contain numerous rare phases including dienerite (Ni,As), shadlunite [(Pb,Cd)(Fe,Cu)$J, altaite (PbTe), laurionite [PbCl(OH)] and cotunnite (PbClJ.

Kuhns, M.P., Hauck, S.A. and Barnes, R.J., 1990, Origin and occurrence of platinum group elements, gold and silver, in the South Filson Creek copper-nickel mineral deposit, Lake County, Minnesota. Natural Resources Research Institute, Univ. Minn., Duluth, Tech. Report, NRRUGMIN-TR-89-15, 60p.

Paces, J.B. and Miller, J.D., Jr., 1993, Precise U-Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Midcontinent Rift System. Journal Geophysical Research, v. 98, no. B8, p. 13,997-14,013.

Severson, M., 199 1, Geology, mineralization, and geostatitics of the Minnarnax/Babbitt Cu-Ni Deposit (Local Boy Area), Minnesota. Natural Resources Research Institute Technical Report NRRIITR-91/13a, 96 p.

Severson, M.J. and Barnes, R.J., 1991, Geology, mineralization, and geostatistics of the MinnamaxBabbitt Cu-Ni deposit (Local Boy Area), Minnesota; Part II: Mineralization and geostatistics. Natural Resources Research Institute Technical Report NRRIITR-9 1/13b, 216 p.

chemical weathering and sedimentary processescombined to produce a marked geochemicaldifferentiation among basement rocks, paleosol, andpelitic layers in the Baraboo Quartzite (Table 1).Ca, Na, and K were leached from granite andrhyolite, and Al and Fe were concentrated in thepaleosol (saprolite and soil) and pelitic layers in thesedimentary section, which consisted originally ofkaolinite and variable amounts of silt-size quartzgrains. The most aluminous of three analysed p elitesis listed in Table 1, and two other samples areequally low in Ca, Na, and K During meta-morphism, K was re-introduced into the paleosol by1120-tich fluids that were channeled along thesub-B araboo unconformity; this K-metasomatismwas accompanied by formation of thin diasp ore-pyrophyffite-white mica veins in basal quartzites.

The compositions of Baraboo rocks are projectedinto the system, KASH, in Fig. 1, in which the positionsof selected minerals are also plotted. The KASHequilibrium metamorphic assemblages for these rocksare: granite and rhyolite, quartz + microcline +muscovite; paleosol, quartz + muscovite; metapelite,quartz + pyrophyllite; and hydrothermal veins,pyrophyllite + muscovite + diaspore. Kaolinite ispresent in metap elite and hydrothermal veins, but onlyas a retrograde product. In addition to KASH minerals,granite and rhyolite contain albite, epidote, and chlorite.Hematite is abundant in these rocks, especially inpaleosol and metapelite, and rutile is common inmetapelite.

POST- 1.76 Ga LOW-GRADE METAMORPHISM OF THE BARABOO QUARTZITEMEDA1US, L.G., Jr., BROWN, P.B. and BUNGE, R.J., Dept. of Geology & Geophysics,

Univ. of Wisconsin-Madison, Madison, WI 53706; medarisgeology.wisc.eduIt has long been recognized that the Baraboo Quartzite is folded and metamorphosed, andrecent U-Pb dating of detrital zircons in the quartzite requires that deformation and recrystal-lization were post-1.76 Ga events. Although the structure of the Baraboo syncline has beenwell studied, little attention has been devoted to metamorphism in the Baraboo Range, otherthan identii'ing pyrophyllite in metap elite layers and recognizing that metamorphism waslow-grade. We have undertaken a petrologic investigation of the Baraboo Quartzite andunderlying granite and rhyolite to determine the conditions ofpost-1.76 Ga metamorphism.

Rock types, chemical compositions. and mineral assemblages A well-developed paleosoloccurs in granite and rhyolite at the unconformable base of the Baraboo Quartzite. Intense

Table IChemical Compositions, Baraboo Rocks

avgrhyolite granite

(1) (2)wt%5102Ti021AJ203

Fe203MnO

MgO

CaO

Na20K20P205LOl

72.800.26

13.30

2.99

0.070.141.16

4.933.170.031.25

avg avgsaprolite soil

(10) (5)

70.51 56.410.36 0.87

17.18 20.892.82 10.18

0.00 0.000.07 0.380.14 0.090.34 0.324.75 5.720.13 0.102.51 3.65

98.80 98.62

69.250.25

15.15

3.460.090.891.55

4.403.140.091.53

99.78

pelite(1)

59.701.03

25.40

8.040.01

0.040.030.040.21

0.124.70

99.32Sum 100.10

quartz

veins

K20 A1203 diaspore

Fig. 1 Compositions of Baraboorocks projected into the KAS planeof the system, KASH

89

POST-1.76 Ga LOW-GRADE METAMORPHISM OF THE BARABOO QUARTZITE MEDARIS, L. G., Jr., BROWN, P.B. and BUNGE, R J., Dept. of Geology & Geophysics,

Univ. of Wisconsin-Madison, Madison, WI 53706; medaris@geology.wisc.edu It has long been recognized that the Baraboo Quartzite is folded and metamorphosed, and recent U-Pb dating of detrital zircons in the quartzite requires that deformation and recrystal- lization were post-1.76 Ga events. Although the structure of the Baraboo syncline has been well studied, little attention has been devoted to metamorphism in the Baraboo Range, other than identifying pyrophyllite in metapelite layers and recognizing that metamorphism was low-grade. We have undertaken a petrologic investigation of the Baraboo Quartzite and underlying granite and rhyolite to determine the conditions of post-1.76 Ga metamorphism.

Rock types, chemical compositions, and mineral assemblages A well-developed paleosol occurs in granite and rhyolite at the unconformable base of the Baraboo Quartzite. Intense -

chemical weathering and sedimentary processes combined to produce a marked geochemical differentiation among basement rocks, paleosol, and pelitic layers in the Baraboo Quartzite (Table 1). Ca, Na, and K were leached from granite and rhyolite, and A1 and Fe were concentrated in the paleosol (saprolite and soil) and pelitic layers in the sedimentary section, which consisted originally of kaolinite and variable amounts of salt-size quartz grains. The most aluminous of three analysed pelites is listed in Table 1, and two other samples are equally low in Ca, Na, and K During meta- morphism, K was reintroduced into the paleosol by H,0-rich fluids that were channeled along the sub-Baraboo unconformity; this K-metasomatism was accompanied by formation of thin diaspore- pyrophyllite-white mica veins in basal quartzites.

Table 1 Chemical Compositions, Baraboo Rocks

avg avg avg rhyolite granite saproliie soil

wt% (1) (2) (10) (5)

Si02 72.80 69.25 70.51 56.41 Ti02 0.26 0.25 0.36 0.87 A1203 13.30 15.15 17.18 20.89 Fe203 2.99 3.46 2.82 10.18 MnO 0.07 0.09 0.00 0.00 MgO 0.14 0.89 0.07 0.38 CaO 1.16 1.56 0.14 0.09 Na20 4.93 4.40 0.34 0.32 K20 3.17 3.14 4.75 5.72 P205 0.03 0.09 0.13 0.10 LO1 1.25 1.53 2.51 3.65 Sum 100.10 99.78 98.80 98.62

pelite (1

59.70 1 .a3 25.40 8.04 0.01 0.04 0.03 0.04 0.21 0.12 4.70

99.32

The compositions of Baraboo rocks are projected into the system, KASH, in Fig. 1, in which the positions of selected minerals are also plotted. The KASH microcline

equilibrium metamorphic assemblages for these rocks are: granite and rhyolite, quartz + microcline + muscovite; paleosol, quartz + muscovite; metapelite, quartz + pyrophyllite; and hydrothermal veins, pyrophyllite + muscovite + diaspore. Kaolhute is present in metapelite and hydrothermal veins, but only as a retrograde product. hi addition to KASH minerals, granite and rhyolite contain albite, epidote, and chlorite. Hematite is abundant in these rocks, especially in paleosol and metapelite, and rutile is common in metapelite. Fig. 1 Compositions of Baraboo

rocks projected into the KAS plane of the system, KASH

Metamorphic conditions Mineral reactions and chemographic relations in the system,KASH (calculated for unit activity of H20), are summarized in Figure 1. Minimumtemperatures of metamorphism are defined by the metapelite assemblage, quartz +pyrophyllite, which is stable above the reaction, Kin + Qtz = Pr! + V, and by the absence ofstable kaolinite from all observed T,, assemblages, indicating that temperatures were abovethe stability limit of kaolinite. Maximum temperatures are defined by the absence of kyanitefrom the assemblage, muscovite + pyrophyffite + diaspore, in hydrothermal veins, which isstable below the reaction, Pr! + Dsp = Ky + V, and by the absence kyanite in metapelite,which indicates that temperatures did not exceed the stability limit of pyrophyllite.

Fluid inclusions were analysed from quartz in a folded quartz vein in metapelite. Thequartz contains a single population of abundant aqueous inclusions that have final meltingpoints of-15 to -18°C, corresponding to 18 to 20 equivalent wt% NaCL First melting below-3 5°C suggests the presence of divalent cations. Homogenization temperatures lie between165 and 215°C with a peak in the 175-185°C range.

Intersection of the fluid inclusion isochore with the limiting mineral reactions constrainstemperature-pressure conditions for the Baraboo Quartzite to lie between —320°C, 2.7 kbarand —385°C, 4.0 kbar. Such values correspond to a thermal gradient of 25-30°C/km. which istypical for Barrovian-type metamorphic terranes and is notably elevated over that for stablecratons (--17°C/km at comparable depths).

2

0250 450

Fig. 2 Mineral assemblages and reactions in the system, KASH,a fluid inclusion isochore for quartz in a folded quartz vein,and metamorphic conditions for the Baraboo Quartzite

Conclusions Folding and metamorphism of the Baraboo Quartzite mark an importantpost-l.76 Ga tectonothermal event in the Lake Superior region. Although the age of thisevent remains uncertain, it could be related to an eastern extension of the Mazatzal belt at-1.63 Ga. However, andalusite-bearing assemblages in the Waterloo Quartzite are probablydue to contact metamorphism associated with Wolf River magmatism at —4.43 Ga.

90

8

6

4

300 350 400T,°C

Metamorphic conditions Mineral reactions and chemographic relations m the system, KASH (calculated for unit activity of KO), are summarized in Figure 1. Minimum temperatures of metamorphism are defined by the metapelite assemblage, quartz + pyrophyllite, which is stable above the reaction, K h + Qtz = Prl + V, and by the absence of stable kaolinite fiom all observed T- assemblages, indicating that temperatures were above the stability limit of kaohite. Maximum temperatures are defined by the absence of kyanite fiom the assemblage, muscovite + pyrophyllite + diaspore, in hydrothermal vems, which is stable below the reaction, Prl + Dsp = Ky + V, and by the absence lqmnite in metapelite, which mdicates that temperatures did not exceed the stability limit of pyrophyllite.

a d inclusions were analysed fiom quartz m a folded quartz vein in metapelite. The quartz contains a single population of abundant aqueous mclusions that have final meltmg points of -15 to -lS°C corresponding to 18 to 20 equivalent wt% NaCL First meltmg below -35OC suggests the presence of divalent cations. H o m o g ~ t i o n temperatures lie between 165 and 215OC with a peak in the 175- lW° range.

Intersection of the fluid mclusion isochore with the limiting mineral reactions constrains temperature-pressure conditions for the Baraboo Quartzite to lie between -320°C 2.7 kbar and -385OC, 4.0 kbar. Such values correspond to a thermal gradient of 25-30°C/km which is typical for Barrovian-type metamorphic terranes and is notably elevated over that for stable cratons (-17OCh at comparable depths).

Fig. 2 Mineral assemblages and reactions in the systeml KASH, a fluid inclusion isochore for quartz in a folded quattz vein, and metamorphic conditions for the Barabm Quarfziie

Conclusions Folding and metamorphism of the Baraboo Quartzite mark an important post- 1-76 Ga tectonothermal event in the Lake Superior region. Although the age of this event remains uncertain, it could be related to an eastern extension of the Mazatzal belt at -1.63 Ga. However, andahsite-bearing assemblages in the Waterloo Quartzite are probably due to contact metamorphism associated with Wolf River magmatism at -1.43 Ga.

SVANBERGITE iN THE BARABOO QUARTZITE: SIGNIFICANCE FOR DIAGENETICPROCESSES AND PHOSPHORUS FLUX IN PRECAMBRIAN OCEANS

MEDARIS, L.G., Jr. and FOURNELLE, J.H., Department of Geology & Geophysics,University of Wisconsin-Madison, Madison, WI 53706; medarisgeology.wisc.edu

Authigenic minerals of the beudantite and crandallite groups have recently been recognized aswidespread constituents of Archean to Cretaceous sedimentary rocks (Rasmussen, 1996).Although the abundance of these aluininophosphate-sulfate minerals is small, on the order of0.2 to 0.01 wt% in a given sample, they appear to be more abundant than authigenic apatiteand play an important role in phosphorus balance in the oceans.

The beudantite and crandaiite mineral groups The general formula for the beudantitegroup is AB3(XO4XSO4XOH)6, where A = Ba, Ca, Ce, Pb, Sr; B = Al, Few; and X = As, P;and for the crandallite group is AB3(XO4)2(OH,F)5, where A = Ba, Bi, Ca, Ce, La, Nd, Sr, Th;B = Al, Fe3+; and X = As, P, Si. Electron microprobe analysis of Baraboo svanbergitedemonstrates that its composition can be portrayed by a distorted pyramid (Fig. 1) in whichmembers of the beudantite group lie along the front edge of the pyramid and members of thecrandallite group lie in the fir face (note that Ca and Ba have been combined for projectionpurposes).

Crandallitegroup

SrsAJ3Q'Q4 )2(H)5 . HO

goyazite

(Ca,Ba)A13(P04)(S04)(OH)6

Fig. I Compositional space of Beudantite and Crandallitegroup minerals from Baraboo metapelite

Occurrence and chemical composition of Baraboo svanbergite Small amounts ofsvanbergite are widespread in metap elite layers in the Baraboo Quartzite, where it isassociated with pyrophyllite, quartz, hematite, and rutile. Svanbergite grains are equant andhave diameters of 10-20 microns, but despite their small size, are readily visiblemicroscopically because of their marked difference in relief compared to associatedpyrophyllite. Baraboo svanbergite contains <2 wt% Fe203, <1 wt% As205, and has negligibleamounts of Bi, Pb, Th, Si, and F; its composition is a solid solution ofAAl3(PO4XSO4XOH)6 -AAI3(P04)2(OH)5'H20, where A = Sr> Ca + Ba + REE, which lies near the base of thepyramid in Fig. 1 between svanbergite and goyazite (Fig. 2).

91

florenciteCeAJ3(P04)2(OH)6

SrAI3(P04)(S04 )(OH)6

svanbergite

SVANBERGITE IN THE BAMBOO QUARTZITE: SIGNIFICANCE FOR DIAGENETIC PROCESSES AND PHOSPHORUS FLUX IN PRJXAMBRIAN OCEANS

MEDARIS, L.G.? Jr. and FOURNELLE? J.H., Department of Geology & Geophysics, University of Wisconsin-Madison, Madison? WI 53706; medaris@geology.wisc.edu

Authigenic minerals of the beudantite and crandallite groups have recently been recognized as widespread constituents of Archean to Cretaceous sedimentary rocks (Rasmussen7 1996). Although the abundance of these alumhophosphate-dte minerals is small, on the order of 0.2 to 0.01 wt% in a given sample7 they appear to be more abundant than authigenic apatite and play an important role in phosphorus balance in the oceans.

i%e beudantite and cranddlite mineral wmp The general formula for the beudantite group is AB3(X04)(S04)(OH)6, where A = Ba7 Ca7 Ce7 Pb7 Sr; B = Al, Few; and X = As7 P; and for the crandallite group is AB3(X04)2(O13,F)547 where A = Ba7 Bi, Ca7 Ce, La7 Nd, Sr7 Th; B = Al, Fe3+; and X = As7 P7 Si Electron microprobe analysis of Baraboo svanbergite demonstrates that its composition can be portrayed by a distorted pyramid (Fig. 1) in which members of the beudantite group lie along the fiont edge of the pyramid and members of the crandallite group lie m the fiu face (note that Ca and Ba have been combmed for projection purposes).

florencite ce%p04)2(0H)

Fig. I Compositional space of Beudantite and Crandallite group minerals from Baraboo metapelite

Occurrence and chemical compmition o f Baraboo svanberdte S d amounts of svanbergite are widespread in metapelite layers m the Baraboo Quartzite7 where it is associated with pyrophylbe7 quartz, hematite7 and rutile. Svanbergite grains are equant and have diameters of 10-20 microns7 but despite their small size7 are readily vkiile microscopically because of their marked &erence in relief compared to associated pyrophyllite. Baraboo svanbergite contains < 2 wt% Fe203? 4 wt% 4 O s 7 and has negligiile amounts of Bi, Pb7 Th7 Si, and F; its composition is a solid solution of ALUJPO~)(SO~)(OH)~ - AAl&P04)2(OH)5*q07 where A = Sr Ca + Ba + W E 7 which lies near the base of the pyramid m Fig. 1 between svanbergite and goyazite (Fig. 2).

0 0.2 0.4 0.6 0.8 1.0

"woodhouseite" Sr/(Sr+Ca+Ba) svanbergite

Fig. 2 Projection of mineral compositions onto thebase of the Beudantite-Crandallite prism

The Baraboo svanbergite solid solution rangesfrom almost pure svanbergite (beudantite group) to—70% goyazite (crandallite group) with a concomitantdecrease in Sr/(Sr+Ca+Ba) (Fig. 2), and A-siteoccupancy is dominated by Sr (Fig. 3). Many 10000

svanbergite grains are strongly zoned, with rims beingenriched in crandallite component, in which the sum ofLa203, Ce203, and Nd203 reaches 5.3 wt %,amounting to --27% occupancy of the A-site (Fig. 3).Although other RE elements may be present, they

Ndcan't readily be determined by electron microprobe .

Fig. 4 Representative chondrite-normalizedtechmques due to their low abundances and spectral La, Ce, and Nd contents of Baraboointerferences. The light REE contents in svanbergite svanbergite-goyazite

are 5000 to 40,000 times greater than in chondrites,and La is enriched over Nd by a factor of 2 to 3 (Fig. 4). There is no marked Ce anomaly,although two samples may have a slight Ce enrichment with respect to La and Nd.

Geological significance of svanbergite Precipitation of authigenic aluminophosphate-sulfate minerals has been ascribed to bacterial decomposition of P-bearing organic matter in anAl-rich environment, such as shale, within the zone of sulfate reduction and methanogenesis(Rasmussen, 1996). If so, the occurrence of svanbergite-goyazite in Baraboo metapelitessignifies that such a process may have been operative in mid-Proterozoic time.

The widespread occurrence of authigenic aluniinophosphate-sulfate minerals represents apreviously unrecognized repository for phosphorus in the oceans. Rasmussen has estimatedthat aluminophosphate precipitation accounts for a phosphorus burial flux of 7.6 x 1010 molesyr, which is comparable to that resulting from authigenic phosphates and P-bearingcarbonates (2.2-9.1 x 10" moles yf'). However, Rasmussen used the composition offlorencite (2 moles of P phi) in his calculation, and using the compositional range of Baraboosvanbergite-goyazite (1.0-1.7 moles of P pfli) reduces the burial flux estimate by 15 to 50%.Regardless, aluniinophosphate-sulfate minerals are an important factor in oceanic P-flux.REFERENCE CITEDRasmussen, B. (1996) American Journal of Science, v. 296, 601-632.

"crandallite" goyazite

00+

0U)

0U)

Ba REE

CaCa

Ba

Fig. 3 Proportions of cations in the A-site ofBaraboo svanbergite-goyazite

Sr

92

"crandallite" goyazite

Ba REE

Fig. 3 Promtions of cations in the A-site of

Fig. 2 Projection of mineral compsitions onto the base of the Beudantite-Crandallite prism

The Baraboo svanbergite solid solution ranges fiom almost pure svanbergite (beudantite group) to -70% goyazite (crandallite group) with a concomitant decrease m Sr/(Sr+Ca+Ba) (Fig. 2)? and A-site occupancy is dominated by Sr (Fig. 3). Many svanbergite grains are strongly zoned, with rims bemg enriched in crandallite component7 m which the sum of L%O3? Ce203? and N&03 reaches 5.3 wt %? amounting to -27% occupancy of the A-site (Fig. 3).

- "woodhouseite" Srl(Sr+Ca+Ba) svanbergite

Although other RE elements may be present? they

Bamboo svanbergite-goyazite

lmmT

can't re&ly be determined by electron microprobe Fig. 4 Representative chondritenormalized

techniques due to their low abundances and spectral La, Ce, and Nd contents of Baraboo mterferences. The light REE contents m svanbergite svanbergite-goyazite

are -5000 to 407000 times greater than m chondrites7 and La is enriched over Nd by a fhctor of 2 to 3 (Fig. 4). There is no marked Ce anomaly7 although two samples may have a slight Ce enrichment with respect to La and Nd.

Geolokcal simificance of svanberkte Precipitation of authigenic aluminophosphate- d t e minerals has been ascriied to bacterial decomposition of P-beakg organic matter m an Al-rich environment, such as shale? within the zone of d a t e reduction and methanogenesis (Rasmussen7 1996). If so7 the occurrence of svanbergite-goyazite m Baraboo metapelites signifies that such a process may have been operative m mid-Proterozoic time.

The widespread occurrence of authigenic alumhophosphate-dte minerals represents a previously unrecognized repository for phosphorus m the oceans. Rasmussen has estimated that alumhophosphate precipitation accounts for a phosphorus burial flux of 7.6 x 10" moles

which is comparable to that resultmg fiom authigenic phosphates and P-bearing carbonates (2.2-9.1 x 10" moles yfl). However7 Rasmussen used the composition of florencite (2 moles of P pfb) m his calculation, and using the compositional range of Baraboo svanbergite-goyazite (1.0- 1.7 moles of P pfi) reduces the burial flux estimate by 15 to 50%. Regardless? aluminophosphate-dte minerals are an important fhctor m oceanic P-flux. REFERENCE CITED Rasmussen, B. (1996) American Journal of Science? v. 296? 601-632.

USE OF HIGH-RESOLUTION AEROMAGNETIC DATA FOR REGIONALGEOLOGY INVESTIGATIONS, SOUThEASTERN WISCONSIN (WHERE'S THEKIMBERLITE!)

MIJDREY, M.G., Jr., Wisconsin Geological and Natural History Survey, 3817 MineralPoint Road, Madison, WU 53705-5100, mgmudreyfacstaff.wisc.edu

Between 1876 and 1913, diamonds were found in at least seven localities in southern andcentral Wisconsin. All were found in Pleistocene gravel deposits or Holocene river gravel.The bedrock kimberlite source for these diamonds is unknown, but was presumed to be innorthern Canada, the only area north of Wisconsin previously known to contain kimberlite.With the discovery of the Lake Ellen kimberlite in Iron County, Michigan, Cannon andMudrey (1981) suggested the drift diamonds in Wisconsin may have come from a more localsource.

Carison and Adams (1997) described a kimberlite in southeastern Wisconsin about 280 macross. The preliminary identification was based on drilling small, highly magnetic anomalyidentified from a little known aeromagnetic survey from the 1980s (800-meter flight-linespacing). Only recently have aeromagnetic surveys been sufficiently detailed to determinethe presence of absence of kimberlite in southeastern Wisconsin. Prior to the most recentsurvey, limited flight-line spacing of 1 0-km precluded the identification of strong, smallmagnetic bodies at the shallow bedrock surface.

Analysis of the aeromagnetic survey indicates that flight-line spacing less than 800 m willbe ineffective in the identification of small, highly magnetic kimberlite at thebedrock/surficial material surface in southeastern Wisconsin.

The identification of the kimberlite, and analysis of available aeromagnetic maps, indicatethat other kimberlitic bodies may occur in southeastern Wisconsin and possibly northeasternIllinois and may be the source for the diamond discoveries in Wisconsin and Illinois.However, urbanization in the Milwaukee-Chicago corridor may discourage further geologicand geophysical analysis and competing land use may make further exploration and ultimatedevelopment difficult.REFERENCES:

Cannon, W.F., and Mudrey, M.G., Jr., 1981, The potential for diamond-bearing kimberlitein northern Michigan and Wisconsin: U.S. Geological Survey Circular 842, 15 p.

Carison, S.M. and Adams, G.W., 1997, The diamondiferous Six-Pak UltramaficLamprophyre Diatreme, Kenosha, Wisconsin, U.S.A. (abs.): Institute on Lake SuperiorGeology, Proceedings, Part 1-Program and Abstracts, v. 43, p. 11

Wisconsin, kimberlite, aeromagnetic data

93

USE OF HIGH-RESOLUTION AEROMAGNETIC DATA FOR REGIONAL GEOLOGY INVESTIGATIONS, SOUTHEASTERN WISCONSIN (WHERE'S THE KIMBERLITE!)

MUDREY, M.G., Jr.? Wisconsin Geological and Natural History Survey7 3817 Mineral Point Road? Madison? WU 53705-5 100, mgmudrey@facstaff.wisc.edu

Between 1876 and 1913, diamonds were found in at least seven localities in southern and central Wisconsin. All were found in Pleistocene gravel deposits or Holocene river gravel. The bedrock kimberlite source for these diamonds is unknown7 but was presumed to be in northern Canada? the only area north of Wisconsin previously known to contain kimberlite. With the discovery of the Lake Ellen kimberlite in Iron County? Michigan? Cannon and Mudrey (1981) suggested the drift diamonds in Wisconsin may have come from a more local source.

Carlson and Adams (1997) described a kimberlite in southeastern Wisconsin about 280 m across. The preliminary identification was based on drilling small, highly magnetic anomaly identified from a little known aeromagnetic survey from the 1980s (800-meter flight-line spacing). Only recently have aeromagnetic surveys been sufficiently detailed to determine the presence of absence of kimberlite in southeastern Wisconsin. Prior to the most recent survey? limited flight-line spacing of 10-krn precluded the identification of strong, small magnetic bodies at the shallow bedrock surface.

Analysis of the aeromagnetic survey indicates that flight-line spacing less than 800 m will be ineffective in the identification of small? highly magnetic kimberlite at the bedrock/surficial material surface in southeastern Wisconsin.

