INSTITUTE ON LAKE SUPERIOR...

INSTITUTE ON LAKE SUPERIOR GEOLOGY

PROCEEDINGS

VOLUME 35 MAY 1989

Part 2. Field Trip GuidebookTrip 1. North Shore Rhyolites, Minnesota

Trip 2. Penokean Structural Terranesin East-Central Minnesota

Trip 3. Mellen Complex, WisconsinTrip 4. Archean Gold Occurrences and Their

Structural Settings

35th Annual MeetingMay 3-6, 1989

held atDuluth, Minnesota 55812


PROCEEDINGS

VOLUME 35 MAY 1989

Part 2. Field Trip Guidebook

Trip 1. North Shore Rhyolites, Minnesota Trip 2. Penokean Structural Terranes

in East-Central Minnesota Trip 3. Mellen Complex, Wisconsin

Trip 4. Archean Gold Occurrences and Their Structural Settings

35th Annual Meeting May 3-6, 1989

held at Duluth. Minnesota 55812

Organizing Committee, 35th Annual Meeting, ILSG (1989)

Richard W. Ojakangas, Dept. of Geology, University of Minnesota, Duluth, MN 55812John C. Green, Dept. of Geology, University of Minnesota-Duluth, Duluth, MN 55812

Timothy B. Hoist, University of Minnesota-Duluth, Duluth, MN 55812

Program Chair and Abstract Editor: John C. GreenGuidebook Editor: Timothy B. Hoist

Volume 35 consists of Parts 1 and 2:

1: Abstracts2: Field Trip Guidebooks

Published and Distributed byInstitute on Lake Superior GeologyJ. Kalliokoski, Secretary/Treasurer

Dept. of Geological Engineering, Geology and GeophysicsMichigan Technological University

Houghton, Michigan 49931

ISSN 1042-9964

Organizing Committee, 35th Annual Meeting, ILSG (1989)

Richard W. Ojakangas, Dept. of Geology, University of Minnesota, Duluth, MN 55812 John C. Green, Dept. of Geology, University of Minnesota-Duluth, Duluth, MN 55812

Timothy B. Hoist, University of Minnesota-Duluth, Duluth, MN 55812

Program Chair and Abstract Editor: John C. Green Guidebook Editor: Timothy B. Hoist

Volume 35 consists of Parts 1 and 2:

1: Abstracts 2: Field Trip Guidebooks

Published and Distributed by Institute on Lake Superior Geology J. Kalliokoski, SecretaryITreasurer

Dept. of Geological Engineering, Geology and Geophysics Michigan Technological University

Houghton, Michigan 49931

ISSN 1042-9964

FIELD TRIP GUIDEBOOK

THIRTY-FIFTH ANNUAL MEETING


MAY 3-6, 1989

DULUTH, MINNESOTA

Trip 1: North Shore Rhyolites, MinnesotaLeader: J. C. Green

University of Minnesota DuluthDuluth, Minnesota 55812

Trip 2: Penokean Structural Terranes in East-Central MinnesotaLeader: T. B. Hoist


Trip 3: MeIlen Complex, WisconsinLeaders:

K. W. Klewin, Northern Illinois University, DeKaIb, Illinois 6011 5J. F. Olmsted, SUNY, Plattsburgh, New York 12901

K.E. Seifert, Iowa State University, Ames, Iowa 50011

TrIp 4: Archean Gold Occurrences and Their Structural Settings

Part A (Virginia Horn) Leaders:J. Welsh, Gustavus Adolphus College, St. Peter, Minnesota 56082D. England, Rhude and Fryberger, Inc. Hibbing, Minnesota 55746D. Groves, Newmont Exploration, Ltd., Duluth, Minnesota 55811

E. Levy, University of Minnesota Duluth, Duluth, Minnesota 55812

Part B (Western and Central Vermilion District) Leaders:P. J. Hudleston, University of Minnesota, Minneapolis, Minnesota 55455

D. L. Southwick, Minnesota Geological Survey, Saint Paul, Minnesota 55114R. L. Bauer, University of Missouri, Columbia Missouri 65211

W. Ulland, American Shield Corporation, Duluth, Minnesota 55802

Cover: Anticline in the early Proterozoic Thomson Formation atThomson Dam, view from the east bank of the St. Louis River,

just south of the highway bridge, looking west.Drawing by Wendell Wilson.

FIELD TRIP GUIDEBOOK

THIRTY-FIFTH ANNUAL MEETING


MAY 3-6, 1989

DULUTH, MINNESOTA

Trip 1: North Shore Rhyolites, Minnesota Leader: J. C. Green

University of Minnesota Duluth Duluth, Minnesota 55812

Trip 2: Penokean Structural Terranes in East-Central Minnesota Leader: T. B. Hoist

University of Minnesota Duluth Duluth, Minnesota 5581 2

Trip 3: Mellen Complex, Wisconsin Leaders:

K. W. Klewin, Northern Illinois University, DeKalb, Illinois 601 15 J. F. Olmsted, SUNY, Pittsburgh, New York 12901

K.E. Seifert, Iowa State University, Ames, Iowa 5001 1

Trip 4: Archean Gold Occurrences and Their Structural Settings

Part A (Virginia Horn) Leaders: J. Welsh, Gustavus Adolphus College, St. Peter, Minnesota 56082 D. England, Rhude and Fryberger, Inc. Hibbing, Minnesota 55746 D. Groves, Newmont Exploration, Ltd., Duluth, Minnesota 5581 1

E. Levy, University of Minnesota Duluth, Duluth, Minnesota 55812

Part B (Western and Central Vermilion District) Leaders: P. J. Hudleston, University of Minnesota, Minneapolis, Minnesota 55455

D. L. Southwick, Minnesota Geological Survey, Saint Paul, Minnesota 551 14 R. L. Bauer, University of Missouri, Columbia Missouri 6521 1

W. Ulland, American Shield Corporation, Duluth, Minnesota 55802

Cover: Anticline in the early Proterozoic Thomson Formation at Thomson Dam, view from the east bank of the St. Louis River,

just south of the highway bridge, looking west. Drawing by Wendell Wilson.

TABLE OF CONTENTS

FIELD TRIP 1

Large rhyolites of the Keweenawan North Shore Volcanic GroupJ.C. Green Al - A15

FIELD TRIP 2

Penokean structural terranes in east-central MinnesotaT.B. Hoist Bi - B17

FIELD TRIP 3

Rock Types and relationships of the Mellen Igneous ComplexK.W. Kiewin, J.F. Olmsted, and K.E. Seifert Cl - C15

FIELD TRIP 4

General geology and structure of Archean rocks of the VirginiaHorn Area, Northeastern Minnesota

J.L. Welsh, D.L. England, D.A. Groves, andE. Levy Dl - D13

Archean gold occurrences and their structural settings:western and central Vermilion district

D.L. Southwick, P.J. Hudleston, R.L. Bauer, andW. Ulland D14 - D24

•11

TABLE OF CONTENTS

FIELD TRIP 1

Large rhyolites of the Keweenawan North Shore Volcanic Group .................................................................................... J.C. Green A1 - A1 5

FIELD TRIP 2

Penokean structural terranes in east-central Minnesota ..................................................................................... - T.B. Hoist 61 B17

FIELD TRIP 3

Rock Types and relationships of the Mellen Igneous Complex ........................... K.W. Klewin, J.F. Olmsted, and K.E. Seifert C1 - C15

FIELD TRIP 4

General geology and structure of Archean rocks of the Virginia Horn Area, Northeastern Minnesota

J.L. Welsh, D.L. England, D.A. Groves, and .......................................................................................... E. Levy D l - D l 3

Archean gold occurrences and their structural settings:

I western and central Vermilion district i D.L. Southwick, P.J. Hudleston, R.L. Bauer, and

...................................................................................... W. Ulland D l4 - D24

FIELD TRIP #1

LARGE RHYOLITES OF THE KEWEENAWAN NORTH SHORE VOLCANIC GROUP

Leader: John C. GreenGeology Department, University of Minnesota, Duluth

INTRODUCTION

The North Shore Volcanic Group (NSVG, Fig. 1), one of the products of the greatMidcontinent Rift of North America (Wold and Hinze, 1982; Green, 1983; Van Schmusand Hinze, 1985), contains a wide variety of compositions of plateau lavas (BVSP,1981; Green, 1982a). These range from the predominant olivine tholeiites throughtransitional basalts, basaltic andesites, andesites and ferroandesites to icelandites andrhyolites (Fig. 2). Felsic rocks (icelandites and rhyolites) are unusually abundantcompared to other Keweenawan plateaus (e. g. Portage Lake Volcanics, MamainsePoint Volcanics, Osler Group, Michipicoten Island, etc.) and younger plateau provincesworld-wide; they constitute about 10% of the southwest limb (the section from Duluth toTofte) and about 25% of the northeast limb (Lutsen to Grand Portage), as measured bystratigraphic thickness along the lakeshore (Tables 1 & 2).

Airfall ash and pumice make up a very small percentage of these felsic units, buttheir rarity is probably exaggerated somewhat by their relative susceptibility to erosionand thus their lack of exposure. Small lava flows make up a small portion of the felsicrocks, but the great bulk consist of thick units, several of which can be traced for longdistances.

It is well understood that magma viscosity depends principally on temperature andcomposition; felsic magmas are predictably many orders of magnitude more viscousthan basaltic magmas at equivalent stages of crystallization. The most commonperception and expectation for rhyolites is that they will erupt as either domes or thick,short (high aspect-ratio) flows, or as ash or pumice falls or pyroclastic flows, in whichthe viscous fluid has been blown apart by its rapidly vesiculating and expandingvolatiles. Submarine and thinner subaerial ignimbrites are typically unwelded, whereasthicker subaerial units have a central welded zone (e. g. Ross and Smith, 1961; Fisherand Schmincke, 1984; Cas and Wright, 1987).

Table 3 (from Henry and others, 1988) summarizes the characteristics typical ofpyroclastic flows as contrasted with lava flows and remobilized ash flows(rheoignimbrites).

A-i

, FIELD TRIP #1

LARGE RHYOLITES OF THE KEWEENAWAN NORTH SHORE VOLCANIC GROUP

Leader: John C. Green Geology Department, University of Minnesota, Duluth

INTRODUCTION

The North Shore Volcanic Group (NSVG, Fig. I ) , one of the products of the great Midcontinent Rift of North America (Wold and Hinze, 1982; Green, 1983; Van Schmus and Hinze, 1985), contains a wide variety of compositions of plateau lavas (BVSP, 1981 ; Green, 1982a). These range from the predominant olivine tholeiites through transitional basalts, basaltic andesites, andesites and ferroandesites to icelandites and rhyolites (Fig. 2). Felsic rocks (icelandites and rhyolites) are unusually abundant compared to other Keweenawan plateaus (e. g. Portage Lake Volcanics, Mamainse Point Volcanics, Osier Group, Michipicoten Island, etc.) and younger plateau provinces world-wide; they constitute about 10% of the southwest limb (the section from Duluth to Tofte) and about 25% of the northeast limb (Lutsen to Grand Portage), as measured by stratigraphic thickness along the lakeshore (Tables 1 & 2).

Airfall ash and pumice make up a very small percentage of these felsic units, but their rarity is probably exaggerated somewhat by their relative susceptibility to erosion and thus their lack of exposure. Small lava flows make up a small portion of the felsic rocks, but the great bulk consist of thick units, several of which can be traced for long distances.

It is well understood that magma viscosity depends principally on temperature and composition; felsic magmas are predictably many orders of magnitude more viscous than basaltic magmas at equivalent stages of crystallization. The most common perception and expectation for rhyolites is that they will erupt as either domes or thick, short (high aspect-ratio) flows, or as ash or pumice falls or pyroclastic flows, in which the viscous fluid has been blown apart by its rapidly vesiculating and expanding volatiles. Submarine and thinner subaerial ignimbrites are typically unwelded, whereas thicker subaerial units have a central welded zone (e. g. Ross and Smith, 1961; Fisher and Schmincke, 1984; Cas and Wright, 1987).

Table 3 (from Henry and others, 1988) summarizes the characteristics typical of pyroclastic flows as contrasted with lava flows and remobilized ash flows (rheoignimbrites).

Fig. 1. Generalized geologic map of western Lake Superior area, showing the NorthShore Volcanic Group (NSVG) and field trip stops.

Fig. 2. Alkalies-Iron-Silica diagram for the NSVG showing fields of rhyolites (RHY),icelandites (ICE), andesites (AND), ferroandesites (FA), transitional basalts(TB), olivine tholeiites (OT), and augite-porphyritic transitional basalts (PTB).

A-2

F

Fig. 1. Generalized geologic map of western Lake Superior area, showing the North Shore Volcanic Group (NSVG) and field trip stops.

F

Fig. 2.

A " M

Alkalies-Iron-Silica diagram for the NSVG showing fields of rhyolites (RHY), icelandites (ICE), andesites (AND), ferroandesites (FA), transitional basalts (TB), olivine tholeiites (OT), and augite-porphyritic transitional basalts (PTB).

A - 2

Table 1. Generalized volcanic stratigraphy of the southwest limb (Duluth to Tofte),NSVG. From Green, 1979.

Approx.thickness (as) Lithostratigraphic unit Lithic character

Top Middle Keweenawan1200 Schroeder basalts amygdaloidal ophitic olivine tholeiites>90 Manitou trachybasalt flow red—brown granular trachybasalt to basalt

(more of the Schroeder basalta)>170 Balmore Bay lava. mostly quartz tholeiita., other basalts>90 Palisade rhyolite flow gray to pink, porphyritic rhyolite>200 Baptism River lava. mixed lavas. mostly basalta, one felsite

Beaver Bay intrusive complex1000 Gooseberry River basalta mixed basalta, one felsite

Lafayette Bluff. Silver Creek Cliff intrusions315 Two Harbor, fina—grained basalt. 'm.laphyre.," some quartz tholeiite.550 Larsasont ophtic basaits asygdaloidal ophitic olivine thol.iita

Stony Point-Knife Ia land diabas. intrusion1500 Sucker River basalt. mixed basalta, mostly ophitic1350 Lakewood basalt. mixed basalts and andesites, some felaite.

Lester River diabase sill1100 Lakeside lavas mixed basalts, andasites, falaites

Endion diabas. gill785 Leif Ericgon Park lavaa mixed basalta. andesites

Duluth ComplexLower Keweenawan

370 Ely's Peak basalt. porphyritic, diabasic. and ophitic basalt.8720 Total

Base Nopeming Sandstone quartz sandstone

Anqular unconformit'5iddle Precambrian

Thomson Formation slate and graywack.

Table 2. Generalized volcanic stratigraphy of the northeast limb (Lutsen to GrandPortage), NSVG. From Green, 1979.

Approx. thickness (m) llthostratl9raphic unit Lithic characterMiddle Keweenawan

Lutsen basaltsTerrace Point basalt flowGood Harbor 8ay andasites

Breakwater trachybasalt flow or sillGrand Marais felsitesCroftville basalts and andesitesDevil Track felsitesRed Cliff basaltsKimball Creek felsitesMarr Island lavasNaniboujox basalts

Brule River rhyolitesHovland diabase complex

Lower Keweenawan

Hoviand lavas

Reservation River diabase complex

Red Rock rhyolite flowDeronda Bay andesite flow

Grand Portage basalts

Olivine basalts, olivine tholelitesThomsonite-bearing ophitic basaltBrown, porphyritic andesite, trachyandesiteBrown. coliannar, granular trachybasaltPink, red, gray porphyritic rhyolite and felsiteVarious fine-grained basalts and basaltic andesitesAphyric and porphyritic rhyolite flowsAmygdaloida. ophitic olivine basaltsPink to tan, porphyritic felsites, icelanditesMixed tholelitic basalt, andesites, felsitesGranular-diabasic basaltsPink to gray porphyritic rhyolite

Mixed porphyritic basalt, trachybasalt, andesite. rhyolite(Middle Keweenawan)

Red, porphyritic rhyoliteGray-brown, aphyric andesiteMixed tholeiltic basalts; porphyritic basalts locally at base

Top

310

50

95

110

155

185

310

120-275

400

550

310

1070

1225 (est.)

610

80

1380722U Total flows

Base Disconformity

Puckwunge Formation Cross-bedded quartz sandstone

DisconformityMiddle Precambrian

Rove Formation Shal. and graywacke

A- 3

Table 1. Generalized volcanic stratigraphy of the southwest limb (Duluth to Tofte), NSVG. From Green, 1979.

#

Approx. t h i c k n e s s (m) L i t h o s t r a t i q r a p h i c u n i t L i t h i c c h a r a c t e r

Middle Keweenawan Schroeder b a s a l t s aayqda lo ida l o p h i t i c o l i v i n e t h o l e i i t e m Manitou t r achybasa l t flow red-brown q ranu la r t r a c h y b a s a l t to b a s a l t

(more of t h e Schroeder b a s a l t s ) Belmore Bay l avas most ly q u a r t z t h o l e i i t e m , o t h e r b a s a l t 8 Pa l i s ade r h y o l i t e flow g ray t o p ink, p o r p h y r i t i c r h y o l i t e Baptism River l avas mixed l a v a s , mostly b a s a l t s , one f e l s i t e

Beaver Bay i n t r u s i v e complex Gooseberry River b a s a l t s mixed b a s a l t s , one f e l s i t e

L a f a y e t t e B lu f f , S i l v e r Creek C l i f f i n t r u s i o n s Two Harbors f ine-gra ined b a s a l t s "melaphyres," some q u a r t z t h o l e i i t e s Larsmont o p h i t i c b a s a l t s amyqdaloidal o p h i t i c o l i v i n e t h o l e i i t s

S tony Point-Knife I s l a n d d i abase i n t r u s i o n Sucker River b a s a l t s mixed bamal ts , mostly o p h i t i c Lakewood b a s a l t s mixed b a s a l t s and a n d e s i t e s , some f e l a i t e m

L e s t e r River d i abase sill Lakeaide l avas mixed b a s a l t s , andemitem, f e l m i t e s

Endion diabame m i l l Loif Ericson Park lava* mixed b a s a l t s , a n d e s i t e s

Duluth Complex Lower Keweenawan

370 - Ely ' s Peak bamalts p o r p h y r i t i c , d i a b a s i c , and o p h i t i c b a a a l t s 8 7 2 0 T o t a l

Base Nopeminq Sandstone q u a r t z s ands tone

Table 2. Generalized volcanic stratigraphy of the northeast limb (Lutsen to Grand Portage), NSVG. From Green, 1979.

Approx. thickness (m) L i thost ra t ig raph ic u n i t L i t h k character

TOP Middle Keweenawan

310 Lutsen basal t s O l i v i n e basalts, o l i v i n e t h o l e i i t e s

50 Terrace Point basa l t f low Thomsontte-bearing ophi t i c basa l t

95 Good Harbor b y andts i tes Brown, por0hyr i t iC andesite. trachyandesite

110 Breakwater trachybasalt f low o r s i l l Brown, col&ar. granular trachybasalt

155 Grand Marais f e l s i t e s Pink, red, gray po rphy r i t i c r h y o l i t e and f e l s i t e

185 C r o f t v i l l e basalts and andesites Various f ine-grained basalts and basa l t i c andesites

310 Oevil Track f e l s i tes Aphyric and po rphy r i t i c r h y o l i t e flows

120-275 Red C l i f f basa l ts Afliygda1oida1, oph i t i c o l i v i n e basa l ts

400 Kimball Creek f e l s i t e s Pink t o tan, po rphy r i t i c f e l s i t es , ice land i tes

550 Marr Is land lavas Mixed t h o l e i i t i c basalt , andesltes, f e l s i t e s

310 Naniboujou basal t s Granular-diabdsic basalts

1070 Brule River rhyo l 1 tes Pink t o gray po rphy r i t i c r h y o l i t e

Hovland diabase complex

Lower Keweenawan

1225 (es t . ) H0vldnd lavas Mixed porphyritic basalt , t rachybasalt , andesne, r h y o l i t e

Reservation River diabase complex (Middle Keueenawan)

610 Red Rock r h y o l i t e flow Red, po rphy r i t i c r h y o l i t e

80 Deronda Bay andesite f low Gray-brown, aphyric andesl t e

1380 Grand Portage basal t s Mixed t h o l e i i t i c basalts; po rphy r i t i c basa l ts l o c a l l y a t base 772'6 Total flows

TABLE 3. CHARACTERISTICS COMMONLY USED TO DISCRIMINATERHYOLITE LAVAS FROM IGNIMBRITES

(FROM HENRY ET AL., 1988)

Common in conventional ignimbrites Common in rhyolite lavas and in

rheoignimbrites

Fiamme Flow bandingEutaxitic texture Ramp structuresAbundant lithic fragments Elongated vesiclesNonwelded tops, bottoms, sides AutobrecciasGradual thinning at edges of units Vitrophyres at or near topsWide areal extent of individual units Lengths generally much <20 km.*Low aspect ratio High aspect ratio*Glass shards in thin sectionBroken pheno's, different sizesGas elutriation pipes

*Unlike rhyolite lava flows, rheoignirnbrites may have dimensions similar to those ofother ignimbrites

Recently, many rhyolites have been described which show anomalous behavioraccording to these models. These may cover wide areas with low aspect ratios,indicating high mobility, yet do not show some of the common features of ash-flow tuffs.For some, high alkali content (peralkalinity) appears to have been a factor in loweringthe viscosity (e. g. Price and others, 1986). For others, a high eruptive temperature andhigh rate of effusion appear to have been important contributors; an increase of 100degrees C. will lower the viscosity about an order of magnitude (Ekren and others,1984; Bonnichsen and Kauffman, 1987). A relatively low SiO2 content seems to becommon to many high-mobility flows. A survey of widespread lavas and lava-like unitsin southwest Idaho and trans-Pecos Texas (Bonnichsen and Kauffman, 1987 and Henryand others, 1988) shows volatile-free SiO2 contents in the range 68 to 74%, whereasrheoignimbrites in the same areas have S1O2 values of 72 to 77%. Many ash flows,made of hot, relatively low-viscosity froth particles, reconsolidate during and afteremplacement to form a devolatilized, "reconstituted lava" that can flow en masse (arheoignimbrite). This flowage, and any subsequent crystallization or devitrification, canobliterate primary pyroclastic textures. Thus the distinction between the end result ofan explosive eruption and an effusive eruption can become greatly diminished.Bonnichsen and Kauffman (1987) and Bonnichsen and others (1988) have suggestedsome criteria by which they may be recognized, but they and others, notably Henryet al. (1988), have described widespread, low-aspect-ratio units whose origin is stillunclear.

The felsic units in the NSVG show evidence of a wide variety of eruptive andemplacement styles, including large, problematical, low-aspect-ratio flows. This field tripwill examine several units throughout the NSVG section and the physical evidence fortheir mode of emplacement. It is designed as a long one-day trip, starting at Duluthand nearly reaching Grand Portage before returning to Duluth. Descriptions of field tripstops in other lithologies of the North Shore (mainly the plateau basalts) can be foundin Green, 1979 and 1987, and a discussion of the physical volcanology of the NSVGappears in Green, 1989. A regional (1 :250,000) geologic map is also available (Green,1 982b).

Chemical analyses of these felsite units are presented in Table 4.

A-4

TABLE 3. CHARACTERISTICS COMMONLY USED TO DISCRIMINATE RHYOLITE LAVAS FROM IGNIMBRITES

(FROM HENRY ET AL., 1988)

Common in conventional ignimbrites Common in rhyolite lavas and in rheoignimbrites

Fiamme Flow banding Eutaxitic texture Ramp structures Abundant lithic fragments Elongated vesicles Nonwelded tops, bottoms, sides Autobreccias Gradual thinning at edges of units Vitrophyres at or near tops Wide areal extent of individual units Lengths generally much c20 km.* Low aspect ratio High aspect ratio* Glass shards in thin section Broken pheno's, different sizes Gas elutriation pipes

*Unlike rhyolite lava flows, rheoignimbrites may have dimensions similar to those of other ignimbrites

Recently, many rhyolites have been described which show anomalous behavior according to these models. These may cover wide areas with low aspect ratios, indicating high mobility, yet do not show some of the common features of ash-flow tuffs. For some, high alkali content (peralkalinity) appears to have been a factor in lowering the viscosity (e. g. Price and others, 1986). For others, a high eruptive temperature and high rate of effusion appear to have been important contributors; an increase of 100 degrees C. will lower the viscosity about an order of magnitude (Ekren and others, 1984; Bonnichsen and Kauffman, 1987). A relatively low SiO, content seems to be common to many high-mobility flows. A survey of widespread lavas and lava-like units in southwest Idaho and trans-Pecos Texas (Bonnichsen and Kauffman, 1987 and Henry and others, 1988) shows volatile-free SiO, contents in the range 68 to 74%, whereas rheoignimbrites in the same areas have Si02 values of 72 to 77%. Many ash flows, made of hot, relatively low-viscosity froth particles, reconsolidate during and after emplacement to form a devolatilized, "reconstituted lava" that can flow en masse (a rheoignimbrite). This flowage, and any subsequent crystallization or devitrification, can obliterate primary pyroclastic textures. Thus the distinction between the end result of an explosive eruption and an effusive eruption can become greatly diminished. Bonnichsen and Kauffman (1987) and Bonnichsen and others (1988) have suggested some criteria by which they may be recognized, but they and others, notably Henry et al. (1988), have described widespread, low-aspect-ratio units whose origin is still unclear.

The felsic units in the NSVG show evidence of a wide variety of eruptive and emplacement styles, including large, problematical, low-aspect-ratio flows. This field trip will examine several units throughout the NSVG section and the physical evidence for their mode of emplacement. It is designed as a long one-day trip, starting at Duluth and nearly reaching Grand Portage before returning to Duluth. Descriptions of field trip stops in other lithologies of the North Shore (mainly the plateau basalts) can be found in Green, 1979 and 1987, and a discussion of the physical volcanology of the NSVG appears in Green, 1989. A regional (1 :250,000) geologic map is also available (Green, 1982b).

Chemical analyses of these felsite units are presented in Table 4.

Table 4.Chemical analyses of felsic volcanic units at field tripstops, recalculated volatile-free

A B C D E F G H I J

Si02 75.11 75.41 68.30 75.46 74.85 73.08 69.92 63.81 62.07 72.54Ti02 .28 .22 .70 .24 .40 .40 .53 1.11 .99 .25A1203 12.45 11.99 12.76 11.10 11.62 11.76 12.52 12.24 15.09 12.02Fe203 1.58 .84 1.61 1.66 .92 3.13 4.09 8.34 2.79 2.51FeO 1.25 3.04 5.79 2.95 3.33 .74 1.63 2.06 6.00 3.64MnO .03 .01 .09 .05 .04 .04 .10 .12 .18 .05MgO .06 .07 .27 .30 .60 .54 .51 1.43 .88 .21CaO .32 .30 1.95 .51 .88 .51 .51 2.26 3.67 .71Na20 3.28 2.13 2.69 1.96 2.22 3.54 3.43 4.12 3.77 3.861(20 5.59 5.92 5.68 5.72 5.09 5.50 5.37 4.23 4.04 4.15P205 .02 .01 .08 .04 .06 .04 .09 .28 .52 .05Zr02 .03 .06 .12 — .08 — — —

A. Stop 1: Tischer Creek rhyolite (ave. of two analyses).B. Stop 2: 42nd Ave E. ignixnbrite (D-87).C. Stop 2: icelandite flow beneath B (0-91).D. Stop 3: Palisade rhyolite (F-201).E. Stop 5: Silver Beaver felsite (F-278).F. Stop 6: Devil Track rhyolite (ave. of three).G. Stop 7: Kimball Creek rhyolite (ave. of five).H. Stop 8: Rangeline icelandite (MI-2).I. Stop 9: Deronda Bay andesite/icelandite (MC-3b).J. Stop 9: Red Rock rhyolite (MC-7).