The identification of the kimberlite, and analysis of available aeromagnetic maps, indicate that other kimberlitic bodies may occur in southeastern Wisconsin and possibly northeastern Illinois and may be the source for the diamond discoveries in Wisconsin and Illinois. However? urbanization in the Mlwaukee-Chicago corridor may discourage hrther geologic and geophysical analysis and competing land use may make hrther exploration and ultimate development difficult. REFERENCES:

Cannon? W.F., and Mudrey, M.G., Jr., 1981, The potential for diamond-bearing kimberlite in northern Michigan and Wisconsin: U.S. Geological Survey Circular 842, 15 p.

Carlson, S.M. and Adams, G.W., 1997, The diamondiferous Six-Pak Ultramafic Lamprophyre Diatreme, Kenosha, Wisconsin, U.S.A. (abs.): Institute on Lake Superior Geology? Proceedings? Part 1-Program and Abstracts? v. 43, p. 1 1

Wisconsin, kimberlite, aeromagnetic data

XENOLITHOLOGIES AS INDICATORS OF INTRUSION MECHANISMSIN THE WAUSAU SYENITE COMPLEX, WISCONSIN

MYERS, Paul E., Geology Department, University of Wisconsin, Eau Claire, WI 54701

In the area west of Wausau, four alkaline subvolcanic plutons: (1) Stettin, (2) Wausau,(3) Rib Mountain, and (4) Ninemile (Figure), were intruded in a southeastward sequenceinto Lower Proterozoic metavolcanic, metasedimentary, and granitic intrusive rocks(Myers and others, 1984). Whereas the first three syenite plutons are concentricallyzoned and pipelike in structure, the youngest, Ninemile pluton, although possessing anaplitic core rim, is a stock-like body which stoped its way to a shallow depth under thenow-eroded volcanoes. This paper shows the connection between xenolithologies, magmaemplacement, and walirock alteration.

EXPLANATION

qp quartz monzonite porphyry

ng Ninemile granite

ga granite aplite

sy syenites

v volcanic rocks

q quartzite

bs biotite schist

fault

contact, dashed where infened

1

1Scale: -, Zmiles

012-3.

_____________

kilometers

Figure -- Map of Wausau syenite plutons

The Stettin pluton, (Figure) which is the oldest, most alkaline intrusion of theWausau syenite complex, is oval in plan with dimensions of 8.8 x 6.4 km. and has aconcordant NE elongation. Its outer wall zone consists of strongly syenitized volcanicrocks and syenite aplite, which grade inward into concentric, sheet-like masses of gneissicnepheline and tabular syenites. The indistinctly bounded intermediate zone consists mainlyof flow-lineated amphibole syenite, and the cylindrical core, with a diameter of 2 km has arim of lineated nepheline syenite and an inner core of coarse, pyroxene syenite identicalwith that in the intermediate zone. The Stettin pluton is separated from the syenite bodies tothe SE by a fault.

In the Wausau pluton, coarse, massive pyroxene and amphibole syenites form apartial outer rim on the north and east sides. A broad intermediate zone of lensoidal syeniteand quartz syenite are crowded with lenticular xenoliths of mica schist, sillimanite-bearing

94

XENOLITHOLOGIES AS INDICATORS OF INTRUSION MECHANISMS IN THE WAUSAU SYENITE COMPLEX, WISCONSIN

MYERS? Paul E., Geology Department? University of Wisconsin, Eau Claire? WI 54701

In the area west of Wausauy four alkaline subvolcanic plutons: (1) Stettin, (2) Wausau, (3) Rib Mountain, and (4) Ninemile (Figure)? were intruded in a southeastward sequence into Lower Proterozoic metavolcanic, metasedimentary, and granitic intrusive rocks (Myers and others, 1984). Whereas the first three syenite plutons are concentrically zoned and pipelike in structure, the youngest, Ninemile pluton, although possessing an aplitic core rim, is a stock-like body which stoped its way to a shallow depth under the now-eroded volcanoes. This paper shows the connection between xenolithologiesy magma emplacement? and wallrock alteration.

EXPLANATION

qp quartz monzonite porphyry

ng Ninemile granite

ga granite aplite

sy syenites

v volcanic rocks

q quartzite

bs biotite schist

,4' fault

, - 1 contact, dashed where inferred

0 1 2 . Scale: 1-1 mles

?-7 kilometers

Figure -- Map of Wausau syenite plutons

The Stettin plutony (Figure) which is the oldest, most alkaline intrusion of the Wausau syenite complexy is oval in plan with dimensions of 8.8 x 6.4 by and has a concordant NE elongation. Its outer wall zone consists of strongly syenitized volcanic rocks and syenite aplite, which grade inward into concentricy sheet-like masses of gneissic nepheline and tabular syenites. The indistinctly bounded intermediate zone consists mainly of flow-lineated amphibole syenite, and the cylindrical core? with a diameter of 2 km has a rim of heated nepheline syenite and an inner core of coarse, pyroxene syenite identical with that in the intermediate zone. The Stettin pluton is separated from the syenite bodies to the SE by a fault.

In the Wausau pluton, coarse? massive pyroxene and amphibole syenites form a partial outer rim on the north and east sides. A broad intermediate zone of lensoidal syenite and quartz syenite are crowded with lenticular xenoliths of mica schist? sillimanite-bearing

quartzite, and syenitized volcanic rocks. Xenolith volume locally exceeds the volume ofthe quartz syenite. As the contact between the Wausau and Rib Mountain plutons isobscured by a broad strip of Rib River alluvium, its core may not be visible in outcrop.

The Rib Mountain pluton produces a crescentic map pattern with an opening to thesouth where it is engulfed by younger Ninemile granite. The most striking feature of thisconcentric pluton is the 8km ring of very large, lenticular quartzite and mica schistxenoliths embedded in foliated quartz syenites in its intermediate zone.

The Ninemile Pluton, is an elliptical, stock-like body which was intruded at 1500Ma. (Van Schmus and Bickford, 1981) into the core and south rim of the Wausau pluton.According to Anderson (1983) it is comagmatic with rapakivi granites of the Wolf Riverbatholith. Although classed as a granite, the Ninemile pluton is mainly coarse biotite-amphibole monzonite containing up to 30 percent strained, polycrystalline quartzite grainsusually accompanied by occasional mica schist and quartzite xenoliths. A crescentic massof granite aplite defines the core rim of the larger southern lobe of the Ninemile pluton.

The Wausau plutons thus show a distinctive magmatic differentiation sequence,beginning with nepheline syenite in the Stettin pluton and ending with pegmatite dikes inthe Ninemile pluton. As these dikes are shallow-dipping and contain miarolitic cavities withautoclasts of early crystalline phases, Falster (1985) concluded that they crystallizedshallow enough for hydrothermal boiling. Several small porphyritic quartz monzoniteplugs cut post-syenite faults outside the Wausau complex and probably represent the "lastgasp" of the differentiated Ninemile magma.

Xenoliths in the Wausau, Rib Mountain and Ninemile plutons are unassimilatedwallrock fragments carried up and down in the magmas during intrusion. Where there hasbeen considerable vertical transport, as for instance along caldera walls, xenoliths showgreat diversity of protolithology and metamorphic grade. They include upper amphibolite-grade, sillimanite-bearing quartzite, mica schist, and amphibolite, metadiorite, andesiticmetavolcanics and metasediments, and even cognate inclusions of earlier-phase syenites.Their shapes are most commonly lenticular and sheet-like with a preferential orientationparallel to cylindrical walls. Xenoliths are uncommon in the Stettin pluton.

In the Wisonsin River channel east of the power dam at Wausau, the relationshipsbetween xenoliths and flow structures in enclosing felsic and mafic quartz syenites are welldisplayed. Biotite amphibolite xenoliths with swirled lineation, give way southward in theoutcrop to sheet-like masses of felsic and mafic rocks. The folded and fragmented maficxenoliths are biotite-rimmed, showing advaned metasomatic replacement and deformation.The enclosing amphibole quartz syenite has highly discordant flow lineation with swirlsand eddies suggesting considerable viscosity and turbulence. Late-stage syenite pegmatiteveins with quartz cores pobably represent residual liquid segregations along incipientthermal contraction fractures in quartz syenites containing incompletely assimilated quartzitexenoliths. The occurrence of several small xenoliths of unaltered, porphyritic trachyte in thenorth end of the outcrop suggest a downward movement of some of the clasts from theoverlying volcanic pile. As elsewhere in the Wausau syenite complex, the considerableconcentric heterogeneity of xenoliths suggests considerable vertical, somewhat laminartransport of xenoliths along caldera walls as a consequence changes in the level of magmain the conduit. Future field studies should include detailed, comprehensive mapping ofxenolithologies.

References:Anderson, J. L., 1983, Proterozoic anorogenic granite plutonism of North America, Geological

Society of America Memoir 161, P. 133-154.Myers, P.E., Sood, M. K., Berlin, L.A., and Faister, A. U., 1984, The Wausau syenite complex,

central Wisconsin, 30th Annual Institute on Lake Superior Geology, 58 pages.Van Schmus, W.R., Medaris, L.G., and Banks, P.O., 1975, Chronology of Precambrian rocks in

Wisconsin, I: The Wolf River batholith, a rapakivi massif approximately 1500 m.y. old, GeologicalSociety of America, Bulletin, v. 86, p. 907-9 14.

95

quartzite, and syenitized volcanic rocks. Xenolith volume locally exceeds the volume of the quartz syenite. As the contact between the Wausau and Rib Mountain plutons is obscured by a broad strip of Rib River alluvium, its core may not be visible in outcrop.

The Rib Mountain pluton produces a crescentic map pattern with an opening to the south where it is engulfed by younger Ninemile granite. The most striking feature of this concentric pluton is the 8km ring of very large, lenticular quartzite and mica schist xenoliths embedded in foliated quartz syenites in its intermediate zone.

The Ninemile Pluton, is an elliptical, stock-like body which was intruded at 1500 Ma. (Van Schmus and Bickford, 198 1) into the core and south rim of the Wausau pluton. According to Anderson (1983) it is comagmatic with rapakivi granites of the Wolf River batholith. Although classed as a granite, the Ninemile pluton is mainly coarse biotite- amphibole monzonite containing up to 30 percent strained, polycrystalline quartzite grains usually accompanied by occasional mica schist and quartzite xenoliths. A crescentic mass of granite aplite defines the core rim of the larger southern lobe of the Ninemile pluton.

The Wausau plutons thus show a distinctive magmatic differentiation sequence, beginning with nepheline syenite in the Stettin pluton and ending with pegmatite dikes in the Ninemile pluton. As these dikes are shallow-dipping and contain miarolitic cavities with autoclasts of early crystalline phases, Falster (1985) concluded that they crystallized shallow enough for hydrothermal boiling. Several small porphyritic quartz monzonite plugs cut post-syenite faults outside the Wausau complex and probably represent the "last gasp" of the differentiated Ninemile magma.

Xenoliths in the Wausau, Rib Mountain and Ninemile plutons are unassimilated wallrock fragments carried up and down in the magmas during intrusion. Where there has been considerable vertical transport, as for instance along caldera walls, xenoliths show great diversity of protolithology and metamorphic grade. They include upper amphibolite- grade, sillimanite-bearing quartzite, mica schist, and arnphibolite, metadiorite, andesitic metavolcanics and metasediments, and even cognate inclusions of earlier-phase syenites. Their shapes are most commonly lenticular and sheet-like with a preferential orientation parallel to cylindrical walls. Xenoliths are uncommon in the Stettin pluton.

In the Wisonsin River channel east of the power dam at Wausau, the relationships between xenoliths and flow structures in enclosing felsic and mafic quartz syenites are well displayed. Biotite amphibolite xenoliths with swirled lineation , give way southward in the outcrop to sheet-like masses of felsic and mafic rocks. The folded and fragmented mafic xenoliths are biotite-rimmed, showing advaned metasomatic replacement and deformation. The enclosing amphibole quartz syenite has highly discordant flow lineation with swirls and eddies suggesting considerable viscosity and turbulence. Late-stage syenite pegmatite veins with quartz cores pobably represent residual liquid segregations along incipient thermal contraction fractures in quartz syenites containing incompletely assimilated quartzite xenoliths. The occurrence of several small xenoliths of unaltered, porphyritic trachyte in the north end of the outcrop suggest a downward movement of some of the clasts from the overlying volcanic pile. As elsewhere in the Wausau syenite complex, the considerable concentric heterogeneity of xenoliths suggests considerable vertical, somewhat laminar transport of xenoliths along caldera walls as a consequence changes in the level of magma in the conduit. Future field studies should include detailed, comprehensive mapping of xenolithologies .

References: Anderson, J. L., 1983, Proterozoic anorogenic granite plutonism of North America, Geological

Society of America Memoir 161, p. 133-154. Myers, P.E., Sood, M. K., Berlin, L.A., and Falster, A. U., 1984, The Wausau syenite complex,

central Wisconsin, 30th Annual Institute on Lake Superior Geology, 58 pages. Van Schmus, W.R., Medaris, L.G., and Banks, P.O., 1975, Chronology of Precambrian rocks in

Wisconsin, I: The Wolf River batholith, a rapakivi massif approximately 1500 m.y. old, Geological Society of America, Bulletin, v. 86, p. 907-914.

COMPOSITION AND SOURCE(S) OF MIDCONTINENT RIFT LAVAS(CHENGWATANA VOLCANICS) NEAR CLAM FALLS, WISCONSIN

NAIMAN, Zachary J., Department of Geosciences, University of Arizona, Tucson, Ari-zona, 85716, znaiman@geo.arizona.edu; WIRTH, Karl R., Geology Department,Macalester College, St. Paul, Minnesota, 55105, wirth@macalester.edu.

Most studies of the 1.1 Ga Midcontinent rift (MCR) have focused on the well-exposed volcanicsin the Lake Superior region from the central part of the rift. Studies of the Chengwatana Volcanics(CV), the southernmost exposed volcanics of the MCR, provide information about variations inrift processes along the axis of the MCR. Here we report major element, trace element, and Ndisotopic data from mafic and felsic CV flows near Clam Falls, Wisconsin. Approximately 3000meters of mafic volcanics, minor interfiow sediment, and rare rhyolite are exposed in the ClamFalls area. The petrography and geochemistry of the rhyolite are discussed by Abbott et al. (thisvolume). New, precise U-Pb zircon ages (1,102±5 Ma) of rhyolite near Clam Falls (Wirth andGehrels; this volume) provide a means for correlation of the Chengwatana Volcanics fromClam Falls and Taylors Falls with volcanics from other parts of the rift. Recent aeromagneticdata (USGS) suggest that Clam Falls flows are stratigraphically lower and therefore older thanCV flows exposed in the Taylors Falls region.

The mafic volcanics from the Clam Falls region are classified as basalt based on majorand trace element abundances. The basalts are mostly olivine normative, but some flows con-tain small amounts of normative quartz. Most flows have moderately low Mg-numbers (Mg# =Mg/[Mg+Fe2I = 0.40 - 0.58) and Si02 contents (45-53 wt. %) indicating that they have under-gone significant fractionation, similar to flows from the Taylors Falls section of the ChengwatanaVolcanics1. Clam Falls basalts exhibit increasing incompatible element abundances (e.g., P, Ti,Y) with decreasing compatible element abundances (e.g., Mg, Ni, Cr) similar to those of theTaylors Falls region, and can be modelled by fractional crystallization processes2. Clam Fallsflows have Ti02 concentrations which are similar to the low-Ti group (<2.3 wt. %) of basaltsrecognized in the Taylors Falls section1.

Neodymium isotopic analysis of eight basalts and two rhyolites from the Clam Falls areayield initial ENd(1 iOO Ma') values between -2.0 and 3.4. Initial ENd values decrease with strati-graphic height (1!gure 1), and all of the values are in general displaced toward more positivevalues relative to those of the overlying flows of the Taylors Falls section1 (Figure 2). Al-though similarly high Nd values (initial ENd 2) have been observed in other volcanic se-quences in the Midcontinent rift(e.g., Group 6 and 7 flows of 3000

Mamainse Point3, Portage Lake Clam Falls Region

Volcanics and "late basalts" of north- 'a i• basalt -

em Wisconsin4), flows with initial '.' Q r yo ite

ENd values > 2 are rare in the early 2000. U -

stages of rift evolution (time equiva-lent to upper Kallander Creek For-mation or Group 5 lavas). Further- -

more, Taylors Falls flows whichoverlie the Clam Falls basalts (and 1000 -

may be correlative with Portage Lake U

Volcanics and Group 6 flows) have -

epsilon Nd values that are generallymore negative (initial ENd = 45 - 0O1) than has been observed in cor- -2 -1 0 1 2 3 4

relative main stage lavas (initial ENd Initial Ed

96

COMPOSITION AND SOURCE(S) O F MIDCONTINENT RIFT LAVAS (CHENGWATANA VOLCANICS) NEAR CLAM FALLS, WISCONSIN

NAIMAN, Zachary J., Department of Geosciences, University of Arizona, Tucson, Ari- zona, 85716, znaiman@geo.arizona.edu; WIRTH, Karl R., Geology Department, Macalester College, St. Paul, Minnesota, 55 105, wirth@macalester.edu.

Most studies of the 1.1 Ga Midcontinent rift (MCR) have focused on the well-exposed volcanics in the Lake Superior region from the central part of the rift. Studies of the Chengwatana Volcanics (CV), the southernmost exposed volcanics of the MCR, provide information about variations in rift processes along the axis of the MCR. Here we report major element, trace element, and Nd isotopic data from mafic and felsic CV flows near Clam Falls, Wisconsin. Approximately 3000 meters of mafic volcanics, minor interflow sediment, and rare rhyolite are exposed in the Clam Falls area. The petrography and geochemistry of the rhyolite are discussed by Abbott et al. (this volume). New, precise U-Pb zircon ages (1,102± Ma) of rhyolite near Clam Falls (Wirth and Gehrels; this volume) provide a means for correlation of the Chengwatana Volcanics from Clam Falls and Taylors Falls with volcanics from other parts of the rift. Recent aeromagnetic data (USGS) suggest that Clam Falls flows are stratigraphically lower and therefore older than CV flows exposed in the Taylors Falls region.

The mafic volcanics from the Clam Falls region are classified as basalt based on major and trace element abundances. The basalts are mostly olivine normative, but some flows con- tain small amounts of normative quartz. Most flows have moderately low Mg-numbers (Mg# = M ~ / [ M ~ + F ~ + ~ ] = 0.40 - 0.58) and Si02 contents (45-53 wt. %) indicating that they have under- gone significant fractionation, similar to flows from the Taylors Falls section of the Chengwatana Volcanicsl. Clam Falls basalts exhibit increasing incompatible element abundances (e.g., P, Ti, Y) with decreasing compatible element abundances (e.g., Mg, Ni, Cr) similar to those of the Taylors Falls region, and can be modelled by fractional crystallization processes2. Clam Falls flows have Ti02 concentrations which are similar to the low-Ti group (42.3 wt. %) of basalts recognized in the Taylors Falls section1.

Neodymium isotopic analysis of eight basalts and two rhyolites from the Clam Falls area yield initial ENd loo Ma values between -2.0 and ""3.4. Initial ENd values decrease with strati- graphic height ( ̂ igure 1 \ , and all of the values are in general displaced toward more positive values relative to those of the overlying flows of the Taylors Falls section1 (Figure 2). Al- though similarly high Nd values (initial ENd = +2) have been observed in other volcanic se- quences in the Midcontinent rift (e.g., Group 6 and 7 flows of Mamainse ~ o i n t 3 , Portage Lake Volcanics and "late basalts" of north- em ~isconsin4), flows with initial ENd values > +2 are rare in the early stages of rift evolution (time equiva- lent to upper Kallander Creek For- mation or Group 5 lavas). Further- more, Taylors Falls flows which overlie the Clam Falls basalts (and may be correlative with Portage Lake Volcanics and Group 6 flows) have epsilon Nd values that are generally more negative (initial ENd = -4.5 - ""0.1) than has been observed in cor- relative main stage lavas (initial ENd

3000

s w ¥ 6 2000 .- a 0

3 2 .̂ 1000 .- 4-Ã

cd j=! w

0

I rn I I I

basalt  Â

 - -

- -

- rn - Â 0 - Â - I n Â

I I

-2 -1 0 1 2 3 4

Initial &srd

= -l - 4) elsewhere in the rift. 12

Most models of the mag-matic evolution of theMidcontinent rift assume that thelarge volume of magmas wereproduced primarily from an en-riched mantle plume with initialENd near zero3'4. Lavas withnegative ENd values have beenmodelled by contamination ofmantle plume melts with conti-nental lithospheric mantle (ENdnear 9) or with continental crust(ENd < -10). In contrast, lavaswith positive ENd values has beenmodelled by a combination ofenriched mantle plume and de-pleted asthenospheric mantle (ENd > 6) that may have been entrained in the plume head5. Fol-lowing this model, the initial ENd values of Chengwatana basalts suggest a progression of mantlesources from enriched mantle plume+depleted asthenospheric mantle (flows with ENd> 1 nearthe base of the Clam Falls section) to enriched mantle plume that has been variably contami-nated with lithosphere (the remainder of the Clam Falls section and the overlying Taylors Fallssection). The inferred progression of mantle sources observed in the Chengwatana Volcanics(plume+asthenosphere —> plume +1- continental lithosphere) occurs at a time when mantlesources elsewhere in the rift are inferred to be changing from plume+continental lithosphere toplume+asthenosphere. If the depleted mantle component of early Clam Falls flows originatedfrom material that was entrained in the plume head, as has been suggested by several for otherparts of the rift3'4'5 then the proportion of melts produced from entrained asthenospheric mantlein the plume varied in space and time throughout the rift. Alternatively, the depleted mantlecomponent may have been differentially incorporated into plume-generated melts as they trav-eled through the asthenosphere before reaching the continental lithosphere. The relatively lowinitial ENd values observed in flows of the Taylors Falls and upper Clam Falls sections indicategreater involvement of a continental lithosphere (crust and/or mantle) component than has pre-viously been recognized during the early part of "Main Stage" volcanism in the rift. In all ofthese cases, the observed isotopic differences between the Chengwatana Volcanics and othervolcanic sequences of the MCR might be related to the "off-axis" position of the ChengwatanaVolcanics relative to the location of the inferred plume head.

References Cited(1) Wirth, Karl R., Vervoort, Jeffrey D., and Naiman, Zachary J, 1997. The Chengwatana Volcanics, Wisconsinand Minnesota: Petrogenesis of the southernmost volcanic rocks exposed in the Midcontinent rift. CanadianJournal of Earth Science, 34: 536-548. (2) Naiman, Zachary J., 1997. Petrogenesis of the ChengwatanaVolcanics:1.1 Ga Midcontinent rift lavas in Minnesota and Wisconsin. Bachelors Thesis, Macalester College. (3) Shirey,S.B., Klewin, K.W., Berg, J.H., and Carlson, R.W.. 1994. Temporal changes in the sources of flood basalts:isotopic and trace element evidence from the 1100 Ma old Keweenawan Mamainse Point Formation, Ontario,Canada: Geochimica et Cosmochimica Acta, 58: 4475-4490. (4) Nicholson Suzanne W., Shirey, Steven B.,Shultz, Klaus J., and Green, John C., 1997. Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts:implications for multiple mantle sources during rift development. Canadian Journal of Earth Science, 34: 504-520. (5) White, Robert S., 1997. Mantle temperatures and lithospheric thinning beneath the Midcontinent riftsystem: evidence from magmatism and subsidence. Canadian Journal of Earth Science, 34: 464-475.

97

-12 -10 -8 -6 -4

Initial ENd

= -1 - +4) elsewhere in the rift. Most models of the mag-

matic evolution of the Midcontinent rift assume that the large volume of magmas were produced primarily from an en- riched mantle plume with initial eNd near zero3'4. Lavas with negative ENd values have been modelled by contamination of mantle plume melts with conti- nental lithospheric mantle (ENd near -9) or with continental crust (ENd < -10). In contrast, lavas with positive ENd values has been modelled by a combination of enriched mantle plume and de-

Initial £\

pleted asthenospheric mantle (ENd > 6) that may have been entrained in the plume head5. Fol- lowing this model, the initial ENd values of Chengwatana basalts suggest a progression of mantle sources from enriched mantle plume+depleted asthenospheric mantle (flows with > 1 near the base of the Clam Falls section) to enriched mantle plume that has been variably contami- nated with lithosphere (the remainder of the Clam Falls section and the overlying Taylors Falls section). The inferred progression of mantle sources observed in the Chengwatana Volcanics (plume+asthenosphere -> plume +I- continental lithosphere) occurs at a time when mantle sources elsewhere in the rift are inferred to be changing from plume+continental lithosphere to plume+asthenosphere. If the depleted mantle component of early Clam Falls flows originated from material that was entrained in the plume head, as has been suggested by several for other parts of the rift3'4y5 then the proportion of melts produced from entrained asthenospheric mantle in the plume varied in space and time throughout the rift. Alternatively, the depleted mantle component may have been differentially incorporated into plume-generated melts as they trav- eled through the asthenosphere before reaching the continental lithosphere. The relatively low initial EM values observed in flows of the Taylors Falls and upper Clam Falls sections indicate greater involvement of a continental lithosphere (crust andlor mantle) component than has pre- viously been recognized during the early part of "Main Stage" volcanism in the rift. In all of these cases, the observed isotopic differences between the Chengwatana Volcanics and other volcanic sequences of the MCR might be related to the "off-axis" position of the Chengwatana Volcanics relative to the location of the inferred plume head.

References Cited (1) Wirth, Karl R., Vervoort, Jeffrey D., and Naiman, Zachary J, 1997. The Chengwatana Volcanics, Wisconsin and Minnesota: Petrogenesis of the southernmost volcanic rocks exposed in the Midcontinent rift. Canadian Journal of Earth Science, 34: 536-548. (2) Naiman, Zachary J., 1997. Petrogenesis of the ChengwatanaVolcanics: 1.1 Ga Midcontinent rift lavas in Minnesota and Wisconsin. Bachelors Thesis, Macalester College. (3) Shirey, S.B., Klewin, K.W., Berg, J.H., and Carlson, R.W.. 1994. Temporal changes in the sources of flood basalts: isotopic and trace element evidence from the 1100 Ma old Keweenawan Marnainse Point Formation, Ontario, Canada: Geochimica et Cosmochimica Acta, 58: 4475-4490. (4) Nicholson Suzanne W., Shirey, Steven B., Shultz, Klaus J., and Green, John C., 1997. Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development. Canadian Journal of Earth Science, 34: 504- 520. (5) White, Robert S., 1997. Mantle temperatures and lithospheric thinning beneath the Midcontinent rift system: evidence from magmatism and subsidence. Canadian Journal of Earth Science, 34: 464-475.