FIELD TRIP STOPS

STOP 1. Tischer Creek at Superior Street, Duluth.Here the stream has eroded a gorge through the small-jointed Tischer Creekrhyolite, which is approximately 300 m thick. This unit contains many thin,discontinuous color bands or laminae that resemble well-flattened fiamme,and moderately abundant, small xenoliths; both suggest an ignimbrite ratherthan a lava. It also shows occasional open, quartz-lined or quartz-filledtension fractures that must have formed after welding. Devitrification hasdestroyed whatever microscopic shard or pumice textures were once present.The Iaminae (fiamme?) are generally parallel to the base of the unit, butlocally show folding, indicating flow after deposition and welding. This flow isthus probably a "High-temperatureTM type according to Bonnichsen et al.(1988). The base of this unit is intruded (and probably melted) by the thickEndion diabase sill.

A-5

able

S i0, TiO,

-203

FeO Mno MgO CaO Na20 K20 p.0. ZrO,

4.Chemical analyses of felsic volcanic units at field trip stops, recalculated volatile-free .

Stop 1: Stop 2: Stop 2: Stop 3: Stop 5: Stop 6: Stop 7: Stop 8: Stop 9: Stop 9:

Tischer Creek rhyolite (ave. of two analyses). 42nd Ave E. ignimbrite (D-87) . icelandite flow beneath B (D-91). Palisade rhyolite (F-201) . Silver Beaver felsite (F-278). Devil Track rhyolite (ave. of three). Kimball Creek rhyolite (ave. of five). Rangeline icelandite (MI-2). Deronda Bay andesite/icelandite (MC-3b). Red Rock rhyolite (MC-7).

FIELD TRIP STOPS

STOP 1. Tischer Creek at Superior Street, Duluth. Here the stream has eroded a gorge through the small-jointed Tischer Creek rhyolite, which is approximately 300 m thick. This unit contains many thin, discontinuous color bands or laminae that resemble well-flattened fiamme, and moderately abundant, small xenoliths; both suggest an ignimbrite rather than a lava. It also shows occasional open, quartz-lined or quartz-filled tension fractures that must have formed after welding. Devitrification has destroyed whatever microscopic shard or pumice textures were once present. The laminae (fiamme?) are generally parallel to the base of the unit, but locally show folding, indicating flow after deposition and welding. This flow is thus probably a "High-temperaturew type according to Bonnichsen et al. (1988). The base of this unit is intruded (and probably melted) by the thick Endion diabase sill.

STOP 2. Lake Superior shore at foot of 42nd Avenue East, Duluth.At this locality is the contact between two felsite units, an icelandite overlainby a rhyolite (Fig. 3). Where the trail reaches the shore there are abundantexposures of the lower part of a thick (about 60 m) rhyolitic ignimbrite, fullof small (1-4 cm) slabs and chips of what were probably pumice, in afine-grained, devitrified matrix now devoid of recognizable shards. Alterationat different stages, probably including that due to contemporaneousdegassing and later burial metamorphism, has created distinct color contrastsbetween the fragments and matrix. Several xenoliths are present, including afew blobs (bombs) of chilled mafic lava. The pumice fragments are notobviously flattened parallel to the base of the flow, even though this is afairly thick unit. This indicates that it was probably erupted at a relatively lowtemperature (Bonnichsen et al.'s Type L) and did not weld or even compactvery much except at the very base. A very similar texture is found at thebase of the large, Tertiary caldera-filling Gomez Tuff of trans-Pecos Texas(Price et al., 1986).

In the impassable cliff up the shore to the northeast this rock is directlyoverlain by another rhyolite with a distinctly different structure. Above anarrow transition zone of a few centimeters, it contains thin, discontinuouscolor bands and lamellae, rather like the Tischer Creek unit, but with astrong lineation of small wrinkles and open fold axes. These featuressuggest a high-temperature ash flow, resulting in thorough welding and bulkflow, after emplacement, in the direction of the lineations - a rheoignimbriteof Bonnichsen et aL's Type H.

In the other direction (SW) along the beach, the basal 2 or 3 meters ofthe main unwelded ash flow is not exposed, but several loose blocks oflaminated rock, found only in this vicinity, appear to be the stronglycompacted and welded basal portion of this unit.

- roiiLC-I...z— J t-

——

• uQ(óec pt-topt(i(D-87)

- :.., \tcCA(&r .çe& top

4to.' ociâ, %eøtD(D-qI)

Fig. 3. Diagrammatic columnar section at field trip Stop 2, lakeshore at foot of 42ndAve East, Duluth.

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STOP 2. Lake Superior shore at foot of 42nd Avenue East, Duluth. At this locality is the contact between two felsite units, an icelandite overlain by a rhyolite (Fig. 3). Where the trail reaches the shore there are abundant exposures of the lower part of a thick (about 60 m) rhyolitic ignimbrite, full of small (1-4 cm) slabs and chips of what were probably pumice, in a fine-grained, devitrified matrix now devoid of recognizable shards. Alteration at different stages, probably including that due to contemporaneous degassing and later burial metamorphisml has created distinct color contrasts between the fragments and matrix. Several xenoliths are present, including a few blobs (bombs) of chilled mafic lava. The pumice fragments are not obviously flattened parallel to the base of the flowl even though this is a fairly thick unit. This indicates that it was probably erupted at a relatively low temperature (Bonnichsen et aI.3 Type L) and did not weld or even compact very much except at the very base. A very similar texture is found at the base of the large, Tertiary caldera-filling Gomez Tuff of trans-Pecos Texas (Price et ah, 1986).

In the impassable cliff up the shore to the northeast this rock is directly overlain by another rhyolite with a distinctly different structure. Above a narrow transition zone of a few centimeters, it contains thin, discontinuous color bands and lamellae, rather like the Tischer Creek unit, but with a strong lineation of small wrinkles and open fold axes. These features suggest a high-temperature ash flow, resulting in thorough welding and bulk flowl after emplacement, in the direction of the lineations - a rheoignimbrite of Bonnichsen et aL1s Type H.

In the other direction (SW) along the beach, the basal 2 or 3 meters of the main unwelded ash flow is not ex~osed, but several loose blocks of laminated rock, found only compacted and welded basal

in this vkinity,. appear to be the strongly portion of this unit.

Fig. 3. Diagrammatic columnar section at field trip Stop 2, lakeshore at foot of 42nd Ave East, Duluth.

Just beyond this zone is the top of a 130-rn thick porphyritic icelandite. It

is highly vesicular to frothy but not pumiceous, and the top meter or two arepenetrated by arkosic sandstone dikes that filled in open cracks in the flowtop before eruption of the overlying ignimbrite. The vesicles (now largelycalcite amygdules) are stretched to varying degrees, and large flowage foldsare evident. This vesicularity dies out to the southwest as the rock becomesmassive and granular. There is little brecciation. These textures andstructures indicate that this unit was a large and fairly hot and rapidlyemplaced lava flow.

None of these units can be traced inland for more than a few kilometersbecause of glacial drift cover and intersection by large mat ic intrusions.

For more details on these Duluth units see Motamedi, 1984.

STOP 3. Palisade Head, Lake County (Figs 4,5).This scenic promontory is made of a 90+ m thick massive, porphyriticrhyolite. The same unit forms the prominent Shovel Point to the northeast(stratigraphically lower flows are exposed in the Baptism River basin inbetween), and it can be traced from East Beaver Bay (to the southwest) atleast 23 km along strike to the northeast. See also Miller, 1987 and 1988and Miller et al., 1987 for detailed local geology.

Throughout most of its extent the Palisade rhyolite closely resembles thisexposure at Palisade Head: massive, with no evidence of pyroclastic texture,only rare xenoliths, and with only rare broken phenocrysts. Under themicroscope it shows fine-grained, poikilitic quartz pseudomorphic aftertridymite, implying crystallization well above 870 degrees C. At the base, theonly exposure (in the southwest end of this hill) shows flow laminations,locally much folded on a small scale, in the lowest 1 to 3 meters.Elsewhere, larger folds in laminations occur farther above the base. At thetop, the only exposure is vesicular (amygdaloidal) but not pumiceous orbrecciated. These characteristics suggest an origin as a large, unusuallymobile, hot lava flow. However, one outcrop in the interior shows small,discontinuous bands or streaks that suggest relict fiamme, and another (nextstop) contains abundant discontinuous streaks suggesting strongly flattenedfiamme. The unit may be a high-temperature rhebignimbrite.

- or .,reccto.. +opfLdQc &WtrLttOI'%S

rir1esVL(F.wf)

qntL o.ttc iior.$bsLt ou.'

Fig. 4. Diagrammatic columnar section of Palisade rhyolite, field trip Stops 3 and 4.

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STOP 3.

Just beyond this zone is the top of a 130-m thick porphyritic icelandite. It is highly vesicular to frothy but not pumiceous, and the top meter or two are penetrated by arkosic sandstone dikes that filled in open cracks in the flow top before eruption of the overlying ignimbrite. The vesicles (now largely calcite amygdules) are stretched to varying degrees, and large flowage folds are evident. This vesicularity dies out to the southwest as the rock becomes massive and granular. There is little brecciation. These textures and structures indicate that this unit was a large and fairly hot and rapidly emplaced lava flow.

None of these units can be traced inland for more than a few kilometers because of glacial drift cover and intersection by large mafic intrusions.

For more details on these Duluth units see Motamedi, 1984.

Palisade Headl Lake County (Figs 4?5). This scenic promontory is made of a 90+ m thick massive, porphyritic rhyolite. The same unit forms the prominent Shovel Point to the northeast (stratigraphically lower flows are exposed in the Baptism River basin in between)> and it can be traced from East Beaver Bay (to the southwest) at least 23 km along strike to the northeast. See also Miller, 1987 and 1988 and Miller et a!.? 1987 for detailed local geology.

Throughout most of its extent the Palisade rhyolite closely resembles this exposure at Palisade Head: massive, with no evidence of pyroclastic texture, only rare xenoliths, and with only rare broken phenocrysts. Under the microscope it shows fine-grained, poikilitic quartz pseudomorphic after tridymitel implying crystallization well above 870 degrees C. At the basel the only exposure (in the southwest end of this hill) shows flow laminations, locally much folded on a small scale, in the lowest 1 to 3 meters. Elsewhere, larger folds in laminations occur farther above the base. At the top, the only exposure is vesicular (amygdaloidal) but not pumiceous or brecciated. These characteristics suggest an origin as a large, unusually mobile, hot lava flow. However, one outcrop in the interior shows small, discontinuous bands or streaks that suggest relict fiamme? and another (next stop) contains abundant discontinuous streaks suggesting strongly flattened fiamme. The unit may be a high-temperature rheoignimbrite.

- - - - - - - - - - - - - - - - ~ U M py o r breccia + o ~

Fig. 4. Diagrammatic columnar section of Palisade rhyolite, field trip Stops 3 and 4.

Fig. 5. View to the southwest of Palisade Head from the top of Shovel Point (bothmade of columnar-jointed Palisade rhyolite). Lower cliffs in middle distance atright (at Baptism River mouth) are made of the Silver Beaver felsite (Stop 5),stratigraphically beneath the Palisade rhyolite.

STOP 4. Road cuts in Palisade rhyolite, Minn. Highway 1, Illgen City.Here part of the upper portion of this unit is exposed, showing in contrast totypical outcrops, a breccia made of diversely oriented blocks. Each block isitself flow-laminated. The laminations appear discontinuous and may bestrongly flattened fiamme. Clearly the breccia was formed after coolingacross the ductile-brittle transition, during late-stage movement of the flow.

STOP 5. Silver-Beaver felsite at the Baptism River (Fig. 6).(Park at Rest Area at Tettegouche State Park headquarters).

t6ed {- breccu,

Fig. 6. Diagrammatic section of Silver Beaver felsite, Stops 5a and 5b, Baptism River.

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, — ) — dbjO.f 51.4 ,bvded, -gcca.

— — - oaded

(ç. 27 )

= -- ?ç.-— •-'

Fig. 5. View to the southwest of Palisade Head from the top of Shovel Point (both made of columnar-jointed Palisade rhyolite). Lower cliffs in middle distance at right (at Baptism River mouth) are made of the Silver Beaver felsite (Stop 5), stratigraphically beneath the Palisade rhyolite.

STOP 4.

STOP 5.

Road cuts in Palisade rhyolite, Minn. Highway 1, lllgen City. Here part of the upper portion of this unit is exposed, showing in contrast to typical outcrops, a breccia made of diversely oriented blocks. Each block is itself flow-laminated. The laminations appear discontinuous and may be strongly flattened fiamme. Clearly the breccia was formed after cooling across the ductile-brittle transition, during late-stage movement of the flow.

Silver-Beaver felsite at the Baptism River (Fig. 6). (Park at Rest Area at Tettegouche State Park headquarters).

Fig. 6. Diagrammatic section of Silver Beaver felsite, Stops 5a and 5b, Baptism River

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A. Walk across Baptism River on foot bridge, then left through woods on anunofficial trail to a high outcrop overlooking the river mouth at the lakeshore.Here is fairly typical, banded, fine-grained, weakly porphyritic to aphyricfelsite, dipping steeply to the east toward the lake. Across the river the unitis a tuff-breccia at water level. Follow along the top of the shore cliff to thesouth to the first beach-cove. Here the basal portion is exposed, overlying apale, chalky-clayey zone that may be either altered airfall ash or fault gouge.The basal 3 to 5 m is an irregularly fractured, unwelded tuff-breccia orlapilli-tuff containing fragments of various once-glassy felsite textural typeswhich include pumice, spherulites, perlitic cracks, etc., considerably altered tokaolinite and quartz. This zone is overlain by what appears to beagglutinate, containing faint blobby areas (Fig. 7) and grading up into moretypical, weakly flow-banded, uniform, finely crystalline felsite.

Across the beach to the west is a very discordant, steeply SE-dippingcontact where a laminated basaltic andesite overlies the rubbly, sand-matrixtop of another flow. A fault separates these rocks from the felsite.

Return to highway.

Fig. 7. Deformed agglutinate (?) blobs in lower part of the Silver Beaver felsite,lakeshore southwest of Williams Creek, Silver Bay area.

B. After crossing back to the north side of the river, go west across thehighway and up the trail (via stairs) along the top of the bank to a long flightof wooden steps leading down to the river again. The outcrops here arenear the stratigraphic middle of the Silver Beaver felsite. Faulting hasdropped it down relative to its exposures at the river mouth, but it still dipseast. The uniform, typically planar lamination is characteristic, as is thefine-grained (rather than dense-aphanitic), aphyric texture. In thin sectionquartz pseudomorphs after tridymite are abundant, and small (1-3 mm)spherulites are not uncommon.

A-9

A. Walk across Baptism River on foot bridge? then left through woods on an unofficial trail to a high outcrop overlooking the river mouth at the lakeshore. Here is fairly typicall banded, fine-grained, weakly porphyritic to aphyric felsite? dipping steeply to the east toward the lake. Across the river the unit is a tuff-breccia at water level. Follow along the top of the shore cliff to the south to the first beach-cove. Here the basal portion is exposed, overlying a pale? chalky-clayey zone that may be either altered airfall ash or fault gouge. The basal 3 to 5 m is an irregularly fractured? unwelded tuff-breccia or lapilli-tuff containing fragments of various once-glassy felsite textural types which include pumice? spherulitesl perlitic cracks? etc.? considerably altered to kaolinite and quartz. This zone is overlain by what appears to be agglutinate? containing faint blobby areas (Fig. 7) and grading up into more typicall weakly flow-banded? uniform? finely crystalline felsite.

Across the beach to the west is a very discordant? steeply SE-dipping contact where a laminated basaltic andesite overlies the rubbly, sand-matrix top of another flow. A fault separates these rocks from the felsite.

Return to highway.

Fig. 7. Deformed agglutinate (?) blobs in lower part of the Silver Beaver felsite? lakeshore southwest of Williams Creek? Silver Bay area.

B. After crossing back to the north side of the river? go west across the highway and up the trail (via stairs) along the top of the bank to a long flight of wooden steps leading down to the river again. The outcrops here are near the stratigraphic middle of the Silver Beaver felsite. Faulting has dropped it down relative to its exposures at the river mouth, but it still dips east. The uniform? typically planar lamination is characteristic? as is the fine-grained (rather than dense-aphanitic), aphyric texture. In thin section quartz pseudomorphs after tridymite are abundant? and small (1-3 mm) spherulites are not uncommon.

Go downstream (if the water level is low enough), working toward the topof the unit. A few meters from the top the flow starts to acquire small,irregular vesicles, then becomes broken up in a flow-top breccia with lineatedvesicles and ashy gray material between the blocks. This is overlain by alaminated, red tuff only a few cm thick, which lenses in and out. These felsicrocks are then overlain by a thin, amygdaloidal basalt flow with largeplagioclase phenocrysts concentrated at the base; its top is intruded by athin black diabase sill.

The Silver Beaver felsite is interpreted to have begun with eruption ofglassy ash and pumice at a moderate or slow rate, producing an unweldedbase, but then changed to agglutinate as hotter or less volatile-rich materialbecame available. The bulk of the unit was produced as either agglutinateor a hot ash flow that coalesced and flowed as a unit aftersettling/accumulation - perhaps a Very High Temperature (Type V)rheoignimbrite of Bonnichsen et al., 1988. It subsequently crystallized inplace to tridymite and feldspar.

Climb back up to the trail and walk out to the highway.

As the trip progresses up the shore we pass upwards stratigraphically tothe exposed top in the Tofte-Lutsen area in southwestern Cook County. Anunknown thickness of lavas was erupted on top of this sequence and nowunderlies the lake or was eroded off before the Portage Lake Volcanics orUpper Keweenawan sandstones were laid down on top of the NSVG here.Beyond Lutsen we start descending in the "northeast limb". Here the stratastrike more easterly than the lakeshore, and dip south-southeast to south(Fig. 8).

Grand Marais sits largely on another large, small-jointed rhyolite complexwhich has been more easily eroded (to form the harbor) than the overlying"Breakwater trachybasalt". This is a thick transitional basalt unit that makesthe "island" and breakwater and a cuesta to the west.

Fig. 8. Generalized geologic map of the northeastern tip of Minnesota showingcontinuity along strike of some of the major units of the NSVG. (from Green,1979).

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Go downstream (if the water level is low enough)? working toward the top of the unit. A few meters from the top the flow starts to acquire small, irregular vesiclesl then becomes broken up in a flow-top breccia with lineated , vesicles and ashy gray material between the blocks. This is overlain by a laminated, red tuff only a few cm thick, which lenses in and out. These felsic rocks are then overlain by a thinl amygdaloidal basalt flow with large plagioclase phenocrysts concentrated at the base; its top is intruded by a thin black diabase sill.

The Silver Beaver felsite is interpreted to have begun with eruption of glassy ash and pumice at a moderate or slow ratel producing an unwelded base, but then changed to agglutinate as hotter or less volatile-rich material became available. The bulk of the unit was produced as either agglutinate or a hot ash flow that coalesced and flowed as a unit after settling1accumulation - perhaps a Very High Temperature (Type V) rheoignimbrite of Bonnichsen et al., 1988. It subsequently crystallized in place to tridymite and feldspar.

Climb back up to the trail and walk out to the highway.

As the trip progresses up the shore we pass upwards stratigraphically to the exposed top in the Tofte-Lutsen area in southwestern Cook County. An unknown thickness of lavas was erupted on top of this sequence and now underlies the lake or was eroded off before the Portage Lake Volcanics or Upper Keweenawan sandstones were laid down on top of the NSVG here. Beyond Lutsen we start descending in the "northeast limb". Here the strata strike more easterly than the lakeshorel and dip south-southeast to south (Fig. 8).

Grand Marais sits largely on another largel small-jointed rhyolite complex which has been more easily eroded (to form the harbor) than the overlying "Breakwater trachybasalt". This is a thick transitional basalt unit that makes the "island" and breakwater and a cuesta to the west.

Fig. 8. Generalized geologic map of the northeastern tip of Minnesota showing continuity along strike of some of the major units of the NSVG. (from Green, 1 979).

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STOP 6. Devil Track rhyolite, Highway 61 opposite Five Mile Rock east of GrandMarais.This exposure, at an abandoned wave-cut cliff of the 5000-year old Nipissingstage of Lake Superior, is fairly typical of this large rhyolite. This unit can betraced for about 40 km to the west, and is about 250 m thick. Neither itstop or base is exposed, and all outcrops show this weakly flow-laminated,fine-grained, aphyric to very weakly porphyritic character, rather similar to theSilver Beaver felsite. Thin sections here also show quartz paramorphs afterprimary tridymite. Tom Fitz (1988) has found that the grain size of thesetridymite plates increases toward the center of the unit, implying that itcrystallized as a simple cooling unit (Fig. 9). No significant breccia facieshas been found. The wide extent, lack of breccia or other viscous flowfeatures other than rare lineations, aphyric texture, and crystallinity suggestthat this was a very hot pyroclastic flow that completely consolidated, afteremplacement, to a pool of devolatilized rhyolite magma that crystallized inplace.

Assuming that its overall original shape was a segment of a sphere (itappears to pinch out at the western end but is thick through much of therest), and that its center was at the present lakeshore (half now underlies thelake), it would have had an original volume of about 600 km3.

STOP 7. Kimball Creek Felsite, Lake Superior shore north of Red Cliff.Here we are near the top of another very large (366 m thick) rhyolite. Thepoint at Red Cliff to the south is held up by a sequence of overlying olivinetholeiite basalts. This rhyolite is weakly plagioclase-phyric, but otherwise thebulk of this flow is fine-grained, crystalline, and massive, rather like the DevilTrack rhyolite. This unit also shows a network of tridymite paramorphs in thegroundmass (Fig. 10), whose length is greatest near the center of the flow(Fig. 9), again indicating primary crystallization as a simple cooling unit.Here (and elsewhere) near the top, however, it is aphanitic and containsdiscontinuous color bands and lamellae that are probably flattened anddeformed pumice fiamme. These structures are preserved here near the topbecause of the rapid cooling after welding, preventing the primarycrystallization which obliterated them in the interior. Unfortunately the topseveral meters are not exposed, as is typical of these large rhyolites. Onepossible inference is that the top may contain easily weathered and eroded,unwelded pumice and ash. The base is not exposed either, but a fewmeters above, the unit shows flow-folding of thin, discontinuous streaks andlamellae (fiamme?) (Fitz, 1988). This unit is thought to have the same originas the Devil Track rhyolite: a high-temperature rheoignimbrite.

For more details on these two large rheoignimbrites see Fitz (1988).

STOP 8. Rangeline icelandite, 1 1/2 mi. E of Kadunce Creek (River).These road cuts show fairly typical textures and structures of a large (40 m)icelandite lava flow. Toward the eastern end is the massive lower part,aphanitic to fine-grained and porphyritic; round amygdules gradually increasetoward the west. Neither top nor base are exposed, but another icelandite afew miles to the northeast shows strongly stretched amygdules in anaphanitic upper crust. Phenocrysts are of plagioclase, ferroaugite, Fe-olivine,and magnetite.

A-il

STOP 6. Devil Track rhyolite, Highway 61 opposite Five Mile Rock east of Grand Marais. - This exposure, at an abandoned wave-cut cliff of the 5000-year old Nipissing stage of Lake Superior, is fairly typical of this large rhyolite. This unit can be traced for about 40 km to the west, and is about 250 m thick. Neither its top or base is exposed, and all outcrops show this weakly flow-laminated, fine-grained, aphyric to very weakly porphyritic character, rather similar to the Silver Beaver felsite. Thin sections here also show quartz paramorphs after primary tridymite. Tom Fitz (1988) has found that the grain size of these tridymite plates increases toward the center of the unit, implying that it crystallized as a simple cooling unit (Fig. 9). No significant breccia facies has been found. The wide extent, lack of breccia or other viscous flow features other than rare lineations, aphyric texture, and crystallinity suggest that this was a very hot pyroclastic flow that completely consolidated, after emplacement, to a pool of devolatilized rhyolite magma that crystallized in place.

Assuming that its overall original shape was a segment of a sphere (it appears to pinch out at the western end but is thick through much of the rest), and that its center was at the present lakeshore (half now underlies the lake), it would have had an original volume of about 600 km3.

STOP 7. Kimball Creek Felsite, Lake Superior shore north of Red Cliff. Here we are near the top of another very large (366 m thick) rhyolite. The point at Red Cliff to the south is held up by a sequence of overlying olivine tholeiite basalts. This rhyolite is weakly plagioclase-phyric, but otherwise the bulk of this flow is fine-grained, crystalline, and massive, rather like the Devil Track rhyolite. This unit also shows a network of tridymite paramorphs in the groundmass (Fig. lo), whose length is greatest near the center of the flow (Fig. 9), again indicating primary crystallization as a simple cooling unit. Here (and elsewhere) near the top, however, it is aphanitic and contains discontinuous color bands and lamellae that are probably flattened and deformed pumice fiamme. These structures are preserved here near the top because of the rapid cooling after welding, preventing the primary crystallization which obliterated them in the interior. Unfortunately the top several meters are not exposed, as is typical of these large rhyolites. One possible inference is that the top may contain easily weathered and eroded, unwelded pumice and ash. The base is not exposed either, but a few meters above, the unit shows flow-folding of thin, discontinuous streaks and lamellae (fiamme?) (Fitz, 1988). This unit is thought to have the same origin as the Devil Track rhyolite: a high-temperature rheoignimbrite.

For more details on these two large rheoignimbrites see Fitz (1988).

STOP 8. Rangeline icelandite, 1 112 mi. E of Kadunce Creek (River). These road cuts show fairly typical textures and structures of a large (40 m) icelandite lava flow. Toward the eastern end is the massive lower part, aphanitic to fine-grained and porphyritic; round amygdules gradually increase toward the west. Neither top nor base are exposed, but another icelandite a few miles to the northeast shows strongly stretched amygdules in an aphanitic upper crust. Phenocrysts are of plagioclase, ferroaugite, Fe-olivine, and magnetite.

£90.1

0 100 200 300 350Sotto. V.,tiCal dislanc. abov. baa. Top Anhedral quartz masses gradational to groupsL..gth 01 quwtz Cty$ta*. VS. SI:.tlgv.plvc P0S.I0fl I' th• Kanbal COsdit rflyodt•. o fused plates in optical continuity.

.

41 0.15• \

0.0! 4o•Sotto.

30 io oo i \\%\Vtic dI.t.nc. aba.. baa. 0* *t (ml

Groups of plates in optical continuity,L.nqth 0* tZ ay.t. vS. sbaIlg.pNc posltlo.v hI (hs 0svI T,.ck otipalt..

some with slight ra1iatin nattern.

FIG. 9 FIG. 10

Fig. 9. Variation in lengths of tridymite paramorphs with height in the Devil Track andKimball Creek rhyolites (from Fitz, 1988).

Fig. 10. Tabular habits of quartz paramorphs after tridymite in the goundmass of theKimball Creek felsite (from Fitz, 1988).

I - 0 100 200

Bottom 300 350

V u t k l (inane* above b a r of unit (m).

F I G . 9

Anhedral quartz masses gradational t o groups

of fused plates in optical continuity.

Croups of plates in optical continuity,

some w i t h sliqht ra4iatinq y t t e r n .

FIG. 10

Fig. 9. Variation in lengths of tridymite paramorphs with height in the Devil Track and Kimball Creek rhyolites (from Fitz, 1988).

Fig. 10. Tabular habits of quartz paramorphs after tridymite in the goundmass of the Kimball Creek felsite (from Fitz, 1988).

STOP 9. Red Rock Point rhyolite at Deronda Bay, Grand Portage Indian Reservation(Fig. 11).

At this locality a thick (80 m) andesiteficelandite flow (MC-3b, Table 3) isoverlain by a large (>140 m) rhyolite unit (MC-7b), but the actual contact issomewhere under the beach. The beach shingle contains pieces of both,including vesicular andesite with pseudomorphs of cristobalite balls in thestretched vesicles. The top of• the rhyolite is cut out by the ReservationRiver diabase intrusion, which also cuts it off to the west so that its lateralextent is unknown as well.

Reseruho, Rwer D3ig

VeS3tVe , erptc.(MC 7)

çto. tcctsias

(Mc- )

Fig. 11. Diagrammatic section of the Red Rock rhyolite at Stop 9, Deronda Bay.

On the way toward the main cliff outcrop on the south side of the bay,there are instructive low, wet outcrops and shallow underwater "exposures",accessible under favorable lake conditions, of the following gentlysouth-dipping sequence, from bottom to top: a) porphyritic andesite oricelandite, probably part of the MC-3 complex, overlain - and probably bakedby - b) a thin basaltic sill; this is overlain by, and has baked somewhat, c) athin layer of pumice-Iapilli tuft; this is overlain by d) a grayish ash bed 2-4cm thick; then e) a rhyolitic flow-breccia (mostly underwater) made ofvariously-oriented blocks containing stretched vesicles. Unfortunatelyboulders cover the interval, only a few meters thick, between this and thecliff outcrop ahead. This is all made of red-orange, porphyritic rhyolite thatdoes not easily give up clues as to its mode of emplacement. In someareas near the base it appears to be a breccia (simply more of the basalautobreccia of a large lava flow?). In others, faint suggestions of pumicechips and spherulites suggest that perhaps this is the basal unweldedpumice zone of a big ash flow, overlain in the cliff by the devitrified,laminated, welded zone. Because of the clear underlying breccia and thelack of obvious fiamme, I prefer the former model, although this flow doescontain some broken phenocrysts. The pumice-lapilli tuft and ash tuft bedswere air-falls which immediately preceded the rhyolite eruption.

Return to Duluth via Grand Marais.

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STOP 9. Red Rock Point rhyolite at Deronda Bay, Grand Portage Indian Reservation (Fig. 11).

- At this locality a thick (80 m) andesitehcelandite flow (MC-3b, Table 3) is overlain by a large (>I40 m) rhyolite unit (MC-7b), but the actual contact is somewhere under the beach. The beach shingle contains pieces of both, including vesicular andesite with pseudomorphs of cristobalite balls in the stretched vesicles. The top o f the rhyolite is cut out by the Reservation River diabase intrusion, which ako cuts it off to the west so that its lateral extent is unknown as well.

Fig. 11. Diagrammatic section of the Red Rock rhyolite at Stop 9, Deronda Bay.

On the way toward the main cliff outcrop on the south side of the bay, there are instructive low, wet outcrops and shallow underwater "exposures", accessible under favorable lake conditions, of the following gently south-dipping sequence, from bottom to top: a) porphyritic andesite or icelandite, probably part of the MC-3 complex, overlain - and probably baked by - b) a thin basaltic sill; this is overlain by, and has baked somewhat, c) a thin layer of pumice-lapilli tuff; this is overlain by d) a grayish ash bed 2-4 cm thick; then e) a rhyolitic flow-breccia (mostly underwater) made of variously-oriented blocks containing stretched vesicles. Unfortunately boulders cover the interval, only a few meters thick, between this and the cliff outcrop ahead. This is all made of red-orange, porphyritic rhyolite that does not easily give up clues as to its mode of emplacement. In some areas near the base it appears to be a breccia (simply more of the basal autobreccia of a large lava flow?). In others, faint suggestions of pumice chips and spherulites suggest that perhaps this is the basal unwelded pumice zone of a big ash flow, overlain in the cliff by the devitrified, laminated, welded zone. Because of the clear underlying breccia and the lack of obvious fiamme, I prefer the former model, although this flow does contain some broken phenocrysts. The pumice-lapilli tuff and ash tuff beds were air-falls which immediately preceded the rhyolite eruption.

Return to Duluth via Grand Marais.

REFERENCES

Bonnichsen, Bill and Kauffman, D. F., 1987, Physical features of rhyolite lava flows inthe Snake River Plain volcanic province, southwestern Idaho: i Fink, J. H.,editor, The emplacement of silicic domes and lava flows: Geological Societyof America Special Paper 212, p. 119-145.

Bonnichsen, Bill, Leeman, W. P., Jenks, M. D., and Honjo, Norio, 1988, Geologic fieldtrip guide to the central and western Snake River Plain, Idaho, emphasizingthe silicic rocks: in Link, P. K. and Hackett, W. R., editors, Guidebook to thegeology of central and southern Idaho: Idaho Geological Survey Bulletin 27,p. 247-281.

BVSP (Basaltic Volcanism Study Project), 1981, Basaltic Volcanism on the TerrestrialPlanets, Pergamon Press, New York, 1286 p.

Cas, R. A. F. and Wright, J. V., 1987, Volcanic Successions, Modern and Ancient: Allenand Unwin, London, 528 p.

Ekren, E. B., Mcintyre, D. H., and Bennett, E. H., 1984, High-temperature, large-volume,lava-like ash-flow tufts without calderas in southwestern Idaho: U. S.Geological Survey Professional Paper 1272, 76 p.

Fisher, R. V. and Schmincke, H.-U., 1984, Pyroclastic Rocks, Springer-Verlag, NewYork, 472 p.

Fitz, T. J., 1988, Large felsic flows in the Keweenawan North Shore Volcanic Group inCook County, Minnesota: unpub. M. S. thesis, University of Minnesota,Duluth, 180 p.

Green, J. C., 1979, Field trip guidebook for the Keweenawan (Upper Precambrian)North Shore Volcanic Group, Minnesota: Minnesota Geological SurveyGuidebook Series, No. 11, 22 p.

_____

1982a, Geology of Keweenawan extrusive rocks, , Wold, R. J. and Hinze, W.J., editors, Geology and Tectonice of the Lake Superior Basin: GeologicalSociety of America Memoir 156, p. 47-56.

_____

1982b, Two Harbors Sheet, Geologic Map of Minnesota: Minnesota GeologicalSurvey.

_____

1983, Geologic and geochemical evidence for the nature and development ofthe middle Proterozoic (Keweenawan) Midcontinent Rift of North America:Tectonophysics, v. 94, p. 413-437.

_____

1987, Plateau basalts of the Keweenawan North Shore Volcanic Group: jjBiggs, D. L., editor, Centennial Field Guide Vol. 3, Geological Society ofAmerica, p. 59-62.

_____

1989, Physical volcanology of mid-Proterozoic plateau lavas: the KeweenawanNorth Shore Volcanic Group, Minnesota: Geological Society of AmericaBulletin, v. 101, in press.

Henry, C. D., Price, J. G., Rubin, J. N., Parker, D. F., Wolff, J. A., Self, Stephen,Franklin, Richard, and Barker, D. S., 1988, Widespread, lavalike silicicvolcanic rocks of Trans-Pecos Texas: Geology, v. 16, p. 509-512.

Miller, J. D., Jr., 1987, Geology of the Keweenawan (Upper Precambrian) Beaver BayComplex in the vicinity of the Silver Bay, Minnesota, hi Balaban, N., editor,Field Trip Guidebook for selected areas in Precambrian geology ofnortheastern Minnesota: Minn. Geol. Survey Guidebook Series No. 17.

_____

1988, Geologic map of the Silver Bay and Split Rock Point NE quadrangles,Lake County, Minnesota: Minn. Geological Survey Miscellaneous Map SeriesM-65.

A-14

REFERENCES

Bonnichsen, Bill and Kauffman, D. F., 1987, Physical features of rhyolite lava flows in the Snake River Plain volcanic province, southwestern ldaho: & Fink, J. H.* editor, The emplacement of silicic domes and lava flows: Geological Society of America Special Paper 21 2* p. 1 19-1 45.

Bonnichsen, Bill, Leeman, W. P., Jenks, M. D., and Honjo, Norio* 1988, Geologic field trip guide to the central and western Snake River Plain, ldaho, emphasizing the silicic rocks: in Link, P. K. and Hackett, W. R., editors, Guidebook to the geology of centrg and southern ldaho: ldaho Geological Survey Bulletin 27, p. 247-281.

BVSP (Basaltic Volcanism Study Project), 1981, Basaltic Volcanism on the Terrestrial Planets, Pergamon Press, New York, 1286 p.

Cas, R. A. F. and Wright, J. V., 1987, Volcanic Successions, Modern and Ancient: Allen and Unwin, London, 528 p.

Ekren, E. B., Mclntyre, D. H., and Bennett, E. H., 1984, High-temperature, large-volume, lava-like ash-flow tuffs without calderas in southwestern ldaho: U. S. Geological Survey Professional Paper 1272, 76 p.

Fisher, R. V. and Schmincke, H.-U., 1984, Pyroclastic Rocks, Springer-Verlag, New York, 472 p.

Fitz, T. J., 1988, Large felsic flows in the Keweenawan North Shore Volcanic Group in Cook County, Minnesota: unpub. M. S. thesis, University of Minnesota, Duluth, 180 p.

Green, J. C., 1979, Field trip guidebook for the Keweenawan (Upper Precambrian) North Shore Volcanic Group, Minnesota: Minnesota Geological Survey Guidebook Series, No. 11, 22 p.

, 1982a, Geology of Keweenawan extrusive rocks, Wold, R. J. and Hinze, W. J., editors, Geology and Tectonice of the Lake Superior Basin: Geological Society of America Memoir 156, p. 47-56.

1982b, Two Harbors Sheet, Geologic Map of Minnesota: Minnesota Geological Survey.

-9 1983, Geologic and geochemical evidence for the nature and development of the middle Proterozoic (Keweenawan) Midcontinent Rift of North America: Tectonophysics, v. 94* p. 413-437.

, 1987, Plateau basalts of the Keweenawan North Shore Volcanic Group: & Biggs, D. L., editor, Centennial Field Guide Vol. 3, Geological Society of America, p. 59-62.

, 1989, Physical volcanology of mid-Proterozoic plateau lavas: the Keweenawan North Shore Volcanic Group, Minnesota: Geological Society of America Bulletin, v. 101, in press.

Henry, C. D., Price, J. G., Rubin, J. N., Parker, D. F., Wolff* J. A., Self, Stephen, Franklin, Richard, and Barker* D. S., 1988, Widespread, lavalike silicic volcanic rocks of Trans-Pecos Texas: Geology, v. 16, p. 509-512.

Miller* J. D., Jr., 1987, Geology of the Keweenawan (Upper Precambrian) Beaver Bay Complex in the vicinity of the Silver Bay, Minnesota, Balaban, N., editor, Field Trip Guidebook for selected areas in Precambrian geology of northeastern Minnesota: Minn. Geol. Survey Guidebook Series No. 17.

, 1988, Geologic map of the Silver Bay and Split Rock Point NE quadrangles, Lake County, Minnesota: Minn. Geological Survey Miscellaneous Map Series M-65.

Miller, J. D., Jr., Weiblen, P. W. and Green, J. C., 1987, Road log and stopdescriptions for the Beaver Bay Complex, i Balaban, N., editor, Field TripGuidebook for selected areas in Precambrian geology of northeasternMinnesota: Minn. Geol. Survey Guidebook Series No. 17.

Motamedi, Shoaullah, 1984, The Keweenawan lavas in the City of Duluth: unpub. M. S.thesis, Univ. of Minnesota-Duluth, 140 p.

Price, J. G., Henry, C. D., Parker, D. F., and Barker, D. S., 1986, Igneous geology ofTrans-Pecos Texas: Field trip guide and research articles: Guidebook 23,Bureau of Economic Geology, Austin, 360 p.

Ross, C. S. and Smith, R. L., 1961, Ash-flow tufts: their origin, geologic relations andidentification: U. S. Geological Survey Professional Paper 366, 81 p.

Van Schmus, W. A. and Hinze, W. J., The Midcontinent Rift System: Annual Reviewsof Earth and Planetary Science, v. 13, p. 345-383.

Wold, R. J. and Hinze, W. J., 1982, editors, Geology and Tectonics of the LakeSuperior Basin: Geological Society of America Memoir 156, 280 p.

A- 15

~ i l l e r ~ J. Da1 Jr., Weiblen? P. W. and Green* J. C.? 1987! Road log and stop descriptions for the Beaver Bay Complexl Balabanl N.? editor* Field Trip Guidebook for selected areas in Precambrian geology of northeastern Minnesota: Minn. Geol. Survey Guidebook Series No. 17.

Motamedil Shoaullahl 1984? The Keweenawan lavas in the City of Duluth: unpub. M. S. thesis* Univ. of Minnesota-Duluth* 140 p.

Pricel J. G., Henry, C. D.l Parker* D. FSl and Barker* D. S.? 1986? Igneous geology of Trans-Pecos Texas: Field trip guide and research articles: Guidebook 23* Bureau of Economic Geologyl Austin* 360 p.

Rossl C. S. and Smith, R. L.? 19611 Ash-flow tuffs: their origin* geologic relations and identification: U. S. Geological Survey Professional Paper 366! 81 p.

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

Woldl R. J. and Hinze, W. J., 1982* editors, Geology and Tectonics of the Lake Superior Basin: Geological Society of America Memoir 15e1 280 p.

- PENOKEAN STRUCTURAL TERRANES IN EAST-CENTRAL MINNESOTA

A one-day field trip associated with theThirty-Fifth Annual Meeting of theInstitute on Lake Superior Geology

May 3-6, 1989

By

Timothy B. HoistDepartment of Geology


introductionThe Penokean orogeny, which occurred near the close of early Proterozoic time

(1875-1825 Ma., Van Schmus, 1976, 1980, 1981) involved deformation and metamorphismof rocks in Minnesota, Wisconsin, upper Michigan, and the Superior, Southern, andGrenville provinces in Canada (HoIst, 1982; Maass and others, 1980; Cannon, 1973;Brocoum and Dalziel, 1974). Until recently, the Penokean orogeny in Minnesota wasusually interpreted as intracratonic (Morey and Sims, 1976; Sims, 1976; Sims and others,1980) with an emphasis on the role of basement rock involving verticai remobilIzation"(Morey, 1979). Of fundamental importance in this interpretation is the boundary, inwest-central Minnesota, between an ancient (in part 3550 Ma.) gneissic terrane and ayounger (Ca. 2700 Ma.) granite-greenstone terrane (Fig. 1). Morey and Sims (1976)suggested that this boundary is part of a major Precambrian crustal feature more than 1200km long which they called the Great Lakes tectonic zone (Sims and others, 1980). Theynoted that rocks which overlie the granite-greenstone terrane (Animikie Group) are lessdeformed and metamorphosed than those which overlie the Great Lakes tectonic zone andgneissic terrane. Because of this these authors suggested that this early Precambrianboundary acted as a locus for limited intracontinental tectonic movement and risinggeothermal gradients (Morey, 1979).

Recent structural investigations in the early Proterozoic Thomson Formation ineast-central Minnesota (Hoist, 1982, 1984c) reveal evidence for multiple deformation anddocument the existence of northward-directed nappes during the Penokean orogeny. Oneof the possible models of nappe emplacement in Minnesota would be gravity gliding off arising diapir following the suggestions of Morey (1979) and Sims and others (1980) forintracratonic deformation. However, the high strains associated with nappe emplacement(Hoist, 1985a) do not support this idea (see the values of strain above a rising diapir inDixon, 1975). Such strains are more consistent with a plate tectonic model (Hoist, 1985a,1985b). The presence of nappes has also recently been reported further to the east in thePenokean orogenic belt (Sims and others, 1987; Klasner, at al., 1988) and Penokeanvolcanic rocks have been shown to be of island arc affinity (Schulz and others, 1984)resulting in plate tectonic syntheses for several areas of the Penokean orogenic belt.

The growing body of structural and petrologic evidence from Wisconsin (LaBerge andothers, 1984; Sims and others, 1985) and upper Michigan (Cambray, 1977, 1978) isconsistent with the structural evidence in east-central Minnesota for a convergent plateboundary model (HoIst, 1 984a, 1 984b). Recent models of the Penokean orogeny inMinnesota involve a convergent plate margin with subduction of a passive margin, or theclose of a back-arc basin (Hoim, HoIst, and Ellis, 1988; Southwick, Morey, and McSwiggen,1988).

B-i

PENOKEAN STRUCTURAL TERRANES IN EAST-CENTRAL MINNESOTA

A one-day field trip associated with the Thirty-Fifth Annual Meeting of the Institute on Lake Superior Geology

May 3-6* 1989

Timothy B. Holst Department of Geology

University of Minnesota Duluth Duluth1 Minnesota 55812

Introduction The Penokean orogenyl which occurred near the close of early Proterozoic time

(1 875-1 825 Ma.* Van Schmus* 1976, 1980, 1981) involved deformation and metamorphism of rocks in Minnesota, Wisconsin, upper Michigan* and the Superiorl Southern, and Grenville provinces in Canada (Holstl 1982; Maass and others* 1980; Cannon, 1973; Brocoum and Dalziel* 1974). Until recently* the Penokean orogeny in Minnesota was usually interpreted as intracratonic (Morey and Sims, 1976; Sims* 1976; Sims and others, 1980) with an emphasis on the role of basement rock involving "vertical remobilization" (Morey, 1979). Of fundamental importance in this interpretation is the boundary* in west-central Minnesota, between an ancient (in part 3550 Ma.) gneissic terrane and a younger (ca. 2700 Ma.) granite-greenstone terrane (Fig. I). Morey and Sims (1976) suggested that this boundary is part of a major Precambrian crustal feature more than 1200 km long which they called the Great Lakes tectonic zone (Sims and others* 1980). They noted that rocks which overlie the granite-greenstone terrane (Animikie Group) are less deformed and metamorphosed than those which overlie the Great Lakes tectonic zone and gneissic terrane. Because of this these authors suggested that this early Precambrian . boundav acted as a locus for limited intracontinental tectonic movement and rising geothermal gradients (Morey, 1 979).

Recent structural investigations in the early Proterozoic Thomson Formation in east-central Minnesota (Hoist* 1982, 1984c) reveal evidence for multiple deformation and document the existence of northward-directed nappes during the Penokean orogeny. One of the possible models of nappe emplacement in Minnesota would be gravity gliding off a rising diapir following the suggestions of Morey (1979) and Sims and others (1980) for intracratonic deformation. However* the high strains associated with nappe emplacement

I (Holstl 1985a) do not support this idea (see the values of strain above a rising diapir in Dixon, 1975). Such strains are more consistent with a plate tectonic model (Holst, 1985a1 1985b). The presence of nappes has also recently been reported further to the east in the Penokean orogenic belt (Sims and others, 1987; Klasner* at al.l 1988) and Penokean 1 volcanic rocks have been shown to be of island arc affinity (Schulz and others, 1984) resulting in plate tectonic syntheses for several areas of the Penokean orogenic belt.

I The growing body of structural and petrologic evidence from Wisconsin (LaBerge and

others* 1984; Sims and others, 1985) and upper Michigan (Cambray, 1977# 1978) is consistent with the structural evidence in east-central Minnesota for a convergent plate boundary model (Holstl 1984a, 1984b). Recent models of the Penokean orogeny in Minnesota involve a convergent plate margin with subduction of a passive margin* or the close of a back-arc basin (Holm, Holst, and Ellis, 1988; Southwick, Morey* and McSwiggen, 1988).

Figure 1: Generalized geologic map of the Precambrian geology of Minnesota (afterSims and others, 1980; Morey and others, 1982). GLTZ is the Great LakesTectonic Zone.

Geologic SettingThe Precambrian rocks of east-central Minnesota can be divided into four distinct

terranes (Fig. 1): 1) Archean rocks; 2) early Proterozoic stratified rocks; 3) earlyProterozoic plutonic rocks; and 4) middle Proterozoic (Keweenawan) sedimentary andvolcanic rocks (Morey, 1978). A stratigraphic column for east-central Minnesota anddetailed descriptions of the different rock types have been presented by Morey (1978,1979), although recent work suggests a rethinking of some of the correlations (Morey andSouthwick, 1984; Southwick and Morey, 1988; Southwick, Morey and McSwiggen, 1988).

The Archean terrane consists of a northern granite-greenstone belt terrane (ca. 2700Ma.), a southern highly deformed gneissic terrane (in part >3550 Ma.), and a centralsheared, schistose segment (Morey, 1978) also known as the Great Lakes tectonic zone(Sims and others, 1980). Along this zone, granitic plutons (2600 Ma., Sims and others,1980) acted as a weld between the northern and southern segments forming a relativelystable craton by the end of the Archean (Morey, 1978).

Sedimentation into a large basin on this craton, the Animikie basin, began at about2100 Ma. (Van Schmus, 1976). Depositional patterns reflect contrasting tectonic conditionsin the northern and southern segments of the Animikie basin. A relatively thin succession(2-3 km) of predominantly sedimentary rocks (labeled the Animikie Group, Keighin andothers, 1972) was deposited north of the northern front of the Great Lakes tectonic zone,whereas a much thicker and more heterogeneous succession (>6 km) of sedimentary and

B-2

•VP Middi. Prot&ozock" IQneous andk!IJI ssdlm.ntsry rocksI'I Early Prot.rozolc

ntrusivs rocks

r.a 1 Early Protsrozocv- i supracruatals1 1 with Iron formationF::::::1 Archean Qranste—

greenstoA. terran

'1 Arch•an gn.is&c

lOOkm

Figure 1: Generalized geologic map of the Precambrian geology of Minnesota (after Sims and others, 1980; Morey and others, 1982). GLTZ is the Great Lakes Tectonic Zone.

Geoloaic Settinq The Precambrian rocks of east-central Minnesota can be divided into four distinct

terranes (Fig. 1): 1) Archean rocks; 2) early Proterozoic stratified rocks; 3) early Proterozoic plutonic rocks; and 4) middle Proterozoic (Keweenawan) sedimentary and volcanic rocks (Morey, 1978). A stratigraphic column for east-central Minnesota and detailed descriptions of the different rock types have been presented by Morey (1978, 1979), although recent work suggests a rethinking of some of the correlations (Morey and Southwick, 1984; Southwick and Morey, 1988; Southwick, Morey and McSwiggen, 1988).

The Archean terrane consists of a northern granite-greenstone belt terrane (ca. 2700 Ma.), a southern highly deformed gneissic terrane (in part >3550 Ma.), and a central sheared, schistose segment (Morey, 1978) also known as the Great Lakes tectonic zone (Sims and others, 1980). Along this zone, granitic plutons (2600 Ma,, Sims and others, 1980) acted as a weld between the northern and southern segments forming a relatively stable craton by the end of the Archean (Morey, 1978).

Sedimentation into a large basin on this craton, the Animikie basin, began at about 21 00 Ma. (Van Schmus, 1976). Depositional patterns reflect contrasting tectonic conditions in the northern and southern segments of the Animikie basin. A relatively thin succession (2-3 km) of predominantly sedimentary rocks (labeled the Animikie Group, Keighin and others, 1972) was deposited north of the northern front of the Great Lakes tectonic zone, whereas a much thicker and more heterogeneous succession (>6 km) of sedimentary and

volcanic rocks (Animikie and Mule Lacs Groups, Morey, 1978) was deposited south of thisfront (Morey, 1983; Morey and Southwick, 1984). Apparently, subsidence was relativelygreater in the southern part of the basin, particularly over the Great Lakes tectonic zoneand the gneissic terrane. Ojakangas (1983) has inferred the extent of the Animikie basinon the basis of sedimentological and tithological similarities in Minnesota, Wisconsin, andMichigan. Because rocks of the Midcontinent Rift system (middle Proterozoic igneous andsedimentary rocks, Fig. 1) separate the Animikie basin into two physically isolatedsegments, the strata in the northwestern segment are assigned to the Animikie and MuleLacs Groups whereas those in the southeastern segment are assigned to the MarquetteRange Supergroup. Correlations have been made among the Lower Proterozoic beddedrocks in Minnesota, Michigan and Wisconsin (Morey, 1983).

All stops on this field trip have been mapped historically as part of the ThomsonFormation. The southern exposures have recently been assigned to other, as yet unnamedunits of early Proterozoic metasediments and metavolcanics, which are older than theThomson Formation (Southwick and Morey, 1988; Southwick, Morey and McSwiggen, 1988).