THE POWDER MILL GROUP REVISITED: BASAL VOLCANIC ROCKS OF ThEMIDCONTINENT RIFT SYSTEM ON THE SOUTH SHORE OF LAKE SUPERIOR

NICHOLSON, Suzanne W. USGS, National Center, MS 954, Reston, VA 20192and WOODRUFF, Laurel G., USGS, 2280 Woodale Drive, Mounds View MN 55112

In northern Wisconsin and Michigan the Powder Mill Group comprises the oldest volcanicrocks related to the 1.1 Ga Midcontinent rift system (MRS). In the more than twenty yearssince Hubbard (1975) first separated the Powder Mill Group from the overlying PortageLake Volcanics, an abundance of geophysical, structural, geochemical, isotopic, andchronological data for Midcontinent rift rocks has become available. This report integratesnew and existing information to provide a better understanding of the early magmatichistory of the MRS.

The Powder Mill Group consists of the basal Bessemer Quartzite, and the reverselymagnetized igneous rocks of the Siemens Creek Volcanics and overlying Kallander CreekVolcanics. The outcrop distribution extends for more than 180 km along strike, from justwest of the Lake Owen fault near Cable, WI discontinuously eastward to Silver Mountain,Ml. The true extent of the group is unknown because it is either structurally truncated orunconformably overlain by younger rocks. However, seismic reflection proffles suggest thatthe Powder Mill volcanic rocks extend laterally beneath Lake Superior.

The Siemens Creek Volcanics is subdivided informally into upper and lowermembers based on field, geochemical, and isotopic characteristics. The lower member of theSiemens Creek Volcanics is dominantly basalt, but indudes recently recognized basal high-MgO picritic flows. The lower member is thin (<50 m thick), has limited regional extent, andis distinctive because some flows contain augite phenocrysts, uncommon among youngerMRS basalts. Pillows formed in the basal few basalt flows that directly overlie the BessemerQuartzite. Basalt with minor basaltic andesite and andesite dominates the upper member,which is more widespread and thicker (up to 1.5 km) than the lower member. No ages havebeen determined for the Siemens Creek Volcanics, but by correlation with the basal units ofthe Osler Group in Ontario and the Nipigon sills, volcanism probably was initiated about1108 Ma.

High-MgO (picritic) rocks related to the Midcontinent rift magmatism are nowknown to occur near the base of the section in three areas: 1) in the Nipigon area in Ontarioassociated with dikes and sills (Sutdiffe, 1987); 2) at Mamainse Point in the basal 500 m ofthe MRS section (Berg and Kiewin, 1988); and 3) in the Club Lake, WI area west of theMineral Lake intrusion as recently identified picritic flows near the base of the SiemensCreek The high-MgO flows in the Siemens Creek Volcanics are strongly altered toserpentine, chlorite and talc: no primary mineralogy is present. An average of three analysesof these high-MgO flows (Ti02 = 1.95 wt %; A1203 8.89 wt %; MgO = 16.09 wt %) ischemically most similar to an analysis of a Nipigon picritic dike (Ti02 = 1.88 wt %; A1203 =7.72 wt %; MgO = 17.1 wt in having higher Ti02 and lower A1203 and MgO than apicritic average reported from Mamainse Point (Ti02 = 0.90 wt %; A1203 = 9.79 wt %; MgO= 19.89 wt %; Berg and Klewin, 1988). Picritic flows in the Siemens Creek Volcanics showsteep REE-element patterns (CeN/YbN = 18.6), consistent with derivation from partialmelting of an enriched mantle source at a depth of more than about 80 km.

Basalts from the lower Siemens Creek Volcanics show moderate Ti02, lower A1203and higher MgO than basalts from the upper Siemens Creek. REE patterns are steeper(CeN/YbN = 14.2), for the lower Siemens Creek than for overlying units. In addition, the twomembers of the Siemens Creek Volcanics can be distinguished on the basis of Nd isotopiccomposition: lower Siemens Creek basalts have initial Ed = -1 whereas the upper SiemensCreek basalts have initial ENd = -3.6.

The Kallander Creek Volcanics overlies the Siemens Creek Volcanics, and also hasbeen informally divided into upper and lower members. Both members range from basalt toandesite and rhyolite. The lowermost 1.5 km of the Kallander Creek consists of flood basalttypically containing plagioclase phenocrysts, which locally, form large radiating clusters. A

98

THE POWDER MILL GROUP REVISITED: BASAL VOLCANIC ROCKS OF THE MIDCONTINENT RIFT SYSTEM ON THE SOUTH SHORE OF LAKE SUPERIOR

NICHOLSON, Suzanne W. USGS, National Center, MS 954, Reston, VA 20192 and WOODRUFF, Laurel G., USGS, 2280 Woodale Drive, Mounds View MN 55112

In northern Wisconsin and Michigan the Powder Mill Group comprises the oldest volcanic rocks related to the 1.1 Ga Midcontinent rift system (MRS). In the more than twenty years since Hubbard (1975) first separated the Powder Mill Group from the overlying Portage Lake Volcanics, an abundance of geophysical, structural, geochemical, isotopic, and chronological data for Midcontinent rift rocks has become available. This report integrates new and existing information to provide a better understanding of the early magmatic history of the MRS.

The Powder Mill Group consists of the basal Bessemer Quartzite, and the reversely magnetized igneous rocks of the Siemens Creek Volcanics and overlying Kallander Creek Volcanics. The outcrop distribution extends for more than 180 km along strike, from just west of the Lake Owen fault near Cable, WI discontinuously eastward to Silver Mountain, ML The true extent of the group is unknown because it is either structurally truncated or unconformably overlain by younger rocks. However, seismic reflection profiles suggest that the Powder Mill volcanic rocks extend laterally beneath Lake Superior.

The Siemens Creek Volcanics is subdivided informally into upper and lower members based on field, geochemical, and isotopic characteristics. The lower member of the Siemens Creek Volcanics is dominantly basalt, but includes recently recognized basal high- MgO picritic flows. The lower member is thin ( ~ 5 0 m thick), has limited regional extent, and is distinctive because some flows contain augite phenocrysts, uncommon among younger MRS basalts. Pillows formed in the basal few basalt flows that directly overlie the Bessemer Quartzite. Basalt with minor basaltic andesite and andesite dominates the upper member, which is more widespread and thicker (up to 1.5 km) than the lower member. No ages have been determined for the Siemens Creek Volcanics, but by correlation with the basal units of the Osier Group in Ontario and the Nipigon sills, volcanism probably was initiated about 1108 Ma.

High-MgO (picritic) rocks related to the Midcontinent rift magmatism are now known to occur near the base of the section in three areas: 1) in the Nipigon area in Ontario associated with dikes and sills (Sutcliffe, 1987); 2) at Mamainse Point in the basal 500 m of the MRS section (Berg and Klewin, 1988); and 3) in the Club Lake, WI area west of the Mineral Lake intrusion as recently identified picritic flows near the base of the Siemens Creek. The high-MgO flows in the Siemens Creek Volcanics are strongly altered to serpentine, chlorite and talc: no primary mineralogy is present. An average of three analyses of these high-MgO flows (Ti02 = 1.95 wt %; A1203 = 8.89 wt %; MgO = 16.09 wt %) is chemically most similar to an analysis of a Nipigon picritic dike (Ti02 = 1.88 wt %; A&03 = 7.72 wt %; MgO = 17.1 wt %) in having higher Ti02 and lower A1203 and MgO than a picritic average reported from Mamainse Point (Ti02 = 0.90 wt %; A l a = 9.79 wt %; MgO = 19.89 wt %; Berg and Klewin, 1988). Picritic flows in the Siemens Creek Volcanics show steep REE-element patterns (CeN/YbN = 18.6), consistent with derivation from partial melting of an enriched mantle source at a depth of more than about 80 km.

Basalts from the lower Siemens Creek Volcanics show moderate Ti02, lower and higher MgO than basalts from the upper Siemens Creek. REE patterns are steeper (CeN/YbN = 14.2), for the lower Siemens Creek than for overlying units. In addition, the two members of the Siemens Creek Volcanics can be distinguished on the basis of Nd isotopic composition: lower Siemens Creek basalts have initial £v = -1 whereas the upper Siemens Creek basalts have initial EM,, = -3.6.

The Kallander Creek Volcanics overlies the Siemens Creek Volcanics, and also has been informally divided into upper and lower members. Both members range from basalt to andesite and rhyolite. The lowermost 1.5 km of the Kallander Creek consists of flood basalt typically containing plagioclase phenocrysts, which locally, form large radiating clusters. A

rhyolite flow near the top of the lower Kallander Creek gives an age of 1107.3 ± 1.6 Ma(Davis and Green, 1997). The upper member (about 2 km thick) is dominated by andesiteand is considered to represent the extrusive products of a localized magmatic system, theMellen complex (Cannon et al. 1993). A thick laterally extensive rhyolite at the top of theupper member of the Kallander Creek gives an age of 1099.0 ± 2.6 Ma (Zartman et al. 1997).

Basalts of the lower member of the Kallander Creek Volcanics are characterized bytheir high Ti02, and low MgO, Cr and Ni, suggesting strong fractionation. Although the REEpatterns for this group are similar to those for the lower Siemens Creek basalts, theelemental REE abundances are substantially higher for this group. In addition, not only areincompatible trace element ratios such as CeN/YbN and Ta/Th similar for both lowermembers of the Kallander Creek and Siemens Creek Volcanics, their Nd isotopiccompositions are similar as well (initial Nd = -1). In contrast, basalts of the upperKallander Creek show lower incompatible trace element ratios and much less steep REEpatterns than the lower Kallander Creek member, despite the fact that their major elementcompositions are similar. The Nd isotopic composition of the upper Kallander Creek yieldsan initial d = -1.5, slightly lower than the lower member.

The Siemens Creek and Kallander Creek Volcanics represent the initiation of acontinental rifting event that has been attributed to the upweiling of a mantle plume beneaththe Lake Superior region. The diversity of basalt compositions, induding picrites, that arerepresentative of the first several million years of rifting are characteristic of othercontinental rift settings in which mantle plumes play a dominant role. Based on thegeochemical and isotopic characteristics, both the lower Siemens Creek and lower KallanderCreek units are probably the products of varying degrees of partial melting of an enrichedsource (mantle plume) at great depth with the lower Kallander Creek having undergonesignificant fractionation. The upper Siemens Creek member probably represents largerdegrees of partial melting of a plume source at shallower depths, producing melts that mayhave interacted with lithosphere and undergone limited fractionation. The upper KailanderCreek appears to have undergone significant fractionation probably in shallow crustalmagma chambers now seen as layered intrusions.

References:

Berg, J.H, and Kiewin, K.W., 1988, High-MgO lavas from the Keweenawan midcontinent rift nearMamainse Point, Ontario: Geology, v. 16, pp. 1003-1006.

Cannon, W.F., Nicholson, S.W., Zartman, R.E., Peterman, Z.E., and Davis, D.W., 1993, TheKallander Creek volcanics—a remnant of a Keweenawan central volcano centered near Mellen,Wisconsin: Institute on Lake Superior Geology Proceedings, Program and Abstracts, v. 39, p. 20-21.

Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent rift inwestern Lake Superior and implications for its geodynamic evolution: Canadian Journal ofEarth Sciences, v. 34, pp. 476-488.

Hubbard, H.A., 1975, Lower Keweenawan volcanic rocks of Michigan and Wisconsin: U.S.Geological Survey, Journal of Research, v.3, no.5, pp. 529-541.

Sutcliffe, R.H., 1987, Petrology of Middle Proterozoic diabases and picrites from Lake Nipigon,Canada: Contributions to Mineralogy and Petrology, v. 96, pp. 201-211.

Zartman, R. E., Nicholson, S.W., Cannon, W.F., and Morey, G.B., 1997, U-Th-Pb zircon ages of someKeweenawan Supergroup rocks from the south shore of Lake Superior: Canadian Journal ofEarth Sciences, v. 34, pp. 549-561.

99

rhyolite flow near the top of the lower Kallander Creek gives an age of 1107.3 Â 1.6 Ma (Davis and Green, 1997). The upper member (about 2 krn thick) is dominated by andesite and is considered to represent the extrusive products of a localized magmatic system, the Mellen complex (Cannon et al. 1993). A thick laterally extensive rhyolite at the top of the upper member of the Kallander Creek gives an age of 1099.0 Â 2.6 Ma (Zartman et al. 1997).

Basalts of the lower member of the Kallander Creek Volcanics are characterized by their high TiOD and low MgO, Cr and Ni, suggesting strong fractionation. Although the REE patterns for this group are similar to those for the lower Siemens Creek basalts, the elemental REE abundances are substantially higher for this group. In addition, not only are incompatible trace element ratios such as C e N / m and Ta/Th similar for both lower members of the Kallander Creek and Siemens Creek Volcanics, their Nd isotopic compositions are similar as well (initial ENd = -1). In contrast, basalts of the upper Kallander Creek show lower incompatible trace element ratios and much less steep REE patterns than the lower Kallander Creek member, despite the fact that their major element compositions are similar. The Nd isotopic composition of the upper Kallander Creek yields an initial Eyy = -1.5, slightly lower than the lower member.

The Siemens Creek and Kallander Creek Volcanics represent the initiation of a continental rifting event that has been attributed to the upwelling of a mantle plume beneath the Lake Superior region. The diversity of basalt compositions, including picrites, that are representative of the first several million years of rifting are characteristic of other continental rift settings in which mantle plumes play a dominant role. Based on the geochemical and isotopic characteristics, both the lower Siemens Creek and lower Kallander Creek units are probably the products of varying degrees of partial melting of an enriched source (mantle plume) at great depth with the lower Kallander Creek having undergone significant fractionation. The upper Siemens Creek member probably represents larger degrees of partial melting of a plume source at shallower depths, producing melts that may have interacted with lithosphere and undergone limited fractionation. The upper Kallander Creek appears to have undergone significant fractionation probably in shallow crustal magma chambers now seen as layered intrusions.

References:

Berg, J.H., and Klewin, K.W., 1988, High-MgO lavas from the Keweenawan midcontinent rift near Marnainse Point, Ontario: Geology, v. 16, pp. 1003-1006.

Cannon, W.F., Nicholson, S.W., Zartman, R.E., Peterman, Z.E., and Davis, D.W., 1993, The Kallander Creek volcanics-a remnant of a Keweenawan central volcano centered near Mellen, Wisconsin: Institute on Lake Superior Geology Proceedings, Program and Abstracts, v. 39, p. 20- 21.

Davis, D.W., and Green, J.C., 1997, Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution: Canadian Journal of Earth Sciences, v. 34, pp. 476-488.

Hubbard, H.A., 1975, Lower Keweenawan volcanic rocks of Michigan and Wisconsin: U.S. Geological Survey, Journal of Research, v. 3, no. 5, pp. 529-541.

Sutcliffe, R.H., 1987, Petrology of Middle Proterozoic diabases and picrites from Lake Nipigon, Canada: Contributions to Mineralogy and Petrology, v. 96, pp. 201-211.

Zartman, R. E., Nicholson, S.W., Cannon, W.F., and Morey, G.B., 1997, U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior: Canadian Journal of Earth Sciences, v. 34, pp. 549-561.

GIS BASED MINERAL POTENTIAL ANALYSIS FOR LODE-GOLD AND MASSIVESULFIDE DEPOSITS IN AN ARCHEAN TERRANE OF NORTHERN MINNESOTA

DEAN M. PETERSON and DR. RONALD L. MORTON

Economic Volcanology Research Lab, Geology Department, University of Minnesota - Duluth,Duluth, Minnesota, USA 55812

The last decade has been a period of worldwide political and economic change. The globalization ofthe world's economy has created new challenges and opportunities for the mining and mineralexploration industry. Many areas, which previously were strictly the domain of nationalist enterprises(Southeast Asia, South America, and the former Soviet Union), are now open to mineral explorationby companies from North America and Europe. Many exploration companies are focusing largeportions of their resources away from North America and into these recently opened countries. Thisswitch in regional area selection has had a negative impact on the amount of mineral explorationconducted in the Archean terranes of Minnesota. Aside from the changes in global economics, otherspecific reasons for the lack of recent mineral exploration in the Archean terranes of Minnesotainclude the following:

no local prospectors are "beating the bush" and discovering prospects in Minnesota> misconception that mine permitting would be as cumbersome as with Wisconsin VMS deposits> absence of success in previous gold and massive sulfide exploration programs' most mineral industry geologists have little knowledge of the geology of Minnesota, and

the identification of specific exploration target areas are normally not included in geologicalreports and maps

The discovery of economic lode-gold and massive sulfide deposits will only occur followingprolonged exploration in the Archean terranes of Minnesota by the mineral industry. To increaseexploration activity, state agencies in Minnesota have completed extensive geological andgeophysical mapping, completed several mineral potential studies, and currently are promotingMinnesota's geology and mineral potential to the mineral industry. However, because of extensiveglacial cover, an absence of producing mines, and the aforementioned reasons, the mineral industryrequires additional geological incentives in order to start exploration programs in the Archeanterranes of Minnesota. These incentives could include detailed descriptions of known prospects,detailed mineral potential studies, and specific targets and geologic criteria upon which to baseexploration programs.

This study is an attempt to generate specific lode-gold and massive sulfide target areas that themineral industry can use as a premise to begin exploration programs in the state. Current ore depositmodels have been integrated into GIS databases of important Canadian mining camps (Timmins,Kirkland Lake, Hemlo, and Sturgeon Lake (Figure 1)), and lode-gold and massive sulfide explorationmodels developed from detailed spatial analysis. Exploration target areas in the Minnesota study areahave been generated from the integration of these models into a detailed GIS of a large Archeantenane of northern Minnesota (Figure 2).

The methods developed to complete this study comprise five separate, but related themes:

> Thorough research on the theory and methods of ore deposit modeling, and the geological settingof Archean lode-gold and massive sulfide deposits.

100

GIs BASED MINERAL POTENTIAL ANALYSIS FOR LODE-GOLD AND MASSIVE SULFIDE DEPOSITS IN AN ARCHEAN TERRANE OF NORTHERN MINNESOTA

DEAN M. PETERSON and DR. RONALD L. MORTON

Economic Volcanology Research Lab, Geology Department, University of Minnesota - Duluth, Duluth, Minnesota, USA 55812

The last decade has been a period of worldwide political and economic change. The globalization of the world's economy has created new challenges and opportunities for the mining and mineral exploration industry. Many areas, which previously were strictly the domain of nationalist enterprises (Southeast Asia, South America, and the former Soviet Union), are now open to mineral exploration by companies from North America and Europe. Many exploration companies are focusing large portions of their resources away from North America and into these recently opened countries. This switch in regional area selection has had a negative impact on the amount of mineral exploration conducted in the Archean terranes of Minnesota. Aside from the changes in global economics, other specific reasons for the lack of recent mineral exploration in the Archean terranes of Minnesota include the following:

> no local prospectors are "beating the bush" and discovering prospects in Minnesota > misconception that mine permitting would be as cumbersome as with Wisconsin VMS deposits > absence of success in previous gold and massive sulfide exploration programs > most mineral industry geologists have little knowledge of the geology of Minnesota, and > the identification of specific exploration target areas are normally not included in geological

reports and maps

The discovery of economic lode-gold and massive sulfide deposits will only occur following prolonged exploration in the Archean terranes of Minnesota by the mineral industry. To increase exploration activity, state agencies in Minnesota have completed extensive geological and geophysical mapping, completed several mineral potential studies, and currently are promoting Minnesota's geology and mineral potential to the mineral industry. However, because of extensive glacial cover, an absence of producing mines, and the aforementioned reasons, the mineral industry requires additional geological incentives in order to start exploration programs in the Archean terranes of Minnesota. These incentives could include detailed descriptions of known prospects, detailed mineral potential studies, and specific targets and geologic criteria upon which to base exploration programs.

This study is an attempt to generate specific lode-gold and massive sulfide target areas that the mineral industry can use as a premise to begin exploration programs in the state. Current ore deposit models have been integrated into GIs databases of important Canadian mining camps (Tirnrnins, Kirkland Lake, Hernlo, and Sturgeon Lake (Figure I)), and lode-gold and massive sulfide exploration models developed from detailed spatial analysis. Exploration target areas in the Minnesota study area have been generated from the integration of these models into a detailed GIs of a large Archean terrane of northern Minnesota (Figure 2).

The methods developed to complete this study comprise five separate, but related themes:

9 Thorough research on the theory and methods of ore deposit modeling, and the geological setting of Archean lode-gold and massive sulfide deposits.

Standardized geological compilations of four analog mining camps from the Superior Province ofOntario, Canada. These compilations are integrated with Ontario Geologic Survey (OGS) drillhole, geochemical and mineral deposit inventory databases and have been converted into GISformat.

) Standardized geological, geophysical, andgeochemical compilation of the Minnesotastudy area, and the conversion of thiscompilation into GIS format.

Development of mineral exploration modelsand targeting criteria generated fromknowledge-based queries of the analog GISdatasets. These models integrate Archeanlode-gold and massive sulfide deposit modelswith spatial features in the analog GISdatabases. Specific geological criteria havebeen developed that define the location of thelode-gold and massive sulfide deposits withinthe analog GIS datasets.

_____________________________________

> Thorough mineral potential evaluation for lode-gold and massive sulfide deposits in theMinnesota study area. This evaluation is based upon spatial analysis of the Minnesota GISdatabase using targeting criteria developed from the exploration models generated from queries ofthe analog GIS databases.

Ely' Towns

....— Major Shear Zones

Simplified geology and location of USGS 1:24,000 scalequadrangle maps of the Minnesota study area.

101

Figure 1 Location map of the study areas.

10 0 10 20 Kilometers

Figure 2

Simolified Geoloey

Proterozoic Rocks

Wawa SubprovinceVennilion Granhtic Complex

Mammnt&

Greenstone TerraneInternal PlutonsGiants Range Granitic Complex

Standardized geological compilations of four analog mining camps from the Superior Province of Ontario, Canada. These compilations are integrated with Ontario Geologic Survey (OGS) drill hole, geochemical and mineral deposit inventory databases and have been converted into GIs format.

I Standardized geological, geophysical, and geochemical compilation of the Minnesota study area, and the conversion of this compilation into GIs format.

Development of mineral exploration models and targeting criteria generated from knowledge-based queries of the analog GIs datasets. These models integrate Archean lode-gold and massive sulfide deposit models with spatial features in the analog GIs databases. Specific geological criteria have been developed that define the location of the

I lode-gold and massive sulfide deposits within the analog GIs datasets.

Figure 1 Location map of the study areas. I

Thorough mineral potential evaluation for lode-gold and massive sulfide deposits in the Minnesota study area. This evaluation is based upon spatial analysis of the Minnesota GIs database using targeting criteria developed from the exploration models generated from queries of the analog GIs databases.

Ely* Towns

Major S k a ~ Zones

Simvlified Geology = Protemzaic Rocks

Wawa Subpronnce Vemilion G r a ~ t i c Complex Greenstone Temne Internal Plutolw Giants Range Gramtic Complex

Figure 2 Simplified geology and location of USGS 1:24,0 scale quadrangle maps of the Minnesota study area.

IMPROVED GEOLOGIC SENSITIVITY AND VULNERABILITY ASSESSMENTS OF GROUNDWATERPOLLUTION POTENTIAL THROUGH APPLICATION OF FUZZY LOGIC

PFANNKUCH, H.O., and PAULSON, Richard, A., Department of Geology and Geophysics,University of Minnesota, 310 Pillsbury Drive S.E. Minneapolis, MN 55455

Vulnerability/sensitivity assessments belong to the general problem class of suitabilityassessments. In general, these are management tools to evaluate the feasibility, reliabilityor risk of using a given physical entity or process. In the environmental or geologic contextit is a decision aid in a resource protection, or management plan.In general, the procedure has three components: a physical model that describes the processin a risk analysis framework, and a methodology to aggregate individual partialvulnerability indices to a global vulnerability index. In the context of a vulnerabilityassessment to groundwater contamination the physical model simulates contaminanttransport along pathways from the land surface to the groundwater body. The pathway issubdivided into a sequence of zones in which different transport processes and parameterspredominate. These compartments are surface parcel, soil zone, vadose zone, capillaryfringe, and the groundwater flow zone which may be further subdivided if more detail iswanted.In a perfectly deterministic world where all transport processes can be described asmathematically exact, where flow boundary geometry, boundary conditions and physico-chemical parameters are known everywhere in the domain of interest, the transport processcan be modeled exactly along continuous pathways, given appropriate computational power.Then, concentration of the contaminant everywhere in space and time is known. Its residencetime in the system and its exposure to and impact of exposure on the target can be quantified.From this, choices about protection of the resource, the environment, or human targets canbe made. This is all that is needed for solving the problem, and no further steps arenecessary.Alas, the world is not isotropic, homogeneous and perfect, and mathematical simulationmodels even less so. Therefore, severely restrictive approximations and simplifyingassumptions have to be introduced to represent the transport process in an half-wayadequate manner. The approximations concern the mathematical description of the process,limited by insufficient and infrequent information, and by inadequate knowledge of impacts.The greater the simplifications, the greater the uncertainty that the model results reflectreality and produce reliable predictions or vulnerability designations.This uncertainty traditionally has been handled by a risk analysis approach. The likelihoodof a contaminant particle reaching the groundwater target and its likelihood to produceunwanted consequences has been expressed in terms of conditional probabilities andstatistical estimates as confidence limits or ranges. Inadequacies in the geologic databasehave been handled by statistical methods such as Monte-Carlo approaches or kriging.A groundwater system is extensive, three dimensional, dynamic and non-homogeneous. Itsproperties and states must be spatially referenced for most efficient analysis and maprepresentation. Georeferencing is best carried out by a Geographic Information System(GIS) which also performs aggregation functions able to handle map overlay-indexprocedures using built in algorithms. The information for each hydrogeologic flowcompartment is represented as an information layer in the GIS, and can be combined into anaggregate or monothematic layer such as a global vulnerability map layer.

Traditional methods of groundwater sensitivity assessment translate the protection affordedby various elements of the geological conditions and materials between the surface and thegroundwater body into vulnerability indices, from which, pollution potential can be

102

IMPROVED GEOLOGIC SENSITIVITY AND VULNERABILITY ASSESSMENTS OF GROUNDWATER POLLUTION POTENTIAL THROUGH APPLICATION OF FUZZY LOGIC

PFANNKUCH, H.O., and PAULSON, Richard, A., Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive S.E. Minneapolis, MN 55455

Vulnerability/sensitivity assessments belong to the general problem class of suitability assessments. In general, these are management tools to evaluate the feasibility, reliability or risk of using a given physical entity or process. In the environmental or geologic context it is a decision aid in a resource protection, or management plan. In generall the procedure has three components: a phvsical model that describes the process in a risk analvsis framework, and a methodology to aggregate individual partial vulnerability indices to a alobal vulnerabilitv index. In the context of a vulnerability assessment to groundwater contamination the physical model simulates contaminant transport along pathways from the land surface to the groundwater body. The pathway is subdivided into a sequence of zones in which different transport processes and parameters predominate. These compartments are surface parcell soil zone, vadose zone, capillary fringe, and the groundwater flow zone which may be further subdivided if more detail is wanted. In a perfectly deterministic world where all transport processes can be described as mathematically exact, where flow boundary geometry, boundary conditions and physico- chemical parameters are known everywhere in the domain of interest, the transport process can be modeled exactly along continuous pathwaysl given appropriate computational power. Then, concentration of the contaminant everywhere in space and time is known. Its residence time in the system and its exposure to and impact of exposure on the target can be quantified, From this, choices about protection of the resource, the environment, or human targets can be made. This is all that is needed for solving the problem, and no further steps are necessary. Alas, the world is not isotropic, homogeneous and perfect, and mathematical simulation models even less so. Therefore, severely restrictive approximations and simplifying assumptions have to be introduced to represent the transport process in an half-way adequate manner. The approximations concern the mathematical description of the process, limited by insufficient and infrequent information, and by inadequate knowledge of impacts. The greater the simplificationsl the greater the uncertainty that the model results reflect reality and produce reliable predictions or vulnerability designations. This uncertainty traditionally has been handled by a risk analysis approach. The likelihood of a contaminant particle reaching the groundwater target and its likelihood to produce unwanted consequences has been expressed in terms of conditional probabilities and statistical estimates as confidence limits or ranges. Inadequacies in the geologic database have been handled by statistical methods such as Monte-Carlo approaches or kriging. A groundwater system is extensive, three dimensional, dynamic and non-homogeneous. Its properties and states must be spatially referenced for most efficient analysis and map representation. Georeferencing is best carried out by a Geographic Information System (GIs) which also performs aggregation functions able to handle map overlay-index procedures using built in algorithms. The information for each hydrogeologic flow compartment is represented as an information layer in the GIsl and can be combined into an aggregate or monothematic layer such as a global vulnerability map layer.

Traditional methods of groundwater sensitivity assessment translate the protection afforded by various elements of the geological conditions and materials between the surface and the groundwater body into vulnerability indices, from which, pollution potential can be

estimated. One such methodology is USEPA's DRASTIC. Many of these procedures lackinternal consistency, and give no explicit rationale for assigning rating indices, rangeintervals and thresholds, weights to the various factors, and leave the choice of an additiveaggregation model unexplained.The Minnesota Department of Natural Resources' (MN DNR) guidelines stand on a muchfirmer conceptual basis. They link geologic sensitivity to travel time of a conservativetracer from the surface to the groundwater. Long travel times are equated to lowsensitivities, and short travel times to high sensitivity. In order to take into account theavailability of information for mappable factors it makes some sweeping simplifications. Ituses three factors: depth to water, geologic material in the vadose zone and material at thewater table and obtains the aggregate index through a binary decision tree procedure.Because of these simplifications the degree of uncertainty and the confidence limits arelarge. Introduction of a fuzzy set methodology will lead to a greater internal consistency ofthe rating process and facilitate the use of vague and descriptive data in a more appropriateway.

Fuzzy logic is a branch of set theory that allows objects degrees of belonging to a set ratherthan a binary yes or no description. Fuzzy numbers are defined by membership functionsthat describe their degree of belonging to one set or another, and thereby are able to dealwith verbal descriptors or linguistic variables, and to take into account the naturalvagueness and uncertainty inherent in geologic data. Fuzzy methodology provides thealgorithms to operate on vague rating classes in a consistent way. Fuzzy methodology willalso give more realistic boundary definitions between rating categories and classes.This research casts the definition of the MN DNR rating factors into a fuzzy context. Thisapproach is capable of: (1) more adequate representation of linguistic variables such aslithologic descriptions from well log information into manipulable index numbers,(2) combining of layer information in a GIS through fuzzy rule based instructions, and(3) to extend the number of factors that go into the overall evaluation to geohydrologic andland use layers.Fuzzy methodology application: The technique for performing a Level 2 geologic sensitivityanalysis as set forth by the MN DNR is relatively simple and straightforward. Thetechniques were intentionally designed so that water resource managers (or others) whomay not have training on, or access to, a geographic information system (GIS), would stillbe able to implement a hand-performed overlay analysis. This can be a very time consumingand tedious task for the analyst. In an effort to speed up and simplify especially thepreliminary assessment or project stage, an Excel spreadsheet based program is beingdeveloped using Microsoft Visual Basic. It accepts the various MN DNR specified vadose zoneparameters under consideration as input. The input data layers include: depth to water tablebelow the land surface, aquifer matrix material type at the water table, cumulativethickness of any low and moderate permeability units in the vadose zone. A second version ofthe program incorporates the principles of fuzzy methodology into the analysis. Thesensitivity ratings produced by the "fuzzy" version are compared to those produced usingthe present MN DNR technique.

103

estimated. One such methodology is USEPA's DRASTIC. Many of these procedures lack internal consistency, and give no explicit rationale for assigning rating indices, range intervals and thresholds, weights to the various factors, and leave the choice of an additive aggregation model unexplained. The Minnesota Department of Natural Resources' (MN DNR) guidelines stand on a much firmer conceptual basis. They link geologic sensitivity to travel time of a conservative tracer from the surface to the groundwater. Long travel times are equated to low sensitivities, and short travel times to high sensitivity. In order to take into account the availability of information for mappable factors it makes some sweeping simplifications. It uses three factors: depth to water, geologic material in the vadose zone and material at the water table and obtains the aggregate index through a binary decision tree procedure. Because of these simplifications the degree of uncertainty and the confidence limits are large. Introduction of a fuzzy set methodology will lead to a greater internal consistency of the rating process and facilitate the use of vague and descriptive data in a more appropriate way.

Fuzzy logic is a branch of set theory that allows objects degrees of belonging to a set rather than a binary yes or no description. Fuzzy numbers are defined by membership functions that describe their degree of belonging to one set or another, and thereby are able to deal with verbal descriptors or linguistic variables, and to take into account the natural vagueness and uncertainty inherent in geologic data. Fuzzy methodology provides the algorithms to operate on vague rating classes in a consistent way. Fuzzy methodology will also give more realistic boundary definitions between rating categories and classes. This research casts the definition of the MN DNR rating factors into a fuzzy context. This approach is capable of: (I) more adequate representation of linguistic variables such as lithologic descriptions from well log information into manipulable index numbers, (2) combining of layer information in a GIs through fuzzy rule based instructions, and (3) to extend the number of factors that go into the overall evaluation to geohydrologic and land use layers. Fuzzy methodology application: The technique for performing a Level 2 geologic sensitivity analysis as set forth by the MN DNR is relatively simple and straightforward. The techniques were intentionally designed so that water resource managers (or others) who may not have training on, or access to, a geographic information system (GIs), would still be able to implement a hand-performed overlay analysis. This can be a very time consuming and tedious task for the analyst. In an effort to speed up and simplify especially the preliminary assessment or project stage, an Excel spreadsheet based program is being developed using Microsoff Visual Basic. It accepts the various MN DNR specified vadose zone parameters under consideration as input. The input data layers include: depth to water table below the land surface, aquifer matrix material type at the water table, cumulative thickness of any low and moderate permeability units in the vadose zone. A second version of the program incorporates the principles of fuzzy methodology into the analysis. The sensitivity ratings produced by the "fuzzy" version are compared to those produced using the present MN DNR technique.

SEISMIC EVIDENCE OF PRE-NICKERSON SEDIMENTS INWESTERN LAKE SUPERIOR

Deborah E. Rausch and Nigel J WattrusLarge Lakes Observatory, University of Minnesota

Duluth, MN 55812

email:nwattrus@dumn.edu

The Superior lobe of the Laurentide Ice Sheet advanced and retreated out of the western basin ofLake Superior several times during the Wisconsin glaciation. Wright et al. (1973) recognized fourseparate phases of ice advance out of the Lake Superior basin, they are (in order of decreasing age):the St. Croix; Automba; Split Rock and Nickerson phases. The tills of the last two phases show anenrichment in clay, which represents the incorporation of proglacial lacustrine sediments with eachadvance. As the ice retreated during the Nickerson phase, a series of proglacial lakes formed againstthe edge of the retreating ice. The position of ice margins bordering these lakes have beendetermined by correlating subaerial moraines on either side of western Lake Superior (Farrand, 1969;Saarnisto, 1974) and Isle Royale (Huber, 1973). A series of subaqueous recessional moraines,presumably associated with this sequence of proglacial lakes, have been described in the western armof Lake Superior near Isle Royale (Landmesser et al., 1982).

The sediment record contained in western Lake Superior is intimately related to the retreat of theLaurentide Ice Sheet. An idealized soft sediment section for western Lake Superior based uponcoring (Farrand, 1963; Dell, 1971; Thomas and Dell, 1978) and high resolution seismic surveying(Johnson, 1980; Scholz, 1985; Anderson, 1997), consists of till overlain by a glaciolacustrine claysequence followed by a thin post-glacial Holocene clay. Recent data collected by the Large LakeObservatory (LLO) suggests the presence of a relict sediment section below the till, presumably fromearlier advances into the Lake Superior basin.

Since 1996, the Large Lakes Observatory (LLO) has collected over 2500 km of high resolutionsingle channel seismic reflection data in western Lake Superior. The surveyed area extends fromDuluth to Isle Royale and southeast to Houghton, Michigan. Most of this new data was collectedwith a ORE Geopulse system. The firing rate was 0.5 seconds and the average vessel speed was 6.5knots. The data were digitally recorded at rate of 0.5 milliseconds/sample for later post-surveyprocessing. Positioning information was derived from the ships GPS navigation system.

Images of the recently acquired data show most of the soft sediment section and bedrock-softsediment interface (acoustic basement). In water depths of less than 100 meters, thedevelopmentlpreservation of the glacial/post-glacial and Holocene clays above the till is negligibledue to current action on the lake floor. In the deeper portions of the lake, the near surface reflectionsare associated with the post-glacial Holocene clays, glaciolacustrine clays and a thin basal till (lessthan six meters thick). Presumably, this glacial/post-glacial sequence is associated with the lastadvance/retreat in the Lake Superior basin (Nickerson/post-Nickerson deposition). Many of recentseismic lines, especially those in the western portion of the survey area, exhibit buried channelscontaining thick sequences of pre-Nickerson deposits (Figure 1). There is often a well definedunconformity separating the pre-Nickerson sediments from the overlying younger section. Theunconformity clearly represents a period of erosion, perhaps associated with the last advance of iceacross the basin.

104

SEISMIC EVIDENCE OF Pm-NICKERSON SEDIMENTS IN WESTERN LAKE SUPERIOR

Deborah E. Rausch and Nigel J Wattrus Large Lakes Observatory, Universiiy of Minnesota

Duluth, MN 558 I2

email:nwattrus@d. umn. edu

The Superior lobe of the Laurentide Ice Sheet advanced and retreated out of the western basin of Lake Superior several times during the Wisconsin glaciation. Wright et al. (1973) recognized four separate phases of ice advance out of the Lake Superior basin, they are (in order of decreasing age): the St. Croix; Automba; Split Rock and Nickerson phases. The tills of the last two phases show an enrichment in clay, which represents the incorporation of proglacial lacustrine sediments with each advance. As the ice retreated during the Nickerson phase, a series of proglacial lakes formed against the edge of the retreating ice. The position of ice margins bordering these lakes have been determined by correlating subaerial moraines on either side of western Lake Superior (Farrand, 1969; Saarnisto, 1974) and Isle Royale (Huber, 1973). A series of subaqueous recessional moraines, presumably associated with this sequence of proglacial lakes, have been described in the western arm of Lake Superior near Isle Royale (Landmesser et al., 1982).

The sediment record contained in western Lake Superior is intimately related to the retreat of the Laurentide Ice Sheet. An idealized soft sediment section for western Lake Superior based upon coring (Farrand, 1963; Dell, 1971; Thomas and Dell, 1978) and high resolution seismic surveying (Johnson, 1980; Scholz, 1985; Anderson, 1997), consists of till overlain by a glaciolacustrine clay sequence followed by a thin post-glacial Holocene clay. Recent data collected by the Large Lake Observatory (LLO) suggests the presence of a relict sediment section below the till, presumably fiom earlier advances into the Lake Superior basin.

Since 1996, the Large Lakes Observatory (LLO) has collected over 2500 km of high resolution single channel seismic reflection data in western Lake Superior. The surveyed area extends from Duluth to Isle Royale and southeast to Houghton, Michigan. Most of this new data was collected with a ORE Geopulse system. The firing rate was 0.5 seconds and the average vessel speed was 6.5 knots. The data were digitally recorded at rate of 0.5 milliseconds/sample for later post-survey processing. Positioning information was derived fiom the ships GPS navigation system.

Images of the recently acquired data show most of the soft sediment section and bedrock-soft sediment interface (acoustic basement). In water depths of less than 100 meters, the development/preservation of the g1aciaVpost-glacial and Holocene clays above the till is negligible due to current action on the lake floor. In the deeper portions of the lake, the near surface reflections are associated with the post-glacial Holocene clays, glaciolacustrine clays and a thin basal till (less than six meters thick). Presumably, this glaciallpost-glacial sequence is associated with the last advancehetreat in the Lake Superior basin (Nickersodpost-Nickerson deposition). Many of recent seismic lines, especially those in the western portion of the survey area, exhibit buried channels containing thick sequences of pre-Nickerson deposits (Figure 1). There is often a well defined unconformity separating the pre-Nickerson sediments from the overlying younger section. The unconformity clearly represents a period of erosion, perhaps associated with the last advance of ice across the basin.

Seismic facies analysis suggests that the pre-Nickerson material is composed of a sequence of glacialtills and glaciolacustrine sediments. Presumably these represent relict sediments preserved fromearlier advances of the Superior Lobe in western Lake Superior. Very few of the cores collected inLake Superior penetrated sediment below the glaciolacustrine varved clays. One long core collectedof Split Rock on the Minnesota North Shore, however, recorded over 190 meters of glacial andglaciolacustrine sediments (Zumberge and Gast, 1961).

References.

Anderson, K.A., 1997, A seismic stratigraphic study of western Lake Superior (M.S. Thesis): University ofMinnesota, Duluth, Minnesota, 82 p.

Dell, C.!., 1971, Late Quaternary sedimentation in Lake Superior (Ph.D. Dissertation): University of Michigan,Ann Arbor, Michigan, 184 p.

Farrand, W.R., 1963, Preliminary report of Lake Superior Coring Program, Core Study, 33 p.Farrand, W.R., 1969, The Quaternary history of Lake Superior: International Association of Great Lakes

Research, p. 181-197.Huber, N.K., 1973, Glacial and postglacial history of Isle Royale National Park, Michigan: U.S. Geological

Survey Professional Paper 754-A.Johnson, T.C., 1980, Late glacial and postglacial sedimentation in Lake Superior based on seismic-reflection

profiles: Quaternary Research, p. 380-391.Landmesser, C.W., Johnson, T.C., and Wold, R.J., 1982, Seismic reflection study of recessional moraines

beneath Lake Superior and their relationship to regional deglaciation: Quaternary Research, p. 173-190.

Scholz, C.A., 1985, Sediment distribution and sedimentation rates in the western arm of Lake Superior using3.5 kHz seismic reflection profiles and 210Pb geochronology (M.S. Thesis): University of Minnesota,Duluth, Minnesota, 129 p.

Thomas, R.L. and Dell, C.I., 1978, Sediments of Lake Superior: Journal of Great Lakes Research, p. 264-275.Wright, H.E., Matsch, C.L., and Cushing, E.J., 1973, Superior and Des Moines Lobes: Geological Society of

America Memoir 136, p. 153-185.Zumberge, J.H., and Gast, P.G., 1961, Geological investigations in Lake Superior: Geotimes, p. 10-13.

sP

Figure 1. Geopulse seismic reflection data collected off the Minnesota North Shore southwest ofGrand Marais, Minnesota. Relict sediments are clearly preserved in a broad channel and areunconformably overlain by younger till and glaciolacustrine sediments of the Nickerson phase.

105

pC"

0C

I I

•1 . -I —-r'" - -

Seismic facies analysis suggests that the pre-Nickerson material is composed of a sequence of glacial tills and glaciolacustrine sediments. Presumably these represent relict sediments preserved from earlier advances of the Superior Lobe in western Lake Superior. Very few of the cores collected in Lake Superior penetrated sediment below the glaciolacustrine vaned clays. One long core collected of Split Rock on the Minnesota North Shore, however, recorded over 190 meters of glacial and glacio1acustrine sediments (Zumberge and Gast, 1961).

References.

Anderson? K.A., 1997, A seismic stratigraphic study of western Lake Superior (M.S. Thesis): University of Mimesota, Duluth, Minnesota, 82 p.

Dell, C.I., 1971, Late Quaternary sedimentation in Lake Superior (Ph.D. Dissertation): University of Michigan, Ann Arbor, Michigan, 184 p.

Farrand, W.R., 1963, Preliminary report of Lake Superior Coring Program, Core Study, 33 p. Farrand, W.R., 1969, The Quaternary history of Lake Superior: International Association of Great Lakes

Research, p. 18 1-1 97. Huber, N.K., 1973, Glacial and postglacial history of Isle Royale National Park, Michigan: U.S. Geological

Survey Professional Paper 754-A. Johnson, T.C., 1980, Late glacial and postglacial sedimentation in Lake Superior based on seismic-reflection

profiles: Quaternary Research, p. 380-391. Landmesser, C.W., Johnson, T.C., and Wold, R.J., 1982, Seismic reflection study of recessional moraines

beneath Lake Superior and their relationship to regional deglaciation: Quaternary Research, p. 173- 190.

Scholz, C.A., 1985, Sediment distribution and sedimentation rates in the western arm of Lake Superior using 3.5 kHz seismic reflection profiles and ' 'qb geochronology (M.S. Thesis): University of Minnesota, Duluth, Minnesota, 129 p.

Thomas, R.L. and Dell, C.I., 1978, Sediments of Lake Superior: Journal of Great Lakes Research, p. 264-275. Wright, H.E., Matsch, C.L., and Cushing, E.J., 1973, Superior and Des Moines Lobes: Geological Society of

America Memoir 136, p. 153-185. Zumberge, J.H.? and Gast, P.G., 1961, Geological investigations in Lake Superior: Geotimes, p. 10-13.

Figure 1. Geopulse seismic reflection data collected off the Minnesota North Shore southwest of Grand Marais, Minnesota. Relict sediments are clearly p r e s e ~ e d in a broad channel and are unconformably overlain by younger till and glaciolacustrine sediments of the Nickerson phase.

PRELIMINARY ORE MINERALOGY OF THE HERONTRACK SILVER-ZINC-COPPEROCCURRENCE, LUMBY LAKE AREA, ONTARIO, CANADA

SAINI-EIDUKAT, Bernhardt, Department of Geosciences, North Dakota StateUniversity, Fargo, ND 58105-5517 USA (sainieid@badlands.nodak.edu).BERNATCHEZ, Raymond, Atikokan Resources, Inc., Box 1376, 126 WillowRoad, Atikokan, Ontario, Canada (rbernatc@atikokan.lakeheadu.ca)

The Herontrack silver-zinc-copper occurrence occurs near Herontrack Lake in the Lumby

Lake greenstone belt. It lies in the southwestern part of the Archean Superior Province, near

Atikokan, Ontario. Recently discovered in the region of the famous Steep Rock iron mine,

the Herontrack occurrence contains numerous stratiform mineralized horizons, some of

which contain significant Ag mineralization.

The Lumby Lake greenstone belt occurs in the Wabigoon subprovince, near its border

with the Quetico subprovince (Fig. 1, from Davis and Jackson, 1988). It comprises a 60 km x

20 km synclinal supracrustal assemblage of mafic, ultramafic (komatiitic) and felsic

metavolcanic packages with subordinate metasedimentary units (Jackson and Chevalier,

1985). It contacts the older Marmion Lake tonalite batholith to the south, and is intruded by

the younger Norway Lake granite to the north, and by mafic dikes. Felsic units include lapilli

tuffs, quartz porphyries, and rhyolite flow breccias. To the west of the Lumby Lake area, the

northeasterly trending Red Paint Lake fault zone truncates the assemblage.

I '-..-_.4+.' ..L—r..JJ

I

____

*+T— : *

! * + e't *

,./ \ • * + + * Finlays0 Lake '* —* Belt. • * Marmion Lake Batholith . *

A + + * —---—-——- : +* : --+kilornetres

Figure 1. Geologic map of the western Wabigoon subprovince, showing the location of the LumbyLake area. From Davis and Jackson, 1988.

106

PRELIMINARY OREi MINERALOGY OF THE HERONTRACK SILVER-ZINC-COPPER OCCURRENCE, LUMBY LAKE AREA? ONTARIO, CANADA

SAINI-EIDUKAT? Bemhardt, Department of Geoscien~es~ North Dakota State University, Fargo? ND 58105-5517 USA (s~fieid@badlands.nod&.edu). BERNATCHEZ, Raymond, Atikokan Resources, Inc.? Box 1376? 126 Willow Road? Atikokan, Ontario, Canada (rbernatc @atikokan.lakeheadu.ca)

The Herontrack silver-zinc-copper occurrence occurs near Herontrack Lake in the Lumby

Lake greenstone belt. It lies in the southwestern part of the Archean Superior Province, near

Atikokan? Ontario. Recently discovered in the region of the famous Steep Rock iron mine,

the Herontrack occurrence contains numerous stratiform mineralized horizonsy some of

which contain significant Ag mineralization.

The Lumby Lake greenstone belt occurs in the Wabigoon subpro~ince~ near its border

with the Quetico subprovince (Fig. 1 from Davis and Jackson, 1988). It comprises a 60 km x

20 km synclinal supracrustal assemblage of mafic? ultramafic (komatiitic) and felsic

metavolcanic packages with subordinate metasedimentary units (Jackson and Chevalier?

1985). It contacts the older Marmion Lake tonalite batholith to the southy and is intruded by

the younger Norway Lake granite to the north? and by mafic dikes. Felsic units include lapilli

tuffs? quartz porphyriesy and rhyolite flow breccia. To the west of the Lumby Lake area? the

northeasterly trending Red Paint Lake fault zone truncates the assemblage.

Figure 1. Geologic map of the western Wabigoon subprovince, showing the location of the Lumby Lake area. From Davis and Jackson, 1988.

The lack of a metamorphic aureole in the supracrustal sequence against the Marmion

Lake tonalite indicates that the batholith was a basement unit upon which the Lumby Lake,

and possibly the Steep Rock Lake group, were deposited (Davis and Jackson, 1988). Davis

and Jackson (1988) dated felsic samples from the Lumby Lake area and the Marmion Lake

batholith to approximately 3 Ga, an age older than most greenstone belts.

During the mid 1990's, Atikokan Resources, Inc. prospected the Lumby Lake area and

discovered a high-grade silver occurrence on the west side of Herontrack lake. The

occurrence is hosted in a 1.5 km thick east-west trending intermediate to felsic volcanic

sequence in the area between Herontrack and Lumby Lakes (Staargaard, 1997). This

sequence thins to the east and west, indicating it might be an eruptive center. The felsic units

are fragmental, often containing quartz eyes, and can contain significant cherty units. Chip

samples (over one meter) indicate greater than 40 g/tonne Ag.

Thin and polished block sections of ore horizon samples are being investigated using

transmitted and reflected light microscopy analysis to determine the host rock and ore

mineralogy and genesis. The relationship between fold structures recognizable on both

outcrop and thin-section scales and mineralization indicates that the silver mineralization

may be locally remobilized. These fold structures, however, may be related to mafic dike

intrusion and not to regional shear. Mineralization occurs in chert units as disseminated

sphalerite, galena, native silver, acanthite, chalcopyrite and pyrite. Native silver and acanthite

occur as isolated grains 10 to 30 p.m in diameter and as veinlets 10—20 p.m wide. No Au was

detected in the Ag or the acanthite using EDS analysis. Electron microprobe analyses are

being undertaken to determine the amount of Ag contained in galena and trace element

composition of sphalerite. Continued mapping may show that the Herontrack silver

occurrence represents a locally remobilized silver-rich distal portion of a larger massive

sulfide system.

References cited:Jackson, M.C. and Chevalier, P., 1985, Precambrian Geology of the Lumby Lake Area,Western Part, Kenora District; Ontario Geological Survey, Geological Series - PreliminaryMap P.2828.

Davis, D.W., and Jackson, M.C., 1988, Geochronology of the Lumby Lake greenstone belt: a3 Ga complex within the Wabigoon Subprovince, northwest Ontario: Bull. G.S.A., v. 100, p.818-824.

Staargaard, C.F., 1997, Private report to Atikokan Resources, Inc.

107

I

The lack of a metamorphic aureole in the supracrustal sequence against the Marmion

Lake tonalite indicates that the batholith was a basement unit upon which the Lumby Lake,

and possibly the Steep Rock Lake group, were deposited (Davis and Jackson, 1988). Davis

and Jackson (1988) dated felsic samples from the Lumby Lake area and the Mannion Lake

batholith to approximately 3 Ga, an age older than most greenstone belts.

During the mid 1990's, Atikokan Resources, Inc. prospected the Lumby Lake area and

discovered a high-grade silver occurrence on the west side of Herontrack lake. The

occurrence is hosted in a 1.5 km thick east-west trending intermediate to felsic volcanic

sequence in the area between Herontrack and Lumby Lakes (Staargaard, 1997). This

sequence thins to the east and west, indicating it might be an eruptive center. The felsic units

are fragmental, often containing quartz eyes, and can contain significant cherty units. Chip

samples (over one meter) indicate greater than 40 ghonne Ag.