Descriptive Structural GeologyExposures of early Proterozoic metasediments and metavoicanics historically called

the Thomson Formation consist of a thick sequence of interbedded slate, slaty greywacke,metagreywacke with some intercalated volcanics. The southern two-thirds of this region,here designated the southern structural terrane, has a pervasive, nearly bedding-parallelfoliation (S1). It ranges from a slaty cleavage in the north to a schistosity in the south nearthe Denham Formation (Hoist, 1982). Strain analysis (Hoist, 1 985a) has firmly establishedthe tectonic nature of this bedding.paraIlel foliation. Also present in the southern structuralterrane are isoclinal recumbent folds in scales ranging from cms to kms (nappes), witheast-west fold axes (F1). Both the northern and southern structural terranes have beenfolded into gentle to open upright folds. Fold axial surfaces strike east-west and fold axeshave horizontal to gentle plunges either east or west. In the southern structural terranethese upright folds (F2) refold the earlier isoclinal recumbent folds (F1). A well-developedcleavage, vertical or dipping steeply to the south (axial-planar to the upright folds) is presentin both the northern and southern structural terranes. In the northern structural terrane thiscleavage is a well developed continuous slaty cleavage in the finer-grained units. In theunits with graded bedding, and in the units with interlaminated fine and coarse layers, thecontinuous cleavage grades into a disjunctive spaced cleavage (terminology of Powell,1979). The spacing of the cleavage domains ranges from continuous up to 1 cm in someof the most coarse-grained units, but it is rarely over a few mm. Cleavage domainsconstitute from 25% of the rock (in the thick graywacke units) to 100% (in the slates with acontinuous cleavage). Domain shapes range from rough to smooth (smooth shapespredominate) with some anastomosing shapes. Within the microlithons, a weak fabric, atleast, is developed everywhere, and commonly the fabric is strong to complete (terminologyof Powell, 1979). In the southern structural terrane the steep foliation is a well-developedcrenulation cleavage (S2). This S2 cleavage can be discrete but is most commonlytransitional to zonal, or entirely zonai (Gray, 1977; Powell, 1979). Spacing of the cleavagedomains is variable. For the most part the spaced crenulation cleavage strikes east-westand dips steeply to the south or is vertical, and axial-planar to the F2 folds. Around somemicrofolds, however, the spaced crenulation cleavage may fan, or be at a constant angle(up to 40°) to the axial plane on one limb, and axial planar on the other limb. Theintersection of S1 and S2 defines a well developed lineation in the rock, trending east-westwith subhorizontal plunges. Detailed mapping has allowed a boundary to be drawn betweenthe area of a single main Penokean deformation in the north (the northern structuralterrane), and the area of two Penokean-aged deformations in the south (the southernstructural terrane). This boundary was interpreted by Hoist (1984c) as a nappe frontbecause of the abrupt nature of the change across this terrane boundary, and because therefraction pattern in the early foliation just south of this boundary suggests that a nappe

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vhcanic rocks (Animikie and Mille Lacs Groups, Morey, 1978) was deposited south of this front (Morey, 1983; Morey and Southwick, 1984). Apparently, subsidence was relatively greater in the southern part of the basin, particularly over the Great Lakes tectonic zone and the gneissic terrane. Ojakangas (1983) has inferred the extent of the Animikie basin on the basis of sedimentological and lithological similarities in Minnesota, Wisconsin, and Michigan. Because rocks of the Midcontinent Rift system (middle Proterozoic igneous and sedimentary rocks, Fig. 1) separate the Animikie basin into two physically isolated segments, the strata in the northwestern segment are assigned to the Animikie and Mille Lacs Groups whereas those in the southeastern segment are assigned to the Marquette Range Supergroup. Correlations have been made among the Lower Proterozoic bedded rocks in Minnesota, Michigan and Wisconsin (Morey, 1983).

All stops on this field trip have been mapped historically as part of the Thomson Formation. The southern exposures have recently been assigned to other, as yet unnamed units of early Proterozoic metasediments and metavolcanics, which are older than the Thomson Formation (Southwick and Morey, 1988; Southwick, Morey and McSwiggen, 1988).

Descriptive Structural Geoloqy Exposures of early Proterozoic metasediments and metavolcanics historically called

the Thomson Formation consist of a thick sequence of interbedded slate, slaty greywacke, metagreywacke with some intercalated volcanics. The southern two-thirds of this region, here designated the southern structural terrane, has a pervasive, nearly bedding-parallel foliation (SJ. It ranges from a slaty cleavage in the north to a schistosity in the south near the Denham Formation (Holst, 1982). Strain analysis (Holst, 1985a) has firmly established the tectonic nature of this bedding-parallel foliation. Also present in the southern structural terrane are isoclinal recumbent folds in scales ranging from cms to kms (nappes), with east-west fold axes (F,). Both the northern and southern structural terranes have been folded into gentle to open upright folds. Fold axial surfaces strike east-west and fold axes have horizontal to gentle plunges either east or west. In the southern structural terrane these upright folds (F2) refold the earlier isoclinal recumbent folds (F,). A well-developed cleavage, vertical or dipping steeply to the south (axial-planar to the upright folds) is present in both the northern and southern structural terranes. In the northern structural terrane this cleavage is a well developed continuous slaty cleavage in the finer-grained units. In the units with graded bedding, and in the units with interlaminated fine and coarse layers, the continuous cleavage grades into a disjunctive spaced cleavage (terminology of Powell, 1979). The spacing of the cleavage domains ranges from continuous up to 1 cm in some of the most coarse-grained units, but it is rarely over a few mm. Cleavage domains constitute from 25Y0 of the rock (in the thick graywacke units) to 10O0/~ (in the slates with a continuous cleavage). Domain shapes range from rough to smooth (smooth shapes predominate) with some anastomosing shapes. Within the microlithons, a weak fabric, at least, is developed everywhere, and commonly the fabric is strong to complete (terminology of Powell, 1979). In the southern structural terrane the steep foliation is a well-developed crenulation cleavage (SJ. This S2 cleavage can be discrete but is most commonly transitional to zonal, or entirely zonal (Gray, 1977; Powell, 1979). Spacing of the cleavage domains is variable. For the most part the spaced crenulation cleavage strikes east-west and dips steeply to the south or is vertical, and axial-planar to the F2 folds. Around some microfolds, however, the spaced crenulation cleavage may fan, or be at a constant angle (up to 40') to the axial plane on one limb, and axial planar on the other limb. The intersection of S1 and S2 defines a well developed lineation in the rock, trending east-west with subhorizontal plunges. Detailed mapping has allowed a boundary to be drawn between the area of a single main Penokean deformation in the north (the northern structural terrane), and the area of two Penokean-aged deformations in the south (the southern structural terrane). This boundary was interpreted by Holst (1984~) as a nappe front because of the abrupt nature of the change across this terrane boundary, and because the refraction pattern in the early foliation just south of this boundary suggests that a nappe

front must be located in the immediate vicinity to the north.Underlying the southern part of what has historically been called the Thomson

Formation is the early Proterozoic Denham Formation. This has now been broken intoupper and lower members by Southwick, Morey and McSwiggen, (1988). The rocks of thisformation have been multiply deformed and metamorphosed in a fashion similar to theoverlying rocks (Hoim, 1986a, 1986b) and are here considered part of the same nappeterrane. The Denham Formation is a sequence of primarily quartz-rich metasedimentaryrocks (metaarkose, quartzite, mica schist, and garnet-staurolite schist) with minor amountsof marble and volcanic rocks. It has been postulated to be stratigraphically equivalent tothe Chocolay Group of the Marquette Range Supergroup in Michigan (Larue, 1981; Morey,1983). Bedding strikes east-west and dips dominantly steeply. A nearly bedding-parallelfoliation is present everywhere in the Denham Formation. The foliation is refracted at ahigher angle to bedding in the more competent arkosic and quartzitic units. The foliationand bedding have been folded with the development, locally, of a crenulation cleavage.Orientations of axial surfaces to these folds vary from horizontal to vertical and strikeeast-west.

The Denham Formation also contains a very well-developed, nearly horizontal,east-west mineral and crenulation lineation. Chocolate-tablet boudinage of quartz veinsoccurs parallel to bedding throughout the area.

The basement to this terrane is the Archean (2700 Ma.) McGrath Gneiss whichcontains a poorly to well-developed foliation and a number of cross-cutting shear zones. Itis a coarse-grained, pinkish-gray, biotite gneiss containing megacrysts of microcline. Someof the megacrysts are rounded, giving the appearance of augen, but many are euhedral andoriented obliquely to the foliation while still others have a sigmoidal shape and are alignedin the foliation. Sense of shear from the sigmoidal porphyroclasts (after Simpson andSchmid, 1983) indicate that the foliation developed in a dominantly dextral shear regime.The foliation is commonly well developed, strikes east-west, and varies in dip fromhorizontal to vertical. A nearly horizontal east-west mineral lineation ranging from crude tolocally well developed is also present. The McGrath Gneiss is locally folded and iscross-cut by nearly vertical, commonly anastomosing shear zones that strike generallyeast-west.

In the northern structural terrane there are some late-stage features including kinkbands which deform the steep cleavage in the Thomson Formation. The orientation ofthese kink bands is quite variable, but poles to over 100 kink bands define a singlemaximum on an equal-area projection (Clark, 1985). The gentle dip of the kink bandsindicates a sub-vertical finite compression during their formation, estimated to be about 5%by Clark (1985).

Conditions Of DeformationPetrographic analysis indicates that the basement and cover rocks have undergone

similar conditions of deformation related to the Penokean orogeny. In all rocks analyzed,the predominant deformational processes appear to be normal crystal-plastic type, involvingdynamic recovery and recrystallization. In both basement and cover rocks, quartz hasundergone ductile deformation. Bending of the crystal lattice has produced undulatoryextinction and, locally, deformation bands. Recovery and recrystallization are indicated bythe development of subgrains and strain free new grains in and along the margins of quartzaggregates. Both brittle and ductile deformation processes have occurred in the feldspargrains. Brittle deformation is indicated in the coarser feldspar by fractures along which newgrains have recrystallized. Simultaneous ductile deformation of the finer grained feldspars isindicated by undulatory extinction, recovery, and recrystallization commonly resulting in agranoblastic polygonal texture. Mica grains are relatively strain free in both basement andcover rocks, probably because of recrystallization. The textures described here indicatedominantly ductile deformation under moderate to high temperature and moderate pressureconditions.

B-4

/Â

front must be located in the immediate vicinity to the north. Underlying the southern part of what has historically been called the Thomson

Formation is the early Proterozoic Denham Formation. This has now been broken into upper and lower members by Southwick, Morey and McSwiggen. (1988). The rocks of this formation have been multiply deformed and metamorphosed in a fashion similar to the overlying rocks (Holm, 1986a, 1986b) and are here considered part of the same nappe terrane. The Denham Formation is a sequence of primarily quartz-rich metasedimentary rocks (metaarkose, quartzite, mica schist, and garnet-staurolite schist) with minor amounts of marble and volcanic rocks. It has been postulated to be stratigraphically equivalent to the Chocolay Group of the Marquette Range Supergroup in Michigan (Larue, 1981; Morey, 1983). Bedding strikes east-west and dips dominantly steeply. A nearly bedding-parallel foliation is present everywhere in the Denham Formation. The foliation is refracted at a higher angle to bedding in the more competent arkosic and quartzitic units. The foliation and bedding have been folded with the development, locally, of a crenulation cleavage. Orientations of axial surfaces to these folds vary from horizontal to vertical and strike east-west.

The Denham Formation also contains a very well-developed, nearly horizontal, east-west mineral and crenulation lineation. Chocolate-tablet boudinage of quartz veins occurs parallel to bedding throughout the area.

The basement to this terrane is the Archean (2700 Ma.) McGrath Gneiss which contains a poorly to well-developed foliation and a number of cross-cutting shear zones. It is a coarse-grained, pinkish-gray, biotite gneiss containing megacrysts of microcline. Some of the megacrysts are rounded, giving the appearance of augen, but many are euhedral and oriented obliquely to the foliation while still others have a sigmoidal shape and are aligned in the foliation. Sense of shear from the sigmoidal porphyroclasts (after Simpson and Schmid, 1983) indicate that the foliation developed in a dominantly dextral shear regime. The foliation is commonly well developed, strikes east-west, and varies in dip from horizontal to vertical. A nearly horizontal east-west mineral lineation ranging from crude to locally well developed is also present. The McGrath Gneiss is locally folded and is cross-cut by nearly vertical, commonly anastomosing shear zones that strike generally east-west.

In the northern structural terrane there are some late-stage features including kink bands which deform the steep cleavage in the Thomson Formation. The orientation of these kink bands is quite variable, but poles to over 100 kink bands define a single maximum on an equal-area projection (Clark, 1985). The gentle dip of the kink bands indicates a sub-vertical finite compression during their formation, estimated to be about 5% by Clark (1985).

Conditions Of Deformation Petrographic analysis indicates that the basement and cover rocks have undergone

similar conditions of deformation related to the Penokean orogeny. In all rocks analyzed, the predominant deforrnational processes appear to be normal crystal-plastic type, involving dynamic recovery and recrystallization. In both basement and cover rocks, quartz has undergone ductile deformation. Bending of the crystal lattice has produced undulatory extinction and, locally, deformation bands. Recovery and recrystallization are indicated by the development of subgrains and strain free new grains in and along the margins of quartz aggregates. Both brittle and ductile deformation processes have occurred in the feldspar grains. Brittle deformation is indicated in the coarser feldspar by fractures along which new grains have recrystallized. Simultaneous ductile deformation of the finer grained feldspars is indicated by undulatory extinction, recovery, and recrystallization commonly resulting in a granoblastic polygonal texture. Mica grains are relatively strain free in both basement and cover rocks, probably because of recrystallization. The textures described here indicate dominantly ductile deformation under moderate to high temperature and moderate pressure conditions.

MetamorphismMetamorphism of the early Proterozoic sedimentary rocks in this region increases

from north to south (Keighin and others, 1972). At the type locality near Thomson in thenorthern terrane, the Thomson Formation is metamorphosed to lower greenschist fades(chlorite zone). Within the Thomson Formation there is a progressive increase inmetamorphic grade to lower amphibolite facies (garnet zone) in the south (Morey, 1979).Farther south, the Denham Formation has been metamorphosed to the staurolite zone ofthe amphibolite fades (indicated by the presence of a coarse-grained schist containingquartz÷muscovite+biotite÷garnet÷staurolite). Morey (1983) has mapped the biotite, garnet,and staurolite isograds for early Proterozoic stratifed rocks in Minnesota, illustrating thisprogressive increase in metamorphic grade from north to south and the similarity of thetrend of metamorphic isograds and structural features (see Morey, 1983, fig. 7).

Petrographic analysis reveals further information about the timing of metamorphismand deformation. Progressive metamorphism during the early phase of deformation in theDenham Formation reached the garnet zone of the amphibolite facies as indicated by thepresence of syntectonic garnet porphyroblasts. The peak of metamorphism, however,occurred during or after the later deformation as indicated by staurolite overgrowing both theschistosity and the crenulation cleavage.

Microprobe analysis of garnet shows only slight compositional variation and nosystematic zonation (HoIm, 1 986a). Rocks of the southern structural terrane have recentlybeen studied by Holm and Selverstone (1989). Using thermobarometric techniques theyfind temperature and pressure estimates of final equilibration increasing southward.Staurolite grade samples from just north of the Denham Formation yield final equilibrationtemperatures of 520-590°C and a pressure of about 7 kb (depth of about 25 km).

Petrologic and Geochemical ConstraintsPenokean age volcanic rocks further to the east have been shown to be of Island

arc affinity (Schulz, 1983; Schulz and others, 1984). Horan, Hansen, and Spencer (1987)suggest that the early Proterozoic intrusive rocks in central Minnesota were formed at aconvergent plate margin, based on isotopic and trace element analysis. They suggest thata gabbro near Mora, Minnesota may have been part of an early Proterozoic ocean crust,and thus may represent part of a suture zone. Recent geochemical and isotopic workreported by Southwick, Morey, and McSwiggen (1988) shows that island arc constituentsare not a major portion of the rocks that now make up the told-and-thrust belt of thePenokean Orogen in Minnesota. They also report that most of the basaltic rocks of theregion have a continental (within plate) affinity, indicating that continental crust played a rolein determining their composition.

Tectonic ModellingHoIm, HoIst and Ellis (1988), using the constraints of structural geology, finite strain

determinations, deformation conditions indicated by microstructures and textures, and theresults of thermobarometry estimates, developed a tectonic model consisting of southwarddirected oblique subduction along the Great Lakes tectonic zone. According to this modelintense deformation occurred in the tootwall of the major thrust, which marked the boundarybetween downgoing and overriding plates during A-type (continental) subduction.Sedimentary rocks deposited on the footwall during loading caused by thrusting eventuallybecame incorporated into the deformation zone. Early formed structures related to footwalldeformation are a dominantly well-developed foliation in the gneiss and isoclinal, recumbentfolds with a bedding-sub-parallel foliation in the southern structural terrane. Progressivemetamorphism during subduction reached the garnet zone of the amphibolite facies.Various deformation inversions show that this early phase of deformation involved extremeflattening (with Z vertical) and large amounts of finite extensional stain in both thenorth-south and east-west directions.

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Metamorohism Metamorphism of the early Proterozoic sedimentary rocks in this region increases

from north to south (Keighin and others, 1972). At the type locality near Thomson in the northern terrane, the Thomson Formation is metamorphosed to lower greenschist facies (chlorite zone). Within the Thomson Formation there is a progressive increase in metamorphic grade to lower amphibolite facies (garnet zone) in the south (Morey, 1979). Farther south, the Denham Formation has been metamorphosed to the staurolite zone of the amphibolite facies (indicated by the presence of a coarse-grained schist containing quartz+muscovite+biotite+garnet+staurolite). Morey (1983) has mapped the biotite, garnet, and staurolite isograds for early Proterozoic stratifed rocks in Minnesota, illustrating this progressive increase in metamorphic grade from north to south and the similarity of the trend of metamorphic isograds and structural features (see Morey, 1983, fig. 7).

Petrographic analysis reveals further information about the timing of metamorphism and deformation. Progressive metamorphism during the early phase of deformation in the Denham Formation reached the garnet zone of the amphibolite facies as indicated by the presence of syntectonic garnet porphyroblasts. The peak of metamorphism, however, occurred during or after the later deformation as indicated by staurolite overgrowing both the schistosity and the crenulation cleavage.

Microprobe analysis of garnet shows only slight compositional variation and no systematic zonation (Holm, 1986a). Rocks of the southern structural terrane have recently been studied by Holm and Selverstone (1989). Using thermobarometric techniques they find temperature and pressure estimates of final equilibration increasing southward. Staurolite grade samples from just north of the Denham Formation yield final equilibration temperatures of 520-590Â° and a pressure of about 7 kb (depth of about 25 km).

Petroloqic and Geochemical Constraints Penokean aae volcanic rocks further to the east have been shown to be of island

arc affinity ( ~ c h u l z , 1983; Schulz and others, 1984). Horan, Hansen, and Spencer (1987) suggest that the early Proterozoic intrusive rocks in central Minnesota were formed at a convergent plate margin, based on isotopic and trace element analysis. They suggest that a gabbro near Mora, Minnesota may have been part of an early Proterozoic ocean crust, and thus may represent part of a suture zone. Recent geochemical and isotopic work reported by Southwick, Morey, and McSwiggen (1988) shows that island arc constituents are not a major portion of the rocks that now make up the fold-and-thrust belt of the Penokean Orogen in Minnesota. They also report that most of the basaltic rocks of the region have a continental (within plate) affinity, indicating that continental crust played a role in determining their composition.

Tectonic Modellinq Holm. Hoist and Ellis

determinations, deformation (1988), using the constraints of structural geology, finite strain

conditions indicated by microstructures and textures, and the results of thermobarometry estimates, developed a tectonic model consisting of southward directed oblique subduction along the Great Lakes tectonic zone. According to this model intense deformation occurred in the footwall of the major thrust, which marked the boundary between downgoing and overriding plates during A-type (continental) subduction. Sedimentary rocks deposited on the footwall during loading caused by thrusting eventually became incorporated into the deformation zone. Early formed structures related to footwall deformation are a dominantly well-developed foliation in the gneiss and isoclinal, recumbent folds with a bedding-sub-parallel foliation in the southern structural terrane. Progressive metamorphism during subduction reached the garnet zone of the amphibolite facies. Various deformation inversions show that this early phase of deformation involved extreme flattening (with Z vertical) and large amounts of finite extensional stain in both the north-south and east-west directions.

Footwafl deformation was followed by imbrication and accretion onto the hanging wallduring uplift associated with continued compression and isostatic rebound. Later formedstructures associated with imbrication and deformation within the hanging wall consist offolding of the foliation and development of shear zones in the McGrath Gneiss and open toclose, upright to overturned folds in the cover rocks of both the southern and northernstructural terranes. The peak metamorphic event (represented by staurolite) occurred afterthe later deformation at temperatures of around 520-590°C and a pressure of 7 kb (depth ofabout 25 km). Increasing temperature associated with decreasing pressure (uplift) isexplained by conductive relaxation caused by crustal thickening and erosion.

Southwick, Morey, and McSwiggen (1988), while noting that several types of platemargin collisional models are possible, favor a collapsing back-arc basin as a model for thePenokean orogeny. They argue that the lithostratigraphic associations of the earlyProterozoic supracrustal rocks in Minnesota, and the geochemical characteristics of thevolcanic rocks are more compatible with a back-arc setting than than with an arc-trenchenvironment.

GUIDE TO FIELD TRIP STOPS

The location of the field trip stops is shown on Figure 2. The field trip begins at theRadisson Duluth Hotel at Superior Street and 5th Avenue West in downtown Duluth,Minnesota. Head southwest on Superior Street and in two blocks join Interstate Highway35 going south. After 14.6 miles on 1-35, take exit 242, turning south (left) on CantonCounty 1. In 2.9 miles, the road crosses a spillway, turns west, and after 0.6 miles,reaches a 1 junction. Turn right (north) on Minnesota Highway 210, which then turns left(west) in less than 0.1 mile. In 0.2 miles, Minnesota Highway 210 crosses the St. LouisRiver. Pull off and park just before reaching the bridge, or cross it and park in the parkingarea on the south side of the road.

STOP 1 THOMSON DAM

This is the type locality of the Thomson Formation, which here consists of slates andmetagraywackes. A variety of sedimentary structures may be seen, including ripple marks,load casts, convolute bedding, cross bedding, graded bedding, and sole marks. Folds withwavelengths from cm to km are present (Figure 3). They are upright to steeply inclined tothe south and have subhorizontal fold axes that trend east-west. Where folds die out alongtrend, axes may plunge east or west as much as 60°. Two such steeply plunging foldsmay be seen in this area. A single axial-planar foliation is present; the foliation rangesfrom a continuous slaty cleavage in the fine-grained units to a disjunctive spaced cleavagein the more coarse graywacke beds. Abundant carbonate concretions and weatheredconcretion voids can be seen flattened in the plane of the cleavage. Strain determinationsin this region using deformed concretions, mud chips, and a thin conglomerate bed to thenorth of the reservoir reveal strain ratios of about X:Y:Z = 7:4:1, with X vertical, Z horizontaland north-south (perpendicular to foliation) and Y horizontal and east-west. Quartz veins,from a few mm to more than a meter in width are present. Some exhibit ptygmatic folding.Basalt dikes of presumed Keweenawan age are also present, as are kink bands andseveral sets of joints. The kink bands dip gently and display a single maximum on anequal-area projection with an average strike of about N7OE and a dip of about 20° to thesouth. Many of the kink bands intersect, but the angle of intersection varies randomlybetween 0 and 30°. No cross-cutting pattern exists within the kink bands (Clark, 1985).

B-6

, Footwall deformation was followed by imbrication and accretion onto the hanging wall

during uplift associated with continued compression and isostatic rebound. Later formed structures associated with imbrication and deformation within the hanging wall consist of folding of the foliation and development of shear zones in the McGrath Gneiss and open to close, upright to overturned folds in the cover rocks of both the southern and northern structural terranes. The peak metamorphic event (represented by staurolite) occurred after the later deformation at temperatures of around 520-590Â° and a pressure of 7 kb (depth of about 25 km). Increasing temperature associated with decreasing pressure (uplift) is explained by conductive relaxation caused by crustal thickening and erosion.

Southwick, Morey, and McSwiggen (1988), while noting that several types of plate margin collisional models are possible, favor a collapsing back-arc basin as a model for the Penokean orogeny. They argue that the lithostratigraphic associations of the early Proterozoic supracrustal rocks in Minnesota, and the geochemical characteristics of the volcanic rocks are more compatible with a back-arc setting than than with an arc-trench environment.

GUIDE TO FIELD TRIP STOPS

The location of the field trip stops is shown on Figure 2. The field trip begins at the Radisson Duluth Hotel at Superior Street and 5th Avenue West in downtown Duluth, Minnesota. Head southwest on Superior Street and in two blocks join Interstate Highway 35 going south. After 14.6 miles on 1-35, take exit 242, turning south (left) on Carlton County 1. In 2.9 miles, the road crosses a spillway, turns west, and after 0.6 miles, reaches a T junction. Turn right (north) on Minnesota Highway 210, which then turns left (west) in less than 0.1 mile. In 0.2 miles, Minnesota Highway 210 crosses the St. Louis River. Pull off and park just before reaching the bridge, or cross it and park in the parking area on the south side of the road.

STOP 1 THOMSON DAM

This is the type locality of the Thomson Formation, which here consists of slates and metagraywackes. A variety of sedimentary structures may be seen, including ripple marks, load casts, convolute bedding, cross bedding, graded bedding, and sole marks. Folds with wavelengths from cm to km are present (Figure 3). They are upright to steeply inclined to the south and have subhorizontal fold axes that trend east-west. Where folds die out along trend, axes may plunge east or west as much as 60'. Two such steeply plunging folds may be seen in this area. A single axial-planar foliation is present; the foliation ranges from a continuous slaty cleavage in the fine-grained units to a disjunctive spaced cleavage in the more coarse graywacke beds. Abundant carbonate concretions and weathered concretion voids can be seen flattened in the plane of the cleavage. Strain determinations in this region using deformed concretions, mud chips, and a thin conglomerate bed to the north of the reservoir reveal strain ratios of about X:Y:Z = 7:4:1, with X vertical, Z horizontal and north-south (perpendicular to foliation) and Y horizontal and east-west. Quartz veins, from a few mm to more than a meter in width are present. Some exhibit ptygmatic folding. Basalt dikes of presumed Keweenawan age are also present, as are kink bands and several sets of joints. The kink bands dip gently and display a single maximum on an equal-area projection with an average strike of about N70E and a dip of about 20' to the south. Many of the kink bands intersect, but the angle of intersection varies randomly between 0 and 30'. No cross-cutting pattern exists within the kink bands (Clark, 1985).

I

4645'

4Tdd. ProtorozoicIgneous Rock.Mddl. Proterozotc5.dim.nt.Thomson FormationNorth.rn T.rran.Thomson FormgtonSouthern TsrronUnnomid UnTt ofMetassd.t,Iitovolcoriic,

Unnamed Pelitic Schst

Denhom FormationUpper MemberDenham FormationLower Member

Figure 2: Geologic map of the area of the field trip (after Southwick, Morey, andMcSwiggen, 1988), showing locations of field trip stops.

Figure 3: Anticline at Thomson Dam, view from the east bank of the St. Louis River, justsouth of the highway bridge, looking west. Drawing by Wendell Wilson.

B- 7

McGrath Gn.Tss

q/s o r Loka

Middle Prolerozotc Igneous Rocks Middle Prolerozofc Sediments Thornson Formotion Northam TÃ§rran

Unnomed Politic Schist

Denham formation Upper Member Denham Formollon Lower Member

Figure 2: Geologic map of the area of the field trip (after Southwick, Morey, and McSwiggen, 1988). showing locations of field trip stops.

A

Figure 3: Anticline at Thomson Dam, view from the east bank of the St. Louis River, just south of the highway bridge, looking west. Drawing by Wendell Wilson.

Driving Directions: Proceed west on Minnesota Highway 210 for about 0.8 mile to the townof Canton. Turn right (north) at the stop sign on to Old Highway 61. Proceed for oneblock and park in either the convenience store parking lot, or the large lot on the right sideof the road. Walk north to first road cut outcrop on west side of road.

STOP 2 CARLTON ROAD CUT

lnterbedded slates and graywackes of the Thomson Formation here dip to the south.A subvertical cleavage is present. A deformed load cast is present, flattened in thecleavage plane. Numerous deformed concretions with quite a range in size are present inboth slate and graywacke beds. The north-south vertical joint face is the approximate XZplane of the strain ellipsoid in the northern structural terrane of the Thomson Formation.