Thin and polished block sections of ore horizon samples are being investigated using

transmitted and reflected light microscopy analysis to determine the host rock and ore

mineralogy and genesis. The relationship between fold structures recognizable on both

outcrop and thin-section scales and mineralization indicates that the silver mineralization

may be locally remobilized. These fold structures, however, may be related to mafic dike

intrusion and not to regional shear. Mineralization occurs in chert units as disseminated

sphalerite, galena, native silver, acanthite, chalcopyrite and pyrite. Native silver and acanthite

occur as isolated grains 10 to 30 pm in diameter and as veinlets 10 - 20 pm wide. No Au was

detected in the Ag or the acanthite using EDS analysis. Electron microprobe analyses are

being undertaken to determine the amount of Ag contained in galena and trace element

composition of sphalerite. Continued mapping may show that the Herontrack silver

occurrence represents a locally remobilized silver-rich distal portion of a larger massive

sulfide system.

References cited: Jackson, M.C. and Chevalier, P., 1985, Precambrian Geology of the Lumby Lake Area, Western Part, Kenora District; Ontario Geological Survey, Geological Series - Preliminary Map P.2828.

Davis, D.W., and Jackson, M.C., 1988, Geochronology of the Lumby Lake greenstone belt: a 3 Ga complex within the Wabigoon Subprovince, northwest Ontario: Bull. G.S.A., v. 100, p. 8 18-824.

Staargaard, C.F., 1997, Private report to Atikokan Resources, Inc.

ALTERATION AND METAMORPHISM IN AN ARCHEAN LODE GOLD DEPOSIT,KItEMZAR MINE, GOUDREAU-LOCHALSH GOLD CAMP, ONTARIO

SALO, R.W. and KISSIN, S.A., Department of Geology, Lakehead University,Thunder Bay, Ontario, P7B SE!

The Kremzar Mine is located 45km northeast of Wawa, Ontario in the MichipicotenGreenstone Belt. The Kremzar is one of several former gold-producing mines in the vicinityincluding the Magino and Edwards Mines, as well as a number of smaller properties. Themine was most recently operated by Canamax Resources Inc. in the late 1980s, with a totalproduction of 46798 oz Au (Domville, 1998). The present study was undertaken while theproperty is under redevelopment as the Island Gold Project of Patricia Mines Inc.

The No.2 Zone, a gold-bearing vein system well exposed on the surface, was channel-sampled in order to study the alteration associated with the quartz-carbonate vein.Petrographic studies were carried out on 25 samples collected at O.5m intervals; 13 of thesewere selected for whole-rock analyses of major, minor and trace elements by XRF andNAA, respectively.

The vein system cuts metavolcanic rocks of intermediate composition, which containthe chlorite zone mineral assemblage chiorite-sericite-albite. Alteration is marked byintense carbonate and quartz flooding, as well as a number of changes in accessorymineralogy. Primary ilmenite is progressively replaced by sphene, and allanite occurs onlywithin the alteration zone. Zoisite, not present in the country rock, is abundant in thealtered zone. Weakly pleochroic, fine-grained biotite is closely associated with quartzveining and flooding.

Among major components, CaO increases inward toward the vein system, whereasnormally mobile components Na20 and K20 show little variation. Ti02 decreases slightly,although it usually immobile. Among trace elements, W shows a marked increase.Although REE are considered to be immobile in gold vein systems (McCuaig & Kerrich,1994), the light REEs, La and Ce, are markedly enriched in the alteration zone.

The presence of euhedral zoisite, minor wollastonite and prehnite and abundantcalcite produce an assemblage resembling that of a low-grade skarn. The skarnoidassemblage is the result of regional metamorphism subsequent to formation of the veinsystem, which has operated on the compositionally modified and carbonate-enrichedalteration zone. Multiple metamorphic events are consistent with three deformationalregimes documented by Arias & Helmstaedt (1989).

ReferencesArias, Z.G. & Helmstaedt, H., 1989. Grant 343 Structural evolution of central and east-

central Michipicoten (Wawa) Greenstone Belt, Superior Province, p. 210-226 jMilne, V.G., ed., Geoscience Research Grant Program, Summary of Research 1988-1989, Ont. Geol. Surv. Misc. Pap. 143, 237 p.

Domville, J., 1998. Patricia Mines. Ont. Prospector 1998, p. 3 1-32.McCuaig, T.C. & Kerrich, R., 1994. P-T-t-deformation-fluid characteristics of lode gold

deposits: Evidence from alteration systematics, p. 339-380 j Lentz, D.R., ed.,Alteration Processes Associated with Ore-forming Processes. Geol. Assoc. Can. ShortCourse Notes, v. 11, 467p.

*student author

108

ALTERATION AND METAMORPHISM IN AN ARCHEAN LODE GOLD DEPOSIT, KRJZMZAR MINE, GOUDREAU-LOCHALSH GOLD CAMP, ONTARIO

SALO, RW.* and KISSIN, S.A., Department of Geology, Lakehead University, Thunder Bay, Ontario, P7B SEl

The Kremzar h4he is located 45km northeast of Wawa, Ontario in the Michipicoten Greenstone Belt. The Kremzar is one of several former gold-producing mines in the vicinity including the Magino and Edwards Mines, as well as a number of smaller properties. The mine was most recently operated by Canamax Resources hc. in the late 1980~~ with a total production of 46 798 oz Au (Domville, 1998). The present study was undertaken while the property is under redevelopment as the Island Gold Project of Patricia Mines Inc.

The N0.2 Zone, a gold-bearing vein system well exposed on the surface, was channel- sampled in order to study the alteration associated with the quartz-carbonate vein. Petrographic studies were carried out on 25 samples collected at OSm intervals; 13 of these were selected for whole-rock analyses of major, minor and trace elements by and NAA, respectively.

The vein system cuts metavolcanic rocks of intermediate composition, which contain the chlorite zone mineral assemblage chlorite-sericite-albite. Alteration is marked by intense carbonate and quartz flooding, as well as a number of changes in accessory mineralogy. Primary ilmenite is progressively replaced by sphene, and allanite occurs only within the alteration zone. Zoisite, not present in the country rock, is abundant in the altered zone. Weakly pleochroic, fie-grained biotite is closely associated with quartz veining and flooding.

Among major components, CaO increases inward toward the vein system, whereas normally mobile components Na20 and K20 show little variation. Ti02 decreases slightly, although it usually immobile. Among trace elements, W shows a marked increase. Although REE are considered to be immobile in gold vein systems (McCuaig & Kemch, 1994), the light REEs, L a and Ce, are markedly enriched in the alteration zone.

The presence of euhedral zoisite, minor wollastonite and prehnite and abundant calcite produce an assemblage resembling that of a low-grade skarn. The skarnoid assemblage is the result of regional metamorphism subsequent to formation of the vein system, which has operated on the compositionally modified and carbonate-enriched alteration zone. Multiple metamorphic events are consistent with three deformational regimes documented by Arias & Helmstaedt (1989).

References Arias, Z.G. & Helmstaedt, H., 1989. Grant 343 Structural evolution of central and east-

central Michipicoten (Wawa) Greenstone Belt, Superior Province, p. 210-226 h Milne, V.G., ed., Geoscience Research Grant Program, Summary of Research 1988- 1989, Ont. Geol. Sum. Misc. Pap. 143, 237 p.

Domville, J., 1998. Patricia Mines. Ont. Prospector 1998, p. 31-32. McCuaig, T.C. & Kemch, R., 1994. P-T-t-deformation-fluid characteristics of lode gold

deposits: Evidence from alteration systematics, p. 339-380 Lentz, D.R., ed., Alteration Processes Associated with Ore-forming Processes. Geol. Assoc. Can. Short Course Notes, v. 11, 467p.

*student author

—

ADDITIONAL PALEOMAGNETIC RESULTS FOR A 1500 Ma MAFIC DIKE ATWATERLOO WISCONSIN

Schaper*, D. , Suess*, W. , Katzer*, L. ,and Kean, W. Department ofGeosciences,University of Wisconsin—Milwaukee P.O. Box 413,Milwaukee, Wi. 53201 (* student authors)

Recent expansion of the Michels Materials Quartzite Quarry(Gillen Quarry) at Waterloo Wi. has exposed additional outcropsof a near vertical mafic dike which cuts the quartzite in aNorth-South direction. The new exposure is approximately 1.5meters wide and about 50 meters long located on the NE topsurface of the quarry. The other exposure previously reported byKean(1994) is on a vertical wall in the Southwest corner of thequarry (Luther,1997). The two exposures are on line and appear tobe the same dike. The age of the dike is assumed to be 1500 Ma,which is the age of mafic material from a drill core in thenearby Portland Quarry (Aldrich et.al., 1959) and similar to theage of a pegmatite (1440 Ma, Aldrich 1959) in the area.Luther(personal communication, 1994) identified the dike asbasalt which has been metamorphosed to possibly greenshistfades.

Thirty six cores were collected from both locations, aswell as from the surrounding quartzite. The edges of the dikeshow evidence of weathering and mineral alteration, so themajority of the mafic samples were collected from the center ofthe dike. Cores from all sites were subjected to detailed thermaldemagnetization to either 600° or 750° C. Several dike sampleswere also A.F. demagnetized to lOOmT. The demagnetizationcharacteristics and the Saturation Isothermal RemanentMagnetization characteristics (SIRN) indicate that magnetite isthe primarily carrier of the magnetism. There is one primarymagnetic direction for the dike which is removed with thermaldemagnetization to 600°C. The magnetism of the quartzite iscarried by hematite, as is evident from thermal demagnetizationto 750°C., and SIRM studies.The magnetic directions of allsamples including those reported by Kean, 1994, are presented inFigure 1. The dike shows negative inclinations (-30° to -50°)anddeclinations in the NNW-NNE direction. The quartzites havepositive inclinations (30° to 40) and westerly declinations,which is similar to the earlier results of Mercer(1984). Themagnetic directions for the dike show streaking, which mayrepresent an alteration component in some of the samples.Nonetheless, the overall direction provides a pole positionconsistent with other 1400—1600 Ma. rocks in the Lake Superiorregion,and significantly different than that of the quartzite.

109

References:

Aldrich,L.T., Wetherill,G.W., Bass, MN.., Compston, W,Davis,G.L., and Tilton,G.R.,1959, Mineral age measurements:Carnegie Institute Washington Year Book. 58, p.245—247.

Kean, WF., 1994, Paleomagnetism of a 1500 Ma. mafic dike atWaterloo Wi. Institute on Lake Superior Geology abstractsand proceedings, Vol.40, p.17.

Luther, F., 1997, The precambrian Waterloo quartzite, Dodge andJefferson Counties, Wisconsin—petrology, structure, andindustrial uses, Field Trip No.5, in Guide to Field TripsWisconsin and Adjacent Areas of Minnesota. 31st annualmeeting of the NCGSA, Madison , Wi. Ed. M.G. Mudrey, Jr.

Mercer,. D., 1984, Paleomagnetism of the Baraboo Quartzite, tJW—Milwaukee M.S. thesis, 294 p.

FIGURE 1

MAGNETIC DIRECTIONS FOR WATERLOO BASALTIC DIKE AND WATERLOOQUARTZITE. DIKE MATERIAL HAS NEGATIVE INCLINATIONS AND NORTHERLY

DECLINATIONS. THE QUARTZITE HAS POSITIVE INCLINATIONS ANDWESTERLY DECLINATIONS. THE DATA REPRESENTS TWO DIFFERENT

EXPOSURES IN THE SAME QUARRY

110

o EXPOSIJ( NES DC smir.£ H(Z EflU( 1(6 IPC RA.Ts. HORIZ EXPO$UI( P06 DC OUNflZITE* WRT OPOSU( P06 DC IW1TZITE

N

References :

Aldrich,L.T., ~etheri1l~G.W.~ Bass, M.N., Compston, W., Davis, I

G.L., and Tilton,GÈR.,1959 Mineral age measurements: Carnegie Institute Washington Year Book 58, p.245-247.

Kean, W.F., 1994, Paleomagnetism of a 1500 Ma. mafic dike at Waterloo Wi. Institute on Lake Superior Geology abstracts and proceedings, Vol.40, p.17.

Luther, F., 1997, The precambrian waterloo quartzite, Dodge and Jefferson Counties, Wisconsin-petrology, structure, and industrial uses, Field Trip No.5, in Guide to Field Trips in Wisconsin and Adjacent Areas of Minnesota. 31st annual meeting of the NCGSA, Madison , Wi. Ed. MçG Mudrey, Jr.

Mercer, D., 1984, Paleomagnetism of the Baraboo Quartzite, UW- Milwaukee M.S. thesis, 294 p.

FIGURE 1

MAGNETIC DIRECTIONS FOR WATERLOO BASALTIC DIKE AND WATERLOO QUARTZITE. DIKE MATERIAL HAS NEGATIVE INCLINATIONS AND NORTHERLY

DECLINATIONS. THE QUARTZITE HAS POSITIVE INCLINATIONS AND WESTERLY DECLINATIONS. THE DATA REPRESENTS TWO DIFFERENT

EXPOSURES IN THE SAME QUARRY

METAMORPHISM, HYDROTHERMAL ALTERATION AND LATERITICWEATHERING OF DRILLED MRS VOLCANIC ROCKS IN IOWA

SCHMIDT, Susanne Th., Mineralogisch-Petrographisches Institut,Bernoullistr. 30, CH 4056 Basel, Switzerland,schmidts@ubaclu.unibas.ch; SEIFERT, Karl, Departement of Geological& Atmospheric Sciences, Iowa State University, Ames, IA 50011,kseifert@pop-2.iastate.edu.

Well cores and cuttings from deep wells into the Precambrian igneous rocksof the buried Midcontinent Rift System in Iowa (Iowa horst, Anderson, 1990)were studied to identify primary and secondary mineral assemblages and thebulk geochemical composition was determined by instrumental neutronactivation analysis (INAA) for trace elements and by inductively coupledplasma (ICP) analysis for major elements. The results indicate a complexalteration history with metamorphic and hydrothermal stages, followed bylateritic weathering on the Precambrian erosion surface. Minerals wereanalyzed by electron microprobe analysis, clay minerals by X-ray diffraction(air-dried and glycolated), and the isotopic composition of calcite wasdetermined.

The large majority of these Midcontinent Rift samples were originallybasalts or diabases (Seifert & Anderson, 1996) compositionally similar tointermediate olivine tholeiites as described by Brannon (1984) from theMidcontinent Rift or North Shore Volcanic Goup (NSVG) exposed north ofDuluth in Minnesota. Although alteration has greatly modified the primarycomposition and texture of most samples, relicts of the original magmaticmineral assemblage can still be observed in some samples. Based onamygdule frequency and degree of alteration it is possible to differentiatebetween the various morphological flow units. Flow tops with a largenumber of amygdules and highest degree of alteration can be distinguishedfrom massive flow interiors without any amygdules and a less intensivealteration. In some flow tops no primary igneous texture is visible. Massiveflow interiors show various degrees of alteration but the primary texture isoften preserved.

A relativly homogenous regional metamorphic alteration pattern isobserved based on the highly amygdaloidal flow tops and amygdule minerals.In addition, some massive flow interiors have preserved part of the earlyalteration history. The assemblage epidote-Fe-rich-chlorite-albite-quartz±pumpellyite±sericite is characteristic for all studied drill sites of theIowa horst and indicates conditions of the beginning greenschist facies. Thesame facies is also observed in the lowermost part of the ca 8 km thick NSVGin Minnesota near Duluth which is interpreted to be the result of burialmetamorphism (Schmidt, 1990; 1993).

111

I

METAMORPHISM, HYDROTHERMAL ALTERATION AND LATERITIC WEATHERING OF DRILLED MRS VOLCANIC ROCKS IN IOWA

SCHMIDT/ Susanne Th.! Mineralogisch-Petrographisches Institut/ Bernoullistr. 30/ CH 4056 Basel/ Switzerland/ schmidts~ubac1u.unibas.ch; SEIFERTt Karl! Departement of Geological & Atmospheric Sciences/ Iowa State University/ Ames/ IA 500111 kseifertapop-2.iastate.edu.

Well cores and cuttings from deep wells into the Precambrian igneous rocks of the buried Midcontinent Rift System in Iowa (Iowa horst/ Anderson/ 1990) were studied to identify primary and secondary mineral assemblages and the bulk geochemical composition was determined by instrumental neutron activation analysis (INAA) for trace elements and by inductively coupled plasma (ICP) analysis for major elements. The results indicate a complex alteration history with metamorphic and hydrothermal stages! followed by lateritic weathering on the Precambrian erosion surface. Minerals were analyzed by electron microprobe analysis/ clay minerals by X-ray diffraction (air-dried and gly~olated)~ and the isotopic composition of calcite was determined.

The large majority of these Midcontinent Rift samples were originally basalts or diabases (Seifert & Anderson/ 1996) compositionally similar to intermediate olivine tholeiites as described by Brannon (1984) from the Midcontinent Rift or North Shore Volcanic Goup (NSVG) exposed north of Duluth in Minnesota. Although alteration has greatly modified the primary composition and texture of most samples/ relicts of the original magmatic mineral assemblage can still be observed in some samples. Based on amygdule frequency and degree of alteration it is possible to differentiate between the various morphological flow units. Flow tops with a large number of amygdules and highest degree of alteration can be distinguished from massive flow interiors without any amygdules and a less intensive alteration. In some flow tops no primary igneous texture is visible. Massive flow interiors show various degrees of alteration but the primary texture is often preserved.

A relativly homogenous regional metamorphic alteration pattern is observed based on the highly amygdaloidal flow tops and amygdule minerals. In addition/ some massive flow interiors have preserved part of the early alteration history. The assemblage epidote-Fe-rich-chlorite-albite- quartz~umpellyite~ericite is characteristic for all studied drill sites of the Iowa horst and indicates conditions of the beginning greenschist facies. The same facies is also observed in the lowermost part of the ca 8 km thick NSVG in Minnesota near Duluth which is interpreted to be the result of burial metamorphism (Schmidt/ 1990; 1993).

The Sharp #1 core documents the complex alteration history that affectedat least part of the Midcontinent Rift System in Iowa. A sequence ofsecondary alteration stages can be established. In the massive flow interior ofSharp #1 (sample 2206.4), an early Fe-poor, Si-rich phyllosilicate replacespyroxene along grain boundaries. Fe-poor, Si-rich phyllosilicates (smectites)are also reported from massive flow interiors of the NSVG indicating anearlier alteration stage under lower temperatures (Schmidt & Robinson,1997). The Fe-poor, Si-rich phyllosilicate is in turn replaced by a late Fe-richchlorite which also occurs in veinlets. This Fe-rich chlorite also forms part ofthe epidote-Fe-rich-chlorite-albite-quartz ± pumpellyite ± sericite assemblagein the amygdaloidal flow tops. In samples, a late potassic alteration is presentwith K-feldspar as a frequent overgrowth not only of silicates (albite andpyroxene), but also of chalcopyrite.

All samples which were derived directly from the original Precambrianerosion surface are partly intensively altered to kaolinite which is attributedto a tropical lateritic weathering (Seifert & Anderson, 1996). This correlateswell with the suggested position of Iowa near the equator at the end of thePrecambrian time. No kaolinite is present deeper in the drill cores.

Anderson, R.R. (1992) The Midcontinent Rift of Iowa. Ph. D. thesis,University of Iowa, Iowa City, 324p.

Brannon, J.C. (1984) Geochemistry of successive lava flows of theKeweenawan North Shore Volcanic Group. Ph. D. Thesis, WashingtonUniversity, St.Louis, 3l2p.

Schmidt, S.Th. (1990) Alteration under conditons of burial metamorphism inthe North Shore Volcanic Group, Minnesota - Mineralogical andgeochemical zonation. Heidelberger GeowisscnschaftlicheAbhandlungen, Band 41, 309 p.

Schmidt, S.Th. (1993) Regional and local patterns of low-grademetamorphism in the North Shore Volcanic Group, Minnesota, USA.Journal of metamorphic Geology, 11, 401-414.

Schmidt, S.Th. & Robinson, D. (1997) Metamorphic grade and porosity andpermeability controls on mafic phyllosilicate distributions in a regionalmetamorphic zeolite to greenschist facies transition of the North ShoreVolcanic Group. Geological Society of America, Bulletin, 109, 683-697.

Seifert, K. & Anderson, R.R. (1996) Geochemistry of buried Midcontintent Riftvolcanic rocks in Iowa: Data from well samples. Jour. Iowa Acad. Sci. 103(3-4), 63-7.

112

CRUSTAL RECYCLING IN THE EVOLUTION OF THE PENOKEAN OROGEN: ISOTOPICEVIDENCE FOR ARCHEAN CONTRIBUTIONS TO CRUSTAL GROWTH IN THEPEMBINE-WAUSAU TERRANE, NORTHERN WISCONSIN

Schulz, Klaus J. and Ayuso, Robert A., U.S. Geological Survey, 954 National Center, Reston,VA 20192 (kschulz@usgs.gov)

Continental growth models depend on information regarding the age of basement in orogenicbelts and on estimates of the amount of crustal recycling involved in their evolution. The distributionand role of Archean basement in the Early Proterozoic Penokean orogen in the Lake Superior regionhas long been a topic of debate and speculation. Early workers viewed the Early Proterozoic history ofthe region in terms of intracratonic deposition and reactivation of Archean crust of the SuperiorProvince (e.g., Sims, 1976). More recently there has been a general consensus that the EarlyProterozoic rocks evolved through plate tectonic processes including continental rifling and subduction,with formation of significant volumes of juvenile crust that was accreted to the Archean craton(Hoffman, 1988; Sims and others, 1989). However, isotopic studies on rocks from the Wisconsinmaginatic terranes have again raised questions about the distribution and role of Archean crust in thePenokean orogen (Barovich and others, 1989; Van Wyck and Johnson, 1997). In particular, Van Wyckand Johnson (1997) proposed that the Penokean orogen evolved through back-arc rifling of the southernSuperior Province, followed by collision of a continental arc terrane (the Marshfield terrane) from thesouth.

One of the primary lines of evidence used by Van Wyck and Johnson (1997) to support a modelof continental back-arc rifling for the Pembine-Wausau terrane was an inferred correlation between thedegree of crustal contamination and distance from the northern margin of the terrane, the Niagara faultzone, in which contamination decreased southward (i.e., €Nd(T) values become more positivesouthwards). To test this model for the Pembine-Wausau terrane and further refme estimates for theinvolvement of older crust, 14 volcanic rocks and 14 granitic rocks from across the Pembine-Wausauterrane in northern Wisconsin were analyzed for Nd and/or Pb isotopes. These samples, collected fromthe three principal outcrop areas in northern Wisconsin—the Dunbar-Pembine area located innortheastern Wisconsin just south of the Niagara fault zone, the Monico area located about 45 km southof the Niagara fault zone in northcentral Wisconsin, and the Marathon County area located at thesouthern margin of the Pembine-Wausau terrane in central Wisconsin—significantly expand thesampling density in these three areas and for the first time provide isotope data for several volcanicunits within the Pembine-Wausau terrane. Results are summarized in the table below.

The analyzed mafic volcanic rocks from the Monico and Marathon County areas are light REE-enriched but have positive ENd(T) from +1.3 to +3 and relatively primitive p values. These Nd isotopedata are slightly more enriched than the Nd isotope results of Beck and Murthy (1991) for theQuinnesec basalts (€Nd(T) +4.2) from the Pembine area and suggest derivation from depleted EarlyProterozoic mantle with possibly a small (�10%) addition of older crustal components. In contrast tothe mafic volcanic rocks, the felsic volcanic rocks from throughout the Pembine-Wausau terrane haveeNd(T) values ranging from —0 to -4 and relatively high p values >10 suggesting variable but significantinput of older, probably Archean crustal components. Surprisingly, the felsic volcanic rocks from theMonico area show the greatest crustal contamination. The granitic rocks, like the felsic volcanic rocks,also have mostly negative ENd(T) values but show a greater range from —0 to —7.4, and relatively high pvalues >10; younger granites tend to have the most negative €Nd(T) and highest p values in both theDunbar-Pembine and Marathon County areas. The isotope data lie along mixing lines between depletedEarly Proterozoic mantle and Archean Superior Province crust; mixing models suggest from 20 to>70% Archean crustal contamination for felsic volcanic and granitic rocks.

113

CRUSTAL RECYCLING IN THE EVOLUTION OF THE PENOKEAN OROGEN: ISOTOPIC EVIDENCE FOR ARCHEAN CONTRIBUTIONS TO CRUSTAL GROWTH IN THE PEMBINE-WAUSAU TERRANE, NORTHERN WISCONSIN

I Schulz, Klaus J. and Ayuso, Robert A., U.S. Geological Survey, 954 National Center, Reston, VA 20 192 (kschulz@usgs.gov)

Continental growth models depend on information regarding the age of basement in orogenic belts and on estimates of the amount of crustal recycling involved in their evolution. The distribution and role of Archean basement in the Early Proterozoic Penokean orogen in the Lake Superior region has long been a topic of debate and speculation. Early workers viewed the Early Proterozoic history of the region in terms of intracratonic deposition and reactivation of Archean crust of the Superior Province (e.g., Sims, 1976). More recently there has been a general consensus that the Early Proterozoic rocks evolved through plate tectonic processes including continental rifting and subduction, with formation of significant volumes of juvenile crust that was accreted to the Archean craton (Hoffinan, 1988; Sirns and others, 1989). However, isotopic studies on rocks from the Wisconsin magmatic terranes have again raised questions about the distribution and role of Archean crust in the Penokean orogen (Barovich and others, 1989; Van Wyck and Johnson, 1997). In particular, Van Wyck and Johnson (1997) proposed that the Penokean orogen evolved through back-arc rifting of the southern Superior Province, followed by collision of a continental arc terrane (the Marshfield terrane) from the south.

One of the primary lines of evidence used by Van Wyck and Johnson (1997) to support a model of continental back-arc rifting for the Pembine-Wausau terrane was an inferred correlation between the degree of crustal contamination and distance from the northern margin of the terrane, the Niagara fault zone, in which contamination decreased southward (i.e., eNd(T) values become more positive southwards). To test this model for the Pembine-Wausau terrane and further refine estimates for the involvement of older crust, 14 volcanic rocks and 14 granitic rocks from across the Pembine-Wausau terrane in northern Wisconsin were analyzed for Nd andlor Pb isotopes. These samples, collected from the three principal outcrop areas in northern Wisconsin-the Dunbar-Pembine area located in northeastern Wisconsin just south of the Niagara fault zone, the Monico area located about 45 krn south of the Niagara fault zone in northcentral Wisconsin, and the Marathon County area located at the southern margin of the Pembine-Wausau terrane in central Wisconsin-significantly expand the sampling density in these three areas and for the first time provide isotope data for several volcanic units within the Pembine-Wausau terrane. Results are summarized in the table below.

The analyzed mafic volcanic rocks from the Monico and Marathon County areas are light REE- enriched but have positive ENd(T) from +1.3 to +3 and relatively primitive p values. These Nd isotope data are slightly more enriched than the Nd isotope results of Beck and Murthy (199 1) for the Quinnesec basalts (eNd(T) - +4.2 ) from the Pembine area and suggest derivation from depleted Early Proterozoic mantle with possibly a small (510%) addition of older crustal components. In contrast to the mafic volcanic rocks, the felsic volcanic rocks from throughout the Pembine-Wausau terrane have eNd(T) values ranging from -0 to -4 and relatively high p values >10 suggesting variable but significant input of older, probably Archean crustal components. Surprisingly, the felsic volcanic rocks from the Monico area show the greatest crustal contamination. The granitic rocks, like the felsic volcanic rocks, also have mostly negative eNd(T) values but show a greater range from -0 to -7.4, and relatively high p values >lo; younger granites tend to have the most negative e ~ d ( T ) and highest p values in both the Dunbar-Pembine and Marathon County areas. The isotope data lie along mixing lines between depleted Early Proterozoic mantle and Archean Superior Province crust; mixing models suggest from 20 to >70% Archean crustal contamination for felsic volcanic and granitic rocks.

Areas ENd(T) TDM (in Ga) p (8U/204Pb)Dunbar-Pembine (North)• Pemene Rhyolite (2)• Dunbar Dome (5)• Bush Lake Granite (1)• Twelve Foot Falls (1)

Quartz Diorite

— +0.35-0.9 to -4.2

.7.4-3.2

2.32.2 to 2.9

2.92.8

1210.411.5

Monico• Basalt (2)• Dacite/Rhyolite (3)• Quartz Porphyry (1)

+2.1-3.1•1

2.1—2.6

2.5

— 9.8

11

9.4

Marathon County (South)• Basalt/Andesite (4)• Dacite/Rhyolite (3)• Pre/Syn-Tectonic Granite (3)• Syn/Post-Tectonic Granite (3)

+3 to +1.3-0.8 to 2.0

4.042

2.0 to 2.22.2 to 2.3

2.32.8

9.810.610.9

.-. 13.2

The new data presented here do not support a correlation of increasing 6Nd with distance fromthe Niagara fault zone. The felsic volcanic rocks from throughout the terrane show isotopic evidencefor crustal contamination with the greatest crustal component in rhyolites from the Monico area. Thegranitic rocks from throughout the terrane also show isotopic evidence for significant crustalcontamination. However, the isotope data do suggest that crustal contamination was greatest for syn- topost-tectonic granites emplaced near the northern and southern margins of the Pembine-Wausau terrane.

The new isotope data for the Pembine-Wausau terrane suggest that: (1) input of Archean crustalcomponents to Penokean crust formation was greater and more widely distributed than previouslyrecognized; (2) there is no clear correlation between ENd and distance from the northern (Niagara)suture zone, although contamination appears to have been greatest for syn- to post-tectonic granitesemplaced near the margins of the terrane; and (3) the high levels of crustal components shown by thefelsic igneous rocks probably result from crustal assimilation and mixing and not from contamination ofa depleted mantle source by subduction of Archean derived sediments.

References citedBarovich, KM. Patchett, P.J., Peterman, Z.E., and Sims, P.K., 1989, Nd isotopes and the origin of 1.9-1.7 Ga

Penokean continental crust of the Lake Superior region: Geological Society of America Bulletin, v. 101,p. 333-338.

Beck, W. and Murthy, V.R., 1991, Evidence for continental crustal assimilation in the Hemlock Formation floodbasalts of the Early Proterozoic Penokean Orogen, Lake Superior region: U.S. Geological SurveyBulletin 1904-I, 25p.

Hoffman, P.F., 1988, United plates of America, the birth of craton: Early Proterozoic assembly and growth ofLaurentia: Annual Reviews of Earth and Planetary Sciences, v. 16, p. 543-603.

Sims, P.K., 1976, Precambrian tectonics and mineral deposits, Lake Superior region: Economic Geology, v. 71, p.1092-1177.

Sims, P.K., Van Schrnus, W.R., Schulz, K.J., and Petennan, Z.E., 1989, Tectono-stratigraphic evolution of theEarly Proterozoic Wisconsin magmatic terranes of the Penokean Orogen: Canadian Journal of EarthSciences, v. 26, p. 2145-2158.

Van Wyck, N. and Johnson, C.M., 1997, Common lead, Sm-Nd, and U-Pb constraints on petrogenesis, crustalarchitecture, and tectonic setting of the Penokean orogeny (Paleoproterozoic) in Wisconsin: GeologicalSociety of America Bulletin, v. 109, p. 799-808.

114

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The new data presented here do not support a correlation of increasing eNd with distance from the Niagarafault zone. The felsic volcanic rocks from throughout the terrane show isotopic evidence for crustal contamination with the greatest crustal component in rhyolites from the Monico area. The granitic rocks from throughout the terrane also show isotopic evidence for significant crustal contamination. However, the isotope data do suggest that crustal contamination was greatest for syn- to post-tectonic granites emplaced near the northern and southern margins of the Pembine-Wausau terrane.

" -

I Areas

Dunbar-Pembine (North) Pemene Rhyolite (2) Dunbar Dome (5) Bush Lake Granite (1) Twelve Foot Falls (1) Quartz Diorite

Monico Basalt (2) DaciteRhyolite (3) Quartz Porphyry (1)

Marathon County (South) BasaltIAndesite (4) DaciteIRhyolite (3) PrefSyn-Tectonic Granite (3) SynPost-Tectonic Granite (3)

The new isotope data for the Pembine-Wausau terrane suggest that: (1) input of Archean crustal components to Penokean crust formation was greater and more widely distributed than previously recognized; (2) there is no clear correlation between EN^ and distance from the northern (Niagara) suture zone, although contamination appears to have been greatest for syn- to post-tectonic granites emplaced near the margins of the terrane; and (3) the high levels of crustal components shown by the felsic igneous rocks probably result from crustal assimilation and mixing and not from contamination of a depleted mantle source by subduction of Archean derived sediments.

References cited Barovich, KM., Patchett, P.J., Petennan, Z.E., and Sims, P.K., 1989, Nd isotopes and the origin of 1.9-1.7 Cia

Penokean continental crust of the Lake Superior region: Geological Society of America Bulletin, v. 101, p. 333-338.

Beck, W. and Murthy, V.R, 1991, Evidence for continental crustal assimilation in the Hemlock Formation flood basalts of the Early Proterozoic Penokean Orogen, Lake Superior region: U.S. Geological Survey Bulletin 1904-1,25p.

Hoffinan, P.F., 1988, United plates of America, the birth of craton: Early Proterozoic assembly and growth of Laurentia: Annual Reviews of Earth and Planetary Sciences, v. 16, p. 543-603.

Sims, P.K., 1976, Precambrian tectonics and mineral deposits, Lake Superior region: Economic Geology, v. 71, p. 1092-1 177.

Sims, P.K., Van Schmus, W.R, Schulz, KJ., and Peterman, Z.E., 1989, Tectono-stratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean Orogen: Canadian Journal of Earth Sciences, v. 26, p. 2145-2158.

Van Wyck, N. and Johnson, C.M., 1997, Common lead, Sm-Nd, and U-Pb constraints on petrogenesis, crustal architecture, and tectonic setting of the Penokean orogeny (Paleoproterozoic) in Wisconsin: Geological Society of America Bulletin, v. 109, p. 799-808.

,

e~d(T.1

- M.35 -0.9 to -4.2

-7.4 -3.2

+2.1 -3.1 -4.7

+3 to +1.3 -0.8 to -2.0 - -1.0 - -4.2

TDM (in Ga)

2.3 2.2 to 2.9

2.9 2.8

2.1 - 2.6 2.5

2.0 to 2.2 2.2 to 2.3 - 2.3 - 2.8

p (*8~/20"~b)

- 12 - 10.4 11.5 -

- 9.8 - 11 9.4

- 9.8 - 10.6 - 10.9 - 13.2

ND ISOTOPE EVIDENCE FOR MIDDLE AND EARLY ARCHEAN CRUSTIN THE WAWA SUBPROVINCE OF THE SUPERIOR PROVINCE, MICHIGAN, U.S.A.

Sims, P.K., Neymark, L.A., and Peterman, Z.E., U.S. Geological Survey;Kotov, A.B., Institute of Precambrian Geology and Geochronology, St. Petersburg, Russia

New Sm-Nd isotopic data on Late Archean granitic rocks from the Wawa subprovince (a granite-greenstoneterrane) of the Superior province, northern Michigan, indicate a Middle to Early Archean crustal source for the rocks.At 2.7 Ga d values vary from -6.4 to +3.3, which correspond to depleted mantle model ages between 2.7 and 3.6 Ga(see table). Previous Nd-isotope studies in granite-greenstone terranes in the southern part of the Superior provincehave indicated that most granitoid rocks are juvenile, i.e. mantle derived, with little identifiable participation from oldercrust or recycled material.

Samples were taken from the Puritan batholith (Fig. I), which lies astride the Michigan - Wisconsin border, andthe northern complex of the Marquette district (Fig. 2); both igneous bodies have crystallization ages of —2.7 Ga.

Candidates for the source of the Middle and Early Archean rocks in the Michigan segment of the Wawasubprovince have not been identified, but these rocks mainly constitute ancient continental crust, perhaps a proto-cratonblock like that in the Sachigo and Minto subprovinces (Percival and other, 1994) to the north.

Not unexpectedly, high-grade gneisses from the adjacent Minnesota River Valley subprovince, to the south, whichcontains the oldest rocks in the Superior province, have comparable old Nd-depleted mantle model ages (see table),ranging from 2.9 to 3.3 Ga.

Table. Sm-Nd isotope data for granites from Wawa Subprovince andfelsic gneisses and granites from MinnesotaRiver Valley Subprovince.

Sample T(Ga) Sm Nd '47Sm/ 143Ndt d(t) d(O) TDM TDM(ppm) (ppm) '44Nd '44Nd

Wawa Subprovince (Granite-Greenstone Terrane)182B 2.7 8.88 40.9 0.0980 0.510695 (13) -3.7 -37.9 3202 3319183B 2.7 19.7 98.2 0.1214 0.511072 (5) -4.5 -30.5 3394 3384198 2.7 5.70 32.2 0.1074 0.510842 (8) -4.1 -35.0 3276 3352215B 2.7 2.76 17.0 0.0982 0.510672 (13) -4.2 -38.4 3237 33610919-1-89 2.7 4.06 30.2 0.0815 0.510594 (12) 0.1 -39.9 2922 30120919-3-89 2.7 8.08 57.7 0.0925 0.510707 (6) -1.5 -37.7 3044 31440920-4-89 2.7 3.56 17.9 0.1205 0.511397(10) 2.3 -24.2 2840 2839l36R 2.7 0.79 3.11 0.1545 0.511781 (9) -2.1 -16.7 3478 3193M169 2.7 1.60 11.5 0.0848 0.5 10708 (10) 1.2 -37.6 2863 2924Ml70 2.7 2.98 17.4 0.1041 0.511040(10) 1.0 -31.2 2908 2943177 2.7 2.53 13.8 0.1107 0.510912 (10) -3.8 -33.7 3277 3334179 2.7 1.01 4.67 0.1315 0.511415(9) -1.2 -23.9 3182 3123

D1044 2.7 4.10 16.7 0.1486 0.511572(7) -4.1 -20.8 3646 3358D1731 2.7 2.08 10.5 0.1200 0.511070(7) -4.0 -30.6 3347 3347D2493 2.7 1.26 6.48 0.1176 0.511266(7) 0.7 -26.8 2959 2966D2494 2.7 5.06 33.6 0.0913 0.510928 (7) 3.3 -33.4 2744 2758D2495 2.7 5.60 33.6 0.1012 0.510792(4) -2.9 -36.0 3163 3256

Minnesota River Valley Subprovince (Gneiss Terrane)34-90 2.8 6.22 44.9 0.0841 0.510600(8) 0.8 -39.8 2971 3037161-B 2.8 2.35 12.8 0.1117 0.510993(11) -1.5 -32.1 3192 3226211 2.8 7.36 38.2 0.1170 0.511027(10) -2.7 -31.4 3309 3325213 2.8 3.80 21.2 0.1089 0.511018(8) 0.0 -31.6 3072 3104

Tilden Granite3-90 2.6 10.5 59.2 0.1078 0.510791 (8) -6.4 -36.0 3358 3456

Gwinn Granite226-89 2.6 5.65 31.6 0.1082 0.511006(10) -2.3 -31.8 3068 3127

*) Data corrected to La Jolla '43Nd/'44Nd=0.5 11860, 2 within-run errors are given in parentheses.* *) Two-stage Nd-model age assuming 47Sm/'41Nd=0. 12 in the source of rocks before T Ga ago.

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ND ISOTOPE EVIDENCE FOR MIDDLE AND EARLY ARCHEAN CRUST IN THE WAWA SUBPROVINCE OF THE SUPERIOR PROVINCE, MICHIGAN, U.S.A.

Sims, P.K., Neymark, L.A., and Petennan, Z.E., U.S. Geological Survey; Kotov, A.B., Institute of Precambrian Geology and Geochronology, St. Petersburg, Russia

New Sm-Nd isotopic data on Late Archean granitic rocks from the Wawa subprovince (a granite-greenstone terrane) of the Superior province, northern Michigan, indicate a Middle to Early Archean crustal source for the rocks. At 2.7 Ga pNd values vary from -6.4 to +3.3, which correspond to depleted mantle model ages between 2.7 and 3.6 Ga (see table). Previous Nd-isotope studies in granite-greenstone terranes in the southern part of the Superior province have indicated that most granitoid rocks are juvenile, i.e. mantle derived, with little identifiable participation from older crust or recycled material.

Samples were taken from the Puritan batholith (Fig. I ) , which lies astride the Michigan - Wisconsin border, and the northern complex of the Marquette district (Fig. 2); both igneous bodies have crystallization ages of -2.7 Ga.

Candidates for the source of the Middle and Early Archean rocks in the Michigan segment of the Wawa subprovince have not been identified, but these rocks mainly constitute ancient continental crust, perhaps a proto-craton block like that in the Sachigo and Minto subprovinces (Percival and other, 1994) to the north.

Not unexpectedly, high-grade gneisses from the adjacent Minnesota River Valley subprovince, to the south, which contains the oldest rocks in the Superior province, have comparable old Nd-depleted mantle model ages (see table), ranging from 2.9 to 3.3 Ga.

Table. Sm-Nd isotope data for granites from Wawa Subprovince and felsic gneisses and granites from Minnesota River Valley Suburovince.

Wawa Subprovince (Granite-Greenstone Terrane)

Minnesota River Valley Subprovince (Gneiss Terrane) 6.22 44.9 0.0841 0.5 10600 (8) 0.8 2.35 12.8 0.11 17 0.510993 (1 1 ) -1.5 7.36 38.2 0.1 170 0.5 1 1027 (10) -2.7 3.80 21.2 0.1089 0.511018(8) 0.0

Tilden Granite 10.5 59.2 0.1078 0.5 1079 1 (8) -6.4

Gwinn Granite 5.65 31.6 0.1082 0.511006(10) -2.3

*) Data corrected to La Jolla '"Nd/'"Nd=0.5 1 1860, 2,-r within-run errors are given in parentheses. **) Two-stage Nd-model age assuming '"Sm/'"Nd=O. 12 in the source of rocks before T Ga ago.

GLTZ,Ma,

Mar,Me,

MIDDLE PROTEROZOIC (1.600-900 Ma)

Jacobsville Sandstone

EARLY PROTEROZOIC (2,500—I .600 Ma)

'.k)lcanic and granitoid rocks of Wisconsinmagmatic zone (—1.880—1,860 Ma)

Sedimentary and volcanic rocks of MarquetteRange Supergroup

ARCHEAN (2,500 Ma and older)

Puritan batholith (—2,700 Ma)

Granitoid rocks of northern complex ofMarquette district (—2,700 Ma)

Metavolcajijc rocks

Gneiss and amphibolite (2,750—2,640 Ma)

Migmatitic gneiss of Minnesota River Valleysubprovince (3,550—2,800 Ma)

Great Lakes tectonic zoneMarquette, MichiganMarenisco, MichiganMellen, Wisconsin

Sm—Nd age locality

I

Figure 1. Geologic map of Precambrian rocks in northern Michigan and adjacentWisconsin, showing localities of Sm-Nd samples. Figure 2 is an enlargement of area out-lined south of Marquette.

EXPLANA11ON

________________________________

Paleozoic rocks, undivided

Rocks of midcontinent rift system (ca. 1,100 Ma)

EXPLANATION—Contact 1361

Approximate boundarybetween Wawa andMinnesotaRivervalley PRO TEROZOIC .subprovinces ROCKS

'—MylondeSm-Nd agelocality

. Wawa / SANDPalmer Sub- COVER

province

1618 ' B828

•3.9034-90 '

Minnesota A Valley '\\subprovince 21

21

5 MILES

5 KILOMETERS

Figure 2. Geologic sketch map of Precambrianrocks in an area south of Marquette, showinglocalities of Sm-Nd samples. The Great Lakestectonic zone separates granite-greenstone ter-rane (Wawa subprovince) from gneiss terrane(Minnesota River Valley subprovince).

116

W" 89' I

88O 1

LAKE SUPERIOR

47%

Figure 1. Geologic map of Precambrian rocks in northern Michigan and adjacent Wisconsin, showing localities of Sm-Nd samples. Figure 2 is an enlargement of area out-

-

lined south of Marquette.

EXPLANATION

n Paleozoic rocks, uncfwidd

MIDDLE PROTEROZOIC (1,600-900 Ma)

Jacobsville Sandstone

Rocks of midcontinent rift system (ca. 1 ,I 00 Ma)

EARLY PROTEROZOIC (2,500-1,600 Ma)

Volcanic. and granitoid rocks of Wisconsin magmatlc zone (-1.880-1.860 Ma) Sedimentary and volcanic rocks of Marquette

n Range Supergroup

ARCHEAN (2,500 Ma and older)

Puritan batholiih (-2,700 Ma)

Granitoid rocks of northern complex of Marquette district (-2,700 Ma)

2:' Metavolcanic rocks 3 " 7 -

1 Gneiss and amphiboliie (2.750-2.640 Ma)

mat3ic gneiss of Minnesota River Valley prwlnce (3,550-2,800 Ma)

GLTZ, Great Lakes tectonic zone Ma, Marquette, Michigan Mar, Marenism, Michigan Me, Mellen, Wisconsin

Sm-Nd age locality

Figure 2. Geologic sketch map of Precambrian rocks in an area south of Marquette, showing localities of Sm-Nd samples. The Great h k e s tectonic zone separates granite-greenstone ter- rane (Wawa subprovince) from gneiss terrane (Minnesota River Valley subprovince).

A THIN VISCOUS SHEET APPROACH TO INVESTIGATE THE POSTRIFT EVOLUTION OF THE MIDCONTINENT RIFT SYSTEM UNDER

THE INFLUENCE OF GRENVILLE OROGENY

Soofi, M. A., and King, S. D., Department of Earth and Atmospheric Sciences, PurdueUniversity, West Lafayette, Indiana 47907

The cause of the termination of rifting along the Midcontinent Rift system (MCR)and the presence of reverse faults along the rift margins are still troublesome for theinvestigators of the MCR. Suggestions for the cause of rifting range from a passivemechanism, where processes at plate boundaries were responsible (e.g. Hinze et al.,1982), to an active mechanism, where a mantle plume ascended and initiated rifting (e.g.Cannon and Hinze, 1992). Similarly, the formation of reverse faults has been suggestedto be a consequence of plate flexure under the loading due to rift related rocks (e.g.Nyquist, 1986), or an effect of compressive stresses from the plate boundaries, whichnot only terminated the rifting but also caused thrusting along the original rift boundingnormal faults (e.g. Cannon, 1994).

This study focuses on the evolution of the MCR under the influence of plate bound-ary forces, particularly the compressive forces from the Grenville Orogeny. We areusing the thin viscous sheet model (TVS) of England and McKenzie (1983) which isimplemented in a finite element code by Houseman and England (1986). This modelhas been used in the study of India-Asia collision (England and Houseman, 1986) andArabia-Eurasia convergence (Sobouti and Arkani-Hamed, 1996), where it has success-fully reproduced the topography and deformation observed in the overriding Eurasianplate. The model uses incompressible, vertically averaged, power-law rheology for thelithosphere.

For a given geometry, boundary conditions, stress-strain exponent (n) and ArgandNumber (Ar) values, the model calculates crustal thickness, stresses, strain rates androtation. These can then be correlated with topography and deformation in the overriding plate. A continuous medium is assumed such that there is no discontinuity in thevelocity field. Fault planes cannot be defined explicitly but, information on the type offaulting can be inferred from the stress and strain rate distribution.

Our results indicate that even in the case of a very weak lithosphere (i.e. n=10)stresses from the Grenille Front (GF) can be transmitted inland to interact with theprocesses that were occuring along the MCR. Also, depending on the size of the collidingmicrocontinents (i.e. indenters) and their position along the GF it is possible that theMCR was subjected to different magnitude of stresses along its length and it evolvedthrough the superposition of these stresses. The varying degree of thrust faulting ob-served along the MCR could be a manifestation of such a collision style. The modelsalso predict significant thickening of crust next to the indenter, the thickening decreasesaway from the collision boundary. For a comparatively rigid lithosphere (n=1,Ar=1)crustal thickening is as much as 2 km where as for a weak lithosphere (n=10,Ar=1)crustal thickening is as much as 19 km. Assuming crustal density of 2700 kg/m3 andmantle density of 3300 kg/rn3 and assuming the surface of the 35 km thick crust tobe at the sea level the change in crustal elevation is 0.36 km for rigid lithosphere and3.4 km for the relatively weak lithosphere. The high elevations predicted by the TVScan initiate local extension, as suggested by England and McKenzie (1982) for the Ti-betan plateau. Such a mechanism may had been responsible for the late stage extensionreported for the Grenville Orogeny (e.g. Easton, 1992).

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A THIN VISCOUS SHEET APPROACH T O INVESTIGATE THE POST RIFT EVOLUTION O F T H E MIDCONTINENT RIFT SYSTEM UNDER

THE INFLUENCE O F GRENVILLE OROGENY

Soofi, M. A., and King, S. D., Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907

The cause of the termination of rifting along the Midcontinent Rift system (MCR) and the presence of reverse faults along the rift margins are still troublesome for the investigators of the MCR. Suggestions for the cause of rifting range fiom a passive mechanism, where processes at plate boundaries were responsible (e.g. Hinze et al., 19821, to an active mechanism, where a mantle plume ascended and initiated rifting (e.g. Cannon and Hinze, 1992). Similarly, the formation of reverse faults has been suggested to be a consequence of plate flexure under the loading due to rift related rocks (e.g. Nyquist, 19861, or an effect of compressive stresses from the plate boundaries, which not only terminated the rifting but also caused thrusting along the original rift bounding normal faults (eg. Cannon, 1994).

This study focuses on the evolution of the MCR under the influence of plate bound- ary forces, particularly the compressive forces from the Grenville Orogeny. We are using the thin viscous sheet model (TVS) of England and McKenzie (1983) which is implemented in a finite element code by Houseman and England (1986). This model has been used in the study of India-Asia collision (England and Houseman, 1986) and Arabia-Eurasia convergence (Sobouti and Arkani-Hamed, 1996), where it has success- fully reproduced the topography and deformation observed in the overriding Eurasian plate. The model uses incompressible, vertically averaged, power-law rheology for the lithosphere.

For a given geometry, boundary conditions, stress-strain exponent (n) and Argand Number (Ar) values, the model calculates crustal thickness, stresses, strain rates and rotation. These can then be correlated with topography and deformation in the over riding plate. A continuous medium is assumed such that there is no discontinuity in the velocity field. Fault planes cannot be defined explicitly but, information on the type of faulting can be inferred from the stress and strain rate distribution.

Our results indicate that even in the case of a very weak lithosphere (i.e. n=lO) stresses from the Grenille Eont (GF) can be transmitted inland to interact with the processes that were occuring along the MCR. Also, depending on the size of the colliding microcontinents (i.e. indenters) and their position along the GF it is possible that the MCR was subjected to difFerent magnitude of stresses along its length and it evolved through the superposition of these stresses. The varying degree of thrust faulting ob- served along the MCR could be a manifestation of such a collision style. The models also predict significant thickening of crust next to the indenter, the thickening decreases away fiom the collision boundary. For a comparatively rigid lithosphere (n=l,Ar=l) crustal thickening is as much as 2 km where as for a weak lithosphere (n=lO,Ar=l) crustal thickening is as much as 19 km. Assuming crustal density of 2700 kg/m3 and mantle density of 3300 kg/m3 and assuming the surface of the 35 km thick crust to be at the sea level the change in crustal elevation is 0.36 km for rigid lithosphere and 3.4 km for the relatively weak lithosphere. The high elevations predicted by the TVS can initiate local extension, as suggested by England and McKenzie (1982) for the Ti- betan plateau. Such a mechanism may had been responsible for the late stage extension reported for the Grenville Orogeny (e.g. Easton, 1992).

We are also studying the effect of oblique convergence along the GF on the evolu-tion of the MCR. For fixed boundary conditions and rheology, the length scale of de-formation decreases with increasing obliquity. As discussed for the normal convergence,microcontinents of various sizes converging obliquely along the GF can produce stressesof different magnitudes along the MCR. Oblique convergence produces same pattern ofcrustal thickening as observed for normal convergence except that it is not symmetric.One significant difference between oblique and normal convergence is the significant areaof crustal thinning next to the colliding microcontinent (indenter). This thinning rep-resent extension in the over riding plate. For normal convergence the thinning is verylocalised and produced in a very small area that does not changes significantly withrheology. For oblique convergence this area of extension increases for weaker rheology(increasing n value). We hypothesize that oblique convergence, together with a weaklithosphere (due to thermal anomaly resulting from subduction), may had helped, if notinitiated, the rifting process.

REFERENCES CITED

Cannon, W. F., 1994, Closing of the Midcontinent rift - A far-field effect of Grenvilliancompression: Geology, v. 22, p. 155-158.