Driving Directions: Reurn south one block and turn right (west) on Minnesota Highway 210.Proceed about 3 miles to the overpass intersection with Interstate Highway 35. Proceedunder the freeway and turn left on entrance ramp to 1-35 South. Proceed for approximately26 miles and take exit 209, Sturgeon Lake. Turn right and proceed into the town ofSturgeon Lake to the I junction. Turn left on Highway 61 and proceed to the first rightturn, which crosses the abandoned railroad track and then turns left to the center of town.In one block turn right (at the intersection with a bar and a bank on the corners) and followPine County 46 out of Sturgeon Lake. The road winds past an old school and heads westfor about a mile, then bends sharply to the right and heads north for a half mile, and thenbends sharply left and runs due west for a number of miles. About 3.2 miles out ofSturgeon Lake (or 1.7 miles west of the second sharp bend in the road) there is a bridgeover the Kettle River. Park and proceed to road cuts on the west side of the river.

STOP 3 KETTLE RIVER ROAD CUT

This bridge was constructed in the mid 1980s (the old bridge is in a farmer's fieldjust to the northeast) and a new road cut exposes metasedimentary rocks on both the northand south sides of the road. The rocks here have been mapped historically as ThomsonFormation, but are part of the "unnamed pelitic schist" unit (Pps) of Southwick, Morey andMcSwiggen (1988). They are part of the southern structural terrane of the rocks deformedduring the Penokean orogeny in east-central Minnesota, and are part of the Moose Lake -Glen Township structural panal of Southwick, Morey and McSwiggen (1988). The rockshere are of garnet metamorphic grade, and two foliations are observable. The earlyfoliation is subparallel to bedding, and the later crenulation cleavage is subvertical andtrends east-west. A lineation exists, parallel to the intersection of the two foliations. Recentthermobarometric work on samples from this outcrop (HoIm and Selverstone, 1989) yieldedfinal equilibration temperature estimates of 470-520°C and a pressure of around 6 kb.

Driving Directions: Continue west on Pine County 46 for about 2.8 miles. Turn left (south)on a dirt road. Follow this road south for 1.5 miles. Turn left (east) on another dirt road.After 1 mile, the "mainTM dirt road turns to the right (south), but continue straight ahead(east) on the "two-track" lane. In .3 or .4 mile there is a railroad crossing. Park near, butnot on the railroad tracks, and walk along the tracks to the northeast. There is a longrailroad cut about .3 mile northeast of the road/railroad intersection.

8-8

Driving Directions: Proceed west on Minnesota Highway 210 for about 0.8 mile to the town of Carlton. Turn right (north) at the stop sign on to Old Highway 61. Proceed for one block and park in either the convenience store parking lot, or the large lot on the right side of the road. Walk north to first road cut outcrop on west side of road.

STOP 2 CARLTON ROAD CUT

Interbedded slates and graywackes of the Thomson Formation here dip to the south. A subvertical cleavage is present. A deformed load cast is present, flattened in the cleavage plane. Numerous deformed concretions with quite a range in size are present in both slate and graywacke beds. The north-south vertical joint face is the approximate XZ plane of the strain ellipsoid in the northern structural terrane of the Thomson Formation.

Driving Directions: Reurn south one block and turn right (west) on Minnesota Highway 210. Proceed about 3 miles to the overpass intersection with Interstate Highway 35. Proceed under the freeway and turn left on entrance ramp to 1-35 South. Proceed for approximately 26 miles and take exit 209, Sturgeon Lake. Turn right and proceed into the town of Sturgeon Lake to the T junction. Turn left on Highway 61 and proceed to the first right turn, which crosses the abandoned railroad track and then turns left to the center of town. In one block turn right (at the intersection with a bar and a bank on the corners) and follow Pine County 46 out of Sturgeon Lake. The road winds past an old school and heads west for about a mile, then bends sharply to the right and heads north for a half mile, and then bends sharply left and runs due west for a number of miles. About 3.2 miles out of Sturgeon Lake (or 1.7 miles west of the second sharp bend in the road) there is a bridge over the Kettle River. Park and proceed to road cuts on the west side of the river.

STOP 3 KETTLE RIVER ROAD CUT

This bridge was constructed in the mid 1980s (the old bridge is in a farmer's field just to the northeast) and a new road cut exposes metasedimentary rocks on both the north and south sides of the road. The rocks here have been mapped historically as Thomson Formation, but are part of the "unnamed pelitic schist" unit (Pps) of Southwick, Morey and McSwiggen (1988). They are part of the southern structural terrane of the rocks deformed during the Penokean orogeny in east-central Minnesota, and are part of the Moose Lake - Glen Township structural panal of Southwick, Morey and McSwiggen (1988). The rocks here are of garnet metamorphic grade, and two foliations are observable. The early foliation is subparallel to bedding, and the later crenulation cleavage is subvertical and trends east-west. A lineation exists, parallel to the intersection of the two foliations. Recent thermobarometric work on samples from this outcrop (Holm and Selverstone, 1989) yielded final equilibration temperature estimates of 470-520Â° and a pressure of around 6 kb.

Driving Directions: Continue west on Pine County 46 for about 2.8 miles. Turn left (south) on a dirt road. Follow this road south for 1.5 miles. Turn left (east) on another dirt road. After 1 mile, the "main" dirt road turns to the right (south), but continue straight ahead (east) on the "two-track" lane. In .3 or .4 mile there is a railroad crossing. Park near, but not on the railroad tracks, and walk along the tracks to the northeast. There is a long railroad cut about .3 mile northeast of the road/railroad intersection.

STOP 4 DENHAM CEMETERY RAILROAD CUT

Rocks exposed along this cut and another similar cut one-halt mile to the southwesthave historically been called the southernmost exposures of Thomson Formation. As at thelast stop these rocks have recently been designated part of an unnamed peletic schist unit(Pps), part of the Moose Lake - Glen Township structural panal by Southwick, Morey, andMcSwiggen (1988). A number of F2 folds are exposed in this cut. Fold axial surfaces arevertical and east-west. Fold axes are subhorizontal. The S, foliation which is subparallel tobedding and the S2 crenulation cleavage (axial-planar to the F2 folds) are most easily seenin the more fine-grained schistose units. Some of the metagraywacke units are nearlyquartzites here. Again there is a east-west subhorizontai lineation. There are quite anumber of early quartz veins which exhibit boudinage. The rocks here are staurolite gradeand garnets are visible in many samples. Based on petrographic study, Hoim, Hoist andEllis (1988) suggested that the garnets were synkinematic with the early foliation during aprogressive metamorphic event, and that the thermal peak of metamorphism occurred afterthe later phase of deformation, as indicated by staurolite porphyroblasts which overprint boththe S1 and S2 foliations. Thermobarometric analysis of samples from this exposurecontaining the assemblage staurolite÷garnet÷plagioclase÷chlorite+ muscovite+biotite+quartzhave yielded final equilibration temperatures of 520-590°C and pressures of about 7 kb(Holm and Selverstone, 1989).

Driving Directions: Return over the same route to the town of Sturgeon Lake. Aftercrossing the abandoned railroad grade in the middle of town turn left (northeast) onHighway 61. Follow Highway 61 northeast for about 5.2 miles. Turn left at the intersectionand follow Highway 61 into Moose Lake. In about one-half mile turn left (west) at the stoplight in the center of town and follow Highway 27 about one-quarter mile up the hill to therailroad crossing. Park in the lot at the old train station on the west side of the road, andthe south side of the railroad tracks. Walk along the tracks to the northeast to somerailroad cut exposures.

STOP 5 MOOSE LAKE RAILROAD CUT

These exposures of metasedimentary rocks are also part of the unnamed peliticschist unit (Pps) of the Moose Lake - Glen Township structural panal of Southwick, Morey,and McSwiggen (1988), and have historically been mapped as Thomson Formation. In thelast decade this cut has become distinctly more overgrown, and it may be necessary todisplace some vegetation to see some of the structural features here. The S1

bedding-parallel foliation is well developed in the rocks along this cut. Early quartz veinsshow boudinage features documenting finite entensional strain within the plane of the earlyS1 foliation. These features can be seen on outcrop faces in several orientations showingthat the extension within the plane of this foliation occurred in all directions. A minerallineation defined by muscovite streaks can be seen on the north side of the tracks at one ofthe first exposures as you walk in along the tracks. Several F2 folds of various scales arepresent. At least one of these folds has an axial plane which dips about 60° to the south,and the S2 crenulation cleavage is axial planar to the fold. In some exposures along thiscut there are two sets of crenulation cleavages, both of which are vertical. A (presumed)Keweenawan basalt dike, vertical and trending approximately N8OE is exposed in this cut.

Driving Directions: Continue west on Highway 27 for 4.0 miles. Turn right (north) off theroad into the farm driveway and park. After asking permission at the house, walk northfrom the house, just east of a line of trees along the edge of the farmer's field to the banksof Glaisby Brook.

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/

STOP 4 DENHAM CEMETERY RAILROAD CUT

Rocks exposed along this cut and another similar cut one-half mile to the southwest have historically been called the southernmost exposures of Thomson Formation. As at the last stop these rocks have recently been designated part of an unnamed peletic schist unit (Pps), part of the Moose Lake - Glen Township structural panal by Southwick, Morey, and McSwiggen (1988). A number of F2 folds are exposed in this cut. Fold axial surfaces are vertical and east-west. Fold axes are subhorizontal. The Sl foliation which is subparallel to bedding and the S2 crenulation cleavage (axial-planar to the F2 folds) are most easily seen in the more fine-grained schistose units. Some of the metagrayacke units are nearly quartzites here. Again there is a east-west subhorizontal lineation. There are quite a number of early quartz veins which exhibit boudinage. The rocks here are staurolite grade and garnets are visible in many samples. Based on petrographic study, Holm, Holst and Ellis (1988) suggested that the garnets were synkinematic with the early foliation during a progressive metamorphic event, and that the thermal peak of metamorphism occurred after the later phase of deformation, as indicated by staurolite porphyroblasts which overprint both the S, and S2 foliations. Thermobarometric analysis of samples from this exposure containing the assemblage staurolite+garnet+plagioclase+chlorite+ muscovite+biotite+quartz have yielded final equilibration temperatures of 520-590Â° and pressures of about 7 kb (Holm and Selverstone, 1989).

Driving Directions: Return over the same route to the town of Sturgeon Lake. After crossing the abandoned railroad grade in the middle of town turn left (northeast) on Highway 61. Follow Highway 61 northeast for about 5.2 miles. Turn left at the intersection and follow Highway 61 into Moose Lake. In about one-half mile turn left (west) at the stop light in the center of town and follow Highway 27 about one-quarter mile up the hill to the railroad crossing. Park in the lot at the old train station on the west side of the road, and the south side of the railroad tracks. Walk along the tracks to the northeast to some railroad cut exposures.

STOP 5 MOOSE LAKE RAILROAD CUT

These exposures of metasedimentary rocks are also part of the unnamed pelitic schist unit (Pps) of the Moose Lake - Glen Township structural panal of Southwick, Morey, and McSwiggen (1988), and have historically been mapped as Thomson Formation. In the last decade this cut has become distinctly more overgrown, and it may be necessary to displace some vegetation to see some of the structural features here. The Sl bedding-parallel foliation is well developed in the rocks along this cut. Early quartz veins show boudinage features documenting finite entensional strain within the plane of the early S, foliation. These features can be seen on outcrop faces in several orientations showing that the extension within the plane of this foliation occurred in all directions. A mineral lineation defined by muscovite streaks can be seen on the north side of the tracks at one of the first exposures as you walk in along the tracks. Several F2 folds of various scales are present. At least one of these folds has an axial plane which dips about 60' to the south, and the S2 crenulation cleavage is axial planar to the fold. In some exposures along this cut there are two sets of crenulation cleavages, both of which are vertical. A (presumed) Keweenawan basalt dike, vertical and trending approximately N8OE is exposed in this cut.

Driving Directions: Continue west on Highway 27 for 4.0 miles. Turn right (north) off the road into the farm driveway and park. After asking permission at the house, walk north from the house, just east of a line of trees along the edge of the farmer's field to the banks of Glaisby Brook.

STOP 6 (ALTERNATE STOP) GLAISBY BROOK

Rocks of these exposures along Glaisby Brook have also been mapped historicallyas Thomson Formation. They are part of a unnamed unit of metasedimentary andmetavolcanic rocks (Pgvi) of Southwick, Morey, and McSwiggen (1988), but are still part oftheir Moose Lake - Glen Township structural panal. Although the exposure is overgrownand lichen-covered, it is possible to see an isoclinal recumbent fold in this outcrop, withseveral minor folds of normal vergence relationship (Figure 4). This is the largest exampleof an F, fold found to date in this region.

Driving Directions: Return to the town of Moose Lake following Highway 27 back to theeast for just over 4 miles. Turn left (northeast) at the stop light, and follow Highway 61 tothe northeast (you need to turn left off the "maine road that goes to the freeway). FollowHighway 61 to the northeast passing through the town of Barnum in just over 4 miles.Continue for another 6 miles on Highway 61 to the village of Mahtowa. Turn left(northwest) on Canton County 4 at Mahtowa. The road turns north after 0.6 miles, andthen west after another 0.9 miles. Just after the turn to the west, turn right (north) ontoCarlton County 7 toward Park Lake. Follow Canton County 7 for 1.7 miles and Park.Proceed west and slightly south into the woods to a series of linear east-west outcropridges.

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Figure 4: Sketch of F, fold at the Glaisby Brook outcrop about 4 miles west of MooseLake.

STOP 6 (ALTERNATE STOP) GLAISBY BROOK

Rocks of these exposures along Glaisby Brook have also been mapped historically as Thomson Formation. They are part of a unnamed unit of metasedimentary and metavolcanic rocks (Pgvi) of Southwick, Morey, and McSwiggen (1988), but are still part of their Moose Lake - Glen Township structural panal. Although the exposure is overgrown and lichen-covered, it is possible to see an isoclinal recumbent fold in this outcrop, with several minor folds of normal vergence relationship (Figure 4). This is the largest example of an Fl fold found to date in this region.

Figure 4: Sketch of Fl fold at the Glaisby Brook outcrop about 4 miles west of Moose Lake.

Driving Directions: Return to the town of Moose Lake following Highway 27 back to the east for just over 4 miles. Turn left (northeast) at the stop light, and follow Highway 61 to the northeast (you need to turn left off the "mainn road that goes to the freeway). Follow Highway 61 to the northeast passing through the town of Barnum in just over 4 miles. Continue for another 6 miles on Highway 61 to the village of Mahtowa. Turn left (northwest) on Carlton County 4 at Mahtowa. The road turns north after 0.6 miles, and then west after another 0.9 miles. Just after the turn to the west, turn right (north) onto Carlton County 7 toward Park Lake. Follow Carlton County 7 for 1.7 miles and Park. Proceed west and slightly south into the woods to a series of linear east-west outcrop ridges.

STOP 7 PARK LAKE

Rocks at this locality are part of the Thomson Formation, and contain two distinctfoliations. This region has been mapped as Pvt2 by Southwick, Morey, and Mcswiggen(1988): Thomson Formation with two foliations. The dominant foliation here is a cleavagethat is parallel to bedding (S1). The bedding (S0) and cleavage (S1) havebeen folded into open, upright subhorizontal folds (F2) that trend east-west. A crenulationcleavage (S2) is axial planar to these folds. In this area the crenulation cleavage (S2) ispresent in most outcrops except for some of the graywacke units. The bedding-parallelfoliation (S1) is present everywhere except for a very few coarse graywacke beds. At theextreme southwest part of this exposure south of Park Lake, small scale isoclinal,recumbent folds (F1) are present, to which the bedding- parallel foliation is axial planar(Figure 5). The S, foliation is at high angles to bedding only in the (rare) hinges of theseF, folds. Some of the F1 folds have been refolded by F2 folds resulting in a Ramsay type 3interference pattern.

Figure 5: Sketch of a small fold at stop 7, Park Lake. Bedding (S0) shown by the silt bedis folded into an isoclinal recumbent fold (F1) with axial planar slaty cleavage (S1) parallelwith bedding on the limbs of the fold. Bedding and the S, foliation are folded by laterupright folds (F2) with the development of type 3 interference patterns (Ramsay, 1967) anda vertical crenulation cleavage (S2).

Driving Directions: Return south by the same route (Carlton County 7 and 4) to the villageof Mahtowa. Turn left (northeast) on Highway 61. Proceed northeast on Highway 61,passing through the village of Atkinson, and proceed under the freeway bridge 8.1 milespast Mahtowa. At the intersection which is 1.05 miles past the freeway bridge, turn right(south) and proceed 0.7 miles south on Gillogly Road to an outcrop on the left (east) sideof the road, at the top of a hill.

B-il

N S

1lcm

,

STOP 7 PARK LAKE

Rocks at this locality are part of the Thomson Formationl and contain two distinct foliations. This region has been mapped as Pv& by Southwick, Morey, and Mcswiggen (1988): Thomson Formation with two foliations. The dominant foliation here is a cleavage that is parallel to bedding (Sl). The bedding (So) and cleavage (St) have been folded into open, upright subhorizontal folds (FJ that trend east-west. A crenulation cleavage (S2) is axial planar to these folds. In this area the crenulation cleavage (SJ is present in most outcrops except for some of the graywacke units. The bedding-parallel foliation (S,) is present everywhere except for a very few coarse graywacke beds. At the extreme southwest part of this exposure south of Park Lakel small scale isoclinal, recumbent folds (Fl) are present, to which the bedding- parallel foliation is axial planar (Figure 5). The S, foliation is at high angles to bedding only in the (rare) hinges of these F, folds. Some of the Fl folds have been refolded by F2 folds resulting in a Ramsay type 3 interference pattern.

Figure 5: Sketch of a small fold at stop 7, Park Lake. Bedding (So) shown by the silt bed is folded into an isoclinal recumbent fold (Fl) with axial planar slaty cleavage (Sl) parallel with bedding on the limbs of the fold. Bedding and the S1 foliation are folded by later upright folds (F2) with the development of type 3 interference patterns (Ramsay, 1967) and a vertical crenulation cleavage (SJ.

Driving Directions: Return south by the same route (Carlton County 7 and 4) to the village of Mahtowa. Turn left (northeast) on Highway 61. Proceed northeast on Highway 61, passing through the village of Atkinsonl and proceed under the freeway bridge 8.1 miles past Mahtowa. At the intersection which is 1.05 rniles past the freeway bridge, turn right (south) and proceed 0.7 miles south on Gillogly Road to an outcrop on the left (east) side of the roadl at the top of a hill.

STOP 8 GILLOGLY ROAD

At earlier stops on this field trip we have seen evidence for two periods of foldingand foliation development in the southern structural terrane, and one main period of foldingand foliation in the northern terrane. Hoist (1984) argued that the early period of folding inthe southern terrane also involved the emplacement of nappes. The evidence he cited fornorthward-directed nappes in the southern terrane included lithologic differences betweenthe two terranes (also see Morey and Southwick, 1984, and Southwick, Morey andMcswiggen, 1988), and the pervasive nature of the S1 foliation in the area of its occurrence.Facing directions of F2 folds (Figure 6) also indicate that a very large area of the southernterrane is on the upper limb of an F, fold. The refraction pattern of the St foliation patternin the region of this outcrop also suggests the existence ot northward-directed nappes in thesouthern terrane, as explained below.

Several graded beds in the Thomson Formation can be seen at this locality strikingeast-west and dipping to the north. In the finer grained tops of these beds, a cleavage (S1)can be observed. It is very gently folded, with horizontal axes trending east-west. lithecleavage is traced toward the bottom of a bed, it is seen to change its orientation markedly,and it becomes a spaced cleavage, dipping moderately to the south in the bottom part ofthe bed. A crenulation cleavage (S2), vertical or very steeply dipping to the south, can beseen in the upper portion of the beds. Mud chips that are flattened in the early S, foliationare present. A line drawing of these relationships is shown in Figure 7. To interpret thepattern of this outcrop, it is useful to determine the orientation of S0 and S1 after the earlyphase of deformation, but prior to the later phase of deformation, which produced F2 folds,the S2 crenulation cleavage, and (very importantly for our puposes here) deformed the earlyS1 cleavage, changing the geometry of bedding/cleavage vergence patterns. This can bedone, at least to a fairly good approximation because we have strain data for both thesouthern and northern structural terranes in the region (Hoist, 1985a). Results of straindeterminations in the southern terrane give the finite strain which resulted from thesuperposition of the strain of the second deformation upon the strain of the firstdeformation. As the only deformational structures present in the northern terrane are thoseof the second deformation, the strain measured there represents the strain associated withthe second deformation. By removing that strain from the finite strain measured in thesouth, we can approximate the strain associated with the first deformation in the southernterrane (Hoist, 1985a; Hoim, HoIst, and Ellis, 1988). This is illustrated in Figure 8. Byremoving the second deformation strain from this outcrop we can reconstruct thebedding(S0) /cteavage(S1) geometry prior to the second deformation. The technique isillustrated in Figure 9. The graded sequence is generalized to alternating coarse and finelayers, with the attitude of bedding and the S1 cleavage (in the finest and coarsest parts ofthe beds) as seen now in outcrop (Figure 9A). Removal of the strain associated with thesecond deformation allows a reconstruction of the geometrical relationships of thesefeatures prior to the second deformation (Figure 9B). The resulting bedding/cleavagevergence pattern (cleavage refraction pattern) is that to be expected on the upper limb andnear the hinge of a large-scale isoclinal, recumbent fold (Figure 10). In the entire region ofthe the southern terrane to the south, the S1 cleavage is seen to be subparallel to thebedding. Significantly that is not true here, and in the outcrop 1 km to the north, along anabandoned railroad near Highway 61, the cleavage/bedding vergence pattern is the sameas seen in this outcrop, although there are no graded beds there. Further, these twooutcrops are very close to the boundary between the northern and southern structuralterranes.

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STOP 8 GILLOGLY ROAD

At earlier stops on this field trip we have seen evidence for two periods of folding and foliation development in the southern structural terranel and one main period of folding and foliation in the northern terrane. Holst (1984) argued that the early period of folding in the southern terrane also involved the emplacement of nappes. The evidence he cited for northward-directed nappes in the southern terrane included lithologic differences between the two terranes (also see Morey and Southwick, 1984, and Southwick, Morey and Mcswiggen, 1988), and the pervasive nature of the Sl foliation in the area of its occurrence. Facing directions of F2 folds (Figure 6) also indicate that a very large area of the southern terrane is on the upper limb of an F, fold. The refraction pattern of the Sl foliation pattern in the region of this outcrop also suggests the existence of northward-directed nappes in the southern terrane, as explained below.

Several graded beds in the Thomson Formation can be seen at this locality striking east-west and dipping to the north. In the finer grained tops of these beds, a cleavage (St) can be observed. It is very gently folded, with horizontal axes trending east-west. If the cleavage is traced toward the bottom of a bedl it is seen to change its orientation markedlyl and it becomes a spaced cleavage, dipping moderately to the south in the bottom part of the bed. A crenulation cleavage (SJ, vertical or very steeply dipping to the south, can be seen in the upper portion of the beds. Mud chips that are flattened in the early Sl foliation are present. A line drawing of these relationships is shown in Figure 7. To interpret the pattern of this outcrop, it is useful to determine the orientation of So and Sl after the early phase of deformation, but prior to the later phase of deformation, which produced F2 folds, the S2 crenulation cleavagel and (very importantly for our puposes here) deformed the early S, cleavage, changing the geometry of beddinglcleavage vergence patterns. This can be done, at least to a fairly good approximation because we have strain data for both the southern and northern structural terranes in the region (Holst, 1985a). Results of strain determinations in the southern terrane give the finite strain which resulted from the superposition of the strain of the second deformation upon the strain of the first deformation. As the only deformational structures present in the northern terrane are those of the second deformation, the strain measured there represents the strain associated with the second deformation. By removing that strain from the finite strain measured in the south, we can approximate the strain associated with the first deformation in the southern terrane (Holst, 1985a; Holm, Holst, and Ellis, 1988). This is illustrated in Figure 8. By removing the second deformation strain from this outcrop we can reconstruct the bedding(So) /cleavage(S,) geometry prior to the second deformation. The technique is illustrated in Figure 9. The graded sequence is generalized to alternating coarse and fine layers, with the attitude of bedding and the Sl cleavage (in the finest and coarsest parts of the beds) as seen now in outcrop (Figure 9A). Removal of the strain associated with the second deformation allows a reconstruction of the geometrical relationships of these features prior to the second deformation (Figure 98). The resulting beddinglcleavage vergence pattern (cleavage refraction pattern) is that to be expected on the upper limb and near the hinge of a large-scale isoclinal, recumbent fold (Figure 10). In the entire region of the the southern terrane to the south, the S, cleavage is seen to be subparallel to the bedding. Significantly that is not true here, and in the outcrop 1 km to the north, along an abandoned railroad near Highway 61, the cleavagebedding vergence pattern is the same as seen in this outcrop, although there are no graded beds there. Further, these two outcrops are very close to the boundary between the northern and southern structural terranes.

Figure 6: Illustration ofaxial plane of F1 fold.cleavage. Small arrowsF2 folds.

Figure 7:crenulation

facing direction of F2 folds (after Borradaile, 1976). Dashed line isFoliation shown diagrammatically is S2 axial planar crenulation

indicate stratigraphic tops, large arrows indicate facing directions of

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"so

Si

I

Line drawing of geometrical relations of bedding (S0), early foliation (S1), andcleavage (S2) at Gillogly Road outcrop (Stop 8). Hammer handle is 40 cm long.

Figure 6: Illustration of facing direction of F2 folds (after Borradaile, 1976). Dashed line is axial plane of F, fold. Foliation shown diagrammatically is S2 axial planar crenulation cleavage. Small arrows indicate stratigraphic topsl large arrows indicate facing directions of F2 folds.

Figure 7: crenulation

Line drawing of geometrical relations of beddin; (So), early foliation cleavage (SJ at Gillogly Road outcrop (Stop 8). Hammer handle is 40

(SJ1 and cm long.

N X:Y:Z t5 1:02

'IINorIh. Aria

S.cond Dito,maion Str.àn

pj X:YZ:4.2t0.3

SOuIh.rn AriaTolal Strain (both diformailons)

N YZ:t2:1:

23.1t

I, t

NZl0

LxiIod.i H

Inh.r.d Strain 01 FIrst Oslorinatlon

Figure 8: Representative block diagrams of the state of finite strain in the northern andsouthern structural terranes, and the strain inferred to have been associated with the firstdeformation.

Figure 9: Bedding and cleavage at the Gillogly Road outcrop (Stop 8) (generalized toalternating coarse and fine layers instead of graded beds) showing the present geometry (A)and geometry after removal of the strain associated with the second phase of seformation(B).

Figure 10: Position on early large-scalerefraction pattern shown in Figure 9B.

isoclinal recumbent fold (nappe) of cleavage

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Northern A r m

Second Oatormallon Strain Soulharn A r c

Total Strain (both d ~ f O r m ~ l l O n a l

Modal I Modal K

Inlurad Strain of FIfl DaforiMllon

Figure 8: Representative block diagrams of the state of finite strain in the northern and southern structural terranes, and the strain inferred to have been associated with the first deformation.