Cannon, W. F., and Hinze, W. J., 1992, Speculations on the origin of the North Amer-ican Midcontinent rift, in Ziegler, P. A., ed., Geodynaniics of rifting, Volume 2.Case History Studies on Rifts: North and South America and Africa: Tectono-physics, v. 213, p. 49-55.

Easton, R. M., 1992, The Grenville Province and the Proterozoic history of Centraland Southern Ontario, in Thurston, P. C., Williams, H. R., Sutcliffe, R. H., andStott, G. M., eds., Geology of Ontario, Ontario Geological Survey, Special Volume4, Part 2, p. 715-904.

England, P., and Houseman, G., 1986, Finite strain calculations of continental defor-mation 2. Comparison with the India-Asia collision zone: Journal of GeophysicalResearch, v. 91, p. 3664-3676.

England, P., and McKenzie, D., 1983, Correction to,: A thin viscous sheet model forcontinental deformation: Geophysical Journal of the Royal Astromomical Society,v. 73, p. 523-532.

Hinze, W. J., Wold, R. J., and O'Hara, N. W., 1982, Gravity and magnetic anomalystudies of Lake Superior, in Wold, R. J., and Hinze, W. J., eds., Geology andtectonics of the Lake Superior basin: Geological Society of America Memoir 156,p. 203-222.

Houseman, G., and England, P., 1986, Finite strain calculations of continental deforma-tion 1. Method and general results for convergent zones: Journal of GeophysicalResearch, v. 91, p. 3651-3663.

Nyquist, J. E., 1986, Thermal and mechanical models of the Mid-Continent rift [Ph.D.thesis]: University of Wisconsin-Madison, 193p.

Sobouti, F., and Arkani-Hamed, J., 1996, Numerical modelling of the deformation ofthe Iranian plateau: Geophysical Journal International, v. 126, p. 805-818.

118

We are also studying the effect of oblique convergence along the GJ? on the evolu- tion of the MCR. For fixed boundary conditions and rheology, the length scale of d e formation decreases with increasing obliquity. As discussed for the normal convergence, microcontinents of various sizes converging obliquely along the GF can produce stresses

I of different magnitudes along the MCR. Oblique convergence produces same pattern of crustal thickening as observed for normal convergence except that it is not symmetric. One significant difference between oblique and normal convergence is the significant area of crustal thinning next to the colliding microcontinent (indenter). This thinning rep- resent extension in the over riding plate. For normal convergence the thinning is very localised and produced in a very small area that does not changes significantly with rheology. For oblique convergence this area of extension increases for weaker rheology (increasing n value). We hypothesize that oblique convergence, together with a weak lithosphere (due to thermal anomaly resulting fiom subduction), may had helped, if not initiated, the rifting process.

REFERENCES CITED

Cannon, W. F., 1994, Closing of the Midcontinent rift - A far-field effect of Grenvillian compression: Geology, v. 22, p. 155-158.

Cannon, W. F., and Hinze, W. J., 1992, Speculations on the origin of the North Amer- ican Midcontinent rift, an Ziegler, P. A., ed., Geodynamics of rifting, Volume 2. Case History Studies on Rifts: North and South America and Africa: Tectono- physics, v. 213, p. 49-55.

Easton, R. M., 1992, The Grenville Province and the Proterozoic history of Central and Southern Ontario, an Thurston, P. C., Williams, H. R., Sutcliffe, R. H., and Stott, G. M., eds., Geology of Ontario, Ontario Geological Survey, Special Volume 4, Part 2, p. 715-904.

England, P., and Houseman, G., 1986, Finite strain calculations of continental defor- mation 2. Comparison with the India-Asia collision zone: Journal of Geophysical Research, v. 91, p. 3664-3676.

England, P., and McKenzie, D., 1983, Correction to: A thin viscous sheet model for continental deformation: Geophysical Journal of the Royal Astromomical Society, v. 73, p. 523-532.

Hinze, W. J., Wold, R. J., and O'Hara, N. W., 1982, Gravity and magnetic anomaly studies of Lake Superior, an Wold, R. J., and Hinze, W. J., eds., Geology and tectonics of the Lake Superior basin: Geological Society of America Memoir 156, p. 203-222.

Houseman, G., and England, P., 1986, Finite strain calculations of continental deforma- tion 1. Method and general results for convergent zones: Journal of Geophysical Research, v. 91, p. 3651-3663.

Nyquist, J. E., 1986, Thermal and mechanical models of the Mid-Continent rift [Ph.D. thesis]: University of Wisconsin-Madison, 193p.

Sobouti, F., and Arkani-Hamed, J., 1996, Numerical modelling of the deformation of the Iranian plateau: Geophysical Journal International, v. 126, p. 805-818.

PALEOMAGNETIC STUDIES OF A PROTEROZOIC PORPHYRITIC DIABASEDIKE, PIFHER AND IRWIN TOWNSHIPS, LAKE NIPIGON DISTRICT,ONTARIO

Thomas*, C., Kean, W., Department of Geosciences,University of Wisconsin—Milwaukee, Milwaukee, WI 53201and Luther, F., Geology Department, UW—Whitewater,Whitewater, WI 53190 (* student author)

This approximately 50 m wide porphyritic diabase dike,outcropping in western Irwin and Pifher Townships in theNipigon district, is locally known as greenspar. The dike hasa characteristic greenish mottled appearance in outcrop as aresult of saussaritized plagioclase glomerophenocrysts in adiabase groundinass (Luther,1997). The dike strikes due northand dips vertically, and is found as segments, off—set byeast—west faulting(Mackasey,1975). There are no radiometricdates on this rock or associated rocks, although, it is cut bya large middle or late Proterozoic sill in IrwinTownship (Mackasey, 1975).

Paleomagnetic studies were completed on 2—3 cores fromeach of 5 locations. Each location represents one dikesegment. Samples from each location were subjected to bothalternating field (A.F.) and thermal demagnetization studies.The samples show one primary magnetic direction which isremoved at demagnetization temperatures of 5700 C. or by A.F.fields of 60 mT. All but one location shows normal polaritywith northwesterly declinations(200°-300°) and inclinations of40°-80°. The northern most section of the dike which we sampledis reversally magnetized with declinations of about 90°—100°and inclinations of —75°. These magnetic directions areconsistent with Keweenawan age paleomagnetic directions in theLake Superior Region.

References

Luther, F.,1997, The Petrology of Greenspar: A ProterozoicPorphyritic Diabase Dike; Pifher and Irwin Townships, LakeNipigon District, Ontario. Institute on Lake Superior Geologyabstracts and proceedings , Vol.43.

Mackasey, W.O.,1975, Geology of Dorothea, Sandra, and IrwinTownships, District of Thunder Bay; Ontario division of Mines,rpt 122 with map 2294, 83 p.

119

U

PALEOMAGNETIC STUDIES OF A PROTEROZOIC PORPHYRITIC DIABASE DIKEf PIFHER AND IRWIN TOWNSHIPS LAKE NIPIGON DISTRICT ONTARIO

Th~mas*~ CSf Keanf W., Department of Geosciencesf University of Wisconsin-Milwaukeef Milwaukeef WI 53201 and Lutherf Fef Geology Departmentf UW-Whitewaterf Whitewaterf WI 53190 ( * student author)

This approximately 50 m wide porphyritic diabase dikef outcropping in western Irwin and Pifher Townships in the Nipigon districtf is locally known as greenspar. The dike has a characteristic greenish mottled appearance in outcrop as a result of saussaritized plagioclase glomerophenocrysts in a diabase groundmass (Lutherf1997). The dike strikes due north and dips verticallyf and is found as segmentsf off-set by east-west faulting(Mackasey,l975). There are no radiometric dates on this rock or associated rocks, althoughf it is cut by a large middle or late Proterozoic sill in Irwin Township (Mackaseyf 1975) .

Paleomagnetic studies were completed on 2-3 cores from each of 5 locations. Each location represents one dike segment. Samples from each location were subjected to both alternating field (A.F.) and thermal demagnetization studies. The samples show one primary magnetic direction which is removed at demagnetization temperatures of 570 C. or by A.F. fields of 60 mT. All but one location shows normal polarity with northwesterly declinations(200~-300') and inclinations of 40'-80° The northern most section of the dike which we sampled is reversally magnetized with declinations of about 90'-100~ and inclinations of -75'- These magnetic directions are consistent with Keweenawan age paleomagnetic directions in the Lake Superior Region.

References

Lutherf F.f1997f The Petrology of Greenspar: A Proterozoic Porphyritic Diabase Dike; Pifher and Irwin Townshipsf Lake Nipigon Districtf Ontario. Institute on Lake Superior Geology abstracts and proceedings Vo1.43.

Mackaseyf W.0.f1975f Geology of Dorothear Sandra, and Irwin Townshipsf District of Thunder Bay; Ontario division of Minesf rpt 122 with map 2294# 83 p.

A GRAVITY, MAGNETIC, AND STRUCTURAL STUDY OF THE WAKEMUP BAYTONALITE, MINNESOTA

TIKOFF, Basil, Department of Geology and Geophysics, Rice University, Houston, TX77005-1892, USA, BAUER, Robert Department of Geological Sciences, University ofMissouri, Columbia, MO, 65211, USA, VIGNERESSE, Jean-Louis Vigneresse,CREGU, 54501 Vandoeuvre, Nancy Cedex, France, and HAGGEMAN, Nick,Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN55455, USA

The Wakemup Bay tonalite is a small intrusive body that intrudes amphibolite-grade biotiteschist on the extreme southern edge of the Quetico sub-province in the western part of LakeVermilion in Minnesota. The tonalite is part of the Wakemup Bay block, which is bounded bythe Vermilion fault to the north and the Haley fault to the south. The Haley fault has a dip-slipcomponent that separates amphibolite-grade rocks of the Quetico sub-province to the northfrom greenschist-grade rocks of the Wawa sub-province to the south. The primary purpose ofour investigation was to evaluate the mechanism of emplacement of the Wakemup Bay plutonand its relationship to local doming by determining its 3D shape and internal magnetic fabric.We conducted a gravity survey over the pluton, and use the gravity inversion to evaluate thepluton shape. We collected anisotropy of magnetic susceptibility (AMS) data to evaluate themagnetic fabric of the pluton.

Bauer (1985, 1986) reports previous structural analysis and mapping of the WakemupBay pluton. The pluton comprises a medium-grained biotite tonalite, with sphene, apatite,magnetite, and zircon as accessory minerals. The tonalite typically has a layered appearance,and the microstructures throughout the pluton, particularly at the pluton margin, indicate solid-state deformation. Foliation dips outward (20-60°) from the middle of the pluton, in aconcentric pattern. Biotite schist - consisting dominantly of plagioclase, quartz, and biotite -surrounds the tonalite on all sides, and has foliation that also dips away from the center of thepluton on all sides. On a more regional scale, the foliations of the schist defines a doublyplunging, EW-trending anticline with moderately dipping limbs. The tonalite is spatiallyassociated with the hinge area of this fold, which B auer (1985, 1986) interpreted as an F3 foldin the local deformation sequence. A hornblende diorite unit that intruded the biotite schistoutcrops on both the north and south sides of the tonalite. The center of the pluton is capped bya roof of walirock that extends —40 m above the tonalite contact. The roof consists of both thebiotite schist and a relatively flat-lying layer of the hornblende diorite.

The Anisotropy of Magnetic Susceptibility (AMS) is a technique that is widely used inthe study of granitic fabrics. The magnetic susceptibility is defined by M = K x H, where M isthe induced magnetization and H is the inducing magnetic field. The variation of this magneticsusceptibility with the sample placed in different orientations produces an AMS ellipsoid,similar to the finite strain ellipsoid. The principle AMS axes are kmax > kint > kmin, whichmay be interpreted in terms of a magnetic foliation (kmax - kint plane) or magnetic lineation(kmax orientation). The bulk susceptibility values varied widely in the Wakemup Bay tonalite(5 x 10-4 to 10-1 SI), interpreted as representing the presence or absence of magnetite. TheAMS foliation essentially parallels the measured field foliation. The major insight from theAMS study comes from the orientation of the magnetic lineation. Throughout the WakemupBay tonalite, the magnetic lineation is oriented dominantly EW, plunges shallowly, and isgenerally parallel to the long direction of the Wakemup Bay pluton and parallel to the trace ofthe F3 fold hinge. This observation has significance with respect to emplacement mechanism.

We selected the Wakemup tonalite for our detailed gravity study because: 1) It hadprevious structural mapping (Bauer, 1985); 2) It has a single surrounding lithology with asignificant and consistent density contrast (2.67 ± 0.04 for the tonalite and 2.75 ± 0.04 for thesurrounding schists); and 3) A walirock roof exists over the center of the pluton, despite therelatively low relief of the area (<40 m). Thus, the depth recorded by the gravity inversionrepresents the true depth of the pluton, within the limitations of the gravity inversion technique(e.g., method of Vigneresse, 1995). To achieve resolution obtained in structuralmeasurements, 142 gravity stations were collected on the Wakemup pluton and immediate

120

., A GMVITY, MAGNETIC, AND STRUCTURAL STUDY OF TJ3E WAKEMUP BAY TONALITE, MINNESOTA

TIKOFF, Basil, Department of Geology and Geophysics, Rice University, Houston, TX 77005- 1892, USA, BAUER, Robert Department of Geological Sciences, University of

I Missouri, Columbia, MO, 652 1 1, USA, VIGNERESSE, Jean-Louis Vigneresse, CREGU, 54501 Vandoeuvre, Nancy Cedex, France, and HAGGEMAN, Nick, Department of Geology and Geophysics, University of Mimesota, Minneapolis, MN 55455, USA

The wakemuP Bay tonalite is a small intrusive body that intrudes amphibolite-grade biotite schist on the extreme southern edge of the Quetico sub-province in the western part of Lake Vermilion in Minnesota. The tonalite is part of the Wakemup Bay block, which is bounded by the Vermilion fault to the north and the Haley fault to the south. The Haley fault has a dip-slip component that separates amphibolite-grade rocks of the Quetico sub-province to the north from greenschist-grade rocks of the Wawa sub-province to the south. The primary purpose of our investigation was to evaluate the mechanism of emplacement of the Wakemup Bay pluton and its relationship to local doming by determining its 3D shape and internal magnetic fabric. We conducted a gravity survey over the pluton, and use the gnvity inversion to evaluate the pluton shape. We collected anisotropy of magnetic susceptibility (AMS) data to eval~late the mdgnetic fabric of the pluton.

Bauer (1985, 1986) reports previous structural analysis and mapping of the Wakemup Bay pluton. The pluton comprises a medium-grained biotite tonalite, with sphene, apatite, magnetite, and zircon as accessory minerals. The tonalite typically has a layered appearance, and the microstructures throughout the pluton, particularly at the pluton margin, indicate solid- state deformation. Foliation dips outward (20-60') from the middle of the pluton, in a concentric pattern. Biotite schist - consisting dominantly of p1agioclase7 quartz, and biotite - surrounds the tonalite on all sides, and has foliation that also dips away from the center of the pluton on all sides. On a more regional scale, the foliations of the schist defines a doubly plunging, EW-trending anticline with moderately dipping limbs. The tonalite is spatially associated with the hinge area of this fold, which Bauer (1985, 1986) interpreted as an F3 fold '

in the local deformation sequence. A hornblende diorite unit that intruded the biotite schist outcrops on both the north and south sides of the tonalite. The center of the pluton is capped by a roof of wallrock that extends -40 m above the tonalite contact. The roof consists of both the biotite schist and a relatively flat-lying layer of the hornblende diorite.

The Anisotropy of Magnetic Susceptibility (AMS) is a technique that is widely used in the study of granitic fabrics. The magnetic susceptibility is defmed by M = K x H, where M is the induced magnetization and H is the inducing magnetic field. The variation of this magnetic susceptibility with the sample placed in different orientations produces an AMS ellipsoid, similar to the finite strain ellipsoid. The principle AMS axes are kmax > kint > kmin7 which may be interpreted in terms of a magnetic foliation (kmax - kint plane) or magnetic lineation (kmax orientation). The bulk susceptibility values varied widely in the Wakemup Bay tonalite (5 x 10-4 to 10-1 SI), interpreted as representing the presence or absence of magnetite. The AMS foliation essentially parallels the measured field foliation. The major insight from the AMS study comes from the orientation of the magnetic lineation. Throughout the Wakemup Bay tonalite, the magnetic lineation is oriented dominantly EW, plunges shallowly, and is generally parallel to the long direction of the Wakemup Bay pluton and parallel to the trace of the F3 fold hinge. This observation has significance with respect to emplacement mechanism.

We selected the Wakemup tonalite for our detailed gravity study because: 1) It had previous structural mapping (Bauer, 1985); 2) It has a single surrounding lithology with a significant and consistent density contrast (2.67 k 0.04 for the tonalite and 2.75 k 0.04 for the surrounding schists); and 3) A wallrock roof exists over the center of the pluton, despite the relatively low relief of the area (< 40 m). Thus, the depth recorded by the gravity inversion represents the true depth of the pluton, within the limitations of the gravity inversion technique (e.g., method of Vigneresse, 1995). To achieve resolution obtained in structural measurements, 142 gravity stations were collected on the Wakemup pluton and immediate

surroundings using a Lacoste & Romberg, model G gravitimeter. Laser theodolite fromexisting benchmarks and the controlled-elevation shoreline Of Lake Vermilion provided thecritical elevation controls. A map of the Bouguer anomalies is provided (Fig. la). Using agravimetric three-dimensional iterative technique, resulted in a good first-order picture of thepluton (Fig. ib). Most of the pluton is very thin, less than 0.5 km thick. There are two rootzones, both of which indicate depths of up to 4.0 km, using the calculated densities. The depthanomaly in the southwest part of the pluton has a linear, NW trend and is approximately 5 kmlong. It lies adjacent to the SW part of the pluton and is thus adjacent to the Haley fault. Thedepth anomaly in the center of the pluton is slightly less deep (—3 km). It also contains a NWtrend, although the depth is clearly at a maximum in the SE part of the NW-SE oriented trend.It is interesting to note that the deepest portion of the tonalite does not sit under the presentexposure of the pluton.

As a result of the structural geology, magnetic analysis, and gravity measurements, theWakemup Bay pluton is interpreted as intruding a broad anticlinal hinge, during the foldingevent (F3). This was one of the possibilities suggested by Bauer (1986) and consistent with thevery thin nature of the pluton (<0.5 km). The walirock preservation in the roof represents theanticlinal hinge. The hornblende diorite unit on the north side, south side, and above the plutonis a good indication of this situation. The two NW-root zones were presumably were feederzones for the magma, although given the very high exposure level this claim is impossible tosupport by geological or magnetic measurements. The feeder zones may have initiated on NWoriented fractures. This type of cross-faulting is commonly seen in folds, particularly thosewith a component of hinge-parallel extension. The large southern root zone may have affectedthe subsequent movement of the Wakemup block, between the Haley and Vermilion faults.

The Haley fault occurs just south of the deep southern root zone, and the root of thetonalite may have acted as a strong heterogeneity in the schist. The Haley fault showsindications of some dextral strike-slip faulting, which is inferredto have occurred after itsearlier normal movement (e.g., Bauer, 1986). This type of small rigid block moving betweenmajor fault systems is inferred in other orogenic belts.REFERENCESBauer, R.L., 1985, Norwegian Bay Quadrangle, St. Louis county, Minnesota. MinnesotaGeological Survey, Miscellaneous Map Series, Map M-59, 1:24,000.Bauer, R.L., 1986, Multiple folding and pluton emplacement in Archean migmatites of thesouthern Vermilion granitic complex, northeastern Minnesota. Canadian Journal of EarthSciences, v. 23, p. 1753-1764.

5314.00

5312.00

5310.00

5308.00

5306.0

5304.00

5302.00

53002.00 524.00 526.00 528.00 530.00 532.00 534.00 536.00 538.00 540.00 542.00 544.00

Fig. 1. Gravity inversion model for Wakemup Bay tonalite. Heavy dashed line representscurrent outcropping of the pluton. Light lines are inferred depth of the pluton, contoured for0.5 km. The tonalite is generally a thin sheet (<0.5 km), with two deep root zones.

121

I

surroundings using a Lacoste & Romberg, model G gravitimeter. Laser theodolite from existing benchmarks and the controlled-elevation shoreline Of Lake Vermilion provided the critical elevation controls. A map of the Bouguer anomalies is provided (Fig. la). Using a gravirnetric three-dimensional iterative technique, resulted in a good first-order picture of the pluton (Fig. lb). Most of the pluton is very thin, less than 0.5 km thick. There are two root zones, both of which indicate depths of up to 4.0 km, using the calculated densities. The depth anomaly in the southwest part of the pluton has a linear, NW trend and is approximately 5 krn long. It lies adjacent to the SW part of the pluton and is thus adjacent to the Haley fault. The depth anomaly in the center of the pluton is slightly less deep (-3 km). It also contains a NW trend, although the depth is clearly at a maximum in the SE part of the NW-SE oriented trend. It is interesting to note that the deepest portion of the tonalite does not sit under the present exposure of the pluton.

As a result of the structural geology, magnetic analysis, and gravity measurements, the Wakemup Bay pluton is interpreted as intruding a broad anticlinal hinge, during the folding event (F3). This was one of the possibilities suggested by Bauer (1986) and consistent with the very thin nature of the pluton ( ~ 0 . 5 km). The wallrock preservation in the roof represents the anticlinal hinge. The hornblende diorite unit on the north side, south side, and above the pluton is a good indication of this situation. The two NW-root zones were presumably were feeder zones for the magma, although given the very high exposure level this claim is impossible to support by geological or magnetic measurements. The feeder zones may have initiated on NW oriented fractures. This type of cross-faulting is commonly seen in folds, particularly those with a component of hinge-parallel extension. The large southern root zone may have affected the subsequent movement of the Wakemup block, between the Haley and Vermilion faults.

The Haley fault occurs just south of the deep southern root zone, and the root of the tonalite may have acted as a strong heterogeneity in the schist. The Haley fault shows indications of some dextral strike-slip faulting, which is inferred to have occurred after its earlier normal movement (e.g., Bauer, 1986). This type of small rigid block moving between major fault systems is inferred in other orogenic belts. REFERENCES Bauer, R.L., 1985, Norwegian Bay Quadrangle, St. Louis county, Minnesota. Minnesota Geological Survey, Miscellaneous Map Series, Map M-59, 1:24,000. Bauer, R.L., 1986, Multiple folding and pluton emplacement in Archean rnigmatites of the southern Vermilion eranitic com~lex, northeastern Minnesota. Canadian Journal of Earth -

Sciences, v. 23, p. 1753-1764. I I I I I I I I I I I I

5300.00-' I I b ! t I I I

522.00 524.00 526.00 528.00 530.00 532.00 534.00 536.00 538.00 540.00 542.00 544.00

Fig. 1. Gravity inversion model for Wakemup Bay tonalite. Heavy dashed line represents current outcropping of the pluton. Light lines are inferred depth of the pluton, contoured for 0.5 km. The tonalite is generally a thin sheet ( ~ 0 . 5 km), with two deep root zones.

THE RELEVANCE OF THE GEOLOGY OF MID-OCEAN RIDGES AND OPHIOLITES TOTHE UNDERSTANDING OF LAYERED INTRUSIONS IN THE MIDCONTINENT RIFT:PART I. GEOMETRY; PART II. INTERNAL STRUCTURE AND PETROLOGY

Tatiana Vislova, Department of Geology and Geophysics, University of Minnesota, USA

Part I. Insights from current research on mid-ocean ridges and ophiolites remain to be appliedto studies of intracontinental rift systems. Dynamic magma chambers where fractionalcrystallization and formation of layered cumulates occur simultaneously with spreading arebeing defined by seismic data for ocean island and mid-ocean ridge systems. But layeredintrusions in continental crust are still generally studied from the point of view of fixedgeometry. However, if we accept the conclusion that the Duluth Complex and associatedintrusions such as the Sonju Lake and the Bald Eagle intrusions formed in an intracontinentalrift system, the concept of dynamic magma chambers during crystallization of layered gabbromust be considered.

There is a growing database on the geometry and dynamic processes of crystallizationand the formation of layered gabbro based on seismic studies along the fast spreading (>-10cm/yr.) East Pacific Rise (Sinton, 1992; Wang, 1996; Barth, 1996) and the slow-spreading(<-5 cm/yr.) Mohns Ridge (Geli, 1994) and Mid-Atlantic Ridge (Rommevaux, 1994). Thesestudies demonstrate that active magma chambers: 1) are segmented and centered undertopographic highs and active vents along the ridge axes (McKenzie, 1997 and Wright, 1995),2) pinch and swell in plan view with typical widths and along-axis lengths of 5 — 15 km (Geli,1994), and 3) are found at depths of 1.5 — 2.5 km below the seafloor. These features are bestdeveloped along slow spreading rifts such as the Mohns Ridge (Geli, 1994).

Because ophiolites are fragments of oceanic crust, they provide an opportunity to studythe processes that occur along ridge axes. Recent detailed mapping and interpretation of theOman ophiolite generally confirm the segmental structure of ridge magma chambers asdescribed above (Nicolas, 1996 and references therein).

The geophysical expressions of present-day oceanic ridges and the 1.1 Ga MidcontinentRift in North America show the same segmented bulls-eye structure on a scale of 100-1000km. On a scale of I — 100 km, the geophysical data on active magma chambers along ridgeaxes and the first derivative of the aeromagnetic data on the Bald Eagle Intrusion have asimilar pinch and swell geometry. These similarities on large and small scales imply that ourcurrent understanding of the processes responsible for the structure and petrologic features ofridge magma chambers and layered gabbro in ophiolites can be used to explain the origin ofsimilar features in layered intrusions in intracontinental rifts. The geometries of intrusionsformed in dynamic or static magma chambers are related to spreading rates and continuity ofspreading. With fast, continuous spreading rates it is possible to produce layered gabbro at alateral scale of many kilometers from a relatively narrow, growing magma chamber which isnever more than 1-2 km wide at any given time (e.g., Nicolas, 1996, p.17,842). On the otherhand, with slow, episodic spreading rates the lateral extent of layered gabbro could berestricted to the dimensions of the active magma chambers. Thus the Sonju Lake intrusion,with its sill-like geometry (Miller, 1996), could have formed in a static magma chamber;whereas the Bald Eagle intrusion, with its funnel shape, could have formed in a more dynamicenvironment.Part II. The funnel-shaped layering in the Bald Eagle intrusion contrasts remarkably withnear-horizontal layering of the other intrusions in the Duluth Complex and in the Omanophiolite. Unfortunately the current interpretations of the layering in ophiolites do not provide

122

THE RELEVANCE OF THE GEOLOGY OF MID-OCEAN RIDGES AND OPHIOLITES TO THE UNDERSTANDING OF LAYERED INTRUSIONS IN THE MIDCONTINENT RIFT: PART I. GEOMETRY; PART 11. INTERNAL STRUCTURE AND PETROLOGY

Tatiana Vislova, Department of Geology and Geophysics, University of Minnesota, USA

Part I. Insights from current research on mid-ocean ridges and ophiolites remain to be applied to studies of intracontinental rift systems. Dynamic magma chambers where fractional crystallization and formation of layered cumulates occur simultaneously with spreading are being defined by seismic data for ocean island and mid-ocean ridge systems. But layered intrusions in continental crust are still generally studied from the point of view of fixed geometry. However, if we accept the conclusion that the Duluth Complex and associated intrusions such as the Sonju Lake and the Bald Eagle intrusions formed in an intracontinental rift system, the concept of dynamic magma chambers during crystallization of layered gabbro must be considered.

There is a growing database on the geometry and dynamic processes of crystallization and the formation of layered gabbro based on seismic studies along the fast spreading (>-lo cmlyr.) East Pacific Rise (Sinton, 1992; Wang, 1996; Barth, 1996) and the slow-spreading (<-5 crnlyr.) Mohns Ridge (Geli, 1994) and Mid-Atlantic Ridge (Rommevaux, 1994). These studies demonstrate that active magma chambers: 1) are segmented and centered under topographic highs and active vents along the ridge axes (McKenzie, 1997 and Wright, 1995), 2) pinch and swell in plan view with typical widths and along-axis lengths of 5 - 15 km (Geli, 1994), and 3) are found at depths of 1.5 - 2.5 km below the seafloor. These features are best developed along slow spreading rifts such as the Mohns Ridge (Geli, 1994).

Because ophiolites are fragments of oceanic crust, they provide an opportunity to study the processes that occur along ridge axes. Recent detailed mapping and interpretation of the Oman ophiolite generally confirm the segmental structure of ridge magma chambers as described above (Nicolas, 1996 and references therein).

The geophysical expressions of present-day oceanic ridges and the 1.1 Ga Midcontinent Rift in North America show the same segmented bulls-eye structure on a scale of 100-1000 km. On a scale of 1 - 100 km, the geophysical data on active magma chambers along ridge axes and the first derivative of the aeromagnetic data on the Bald Eagle Intrusion have a similar pinch and swell geometry. These similarities on large and small scales imply that our current understanding of the processes responsible for the structure and petrologic features of ridge magma chambers and layered gabbro in ophiolites can be used to explain the origin of similar features in layered intrusions in intracontinental rifts. The geometries of intrusions formed in dynamic or static magma chambers are related to spreading rates and continuity of spreading. With fast, continuous spreading rates it is possible to produce layered gabbro at a lateral scale of many kilometers from a relatively narrow, growing magma chamber which is never more than 1-2 km wide at any given time (e.g., Nicolas, 1996, p.17,842). On the other hand, with slow, episodic spreading rates the lateral extent of layered gabbro could be restricted to the dimensions of the active magma chambers. Thus the Sonju Lake intrusion, with its sill-like geometry (Miller, 1996), could have formed in a static magma chamber; whereas the Bald Eagle intrusion, with its funnel shape, could have formed in a more dynamic environment. Part It. The funnel-shaped layering in the Bald Eagle intrusion contrasts remarkably with near-horizontal layering of the other intrusions in the Duluth Complex and in the Oman ophiolite. Unfortunately the current interpretations of the layering in ophiolites do not provide

a ready explanation for the layering in the Bald Eagle intrusion. There is an active debate inthe literature concerning the processes that would lead from the isotropic crystal mushes ofactive ocean-ridge magma chambers to the near-horizontal layering in gabbro of the oceanfloor and in ophiolites (Quick, 1993, and Nicolas, 1996). In essence Quick's analysis impliesthat the layering and the igneous lamination are post-crystallization phenomena, producedwhen crystal mush is carried away from the active magma chamber. If this idea is applicable tothe Duluth Complex, the Bald Eagle intrusion could be a frozen active chamber thus far notobserved in ophiolites. Layered gabbro of adjacent South Kawishiwi intrusion could be theequivalent of layered gabbro in ophiolite produced during spreading away from a ridge axis.

One might expect that the cryptic, compositional variations of cumulus mineralsformed in static chambers might differ from those formed in dynamic chambers. A suggestionof such differences can be seen in Figure 1. All three data sets lie on a common trend. Gabbroin the Oman ophiolite and rocks in the Bald Eagle intrusion show more restricted ranges ofdifferentiation than rocks in the Sonju Lake intrusion, and the Oman magma is the mostprimitive. Additional electron microprobe mineral and minor- and trace- element rock analysesof the Bald Eagle intrusion are planned to evaluate the suggestion that the chemical variations

produced in dynamic chambers are100 definitively distinct from those

established for static chambers.

Fig. 1. Mg/(Mg+Fe) variation in pyroxene andolivine. All the data are electron microprobeanalyses. Sonju Lake intrusion (Miller, 1996);Bald Eagle intrusion - data points are averagesof over 200 analyses of representative samplesof troctolite, olivine and oxide gabbros; Omanophiolite — data from gabbro sills (Korenaga,1997).

References: Barth, G. A., and J. C. Mutter, Variability in oceanic crustal thickness and structure: Multichannelseismic reflection results from the northern East Pacific Rise, J. Geophys. Res., 101, 17,951-17,975, 1996.Geli, L., and V. Renard, Ocean crust formation process at very slow spreading centers: A model for the Mohnsridge, near 72 N, based on magnetic, gravity, and seismic data, J. Geophys. Res., 99, 2995-3013, 1994.Korenaga, J., and P. B. Kelemen, Origin of gabbro sills in the Moho transition zone of the Oman ophiolite:Implications for magma transport in the oceanic lower crust, J. Geophys. Res., 102,27,729-27,749, 1997.McKenzie, D., and D. Fairhead, estimates of the effective elastic thickness of the continental lithosphere fromBouguer and free air gravity anomalies, J. Geophys. Res., 102, 27,523-27,552, 1997.Miller, J. D., and E. M. Ripley, Layered Intrusions of the Duluth Complex, Minnesota, USA, In: RG. Cahom(ed.) Layered Intrusions, Elsevier Science, 1996Nicolas, A., F. Boudier, and B. ildefonse, Variable crustal thickness in the Oman ophiolite: Implication foroceanic crust, J. Geophys. Res., 101, 17,941-17,950, 1996.Quick, J. E., and R. P. Denlinger, Ductile deformation and the origin of layered gabbro in ophiolites, J. Geophys.

Res., 98, 14,015-14,027, 1993.Rommevaux, C., C. Deplus, and P. Patriat, Three-dimensional gravity study of the Mid-Atlantic Ridge: evolutionof the segmentation between 28° and 29°N during the last 10 my, J. Geophys. Res., 99, 3015-3029, 1994.Sinton, J. IvL, and it S. Detrick, Mid-ocean ridge magma chambers, J. Geophys. Res., 97, 197-216, 1992.Wan& X., J. it Cochran and G. A. Barth, Gravity anomalies, crustal thickness, and the pattern of mantle flow atthe fast spreading East Pacific Rise, 9-10 N: evidence for three-dimensional upwelling, J. Geophys. Res., 101,17,927-17,940, 1996.Wright, D. J., it M Haymon, and D. J. Fomari, Crustal fissuring and its relationship to magmatic andhydrothermal processes on the East Pacific Rise crest (9°12' to 54'N), J. Geophys. Res., 100, 6097-6120, 1995.

123

. ::1

70

60 +

___t+

40+SOOjU Lakelnbusion

+ OBald Eagle lnfrusion

0 20 40

Pom % Mg!(Mg+Fe) in olivine

a ready explanation for the layering in the Bald Eagle intrusion. There is an active debate the literature concerning the processes that would lead from the isotropic crystal mushes

References: Barth, G. A , and J. C. Mutter, Variability in oceanic crustal thickness and structure: Multichannel seismic reflection results from the northern East Pacific Rise, J. Geophys. Res., 10 1 17,95 1 - 17-975, 1996. Geli, L., and V. Renard, Ocean crust formation process at very slow spreading centers: A model for the Mohns ridge, near 72 N, based on magnetic, gravity, and seismic data, J. Geophys. Res., 99,2995-3013, 1994. Korenaga, J., and P. B. Kelemen, Origin of gabbro sills in the Moho transition zone of the Oman ophiolite: Implications for magma transport in the oceanic lower cmst, J. Geophys. Res., 102,27,729-27,749, 1997. McKenzie, D., and D. Fairhead, estimates of the effective elastic thickness of the continental lithosphere from Bouguer and free air gravity anomalies, J. Geophys. Res., 102,27,523-27,552, 1997. Miller, J. D., and E. M. Ripley, Layered Intrusions of the Duluth Complex, Minnesota, USA, hi: RG. Cawthorn (ed) Layered Intrusions, Elsevier Science, 1996 Nicolas, A , F. Boudier, and B. Ildefonse, Variable crustal thickness in the Oman ophiolite: Implication for oceaniccrust, J. Geophys. Res., 101, 17,941-17,950, 1996. Quick, J. E., and R P. Denlinger, Ductile deformation and the origin of layered gabbro in ophiolites, J. Geophys. Res., 98, 14,015-14,027, 1993. Ronunevaux, C., C. Deplus, and P. Patriat, Three-dimensional gravity study of the Mid-Atlantic Ridge: evolution of the segmentation between 28O and 2g0N during the last 10 my, J. Geophys. Res., 99,3015-3029,1994. Sinton, J. My and R S. Detrick, Mid-ocean ridge magma chambers, J. Geophys. Res., 97,197-216,1992. Wane, X., J. R Cochran and G. A Barth, Gravity anomalies, crustal thickness, and the pattern of mantle flow at the fast spreading East Pacific Rise, 9-10 N: evidence for three-dimensional upwelling, J. Geophys. Res., 101, 17,927- 17,940, 1996. Wright, D. J., R. M. Haymon, and D. J. Fornari, Crustal fissuring and its relationship to magmatic and hydrothermal processes on the East Pacific Rise crest (9O12' to 5479, J. Geophys. Res., 100,6097-6120, 1995.

in of

active ocean-ridge magma chambers to the near-horizontal layering in gabbro of the ocean floor and in ophiolites (Quick, 1993, and Nicolas, 1996). In essence Quick's analysis implies that the layering and the igneous lamination are post-crystallization phenomena, produced when crystal mush is carried away from the active magma chamber. If this idea is applicable to the Duluth Complex, the Bald Eagle intrusion could be a frozen active chamber thus far not observed in ophiolites. Layered gabbro of adjacent South Kawishiwi intrusion could be the equivalent of layered gabbro in ophiolite produced during spreading away from a ridge axis.

One might expect that the cryptic, compositional variations of cumulus minerals formed in static chambers might differ from those formed in dynamic chambers. A suggestion of such differences can be seen in Figure 1. All three data sets lie on a common trend. Gabbro in the Oman ophiolite and rocks in the Bald Eagle intrusion show more restricted ranges of differentiation than rocks in the Sonju Lake intrusion, and the Oman magma is the most primitive. Additional electron microprobe mineral and minor- and trace- element rock analyses of the Bald Eagle intrusion are planned to evaluate the suggestion that the chemical variations

produced in dynamic chambers are definitively distinct from those established for static chambers.

Fig. 1. Mgl(Mg+Fe) variation in pyroxene and olivine. All the data are electron microprobe analyses. Sonju Lake intrusion (Miller, 1996); Bald Eagle intrusion - data points are averages of over 200 analyses of representative samples of tructolite, olivine and oxide gabbros; Oman ophiolite - data from gabbro sills (Korenaga, 1997).

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..-.....---- L-..-.-.---L-.-..-----^---.....--i- ^<---.. jp.

---.--.....- L--~-.~.---L-..-......L-------.-.~----------

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; + + + , 6 0 . - - - - - - - - - - - ~ - - - - - - - - - - p . + - - - - - - - - ~ . - - - - - - - - - ~ - - - - - - - - - -

 ¥  ¥  ¥  Hi"++--?--.: - - - - - - - - - - ; ..--.----- L - - - - - - - - - - + Sonju Lake intrusion --..-.- d.+f? - - - - - - - - - [ - - - - - - - -

+ I OBald Eagle Intrusion --------.---: ---------.; -----.-- xman ophiolite gabbro SIIIS -

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PRECISE U-Pb ZIRCON AGES OF MIDCONTINENT RIFT RHYOLITE(CHENGWATANA VOLCANICS), CLAM FALLS, WI

WIRTH, Karl R., Geology Department, Macalester College, St. Paul, MN 55105,wirth@macalester.edu; and GEHRELS, George, E., Department of Geosciences, Uni-versity of Arizona, Tucson, AZ 85721, gehrels@geo.arizona.edu.

Recent studies of the southwestern segment of the Keweenawan Midcontinent rift along the St.Croix River have focused on the structural (Leslie et al., 1994), paleomagnetic (Kean et a!.,1997), and magmatic (Wirth et al., 1997) evolution of the poorly-exposed Chengwatana Volcanics(Minnesota and Wisconsin). Comparison of Chengwatana Volcanics from this segment of therift with the those of the better-studied portions of the rift in the Lake Superior region (e.g.,Nicholson et al., 1997) have been hindered by the lack of well-constrained ages for theChengwatana Volcanics. The only previous geochronologic study of these volcanics (Zartmanet al., 1997) resulted in an age of 1094.6 ± 2.1 Ma for rhyolite exposed in the Ashland synclinein northern Wisconsin; rhyolite from drill core in Hudson-Afton Horst (SE of Minneapolis)yielded an anomalously old (1130 Ma) age. Here we report precise U-Pb zircon ages (1102 ± 5Ma) for rhyolites exposed in the lower part of the volcanic section along the Lake Owen Fault,which bounds the eastern margin of the rift. Petrographic and geochemical studies of the rhy-olitic and basaltic flows of this segment of the rift are described by Abbott et al. and Naiman eta!. (both this volume).

Rhyolite exposed one kilometer east of Clam Falls, WI contains phenocrysts of plagio-clase and rare quartz in a spherulitic groundmass of tabular quartz and potassium feldspar. Therhyolite is typically fine- to medium-grained and massive (sample KC-302a), but becomesmore coarse-grained near its base (sample KC-302d) and contains subangular clasts of basalt.Flow structures are not apparent, but the rhyolites are otherwise similar to the large rhyolites ofthe North Shore Volcanic Group. Rhyolite is rare in this portion of the Midcontinent rift and isestimated to compose less than one percent of the exposed volcanic and sedimentary sections.

Zircons from both samples have relatively simple prismatic (length:width ratios = 2:1 to8:1) forms, are translucent, and vary in color from reddish-brown, to honey brown, and to palepink. Dark grains typically contain abundant inclusions and were not selected for analysis.Analyses were conducted by conventional isotope dilution-thermal ionization mass spectrom-etry, as described by Gehrels et a!. (1991). Six euhedral crystals (175-250 jim in length) fromKC-302a were abraded to 75% their original size and were analyzed as individual grains. Threeof the grains yielded concordant ages with mean 2o6Pb*P38U and 2o7Pb*/2o6Pb* ages of 1102Ma; an uncertainty of ±5 Ma (2a) is assigned to this age on the basis of the errors of theindividual determinations, rather than the error of the mean of the three concordant analyses.Three additional grains are slightly discordant, presumably due to small amounts of lead loss.A discordia through these points, projected from 1102 Ma, yields a lower intercept of 196±205Ma (MSWD = 0.15). Analyses of KC-302d were conducted on two multigrain fractions, oneunabraded single grain, and five abraded single grains. The single grains were all —200 .tm inlength originally, and the multigrain fractions consisted of: 1) seven grains measuring —125 jimin length, and 2) thirty grains —80 jim in 1enth. The three abraded grains are analyticallyconcordant and yield mean 2o6Pb*/238U and ZO7pb*/2O6pb* ages of 1101 Ma ± 5 Ma. Threeadditional grains and the two multigrain fractions are slightly discordant; a discordia throughthese points, projected from 1101 Ma, yields a lower intercept of 63 ±217 Ma (MSWD = 0.45).Given the uncertainties of ±5 Ma on each of the ages, the 1 Ma age difference between the twosamples is probably not significant.

Analyses of the Chengwatana rhyolites near Clam Falls indicate that magmatism beganby at least 1102 Ma in this portion of the rift, considerably earlier than the age reported byZartman et al. (1997) for rhyolite (1094.6 ± 2.1 Ma) exposed near the base of the Chengwatana

124

- PRECISE U-Pb ZIRCON AGES OF MIDCONTINENT RIFT RHYOLITE (CHENGWATANA VOLCANICS), CLAM FALLS, WI

WIRTH, Karl R., Geology Department, Macalester College, St. Paul, MN 55105, wirth @macalester.edu; and GEHRELS, George, E., Department of Geosciences, Uni-

I versity of Arizona, Tucson, AZ 85721, gehrels@geo.arizona.edu.

Recent studies of the southwestern segment of the Keweenawan Midcontinent rift along the St. Croix River have focused on the structural (Leslie et al., 1994), paleomagnetic (Kean et al., 1997), and magmatic (Wirth et al., 1997) evolution of the poorly-exposed Chengwatana Volcanics (Minnesota and Wisconsin). Comparison of Chengwatana Volcanics from this segment of the rift with the those of the better-studied portions of the rift in the Lake Superior region (e.g., Nicholson et al., 1997) have been hindered by the lack of well-constrained ages for the Chengwatana Volcanics. The only previous geochronologic study of these volcanics (Zartman et al., 1997) resulted in an age of 1094.6 zk 2.1 Ma for rhyolite exposed in the Ashland syncline in northern Wisconsin; rhyolite from drill core in Hudson-Afton Horst (SE of Minneapolis) yielded an anomalously old (1 130 Ma) age. Here we report precise U-Pb zircon ages (1 102 k 5 Ma) for rhyolites exposed in the lower part of the volcanic section along the Lake Owen Fault, which bounds the eastern margin of the rift. Petrographic and geochemical studies of the rhy- olitic and basaltic flows of this segment of the rift are described by Abbott et al. and Naiman et al. (both this volume).

Rhyolite exposed one kilometer east of Clam Falls, WI contains phenocrysts of plagio- clase and rare quartz in a spherulitic groundmass of tabular quartz and potassium feldspar. The rhyolite is typically fine- to medium-grained and massive (sample KC-302a), but becomes more coarse-grained near its base (sample KC-302d) and contains subangular clasts of basalt. Flow structures are not apparent, but the rhyolites are otherwise similar to the large rhyolites of the North Shore Volcanic Group. Rhyolite is rare in this portion of the Midcontinent rift and is estimated to compose less than one percent of the exposed volcanic and sedimentary sections.

Zircons from both samples have relatively simple prismatic (1ength:width ratios = 2: 1 to 8: 1) forms, are translucent, and vary in color from reddish-brown, to honey brown, and to pale pink. Dark grains typically contain abundant inclusions and were not selected for analysis. Analyses were conducted by conventional isotope dilution-thermal ionization mass spectrom- etry, as described by Gehrels et al. (1991). Six euhedral crystals (175-250 pm in length) from KC-302a were abraded to 75% their original size and were analyzed as individual grains. Three of the grains yielded concordant ages with mean 206~b* /238~ and 207~b*/206~b* ages of 1102 Ma; an uncertainty of k5 Ma (20) is assigned to this age on the basis of the errors of the individual determinations, rather than the error of the mean of the three concordant analyses. Three additional grains are slightly discordant, presumably due to small amounts of lead loss. A discordia through these points, projected from 1102 Ma, yields a lower intercept of 196 5 205 Ma (MSWD = 0.15). Analyses of KC-302d were conducted on two multigrain fractions, one unabraded single grain, and five abraded single grains. The single grains were all -200 pm in length originally, and the multigrain fractions consisted of: 1) seven grains measuring - 125 pm in length, and 2) thirty grains -80 pm in length. The three abraded grains are analytically concordant and yield mean 206~b* /238~ and 2O7pb*/2O6pb* ages of 1101 Ma k 5 Ma. Three additional grains and the two multigrain fractions are slightly discordant; a discordia through these points, projected from 1101 Ma, yields a lower intercept of 63 k 217 Ma (MSWD = 0.45). Given the uncertainties of k5 Ma on each of the ages, the 1 Ma age difference between the two samples is probably not significant.

Analyses of the Chengwatana rhyolites near Clam Falls indicate that magmatism began by at least 1102 Ma in this portion of the rift, considerably earlier than the age reported by Zartman et al. (1997) for rhyolite (1094.6 k 2.1 Ma) exposed near the base of the Chengwatana

1.90 1.98

207pb / 235 u 207Pb / 235 u

section in the southeastern limb of the Ashland syncline. This implies that flows exposed nearClam Falls likely formed contemporaneously with the Upper Kallander Creek Volcanics andthe Mellen Complex of Northern Wisconsin, and the Group 5 flows and Great Conglomerate ofMamainse Point. The 1102 ± 5 Ma age of Chengwatana Volcanics in the Clam Falls regionimplies that the Clam Falls flows may have "reversed" paleomagnetic directions since they arecoeval with the time of the reverse-to-normal magnetic polarity change (1105-1100 Ma) thathas been widely observed throughout the Lake Superior region. New aeromagnetic data (USGS)suggest that Chengwatana Volcanic flows exposed near Taylors Falls, MN, are younger thanthose near Clam Falls. Flows near Taylors Falls have predominantly "normal" paleomagneticdirections (Kean et al., 1997) and are likely younger than the regionally documented reverse-to-normal polarity shift; these flows may be correlative with Portage Lake Volcanics (northernWI) and Group 6 flows (Mamainse Point). A few Taylors Falls flows have "reversed" paleo-magnetic directions (Kean et al., 1997) and record a reversal event that is younger than the

regionally observed reverse-to-normal magneticMinnesota Upper Mich Michipicoten Is.

NW Wisc Mamainse Point polarity change.

References CitedGehrels, G.E., McClelland, W.C., Samson, S.D., and

Patchett, P.J., 1991, Canadian Journal of Earth Sci-ences, v. 28, P. 1285-1300.

Green, J.C. and Fitz, T.J., 1993, Journal of Volcanologyand Geothermal Research, v. 54, p. 177-196.

Kean, W.F., Williams, I., Chan, L., and Feeney, J., 1997,Geophysical Research Letters, v. 24, p. 1523-1526.

Leslie, M., Wetzel, T., Wirth, K.R., and Craddock, J.P.,1994, 40thAnnual Meeting of the Institute on LakeSuperior Geology, Houghton, MI, May 11-14, PartI - Program with Abstracts, v. 40, p. 35-36.

Miller, J.D., Nicholson, S.W., and Cannon, W.F., 1995,Minnesota Geological Survey Guidebook Series, p.1-22.

Nicholson, S.W., Shirey, S.B., Schulz, K.J., Green, J.C.,1997, Canadian Journal of Earth Sciences, v. 34, p.504-520.

Wirth, K.R., Vervoort, J.D., Naiman, Z.J., 1997, CanadianJournal of Earth Sciences, v. 34, no. 4, p. 536-548.

Zartman, R.E., Nicholson, S.W., Cannon, W.F., and Morey,G.B., 1997, Canadian Journal of Earth Sciences, v.34, p. 549-561.

125

U

00en

0.190

0.186

0.182

0.178

0.174

Lower Intercept =196 ±205 Ma (MS WD=0.15)

I I

1.82 1.90 1.98 2.06 1.82

Lower Intercept =

I/ 63±217Ma(MSWD=0.45)

2.06

section in the southeastern limb of the Ashland syncline. This implies that flows exposed near Clam Falls likely formed contemporaneously with the Upper Kallander Creek Volcanics and the Mellen Complex of Northern Wisconsin, and the Group 5 flows and Great Conglomerate of Mamainse Point. The 1102 Â 5 Ma age of Chengwatana Volcanics in the Clam Falls region implies that the Clam Falls flows may have "reversed" paleomagnetic directions since they are coeval with the time of the reverse-to-normal magnetic polarity change (1 105-1 100 Ma) that has been widely observed throughout the Lake Superior region. New aeromagnetic data (USGS) suggest that Chengwatana Volcanic flows exposed near Taylors Falls, MN, are younger than those near Clam Falls. Flows near Taylors Falls have predominantly "normal" paleomagnetic directions (Kean et al., 1997) and are likely younger than the regionally documented reverse- to-normal polarity shift; these flows may be correlative with Portage Lake Volcanics (northern WI) and Group 6 flows (Mamainse Point). A few Taylors Falls flows have "reversed" paleo- magnetic directions mean et al., 1997) and record a reversal event that is younger than the

regionally observed reverse-to-normal magnetic Minnesota ' Upper Mich Michipicoten Is. Wisconsin NW Wisc Mamainse Point polarity change.

References Cited Gehrels, G.E., McClelland, W.C., Samson, S.D., and

Patchett, P.J., 1991, Canadian Journal of Earth Sci- ences, v. 28, p. 1285-1300.

Green, J.C. and Fitz, T.J., 1993, Journal of Volcanology and Geothermal Research, v. 54, p. 177-196.

Kean, W.F., Williams, I., Chan, L., and Feeney, J., 1997, Geophysical Research Letters, v. 24, p. 1523-1526.

Leslie, M., Wetzel, T., Wirth, K.R., and Craddock, J.P., 1994,40thAnnual Meeting of the Institute on Lake Superior Geology, Houghton, MI, May 11-14, Part I - Program with Abstracts, v. 40, p. 35-36.

Miller, J.D., Nicholson, S.W., and Cannon, W.F., 1995, Minnesota Geological Survey Guidebook Series, p. 1-22.

Nicholson, S.W., Shirey, S.B., Schulz, K.J., Green, J.C., 1997, Canadian Journal of Earth Sciences, v. 34, p. 504-520.

Wirth, K.R., Vervoort, J.D., Naiman, Z.J., 1997, Canadian Journal of Earth Sciences, v. 34, no. 4, p. 536-548.

Zartman, R.E., Nicholson, S.W., Cannon, W.F., and Morey, G.B., 1997, Canadian Journal of Earth Sciences, v. 34, p. 549-561.