Figure 9: Bedding and cleavage at the Gillogly Road alternating coarse and fine layers instead of graded beds) and geometry after removal of the strain associated with (6).

outcrop (Stop 8) (generalized to showing the present geometry (A) the second phase of seformation

Figure 10: Position on early large-scale isoclinal recumbent fold (nappe) of cleavage refraction pattern shown in Figure 9B.

Driving Directions: Return north on Gillogly Road 0.7 miles to Highway 61 and turn right(northeast) on Highway 61. Proceed for 1 mile to a road cut with outcrops on both side ofthe road.

STOP 9 HIGHWAY 61 ROAD CUT

The structural features seen in these exposures of the Thomson Formation are thoseseen at the type locality, Thomson Dam (Stop 1). Several upright, subhorizontal folds trendeast-west. An axial planar cleavage is present, vertical or very steeply dipping to the south.Abundant mud chips are flattened in the plane of the cleavage. A faint vertical lineation Isalso present. The mud chips have been used for finite strain estimates. Some examplesof cleavage refraction are present, and they show normal bedding/cleavage vergencerelationships to the upright folds. No evidence has been found at this locality for the earlyfoliation and recumbent folding seen in the southern structural terrane. At this point theboundary between the two structural terranes seems to be rather sharp.

Driving Directions: Proceed northeast along Highway 61 for 1 .9 miles to an intersectionwith Minnesota Highway 210. Turn left (west) on Highway 210 and proceed for 0.1 milesand turn right on the entrance ramp to the freeway, 1-35 North. Return to Duluth onInterstate 35.

REFERENCES CITED

Borradaile, G.,J., 1976, "Structural facing" (Shakleton's rule) and the Paleozoic rocks of theMalaguide Complex near Velez Rubio, SE Spain: Koninklijk Nederlandse Akademievan Wetenschappen, ser. B, v. 79, p. 330-336

Brocoum, S. J., and Dalziel, I. W. D., 1974, The Sudbury Basin, the southern province, theGrenville Front, and the Penokean orogeny: Geological Society of America Bulletin,v. 85, p. 1571-1580.

Cambray, F. W., 1977, Field guide to the Marquette district, Michigan: Michigan BasinGeological Society Annual Meeting, 62 p.

________

1978, Plate Tectonics as a model for the environment of deposition anddeformation of the Early Proterozoic (Precambrian X) of northern Michigan:Geological Society of America Abstracts with Programs, v. 10, p. 376.

Cannon, W. F., 1973, The Penokean orogeny in northern Michigan, Young, G. M., ed.,Huronian stratigraphy and sedimentation: Geological Association of Canada SpecialPaper, v. 12, p. 211-249.

Clark, R.G., 1985, The structural geology of the Thomson Formation; Cloquet and Eskoquadrangles, east-central Minnesota: unpublished MS thesis, University of MinnesotaDuluth, Duluth, Minnesota 114 p.

Dixon, J. M., 1975, Finite strain and progressive deformation in models of diapiricstructures: Tectonophysics, v. 28, p. 89-124.

Gray, D.R., 1977, Morphologic classification of crenulation cleavages: Journal of Geology, v.85, p. 229-235.

HoIm, 0. K., 1986a, A structural investigation and tectonic interpretation of the Penokeanorogeny: east-central Minnesota, [M.S. Thesis]: University of Minnesota Duluth, 114p.

_______

1986b, An account of multiphase deformation during the Penokean orogeny:evidence from analysis of the Archean McGrath Gneiss and overlying EarlyProterozoic metasedimentary rocks in east-central Minnesota: Institute on LakeSuperior Geology Abstracts, v. 32, p. 32-33.

B-15

Driving Directions: Return north on Gillogly Road 0.7 miles to Highway 61 and turn right (northeast) on Highway 61. Proceed for 1 mile to a road cut with outcrops on both side of the road.

STOP 9 HIGHWAY 61 ROAD CUT

The structural features seen in these exposures of the Thomson Formation are those seen at the type locality, Thomson Dam (Stop 1). Several upright, subhorizontal folds trend east-west. An axial planar cleavage is present, vertical or very steeply dipping to the south. Abundant mud chips are flattened in the plane of the cleavage. A faint vertical lineation is also present. The mud chips have been used for finite strain estimates. Some examples of cleavage refraction are present, and they show normal beddinglcleavage vergence relationships to the upright folds. No evidence has been found at this locality for the early foliation and recumbent folding seen in the southern structural terrane. At this point the boundary between the two structural terranes seems to be rather sharp.

Driving Directions: Proceed northeast along Highway 61 for 1.9 miles to an intersection with Minnesota Highway 210. Turn left (west) on Highway 210 and proceed for 0.1 miles and turn right on the entrance ramp to the freeway, 1-35 North. Return to Duluth on Interstate 35.

REFERENCES CITED

Borradaile, G.,J., 1976, "Structural facing" (Shakleton's rule) and the Paleozoic rocks of the Malaguide Complex near Velez Rubio, SE Spain: Koninklijk Nederlandse Akademie van Wetenschappen, ser. B, v. 79, p. 330-336

Brocoum, S. J., and Dalziel, 1. W. D., 1974, The Sudbury Basin, the southern province, the Grenville Front, and the Penokean orogeny: Geological Society of America Bulletin, V. 85, p. 1571-1580.

Cambray, F. W., 1977, Field guide to the Marquette district, Michigan: Michigan Basin Geological Society Annual Meeting, 62 p. , 1978, Plate Tectonics as a model for the environment of deposition and

, deformation of the Early Proterozoic (Precambrian X) of northern Michigan: Geological Society of America Abstracts with Programs, v. 10, p. 376.

I Cannon, W. F., 1973, The Penokean orogeny in northern Michigan, Young, G. M., a., Huronian stratigraphy and sedimentation: Geological Association of Canada Special Paper, v. 12, p. 21 1-249.

I Clark, R.G., 1985, The structural geology of the Thomson Formation; Cloquet and Esko quadrangles, east-central Minnesota: unpublished MS thesis, University of Minnesota Duluth, Duluth, Minnesota 114 p.

Dixon, J. M., 1975, Finite strain and progressive deformation in models of diapiric

I structures: Tectonophysics, v. 28, p. 89-124. Gray, D.R., 1977, Morphologic classification of crenulation cleavages: Journal of Geology, v.

85, p. 229-235.

I Holm, D. K., 1986a, A structural investigation and tectonic interpretation of the Penokean orogeny: east-central Minnesota, [M.S. Thesis]: University of Minnesota Duluth, 114 P.

I , 1986b, An account of multiphase deformation during the Penokean orogeny:

evidence from analysis of the Archean McGrath Gneiss and overlying Early Proterozoic rnetasedimentary rocks in east-central Minnesota: Institute on Lake Superior Geology Abstracts, v. 32, p. 32-33.

I

HoIm, D.K, Hoist, T.B., and Ellis, M., 1988, Oblique subduction, footwall deformation, andimbrication: A model for the Penokean orogeny in east-central Minnesota:Geological Society of America Bulletin, v. 100, P. 1811-1818.

Holm, O.K., and Selverstone, J., 1989, Preliminary constraints on the P-T evolution of thePenokean orogeny: east-central Minnesota: Institute on Lake Superior GeologyAbstracts and Proceedings, v. 35 (this volume).

Hoist, T. B., 1982, Evidence for multiple deformation during the Penokean orogeny in theMiddle Precambrian Thomson Formation, Minnesota: Canadian Journal of EarthSciences, v. 19, p. 2043-2047.

_______

1984a, Penokean tectonics: constraints from the structural geology in east-centralMinnesota: Institute on Lake Superior Geology Abstracts, v. 30, p. 19.

_______

1984b, The Early Proterozoic Penokean orogeny as a convergent plate boundary:Geological Association of Canada Program with Abstracts, v. 9, p. 75.

________

1 984c, Evidence for nappe development during the early Proterozoic Penokeanorogeny, Minnesota: Geology, v. 12, p. 135-138.

_______

1985a, Implications of a large flattening strain for origin of a bedding-parallelfoliation in the Early Proterozoic Thomson Formation, Minnesota: Journal ofStructural Geology, v. 7, p. 375-383.

________

1985b, Strain within early Proterozoic nappes in the Penokean orogenic belt,Minnesota: Geological Society of America Abstracts with Programs, v. 17, p. 293.

Horan, M.F., Hanson, G.N., and Spencer, K.J., 1987, Pb and Nd isotope and trace elementconstraints on the origin of basic rocks in an early Proterozoic igneous complex,Minnesota: Precambrian Research, v. 37, p. 323-342.

Keighin, C. W., Morey, G. B., and Goldich, S. S., 1972, East-central Minnesota i Sims, P.K., and Morey, G. B., eds., Geology of Minnesota: A centennial volume: MinnesotaGeological Survey, p. 240-255.

Kiasner, J.S., Ojakangas, R.W., Schulz, K.J., and LaBerge, G.L., 1988, Evidence fordevelopment of an early Proterozoic overthrust-nappe system in the PenokeanOrogen of northern Michigan: Institute on Lake Superior Geology Abstracts andProceedings, v. 34, p. 56-57.

LaBerge, G. L., Schulz, K. J., and Myers, P. E., 1984, The plate tectonic history ofnorth-central Wisconsin: Institute on Lake Superior Geology Abstracts, v. 30, p.25-27.

Larue, D. K., 1981, The Chocolay Group, Lake Superior region, USA: sedimentologicevidence for deposition in basinal and platform settings on an early Proterozoiccraton: Geological Society of America Bulletin Part I, v. 92, p. 417-435.

Maass, A. S., Medaris, L. G., Jr., and Van Schmus, W. R., 1980, Penokean deformation incentral Wisconsin, fr Morey, G. B., and Hanson, G. N., Selected studies of Archeangneisses and lower Proterozoic rocks, southern Canadian Shield: Geological Societyof America Special Paper 182, p. 147-157.

Morey, G.B., 1978, Lower and Middle Precambrian stratigraphic nomenclature foreast-central Minnesota: Minnesota Geological Survey Report of investigations 21, 52p.

_______

1979, Field trip guidebook for the Precambrain geology of east-central Minnesota:Minnesota Geological Survey, Guidebook Series No. 12, 28 p.

________

1983, Lower Proterozoic stratified rocks and the Penokean orogeny in east-centralMinnesota: Geological Society of America Memoir 160, p. 97-112.

Morey, G. B., and Sims, P. K., 1976, Boundary between two Precambrian W terranes inMinnesota and its geologic significance: Geological Society of America Bulletin, v.87, p. 141-152.

Morey, G. B., Sims, P. K., Cannon, W. F., Mudrey, M. G., Jr., and Southwick, 0. L., 1982,Geologic map of the Lake Superior Region, Minnesota, Wisconsin, and NorthernMichigan: Minnesota Geological Survey State Map Series S-13.

B-16

D.K, Holstl T.B.* and Ellis* M., 1988* Oblique subduction, footwall deformation, and imbrication: A model for the Penokean orogeny in east-central Minnesota: Geological Society of America Bulletinl v. 1 001 p. 181 1-1 81 8. D.K., and Selverstonel J.* 1989# Preliminary constraints on the P-T evolution of the Penokean orogeny: east-central Minnesota: lnstitute on Lake Superior Geology Abstracts and Proceedings, v. 35 (this volume). T. B.* 1982, Evidence for multiple deformation during the Penokean orogeny in the Middle Precambrian Thomson Formation* Minnesota: Canadian Journal of Earth Sciencesl v. 19, p. 2043-2047.

1984al Penokean tectonics: constraints from the structural geology in east-central Minnesota: lnstitute on Lake Superior Geology Abstracts* v. 30, p. 19.

1984b, The Early Proterozoic Penokean orogeny as a convergent plate boundary: Geological Association of Canada Program with Abstracts, v. gI p. 75. , 1 9 8 4 ~ ~ Evidence for nappe development during the early Proterozoic Penokean

orogenyl Minnesota: Geology, v. 12* p. 135-138. 1985a, Implications of a large flattening strain for origin of a bedding-parallel

foliation in the Early Proterozoic Thomson Formation* Minnesota: Journal of Structural Geology, v. 7* p. 375-383,

1985bl Strain within early Proterozoic nappes in the Penokean orogenic belt, Minnesota: Geological Society of America Abstracts with Programs, v. 17* p. 293.

Horan, M.F., Hanson, G.N., and Spencer, K.J., 1987, Pb and Nd isotope and trace element constraints on the origin of basic rocks in an early Proterozoic igneous complex, Minnesota: Precambrian Research, v. 37, p. 323-342.

Keighin, C. W., Morey* G. ELl and Goldich* S. S., 1972, East-central Minnesota @ Sims, P. K., and Morey, G. B., Geology of Minnesota: A centennial volume: Minnesota Geological Survey, p. 240-255.

Klasner* J.S., Ojakangas* R.W.* Schulz, K.J., and LaBerge, G.L., 1988, Evidence for development of an early Proterozoic overthrust-nappe system in the Penokean Orogen of northern Michigan: Institute on Lake Superior Geology Abstracts and Proceedingsl v. 34* p. 56-57.

LaBerge, G. L., SchuIzl K. J., and Myers, P. E.* 1984, The plate tectonic history of north-central Wisconsin: lnstitute on Lake Superior Geology Abstracts, v. 30, p. 25-27.

Larue, D. K.# 1981, The Chocolay Groupl Lake Superior region, USA: sedimentologic evidence for deposition in basinal and platform settings on an early Proterozoic craton: Geological Society of America Bulletin Part I, v. 92, p. 417-435.

Maass* R. S.* Medaris, L. G., Jr., and Van Schmus, W. R.* 1980, Penokean deformation in central Wisconsinl Morey, G. B.* and Hanson, G. N.# Selected studies of Archean gneisses and lower Proterozoic rocks, southern Canadian Shield: Geological Society of America Special Paper 182. p. 147-1 57.

Morey, G.B., 1978* Lower and Middle Precambrian stratigraphic nomenclature for east-central Minnesota: Minnesota Geological Survey Report of Investigations 21, 52 P a

, 1979* Field trip guidebook for the Precambrain geology of east-central Minnesota: Minnesota Geological Surveyl Guidebook Series No. 12* 28 p.

, 1983, Lower Proterozoic stratified rocks and the Penokean orogeny in east-central Minnesota: Geological Society of America Memoir 160, p. 97-1 12.

Morey, G. B.* and Sims* P. K., 1976* Boundary between two Precambrian W terranes in Minnesota and its geologic significance: Geological Society of America Bulletin, v. 87# p. 141-152.

Morey, G. BSl Sims, P. K., Cannon* W, F., Mudrey, M. G,, Jr., and Southwickl 0. L., 1982* Geologic map of the Lake Superior Region* Minnesota, Wisconsin, and Northern Michigan: Minnesota Geological Survey State Map Series S-13.

Morey, G. B., and Southwick, D. K., 1984, Early Proterozoic geology of east-central- Minnesota - a review and reappraisal: Institute on Lake Superior Geology Abstracts,

v. 30, p. 35-36.Ojakangas, R. W., 1983, Tidal deposits in the Early Proterozoic basin in the Lake Superior

region - The Palms and Pokegama Formations: evidence for subtidal-sheifdeposition of Superior-type banded iron formations: Geological Society of AmericaMemoir 160, p. 49-66.

Powell, C. McA., 1979, A morphological classification of rock cleavage: Tectonophysics, v.58, P. 21-34.

Ramsay, J. G., 1967, Folding and Fracturing of Rocks: McGraw Hill, New York, 568 p.Schulz, K.J., 1983, Geochemistry of the volcanic rocks of northeastern Wisconsin: Institute

on Lake Superior Geology Abstracts and Proceedings, v. 29, p.39.Schulz, K. J., LaBerge, G. L., Sims, P. K., Peterman, Z. E., and Klasner, J. S., 1984, The

volcanic-plutonic terrane of northern Wisconsin: implications for early Proterozoictectonism, Lake Superior region: Geological Association of Canada Program withAbstracts, v. 9, p. 103.

Simpson, C., and Schmid, S. M., 1983, An evaluation of criteria to deduce the sense ofmovement in sheared rocks: Geological Society America Bulletin, v. 94, p.1281-1288.

Sims, P. K., 1976, Precambrian tectonics and mineral deposits, Lake Superior region:Economic Geology, v. 71, p. 1092-1118.

Sims, P. K., Card, K. 0., Morey, G. B., and Peterman, Z. E., 1980, The Great Lakestectonic zone - A major crustal structure in central North America: GeologicalSociety of America Bulletin, Pt. 1, v. 91, p. 690-698.

Sims, P. K., Peterman, Z. E., and Schulz, K. J., 1985, The Dunbar Gneiss-granitoid dome:Implications for early Proterozoic tectonic evolution of northern Wisconsin:Geological Society of America Bulletin, v. 96, p. 1101-1112.

Sims, P. K., Peterman, Z. E., Klasner, J. S., Cannon, W. F., and Schulz, K. J., 1987,Nappe development and thrust faulting in the upper Michigan segment of the earlyProterozoic Penokean orogen: Geological Society of America Abstracts withPrograms, v. 19, p. 246.

Southwick, D.L., and Morey, G.B., 1988, Tectonic imbrication and foredeep development inthe Penokean Orogen, east-central Minnesota: Institute on Lake Superior Geology,v. 34, p. 106-107.

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, and accompanying text:Minnesota Geological Survey, Report of Investigations 37, Saint Paul, Minnesota, 25p.

Van Schmus, W. R., 1976, Early and Middle Proterozoic history of the Great Lakes area,North America: Philosophical Transactions of the Royal Society of London, v. 280,p. 605-628.

________

1980, Chronology of igneous rocks associated with the Penokean orogeny inWisconsin: Geological Society of America Special Paper 182, p. 159-168.

________•

1981, Possible interpretations of the Penokean orogeny: International ProterozoicSymposium, University of Wisconsin, Madison, Abstracts Volume, p. 44.

B-17

Moreyl G. Ba1 and Southwick, D. K., 1984, Early Proterozoic geology of east-central Minnesota - a review and reappraisal: lnstitute on Lake Superior Geology Abstractsl V. 301 p. 35-36.

Ojakangasl R. Ws1 1 9831 Tidal deposits in the Early Proterozoic basin in the Lake Superior region - The Palms and Pokegama Formations: evidence for subtidal-shelf deposition of Superior-type banded iron formations: Geological Society of America Memoir 160, p. 49-66.

Powelll C. McA.* 197g1 A morphological classification of rock cleavage: Tectonophysics, v. 58, p. 21-34.

Ramsayl J. G., 1967# Foldina and Fracturina of Rocks: McGraw Hill, New York, 568 p. Schulz, K.JsI 1983* Geochemistry of the volcanic rocks of northeastern Wisconsin: lnstitute

on Lake Superior Geology Abstracts and Proceedingsl v. 2g1 p.39. Schulz, K. Js1 LaBergel G. L., Simsl P. Ks1 Petermanl Z. E., and Klasnerl J. Ss1 1984* The

volcanic-plutonic terrane of northern Wisconsin: implications for early Proterozoic tectonism, Lake Superior region: Geological Association of Canada Program with Abstractsl v. g1 p. 103.

Simpson, C., and Schmid, S. M., 1983* An evaluation of criteria to deduce the sense of movement in sheared rocks: Geological Society America Bulletinl v. 94# p. 1281 -1 288.

Simsl P. Ke1 1976, Precambrian tectonics and mineral deposits, Lake Superior region: Economic Geology, v. 71 p. 1092-1118.

Simsl P. K., Card, K. Dm1 Morey, G. B., and Peterman, Z. E., 1980, The Great Lakes tectonic zone - A major crustal structure in central North America: Geological Society of America Bulletin, Pt. 1, v. 91, p. 690-698.

Sims, P. K., Petermanl Z. El and Schulzl K. Ja1 19851 The Dunbar Gneiss-granitoid dome: Implications for early Proterozoic tectonic evolution of northern Wisconsin: Geological Society of America Bulletin, v. 96, p. 1101-1 112.

Sims, P. Ks1 Petermans Z. Es1 Klasnerl J. S., Cannon, W. F., and Schulz, K. J., 1987, Nappe development and thrust faulting in the upper Michigan segment of the early Proterozoic Penokean orogen: Geological Society of America Abstracts with Programs, v. l g l p. 246.

Southwickl D.Ls1 and Morey, G.B., 1988, Tectonic imbrication and foredeep development in the Penokean Orogen, east-central Minnesota: lnstitute on Lake Superior Geologyl V. 34# p. 106-1 07.

Southwickl D.L., Morey, G. B., and McSwiggen, P.L1 1988, Geologic map (scale 1:250,000) of the Penokean Orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological Surveyl Report of Investigations 37, Saint Paul, Minnesotal 25 P

Van Schmusl W. R.# 1976, Early and Middle Proterozoic history of the Great Lakes area, North America: Philosophical Transactions of the Royal Society of London* v. 2801 p. 605-628.

19801 Chronology of igneous rocks associated with the Penokean orogeny in Wisconsin: Geological Society of America Special Paper 1 8Z1 p. 159-168.

, 1981 Possible interpretations of the Penokean orogeny: International Proterozoic Symposium, University of Wisconsinl Madison, Abstracts Volume, p. 44.

- ROCK TYPES AND RELATIONSHIPS OF THE MELLEN IGNEOUS COMPLEX

A One Day Field Trip to the Keweenawan of the Mellen, Wisconsin Area

Trip LeadersKenneth W. Klewin; Northern Illinois University; Dekalb, IL

James F. Olmsted; SUNY; Plattsburgh, NYKarl E. Seifert; Iowa State University; Ames, IA

Introduction

The purpose of this trip is to provide an opportunity to examine the rock-types, igneousfeatures and relationships of the Mellen Complex. We will examine exposures that provideopportunities to compare this with other layered intrusions and make judgements on conditionsand processes of formation. Critical exposures are often inaccessible for brief visits but thereare many that we think you will find interesting and provide for lively discussion. A fewlocations we consider important but less accessible are noted and will be included if time andconditions permit.

Geology of the Mellen Area

GeneralThe Mellen Complex includes several layered basic and related intrusions emplaced near

the base of the Keweenawan volcanic pile. So far as has been determined the volcanics in theMellen area are Lower Keweenawan nearly conformably overlying the older Proterozoic units ofthe Gogebic Iron Range. The lower contact of the complex is slightly discordant cutting tolower stratigraphic levels westward, beveling across the entire Proterozoic to Archean gneisses.All of the Proterozoic units have been tilted toward the northwest into the Lake Superiorsyncline resulting in northeast strikes and steep northwest dips. Faulting has accompanied thistilting but it is difficult to demonstrate in the field.

Paleomagnetic studies of Books, et.aL(1 966) show normal polarity for the intrusive rocksof the Complex indicating Middle Keweenawan age for acquisition of its magnetism.Green(1982) places the Mellen Gabbro equivalent to the Duluth Complex but it could be slightlyolder. Halls and Pesonen(1982) indicate rocks of the Duluth Complex are found in contact withvolcanics of normal polarity while the Mellen appears to intrude only reversed volcanics. Weonly know that the intrusives acquired their its magnetic polarity during the normal period andthey do not cut Upper Keweenawan sedimentary units.

Intrusive Units

The Mellen Complex includes two layered gabbroic intrusions the Potato River Intrusion(PRI) (Klewin 1987 & in review) on the east and the Mineral Lake Intrusion (MLI) (Olmsted,1969; Seifert, in prep.) to the west. These are separated by the Mellen Granite (MG) whichintruded the gabbro. Contacts between intrusives of the Complex are sharp, often with welldeveloped breccias but chilling is only observed where pre-intrusive rocks are involved. Severalsmaller intrusives include the Rearing Pond (RP) intrusion (Olmsted, 1979) and a number ofsmaller gabbro and granophyre sills at the west end of the complex (Leighton, 1954)

The two larger intrusions have the size and character of layered intrusions throughoutthe world. The PRI is about 3.5 km. wide and extends along strike for over 30 km. The MLI isabout 5 km. thick and about 15 km along strike but may have been faulted off on the west endindicating it's true length was somewhat greater. The RP is a small intrusion of elliptical planwith dimensions about 2 by 4 km.. It appears intruded into the upper part of the MLI butmapping does not indicate timing. A smaller more sill-like unit of similar petrographic character(the Picritic Zone, Figure 1)has intruded near the base of the PRI. Northwest of the MLI areseveral large gabbro and granophyre sills that intrude Lower Keweenawan volcanics or other

c-i

- ROCK TYPES AND RELATIONSHIPS OF THE MELLEN IGNEOUS COMPLEX

A One Day Field Trip to the Keweenawan of the Mellen, Wisconsin Area

Trip Leaders Kenneth W. Klewin; Northern Illinois University; Dekalb, IL

James F. Olmsted; SUNY; Plattsburgh, NY Karl E. Seifert; Iowa State University; Ames, IA

Introduction

The purpose of this trip is to provide an opportunity to examine the rock-types, igneous features and relationships of the Mellen Complex. We will examine exposures that provide opportunities to compare this with other layered intrusions and make judgements on conditions and processes of formation. Critical exposures are often inaccessible for brief visits but there are many that we think you will find interesting and provide for lively discussion. A few locations we consider important but less accessible are noted and will be included i f time and conditions permit,

Geology of the Mellen Area

General The Mellen Complex includes several layered basic and related intrusions emplaced near

the base of the Keweenawan volcanic pile. So far as has been determined the volcanics in the Mellen area are Lower Keweenawan nearly conformably overlying the older Proterozoic units of the Gogebic Iron Range. The lower contact of the complex is slightly discordant cutting to lower stratigraphic levels westward, beveling across the entire Proterozoic to Archean gneisses. All of the Proterozoic units have been tilted toward the northwest into the Lake Superior syncline resulting in northeast strikes and steep northwest dips. Faulting has accompanied this tilting but it is difficult to demonstrate in the field.

Paleornagnetic studies of Books, et.a1.(1966) show normal polarity for the intrusive rocks of the Complex indicating Middle Keweenawan age for acquisition of its magnetism. Green(l982) places the Mellen Gabbro equivalent to the Duluth Complex but it could be slightly older. Halls and Pesonen(l982) indicate rocks of the Duluth Complex are found in contact with volcanics of normal polarity while the Mellen appears to intrude only reversed volcanics. We only know that the intrusives acquired their its magnetic polarity during the normal period and they do not cut Upper Keweenawan sedimentary units.

Intrusive Units

The Mellen Complex includes two layered gabbroic intrusions the Potato River lntrusion (PRI) (Klewin 1987 & in review) on the east and the Mineral Lake Intrusion (MLI) (Olmsted, 1969; Seifert, in prep.) to the west. These are separated by the Mellen Granite (MG) which intruded the gabbro. Contacts between intrusives of the Complex are sharp, often with well developed breccias but chilling is only observed where pre-intrusive rocks are involved. Several smaller intrusives include the Rearing Pond (RP) intrusion (Olmsted, 1979) and a number of smaller gabbro and granophyre sills at the west end of the complex (Leighton, 1954)

The two larger intrusions have the size and character of layered intrusions throughout the world. The PRI is about 3.5 km. wide and extends along strike for over 30 km. The MLI is about 5 km. thick and about 15 km along strike but may have been faulted off on the west end indicating it's true length was somewhat greater. The RP is a small intrusion of elliptical plan with dimensions about 2 by 4 km.. It appears intruded into the upper part of the MLI but mapping does not indicate timing. A smaller more sill-like unit of similar petrographic character (the Picritic Zone, Figure 1)has intruded near the base of the PRI. Northwest of the MLI are several large gabbro and granophyre sills that intrude Lower Keweenawan volcanics or other

intwsions but to the west are also emplaced into older Proterozoic units(Leighton 1954).Leighton (1954) argued on the basis of differences between dip of igneous lamination and modallayering that tilting took place during intrusion. Halls & Pesonen (1982) note that thepaleomagnetic pole positions of the Complex lie on the APW curve without structural correctionindicating tilting prior to acquisition of polarity. This permits tilting prior to or during intrusion butis not conclusive.

PetrologyThe Mellen Complex may be classified into three distinct types of intrusives. The PRI

and MLI are coarse grained feldspathic gabbros that range from olivine rich to anorthositiccompositions (Tables 1 & 2). Plagioclase and olivine are largely cumulus while pyroxenes morecommonly occur as postcumulus oikocrysts. Rhythmic layering is rare but lamination is welldeveloped roughly parallel to the regional structure (Leighton, 1954; Olmsted, 1969; Klewin,1987). The RP and the Picritic Zone of the PRI are more mafic picritic intrusions thatrepresent pulses of unique magma. The RP in particular is somewhat more primitive than theothers having much higher Mg#'s and more calcic plagioclase in addition to higher mafic mineralcontent. The third intrusive type is the Mellen Granite.

Cryptic zoning is observed in the larger intrusions indicating that some in situ.fractionation occurred (Olmsted,1969; 1979; Klewin,1984). These (PRI & MLI) are zoned fromgabbroic or anorthositic gabbro compositions through ferrodiorites to granitic rocks. There issome evidence that excessive amounts of granophyre resulted from addition of crustal minimummelt material (Seifert, in prep).

Two distinct granites occur at Mellen. Brick red granophyre often forms the uppermostunits of the layered intrusions (Olmsted, 1969, Klewin 1987). It is also found intrusive intogabbros or metavolcanics in the western part of the complex (Leighton,1954). Granophyresdisplay spectacular simplectite textures in which quartz occurs intergrown with alkali feldspar inmicrographic, radiating fringe and vermicular patterns (Leighton, 1954). Where these graniticunits are thickened the above textures give way to hypidiomorphic granular with only minorintergrowth features. This is well displayed at a granite boss in the western part of the areacalled St.Peters Dome (sic.)

In contrast with the granophyre is the Mellen Granite. This medium to coarse porphyriticgranite is one of the younger intrusives of the complex.. It has intruded both the PRI on theeast and MLI to the west. On the south it has intruded both Keweenawan volcanics andsiltstones of the underlying Tyler formation. The nature of the upper contact is not well knownas exposure is poor. At this time only preliminary analytical work has been accomplished onthis intrusive so its chemical nature poorly known

Descriptions of the Intrusions

Potato River IntrusionGeneral

The PRI forms the eastern half of the MeIlen Complex (Fig. 1) and will be the subject ofthe first part of the trip. In general, the PRI is poorly exposed, being mostly covered byunconsolidated Pleistocene sediment and heavy forest and many of the exposures are deeplyweathered. Relief in the area is generally less than 150m with the highest hills underlain byresistant lower Keweenawari volcanics. The best exposures of intrusive rocks are scatteredglacially scoured outcrops and along drainages. The overall length of the PRI is 33 km.. It isdivided approximately in half by a fault along the Tyler Forks River into a narrow and a muchwider segment. The width of the intrusion ranges from about 1 km. to 4.5 km. averaging 3.6east of the Tyler Forks River where the trip is focused.

Basal and roof contacts of the PRI are roughly concordant with the enclosing volcanicsand with the regional structure. While the PRI is largely enclosed by volcanics it cuts into theunderlying Proterozoic units near its western end. Rare xenoliths of metabasite have beenfound near the base of the intrusion.

C- 2

intrusions but to the west are also emplaced into older Proterozoic units(Leight0n 1954). Leighton (1954) argued on the basis of differences between dip of igneous lamination and modal layering that tilting took place during intrusion. Halls & Pesonen (1982) note that the paleomagnetic pole positions of the Complex lie on the APW curve without structural correction indicating tilting prior to acquisition of polarity. This permits tilting prior to or during intrusion but is not conclusive.

Petrology The Mellen Complex may be classified into three distinct types of intrusives. The PRI

and MLI are coarse grained feldspathic gabbros that range from olivine rich to anorthositic compositions (Tables 1 & 2). Plagioclase and olivine are largely cumulus while pyroxenes more commonly occur as postcumulus oikocrysts. Rhythmic layering is rare but lamination is well developed roughly parallel to the regional structure (Leighton, 1954; Olmsted, 1969; Klewin, 1987). The RP and the Picritic Zone of the PRI are more mafic picritic intrusions that represent pulses of unique magma. The RP in particular is somewhat more primitive than the others having much higher Mg#'s and more calcic plagioclase in addition to higher mafic mineral content. The third intrusive type is the Mellen Granite.

Cryptic zoning is observed in the larger intrusions indicating that some in situ. fractionation occurred (Olmsted,1969; 1979; Klewin,1984). These (PRI & MLI) are zoned from gabbroic or anorthositic gabbro compositions through ferrodiorites to granitic rocks. There is some evidence that excessive amounts of granophyre resulted from addition of crustal minimum melt material (Seifert, in prep).

Two distinct granites occur at Mellen. Brick red granophyre often forms the uppermost units of the layered intrusions (Olmsted, 1969, Klewin 1987). It is also found intrusive into gabbros or metavolcanics in the western part of the complex (Leighton,l954). Granophyres display spectacular simplectite textures in which quark occurs intergrown with alkali feldspar in micrographic, radiating fringe and vermicular patterns (Leighton, 1954). Where these granitic units are thickened the above textures give way to hypidiomorphic granular with only minor intergrowth features. This is well displayed at a granite boss in the western part of the area called StPeters Dome (sic.)

In contrast with the granophyre is the Mellen Granite. This medium to coarse porphyritic granite is one of the younger intrusives of the complex.. It has intruded both the PRI on the east and MLI to the west. On the south it has intruded both Keweenawan volcanics and siltstones of the underlying Tyler formation. The nature of the upper contact is not well known as exposure is poor. At this time only preliminary analytical work has been accomplished on this intrusive so its chemical nature poorly known

Descriptions of the Intrusions

Potato River Intrusion General

The PRI forms the eastern half of the Mellen Complex (Fig. 1) and will be the subject of the first part of the trip. In general, the PRI is poorly exposed, being mostly covered by unconsolidated Pleistocene sediment and heavy forest and many of the exposures are deeply weathered. Relief in the area is generally less than 150m with the highest hills underlain by resistant lower Keweenawan volcanics. The best exposures of intrusive rocks are scattered glacially scoured outcrops and along drainages. The overall length of the PRI is 33 km.. It is divided approximately in half by a fault along the Tyler Forks River into a narrow and a much wider segment. The width of the intrusion ranges from about 1 km. to 4.5 km. averaging 3.6 east of the Tyler Forks River where the trip is focused.

Basal and roof contacts of the PRI are roughly concordant with the enclosing volcanics and with the regional structure. While the PRI is largely enclosed by volcanics it cuts into the underlying Proterozoic units near its western end. Rare xenoliths of metabasite have been found near the base of the intrusion.

'S ,_

f— / 'S / 'S'5, '5I\ \ / S

/ 5' 5' 5'-5'.'

Pm = Proterozojcmetasedimentary rocks

Ks = Keweenawansedimentary rocks

Ky = Keweenawanvolcanic rocks

Ar = Archean rocks

Ks

Ar

Ar

POTATO RIVER INTRUSION

15

Upper Zone

Main Zone

Picritic Zone

MINERAL LAKE INTRUSION

REARING POND INTRUSION

MELLEN GRANITE

Figure 1

Pm = P r o t e r o z o i c me tased imen ta r y r o c k s

Ks = Keweenawan sed imen ta r y r o c k s

Kv = Keweenawan

POTATO R I V E R I N T R U S I O N

Figure 1 MELLEN G R A N I T E

. - -. . -- -. - . . -

Exposures of plagioclase-rich rocks commonly exhibit lamination of the plagioclase laths.Tabet and Mangham (1978) reported an average orientation of N64oE, 84oNW. Occurrences ofrhythmic layering with average orientation N57oE, 64oNW are restricted to the Picritic Zone nearthe middle of the PRI. Contacts between gabbroic rocks are typically gradational.

The PRI has been divided into several zones based on lithology (Fig.1; Kiewin, inreview). Lowermost is a poorly exposed Border Zone of highly altered olivine gabbro cut byrare contemporaneous or later dikes. Where exposed, the Border Zone is irregular in thickness,ranging up to 2 m. Most of the intrusion is assigned to the Main Zone of medium grained rocksranging from olivine gabbro (at the base ), leucogabbro and quartz gabbro to ferrogabbro(toward the top). The average thickness of the main zone is 3300 m. Present locally at the topof the intrusion is an Upper Zone of intrusive granite and granophyre. On the basis of traceelement geochemistry (Klewin, in review) the granophyric rocks appear genetically related to theMain Zone rocks but coarser grained granitic dikes and irregularly shaped granitic intrusions(large dikes?) near the top of the intrusion west of the Tyler Forks River are not. The thicknessof the Upper Zone ranges from zero locally to 120 m. Finally, midway into the Main Zone is thedistinctive intrusive Picritic Zone consisting of picrite and troctolite. The thickness of the PicriticZone is estimated to be 175 to 200 m. The Main Zone is further divided into Main Zones I, II

and Ill. Main Zone I occurs below the Picritic Zone, is 850 m thick and consists of olivinegabbro and olivine leucogabbro. Main Zone Il, above the Picritic Zone, is 1650 m thick andconsists of olivine gabbro, olivine leucogabbro, gabbro and quartz-bearing leucogabbro. Thearrival of cumulus augite occurs near the top of Main Zone II. The boundary between MainZones Il & Ill is marked by the arrival of cumulus Fe-Ti oxides. Main Zone Ill consists of quartzgabbro, quartz leucogabbro and ferrogabbro. Average thickness is 800 m. The arrival ofcumulus apatite occurs in Main Zone Ill.

Petrography and Mineral Chemistry

Most of the Potato River Intrusion is composed of olivine gabbro and olivine bearingleucogabbro. Primary mineralogy is plagioclase, olivine, clinopyroxene, orthopyroxene, Fe-Tioxides and apatite. Major cumulus minerals are plagioclase and olivine. Plagioclase typicallyforms normally zoned lath-shaped to blocky grains. Olivine is subhedral to anhedral andunzoned. Clinopyroxene is anhedral and forms oikocrysts where it is intercumulus and is oftenmoderately zoned. Orthopyroxene is less common and occurs as inverted pigeonite. Ilmeniteand titanomagnetite occur as small oikocrysts or interstitial patches or as skeletal cumulus grainsin the ferrogabbros. Apatite typically occurs as tiny included grains but is larger and cumulus inthe ferrogabbros. Quartz gabbro is marked by intercumulus micrographic intergrowths of quartzand alkali feldspar. Late stage gabbro also contains Ca-amphibole, and rarely fayatitic olivineand pigeonite.

Excepting the Picritic Zone, plagioclase composition ranges from An71 to An36; olivineFo63 to Fo31 and augite Wo44En44Fsl2 to Wo43En32Fs25. Even in the granophyric rocks theaugite never becomes ferroaugite attesting to the modest degree of iron enrichment in theintrusion.

Main Zone rocks are mostly ortho- and mesocumulates, with a notable lack of adcumulusgrowth of plagioclase or olivine. Adcumulate troctolites, however are found in the Picritic Zone.The granophyric rocks exhibit spectacular radiating sheaves of intergrown quartz and alkalifeldspar. Modal layering is rare but occurs in the Picritic Zone and planar lamination ofplagioclase is common in the gabbros. The universal occurrence of the latter and almost totallack of the former throughout the complex is an interesting point that bears on processes andconditions during emplacement. Contacts between different gabbroic rocks are gradationalsupporting the single magma pulse hypothesis (excepting the Picritic Zone).

As judged by textural and structural relations the parent magma was saturated with botholivine and plagioclase. Subsequent arrivals as cumulus minerals were: augite (after 72%solidified), Fe-Ti oxides (after 75% solidified) and apatite (after 78% solidified). The cumulusminerals seem to come in gradually but this may be more apparent than real due to the verypoor exposure.

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Exposures of plagioclase-rich rocks commonly exhibit lamination of the plagioclase laths. abet and Mangham (1978) reported an average orientation of N64oE, 84oNW. Occurrences of rhythmic layering with average orientation N57oE, 64oNW are restricted to the Picritic Zone near the middle of the PRI. Contacts between gabbroic rocks are typically gradational.

The PRI has been divided into several zones based on lithology (Fig.l; Klewin, in review). Lowermost is a poorly exposed Border Zone of highly altered olivine gabbro cut by rare contemporaneous or later dikes. Where exposed, the Border Zone is irregular in thickness, ranging up to 2 m. Most of the intrusion is assigned to the Main Zone of medium grained rocks ranging from olivine gabbro (at the base ), leucogabbro and quartz gabbro to ferrogabbro (toward the top). The average thickness of the main zone is 3300 m. Present locally at the top of the intrusion is an Upper Zone of intrusive granite and granophyre. On the basis of trace element geochemistry (Klewin, in review) the granophyric rocks appear genetically related to the Main Zone rocks but coarser grained granitic dikes and irregularly shaped granitic intrusions (large dikes?) near the top of the intrusion west of the Tyler Forks River are not. The thickness of the Upper Zone ranges from zero locally to 120 m. Finally, midway into the Main Zone is the distinctive intrusive Picritic Zone consisting of picrite and troctolite. The thickness of the Picritic Zone is estimated to be 175 to 200 m. The Main Zone is further divided into Main Zones I, II and Ill. Main Zone I occurs below the Picritic Zone, is 850 m thick and consists of olivine gabbro and olivine leucogabbro. Main Zone 11, above the Picritic Zone, is 1650 m thick and consists of olivine gabbro, olivine leucogabbro, gabbro and quartz-bearing leucogabbro. The arrival of cumulus augite occurs near the top of Main Zone II. The boundary between Main Zones II & Ill is marked by the arrival of cumulus Fe-Ti oxides. Main Zone 111 consists of quartz gabbro, quartz leucogabbro and ferrogabbro. Average thickness is 800 m. The arrival of cumulus apatite occurs in Main Zone Ill.


Most of the Potato River Intrusion is composed of olivine gabbro and olivine bearing leucogabbro. Primary mineralogy is plagioclase, olivine, clinopyroxene, orthopyroxene, Fe-Ti oxides and apatite. Major cumulus minerals are plagioclase and olivine. Plagioclase typically forms normally zoned lath-shaped to blocky grains. Olivine is subhedral to anhedral and unzoned. Clinopyroxene is anhedral and forms oikocrysts where it is intercumulus and is often moderately zoned. Orthopyroxene is less common and occurs as inverted pigeonite. llmenite and titanomagnetite occur as small oikocrysts or interstitial patches or as skeletal cumulus grains in the ferrogabbros. Apatite typically occurs as tiny included grains but is larger and cumulus in the ferrogabbros. Quartz gabbro is marked by intercumulus micrographic intergrowths of quartz and alkali feldspar. Late stage gabbro also contains Ca-amphibole, and rarely fayalitic olivine and pigeonite.

Excepting the Picritic Zone, plagioclase composition ranges from An71 to An36; olivine Fo63 to Fo31 and augite Wo44En44Fs12 to Wo43En32Fs25. Even in the granophyric rocks the augite never becomes ferroaugite attesting to the modest degree of iron enrichment in the intrusion.

Main Zone rocks are mostly ortho- and mesocumulates, with a notable lack of adcumulus growth of plagioclase or olivine. Adcumulate troctolites, however are found in the Picritic Zone. The granophyric rocks exhibit spectacular radiating sheaves of intergrown quartz and alkali feldspar. Modal layering is rare but occurs in the Picritic Zone and planar lamination of plagioclase is common in the gabbros. The universal occurrence of the latter and almost total lack of the former throughout the complex is an interesting point that bears on processes and conditions during emplacement. Contacts between different gabbroic rocks are gradational supporting the single magma pulse hypothesis (excepting the Picritic Zone).

As judged by textural and structural relations the parent magma was saturated with both olivine and plagioclase. Subsequent arrivals as cumulus minerals were: augite (after 72% solidified), Fe-Ti oxides (after 750h solidified) and apatite (after 78% solidified). The cumulus minerals seem to come in gradually but this may be more apparent than real due to the very poor exposure.

References on the Potato River Intrusion

Kiewin, K.W., 1987, The petrology and geochemistry of the Potato River Intrusion, northernWisconsin. Unpublished Ph.D. thesis, Northern Illinois University, DeKaIb, IL, 357 p.

Klewin, K.W., in review, The petrology of the Proterozoic Potato River layered intrusion, northernWisconsin. Jour. of Petrology.

Mangham, J.R., 1975, The structure and petrology of the eastern Mellen intrusive complex, IronCounty, Wisconsin. M.S. thesis, University of Wisconsin, Madison, 134 p.

Tabet, D.E., 1974, Structure and petrology of the Mellen igneous intrusive complex near Mellen,Wisconsin. M.S. thesis, University of Wisconsin, Madison, 81 p.

Tabet, D.E. and Mangham, J.R., 1978, The geology of the eastern Mellen intrusive complex,Wisconsin. Geoscience Wisconsin, v. 3, .p.1 -19.

Mineral Lake IntrusionGeneral

As noted above the Mellen Complex is divided near its midpoint by the Mellen granite.West of the MG are the Mineral Lake and Rearing Pond intrusions (Olmsted, 1969, 1979) alongwith several smaller sills that make up the western part of the complex.(Leighton 1954).Exposure here is probably somewhat better than that found to the east. Many similarities andseveral important differences may be found between the two halves of the complex. Some ofthese will be considered here.

Like the PRI the Mineral Lake Intrusion is hyperfeldspathic with cumulus plagioctase anddivine. A ubiquitous feature is the alignment of plagioclase laths while modal or rhythmiclayering is uncommon. The lower third of the MLI has been termed anorthositic olivine gabbroand represents that portion in which both olivine and plagioclase are cumulus. Above about1500 m above the base olivine becomes less common and the rock grades to gabbroicanorthosite. This unit, over 2000 m thick is composed of over 80% plagioclase with minorpyroxene and Fe-Ti oxides. In many exposures the rock is true anorthosite and the laminationalmost perfect. In glacially polished exposures where clinopyroxene oikocrysts are abundant therock has a decided spotted appearance.

Near the top of the intrusion the feldspathic rocks give way to ferrodiorite which in turngrades upward through granodioritic to granitic or granophyric units. In the western part of thecomplex are found several areas of abundant granophyre to the point appearing excessive if oneaccepts the hypothesis that their origin is entirely through fractionation of a gabbroic parent.These granophyres are often intrusive into enclosing gabbros often forming spectacular breccias.It is unfortunate that none of these is easily accessible for your inspection. Recent isotope andtrace element studies on the MLI suggest that this evolved felsic section includes a crustalcomponent and was emplaced and differentiated as a separate conformable intrusion at the topof the anorthositic unit (Seifert et.al.,1 985; Seifert, in preparation).

The upper contact of the MLI is with the overlying Rearing Pond intrusion as well aswhat are presumed to be Lower Keweenawan volcanics, now strongly recrystallized by contactmetamorphism. Because of poor exposure it has not been possible to observe contact relationsbetween the MLI and the RP intrusion so relative age is in doubt. It is suspected that like thecase of the Picritic Zone of the PRI the closeness of timing prevented any chilling. The basalcontacts of the MLI with underlying volcanics and hornfels are equally poorly exposed and whatis exposed is often complicated. Considerable reduction of grain size is observed at the basebut composition of these rocks disallow their use as parent magma.


Modal analyses of representative rocks are found in Table 1. At first note the rockslabeled "Chill Zone" appear ordinary candidates but chemistry indicates they are somewhatevolved. Likewise their mineral compositions are evolved in comparison to the rocksimmediately above. Overlying the basal rocks is an divine rich zone(MG-4) that is probably not

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References on the Potato River Intrusion

Klewin, K.W., 1987, The petrology and geochemistry of the Potato River Intrusion, northern Wisconsin. Unpublished Ph.D. thesis, Northern Illinois University, DeKalb, IL, 357 p.

Klewin, K.W., in review, The petrology of the Proterozoic Potato River layered intrusion, northern Wisconsin. Jour. of Petrology.

Mangham, J.R., 1975, The structure and petrology of the eastern Mellen intrusive complex, Iron County, Wisconsin. M.S. thesis, University of Wisconsin, Madison, 134 p.

Tabet, D.E., 1974, Structure and petrology of the Mellen igneous intrusive complex near Mellen, Wisconsin. M.S. thesis, University of Wisconsin, Madison, 81 p.

Tabet, D.E. and Mangham, J.R., 1978, The geology of the eastern Mellen intrusive complex, Wisconsin. Geoscience Wisconsin, v. 3, .p.1-19.

Mineral Lake Intrusion General

As noted above the Mellen Complex is divided near its midpoint by the Mellen granite. West of the MG are the Mineral Lake and Rearing Pond intrusions (Olmsted, 1969, 1979) along with several smaller sills that make up the western part of the complex.(Leighton 1954). Exposure here is probably somewhat better than that found to the east. Many similarities and several important differences may be found between the two halves of the complex. Some of these will be considered here.

Like the PRI the Mineral Lake Intrusion is hyperfeldspathic with cumulus plagioclase and olivine. A ubiquitous feature is the alignment of plagioclase laths while modal or rhythmic layering is uncommon. The lower third of the MLI has been termed anorthositic olivine gabbro and represents that portion in which both olivine and plagioclase are cumulus. Above about 1500 m above the base olivine becomes less common and the rock grades to gabbroic anorthosite. This unit, over 2000 m thick is composed of over 80% plagioclase with minor pyroxene and Fe-Ti oxides. In many exposures the rock is true anorthosite and the lamination almost perfect. In glacially polished exposures where clinopyroxene oikocrysts are abundant the rock has a decided spotted appearance.

Near the top of the intrusion the feldspathic rocks give way to ferrodiorite which in turn grades upward through granodioritic to granitic or granophyric units. In the western part of the complex are found several areas of abundant granophyre to the point appearing excessive if one accepts the hypothesis that their origin is entirely through fractionation of a gabbroic parent. These granophyres are often intrusive into enclosing gabbros often forming spectacular breccias. It is unfortunate that none of these is easily accessible for your inspection. Recent isotope and trace element studies on the MLI suggest that this evolved felsic section includes a crustal component and was emplaced and differentiated as a separate conformable intrusion at the top of the anorthositic unit (Seifert et.a1.,1985; Seifert, in preparation).

The upper contact of the MLI is with the overlying Rearing Pond intrusion as well as what are presumed to be Lower Keweenawan volcanics, now strongly recrystallized by contact metamorphism. Because of poor exposure it has not been possible to observe contact relations between the MLI and the RP intrusion so relative age is in doubt. It is suspected that like the case of the Picritic Zone of the PRI the closeness of timing prevented any chilling. The basal contacts of the MLI with underlying volcanics and hornfels are equally poorly exposed and what is exposed is often complicated. Considerable reduction of grain size is observed at the base but composition of these rocks disallow their use as parent magma.


Modal analyses of representative rocks are found in Table 1. At first note the rocks labeled "Chill Zone" appear ordinary candidates but chemistry indicates they are somewhat evolved. Likewise their mineral compositions are evolved in comparison to the rocks immediately above. Overlying the basal rocks is an olivine rich zone(MG-4) that is probably not

cottinuous although better exposure might refute this. This mafic rich zone grades upward over1000 -1500 m with olivine decreasing and plagioclase increasing. In this portion of the intrusionplagioclase lamination is not well developed. Pyroxene and Fe-Ti oxides are both intercumulusthroughout this level. At about 1500 m above the base a very coarse "pegmatitic' unit isencountered that appears to be roughly parallel with the base. It is in this region only wheremodal layering has been observed. While it is not known if this horizon is continuous it hasbeen mapped over a distance of about three km.. This is about the level where olivinebecomes rare and the rocks above are rich in plagioclase and are laminated.

Olivine, now Fe-rich reappears as a cumulus phase in the ferrodiorite where plagioclase,pyroxene, oxides and apatite are cumulus. The very Fe-,Ti-,P-rich nature of this requires thatfractionation played a substantial role in their development although in addition contamination bycrustal material cannot be ruled out. Upwards, intercumulus granophyre becomes importantwhile amphiboles replace olivine and pyroxene as common phases and modal apatitedecreases. At one level large euhedral zircons are abundant.

Excepting the basal chill zone, plagioclase averages about An60, ranging to rims of An20in the most felsic units. Moderate normal zoning about AnlO is common but less so in themore feldspathic adcumulates Plagioclase in the basal Chill Zone is more strongly zoned buthas an average value of An42. This corresponds with the whole rock chemistry. Olivine rangesfrom Fo65 near the base to Fo25 in the ferrodiorites. The Chill Zone olivines have intermediatevalues. Clinopyroxene values are Wo42En39Fsl9 to Wo35En25Fs4O in the ferrodioritedemonstrating somewhat more Fe-enrichment than in the FRI.

Rearing Pond Intrusion

The Rearing Pond Intrusion is unique in several respects, although it somewhatresembles the Picritic Zone of the PRI. It is emplaced at the top of the MLI and with theexception of its extreme western end where it intrudes metavolcanics it is enclosed entirely byother intrusive rocks. Like the Picritic Zone it is composed of olivine rich rocks and exhibitssome modal layering. Textures and mineralogy were studied by Olmsted, 1979, but much workremains to understand its chemistry. Based on mineralogy it too is differentiated but not to thedegree of the MLI. Olmsted 1979, proposed an elongated funnel shape for the intrusion and onthe basis of orientation of layering suggested that it has experienced the tilting typical of theregion. Near its outer perimeter is found a peridotite layer composed largely of cumulus olivinewith intercumulus clinopyroxene and plagioclase. The uppermost rocks are gabbro and quartzgabbro lying above a large central troctolitic unit.

Abundant olivine in the RP averages about Fo80 and is rather strongly serpentinized.The very high Mg#'s of the rock analyses are in line with this primative composition.Plagioclase composition ranges from An85 in lowermost rocks to An50 in the gabbro.Pyroxenes are equally primative with compositions CPX En49Wo46Fs5 and OPX ranging fromEn80 in the troctolite and peridotite to En68 in gabbro.

While time will not permit extensive study of this intrusion we will make one stop thatwill provide an opportunity to examine two of the three major rocktypes.

References to the Mineral Lake and Rearing Pond Intrusions.

Leighton, M.W., 1954, Petrogenesis of a gabbro-granophyre complex in northern, Wisconsin.BulI.GeoLSoc.America, v.65, p.401-442.

Olmsted, J.F., 1969, Petrology of the Mineral Lake Intrusion, northwestern Wisconsin. inY.W.lsachsen, Origin of Anorthosite and Related Rocks, p.149-162, N.Y.State Mus. andSd. Serv. Memoir.

1979, Crystallization history and textures of the Rearing Pond gabbro, northwesternWisconsin. Am.Mineral., v.64, p844-855.

Seifert, K.E., Peterman,Z.E. and Windom,K.E., 1985, Mineral Lake layered intrusion,NWWisconsin. Geol. Soc. America Abs. with Prog.,17, p.712.

__________

in prep., Geochemistry of the Mineral Lake pluton, NW Wisconsin.

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continuous although better exposure might refute this. This mafic rich zone grades upward over 1000 -1500 m with olivine decreasing and plagioclase increasing. In this portion of the intrusion plagioclase lamination is not well developed. Pyroxene and Fe-Ti oxides are both intercumulus throughout this level. At about 1500 m above the base a very coarse "pegmatitic" unit is encountered that appears to be roughly parallel with the base. It is in this region only where modal layering has been observed. While it is not known if this horizon is continuous it has been mapped over a distance of about three km.. This is about the level where olivine becomes rare and the rocks above are rich in plagioclase and are laminated.

Olivine, now Fe-rich reappears as a cumulus phase in the ferrodiorite where plagioclase, pyroxene, oxides and apatite are cumulus. The very Fe-,Ti-,P-rich nature of this requires that fractionation played a substantial role in their development although in addition contamination by crustal material cannot be ruled out. Upwards, intercumulus granophyre becomes important while amphiboles replace olivine and pyroxene as common phases and modal apatite decreases. At one level large euhedral zircons are abundant.

Excepting the basal chill zone, plagioclase averages about An60, ranging to rims of An20 in the most felsic units. Moderate normal zoning about An10 is common but less so in the more feldspathic adcumulates Plagioclase in the basal Chill Zone is more strongly zoned but has an average value of An42. This corresponds with the whole rock chemistry. Olivine ranges from Fo65 near the base to Fo25 in the ferrodiorites. The Chill Zone olivines have intermediate values. Clinopyroxene values are Wo42En39Fsl9 to Wo35En25Fs40 in the ferrodiorite demonstrating somewhat more Fe-enrichment than in the PRI.

Rearing Pond Intrusion

The Rearing Pond Intrusion is unique in several respects, although it somewhat resembles the Picritic Zone of the PRI. It is emplaced at the top of the MLI and with the exception of its extreme western end where it intrudes metavolcanics it is enclosed entirely by other intrusive rocks. Like the Picritic Zone it is composed of olivine rich rocks and exhibits some modal layering. Textures and mineralogy were studied by Olmsted, 1979, but much work remains to understand its chemistry. Based on mineralogy it too is differentiated but not to the degree of the MLI. Olmsted 1979, proposed an elongated funnel shape for the intrusion and on the basis of orientation of layering suggested that it has experienced the tilting typical of the region. Near its outer perimeter is found a peridotite layer composed largely of cumulus olivine with intercumulus clinopyroxene and plagioclase. The uppermost rocks are gabbro and quartz gabbro lying above a large central troctolitic unit.

Abundant olivine in the RP averages about Fo80 and is rather strongly serpentinized. The very high Mg#'s of the rock analyses are in line with this primative composition. Plagioclase composition ranges from An85 in lowermost rocks to An50 in the gabbro. Pyroxenes are equally primative with compositions CPX En49Wo46Fs5 and OPX ranging from En80 in the troctolite and peridotite to En68 in gabbro.

While time will not permit extensive study of this intrusion we will make one stop that will provide an opportunity to examine two of the three major rocktypes.

References to the Mineral Lake and Rearing Pond Intrusions.

Leighton, M.W., 1954, Petrogenesis of a gabbro-granophyre complex in northern, Wisconsin. Bull.Geol.Soc.America, v.65, p.401-442.

Olmsted, J.F., 1969, Petrology of the Mineral Lake Intrusion, northwestern Wisconsin. in Y.W.Isachsen, Origin of Anorthosite and Related Rocks, p.149-162, N.Y.State Mus. and Sci. Sew. Memoir.

1979, Crystallization history and textures of the Rearing Pond gabbro, northwestern Wisconsin. Am.Mineral., v.64, p844-855.

Seifert, K.E., Peterman,Z.E, and Windom,K.E., 1985, Mineral Lake layered intrusion, NWWisconsin. Geol. Soc. America Abs. with Prog.,17, p.712.

, in prep., Geochemistry of the Mineral Lake pluton, NW Wisconsin.

Field Trip Stop Descriptions

Potato River Intrusion

All of the stops are on county forest land, the roads are quite good there should be nodifficulty with access. Watch for ticks in the spring, logging trucks in the summer, deer huntersin the fall and snowmobiles in the winter. The locations are shown in Fig. 2 which shows aportion of the Saxon 7.5' quadrangle map. The intrusion essentially parallels WI Hwy.77between Mellen and Hurley. The trip begins at the intersection of highways 77 and 122 inUpson, WI.

STOP 1 along Hwy. 122 north of Upson. Picritic Zone rocks.

Proceed north from Upson on Hwy. 122 over the range of hills that marks theKeweenawan, Powdermill Group lavas. When the road straightens to a northerly heading (-4.5mi. N of Upson) look for a gravel road to the right (E). Turn onto the gravel road and park afterabout 50 m. South of the road is a small outcrop of massive troctolite (Tables 1&2) of thePicritic Zone. On the south side of the outcrop the basal contact of the Picritic Zone withaltered pegmatitic gabbro of the Main Zone can be seen. The troctolite is massive and hasadcumulate texture. Plagioclase grains in this rock are equant in shape opposed to lath-shapedas in the other troctolites. Olivine is anhedral and appears to have undergone considerableadcumulus growth. With careful study you can observe small grains of Cr-rich spinel in thetroctolite (>7 wt.% Cr203; whole rock Cr up to 500 ppm). In terms of texture (massive,adcumulate), mineral composition (the most primitive, An70, Fo67), and Cr-spinel; this outcrop isunique in the Potato River Intrusion. It may represent an addition of mafic magma unrelated tothe rest of the Picritic Zone.

Return to Hwy.122 and proceed N about 200 m to a second gravel road, this time to theleft (W), and drive about 200 m W and park. Directly south is another outcrop of Picritic Zonerock. Here again the base of the Picritic Zone with underlying Main Zone pegmatitic rock canbe seen. The Picritic Zone rock in this exposure is an olivine rich picrite containing up to 50modal % olivine. The texture is massive to layered with faint rhythmic layering visible in places.Large oikocrysts of clinopyroxene enclose olivine and plagioclase. Cr-spinel is lacking andwhole rock Cr content is in the range of 150-200 ppm. Nickel content is also low for sucholivine rich rocks in the range of 700-750 ppm. Inasmuch as the An and Fo contents are nothigh relative to the rest of the intrusion, this outcrop may represent a zone of accumulation ofmafic minerals.

STOP 2 Along the Potato River. Top of Main Zone II.

Continue west on the gravel road about three miles where the road ends at aturn-around. Just to the west is the Potato River and a small waterfall. The rock here isquartz-bearing leucogabbro (Tables 1 & 2). In a small outcrop just up from the river thedistinctive reddish color of weathered interstitial Alk-Feldspar can be seen. This rock iscomposed of plagioclase, clinopyroxene, Fe-Ti oxides and interstitial patches of micrographicallyintergrown quartz and alkali-feldspar. The feldspar contains some iron which produces the redcolor when altered. The outcrop in the river provides an excellent example of lamination ofplagioclase grains. The outcrop is cut by aphanitic basaltic dikes. Further north along the riverthe intrusive nature of the Upper Zone rocks can be seen.

STOP 3 Along gravel road west of Upson. Main Zone I.

Retrace the route to Upson via Hwy 122. Turn right (W) on Hwy.77, proceed about 3miles and turn right (N) on a gravel road, proceed — 2,5 miles and park along road. Severallow outcrops along the road are typical of Main Zone I. They are olivine gabbros (Tables 1 &

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Field Trip Stop Descriptions

Potato River Intrusion

All of the stops are on county forest land, the roads are quite good there should be no difficulty with access. Watch for ticks in the spring, logging trucks in the summer, deer hunters in the fall and snowmobiles in the winter. The locations are shown in Fig. 2 which shows a portion of the Saxon 7.5' quadrangle map. The intrusion essentially parallels Wl Hwy.77 between Mellen and Hurley. The trip begins at the intersection of highways 77 and 122 in Upson, Wl.

STOP 1 along Hwy. 122 north of Upson. Picritic Zone rocks.

Proceed north from Upson on Hwy. 122 over the range of hills that marks the Keweenawan, Powdermill Group lavas. When the road straightens to a northerly heading (-4.5 mi. N of Upson) look for a gravel road to the right (E). Turn onto the gravel road and park after about 50 m. South of the road is a small outcrop of massive troctolite (Tables 1&2) of the Picritic Zone. On the south side of the outcrop the basal contact of the Picritic Zone with altered pegmatitic gabbro of the Main Zone can be seen. The troctolite is massive and has adcumulate texture. Plagioclase grains in this rock are equant in shape opposed to lath-shaped as in the other troctolites. Olivine is anhedral and appears to have undergone considerable adcumulus growth. With careful study you can observe small grains of Cr-rich spinel in the troctolite (>7 wt.% Cr203; whole rock Cr up to 500 ppm). In terms of texture (massive, adcumulate), mineral composition (the most primitive, An70, Fo67), and Cr-spinel; this outcrop is unique in the Potato River Intrusion. It may represent an addition of mafic magma unrelated to the rest of the Picritic Zone.

Return to Hwy.122 and proceed N about 200 m to a second gravel road, this time to the left (W), and drive about 200 m W and park. Directly south is another outcrop of Picritic Zone rock. Here again the base of the Picritic Zone with underlying Main Zone pegmatitic rock can be seen. The Picritic Zone rock in this exposure is an olivine rich picrite containing up to 50 modal % olivine. The texture is massive to layered with faint rhythmic layering visible in places. Large oikocrysts of clinopyroxene enclose olivine and plagioclase. Cr-spinel is lacking and whole rock Cr content is in the range of 150-200 ppm. Nickel content is also low for such olivine rich rocks in the range of 700-750 ppm. Inasmuch as the An and Fo contents are not high relative to the rest of the intrusion, this outcrop may represent a zone of accumulation of mafic minerals.

STOP 2 Along the Potato River. Top of Main Zone II.

Continue west on the gravel road about three miles where the road ends at a turn-around. Just to the west is the Potato River and a small waterfall. The rock here is quartz-bearing leucogabbro (Tables 1 & 2). In a small outcrop just up from the river the distinctive reddish color of weathered interstitial Alk-Feldspar can be seen. This rock is composed of plagioclase, clinopyroxene, Fe-Ti oxides and interstitial patches of micrographically intergrown quartz and alkali-feldspar. The feldspar contains some iron which produces the red color when altered. The outcrop in the river provides an excellent example of lamination of plagioclase grains. The outcrop is cut by aphanitic basaltic dikes. Further north along the river the intrusive nature of the Upper Zone rocks can be seen.

STOP 3 Along gravel road west of Upson. Main Zone I.

Retrace the route to Upson via Hwy 122. Turn right (W) on Hwy.77, proceed about 3 miles and turn right (N) on a gravel road, proceed - 2,5 miles and park along road. Several low outcrops along the road are typical of Main Zone 1. They are olivine gabbros (Tables 1 &

C)

Figu

re 2

Table 1 Modes

SAMPLE ROCKTYPE PLAG. OL CPX OPX OXIDE HLBD. BlOT. AP K-SPAR QZ OTHER

MINERAL LAKE INTRUSION'6-5-65" Chill Zone 51.2 3.6 20.2 10.8 5.4 3.8 4.5 TR

Ave.M656-8 Chill Zone 55.5 2.5 *325 5.0 4.0 TRMO-4 OlivineGabbro 20.0 40.0 *38.0 1.0 1.0MO-6 Gabbro 40.0 *59.0 1.0MG—9 Gab.Anorthosite 80.0 *18.0 2.0"11-9" Gab. Anorthosite 79.5 9.3 2.9 5.1 1.4 1.7 1.7 TR"3-10" Gab.Anorthosite 78.0 *127 0.7 9.2 0.4"25-42" Ferrodiorite 61.5 14.3 *11.9 6.2 1,3 1.8 2.9MG-16 Q.Mon2odiorlte 40.0 TR 2.0 30.0 5.0 10.0 13.0MO-i? Q.Monzodiorite 30.0 *8.0 TR 12.0 30.0 20.0MG- 18 Granophyre 3.0 49.0 48.0MO-19 GranopI'i're 2.0 30.0 #68.0

POTATO RIVER INTRUSION84-40D Troctollte 76.0 20.0 3.0 1.083-28C Picrlte 25.0 55.0 15.0 2.0 3.084-43B Q. Leucogabbro 66.0 25.0 2.0 2.0 5.084-71A Granophyre 65.0 5.0 5.0 TR 10.0 5.0 TR84.49A OlivlneOabbro 65.0 15.0 15.0 3.0 2.084-516 LayeredTroctoli 52.0 45.0 1.0 1.0 1.084-53D OlivineGabbro 64,0 18.0 15.0 3.084-54C QuartzGabbro 50.0 40.0 5.0 1.0 4.0

REARING POND INTRUSIONW-39 01 Oabbro 54.6 13.8 25.6 5.4 0.6W- 163 Troctollte 52.8 39.7 5.1 2.2 0.6 0.6W—74 Peridotite 24.3 72.4 0.2 2.2 1.1W-6 Gabbro 55.6 0.5 13.2 15.7 0.3 14.6 0.1 TRW-164 Gabbro 52.7 25.5 7.6 4.3 1.0 3.2 0.4 0.4 3.0 TR

*Iflcludes CPX & OPX undistinguished# Largely altered granophyric material

Page 1

Table 1 Modes

SAMPLE ROCKTY P E P LAG.

MINERAL LAKE INTRUSION 6-5-65" Chi11 Zone 51.2 Ave.MG56-8 Chill Zone 55.5 MG- 4 Olivine Gabbro 20.0 MG-6 Gabbro 40.0 MG-9 Gab. Anorthosite 80.0 " 1 1-9" Gab. Anorthosite 79.5 "3- 10" Gab.Anorthosi te 78.0 "25- 42" Ferrodior ite 61.5 MG- 16 Q. M onzodi o r i te 40.0 MG- 17 Q.Monzodiori te 30.0 MG- 18 Granophyre MG- 19 Granophyre

(-1 I

* POTATO RIVER INTRUSION Troctolite 76.0 Picrite 25.0 Q. Leucogabbro 66.0 Granophyre 65.0 Olivine Oabbro 65.0 Layered Troctoli 52.0 Olivine Gabbro 64.0 Quartz Gabbro 50.0

REARING POND INTRUSION W-39 01 Oabbro 54.6 W-163 Troctolite 52.8 W-74 Peridotite 24.3 W-6 Gabbro 55.6 W-164 Gabbro 52.7

CPX OPX OXIDE HLBD. BIOT. AP K-SPAR QZ OTHER

*Includes CPX & OPX undistinguished Largely altered granophyric material

Table 2 Analyses

Mineral Lake Intrusion Mellen Rearing PondGranite

Oxides MO 55,56 MO - 4 MG - 9 116 - 14 25-42 MO - 16 MG - 17MG - 18 MG - 19 Mean n=4 RP Mag 87-1 1 p58 Ave.

S102 47.70 44.30 50.40 48.30 43.56 56.40 60.00 74.40 75.60 69.26 44.47 37.221102 2.57 1.86 1.23 0.87 3.31 1.75 1.67 0.26 0.16 0.45 1.18 0.32A1203 15.40 9.66 24.10 22.60 13.61 15.20 14.20 12.80 12.40 13.88 18.86 4.30

FeUt 14.60 20.70 5.65 8.42 20.60 12.50 9.46 2.29 1.92 4.00 7.84 16.41MnO 0.21 0.28 0.08 0.10 0.31 0.18 0.13 0.04 0.04 0.07 0.11 0.27MgO 6.62 14.30 2.06 5.42 3.66 2.31 1.91 0.18 0.08 0.40 10.90 29.19CeO 8.76 6.05 10.90 10.40 9.35 5.33 5.27 0.62 0.38 1.45 10.13 2.85Na20 2.75 1.98 4.30 3.86 2.62 3.39 4.15 3.90 4.07 3.47 1.89 0.13K20 0.38 0.42 0.76 0.10 0.42 2.11 2.28 5.47 5.35 5.61 0.36 0.09P205 0.44 0.20 0.39 0.33 1.67 0.54 0.60 0.04 0.03 0.05 0.12 0.01LOl ND ND ND ND ND ND ND ND ND 1.04 ND ND

? TOTAL 99.43 99.75 99.87 100.40 99.11 99.71 99.67 100.00 100.03 99.68 95.86 90.79C

Mg# 45 55 39 53 24 25 26 13 7 15 71 76C/PW Norms

Ap 0.96 0.44 0.85 0.78 3.67 1.18 1.31 0.09 0.07 0.11 0.27 0.02Mt 2.36 3.34 0.92 1.99 3.35 2.02 1.53 0.37 0.30 0.65 1.32 2.91Ii 4.91 3.54 2.34 1.65 6.34 3.33 3.18 0.50 0.31 0.87 2.34 0.67Or 2.25 2.48 4.49 0.59 2.50 12.47 13.49 32.44 32.06 33.56 2.21 0.58Ab 23.35 16.74 30.69 28.80 22.30 28.70 35.17 29.84 32.20 29.73 16.65 1.21An 28.65 16.22 44,21 43.90 24.28 20.04 13.41 2.84 1.13 5.80 43.65 11.96NE 0.00 0.00 3.08 2.05 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00a 0.00 0.00 0.00 0.00 0.00 7.62 10.45 29.48 30.04 21.32 0.00 0.00Di 10.05 10.27 6.12 4.41 9.79 2.48 7.56 0.00 0.52 1.00 6.34 2.82Ity 16.50 8.40 0.00 0.00 13.29 21.46 13.17 3.63 2.55 6.20 2.18 9.4801 10.40 38.14 6.70 15.90 13.83 0.00 0.00 0.00 0.00 0.00 24.58 70.06

11055 ,56,58AVE, ChiliZone, MO-4, OL Gabbro, STOP 7; MG- 14,MG—9. (3ab.Anorthosite, STOPS 9 & 1025—42 Ferrodlorlte, MG — 16, 1 7, Q. Monzogbbro, STOP 11MO— 18, 19,Oranophyre, STOPDRPMAG, Troctollte; 87— liP Peridotite, STOP 1 2

Page 1

Table 2 Analyses

Mineral Lake Intrusion Mellen Granite

Oxides MO55,56 MG-4 MG-9 MG-1425-42 MG-16MG-17MG-18 MG-19 Meann=4 58 Ave.

Si02 47.70 Ti02 2.57 A1203 1 5.40

Feot M no M go CaO Na20 K20 P 205 LO I TOTAL

Rearing Pond

RP Mag

M655,56,58AVE, ChillZone, MO-4,Ol.Gabbro, STOP 7; MG- 14,MG-9, Gab.Anorthosite, STOPS 9 & 10 25-42 Ferrodiorlte, MO - 16, 17,Q. Monzogabbro, STOP 1 1 MO - 18, 19, Oranophyre, STOP D RPMAO, Troctolite; 87- 1 1 P Peridotite, STOP 12

Page 1

Toble 2 Analyses

Potato River Intrusion PRI Estimated Bulk Compositions

84-40D 83-28C 84-43B 84-71A 84—49A 84-S1B 84-53D 84-54C MZ I PZ MZ II MZ III UZ

S102 46.06 40.91 51.10 61.98 47.98 44.35 48.56 49.31 48.64 47.46 50.88 50.74 62.271102 0.41 0.99 1.63 0.77 1.12 0.23 1.62 3.49 0.97 0.75 1.25 3.01 1.71A1203 20.20 6.06 19.62 13.76 19.59 14.36 19.48 15.65 18.70 17.91 20.62 14.87 12.89Fe203 2.27 3.76 1.94 5.23 1.78 2.56 2.89 3.36 1.58 1.90 2.03 4.98 4.31FeO 7.92 23.32 5.72 5.28 8.12 14.56 7.88 9.28 9.10 10.31 6.36 9.78 5.68MnO 0.12 0.33 0.12 0.16 0.12 0.21 0.13 0.17 0.13 0.15 0.12 0.21 0.19MgO 9.16 19.67 3.58 1.07 6.57 14.70 5.40 4.22 7.86 9.50 4.51 3.11 2.01CeO 10.26 4.80 11.73 3.20 11.17 6.75 9.49 10.47 10.13 9.23 10.77 9.01 3.73Na20 2.29 0.86 3.07 3.90 2.61 2.05 2.92 2.74 2.66 2.50 3.06 2.86 3.55K20 0.23 0.16 0.51 3.18 0.44 0.20 0.54 0.61 0.32 0.38 0.47 1.16 3.65P205 0.01 0.06 0.17 0.11 0.12 0.02 0.17 0.29 0.07 0.03 0.13 0.76 0.44LOt ND ND ND ND ND ND ND ND ND ND ND ND ND, TOTAL 98.93 100.92 99.19 98.64 99.62 99.99 99.08 99.59 100.16 100.12 100.20 100.49 100.43

Mg# 66 61 50 18 59 65 52 42 61 62 54 31 31C/PWNormsAp 0.0 0.1 0.4 0.3 0.3 - 0.1 0.4 0.7 0.2 0.1 0.3 1.8 1.0Mt 3.3 6.4 2.8 2.5 2.6 3.7 4.2 4.9 2.3 2.8 2.9 7.2 6.3Ii 0.8 1.9 3.1 1.5 2.1 0.4 3.1 6.6 1.8 1.4 2.4 5.7 3.3Or 1.4 0.9 3.0 19.1 2.6 1.2 3.2 3.6 1.9 2.3 2.8 6.9 21.6Ab 19.4 7.2 26.0 33.6 22.1 17.4 24.7 23.2 22.5 21.2 25.9 24.2 30.0An 44.2 12.1 38.3 10.8 40.4 29.4 38.5 28.6 38.1 36.5 41.1 24.3 8.5NE 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0Q 0.0 0.0 2.4 14.1 0.0 0.0 0.0 3.7 0.0 0.0 0.6 7.0 17.5Dl 5.4 9.1 15.4 4.0 11.6 3.2 6.2 17.6 9.5 7.4 9.4 12.8 5.9Fly 3.7 12.2 7.8 14.1 5.4 4.1 15.8 10.6 9.8 7.1 14.8 10.6 6.501 20.8 49.9 0.0 0.0 12.5 40.5 3.2 0.0 14.0 21.5 0.0 0.0 0.0

84—40D Massive Troctolite, 83-28C Picrite, STOP 1; 83—43B, Q.Leucogabbro, 84—7 1A, Orenophyre, STOP 284—49A, O1.Oabbro, STOP 3; 84-51 B, Layered Troctolite, STOP 4; 84-53D, O1.Gabbro, STOP 5; 84-54C, Q.Gabbro, STOP 6Estimated Bulk Compositions: MZ I,Main Zone I;PZ, Picritic Zone; MZ II,Main Zone II;

MZ lit, Main Zone ill; UZ, Upper Zone

Page 2

Table 2 Analyses

si 02 Ti02 A1203 Fe203 FeO M no M go CaO Na20 K20 P 205 LO I

<-) TOTAL I d - Mg*

Potato R ive r Intrusion PRI Estimated Bulk Compositions

84-40D 83-28C 84-43B 84-71A 84-49A 84-51 B 84-53D 84-54C MZ I PZ M Z I I M Z I I I

84-400 Massive Troctolite, 83-28C Picrite, STOP 1 ; 83-43B, Q.Leucogabbro, 84-7 lA, Oranophyre, STOP 2 84-49A, Ol.Gabbro, STOP 3; 84-5 16, Layered Troctolite, STOP 4; 84-53D, Ol.Oabbro, STOP 5; 84-54C, Q.Gabbro, STOP 6 Estimated Bulk Compositions: MZ I, Main Zone 1; PZ, Picrit ic Zone; MZ 11, Main Zone 11;

MZ 1 1 1 , Ma1 n Zone I I I ; UZ, Upper Zone

2) composed of cumulus plagioclase (An64-62) and olivine (Fo62-57) along with intercumulusclinopyroxene and Fe-Ti oxides.

STOP 4 Along gravel road west of Upson. Rhythmic layering in the Picritic Zone.

Continue about one mile N along the gravel road to an overgrown logging road to theleft. There is active logging in the area and roads spring up from year to year. To distinguishthe correct one there should not be a road to the right directly opposite this one as there are inother places. Park on the overgrown road and walk W up a slight hill to the top (50-1 00 m). Atthe top of the hill turn right (N) into the woods 100-200 m, Here is a low outcrop of layeredtroctolite of the upper part of the Picritic Zone. The distinctive feature of this rock is rhythmicmodal layering. Plagioclase (An67.5) rich and olivine (Fo63.5) rich layers alternate on the scaleof centimeters. Lath-shaped plagioclase grains are laminated in the leucocratic layers. Here theplagioclase and olivine are more calcic and magnesian respectively than in the adjacent MainZone rocks but less so than in the massive troctotites. This part of the Picritic Zone is the onlypart of the intrusion that exhibits rhythmic layering. What do you make of that?

STOP 5 Along gravel road W of Upson. Main Zone II rocks.

Continue along gravel road about 1.5 miles. As the road turns left to round a hill stopwhen you are heading W and park. The hill is underlain by olivine gabbro of the Main Zone II(Tables 1 & 2). This is cut by a typical Keweenawan high Ti-P dike. This is about the highestoccurrence of olivine gabbro in the intrusion. Plagioclase here is not strongly laminated, itscomposition is about An60 and olivine composition is about Fo55.

STOP 6 Along gravel road rest of Upson. Main Zone lii rocks.

Continue on the gravel road for about 1 mile until there is another hilt just to the left. Ifyou reach a small lake on the right you have gone too far. This hill is composed of quartzbearing gabbro of the Main Zone Ill (Tables 1 & 2) also cut by a diabase dike. The rock hereis composed of cumulus plagioclase, clinopyroxene and Fe-Ti oxides with interstitial quartz andalkali feldspar. This is the lower part of the Main Zone Ill; after the appearance of cumulusoxides, but before cumulus apatite.

Return to Hwy.77, turn right (W) toward Mellen which is about 13 miles. On reachingMellen proceed N on Hwy.13 through town about 1 mile. Turn right on Hwy 169 to Gurneyfollowing the signs to Copper Falls State Park where we will eat lunch.

STOP AYou are encouraged to walk the short distance along the trails to and just beyond

Brownstone Falls where the contact between the volcanics (reversed polarity) and the upperKeweenawan may be seen. Continuing along the path beyond the Brownstone Falls you areproceeding up section through the Copper Harbor Conglomerate, the Nonsuch Formation andinto the lower part of the Freda Sandstone. As you will see the units are vertical here withstrike about N6OE. The uppermost volcanics at the Brownstone Falls are rhyolite flows overlyingbasalt. One of us at least (JFO) thinks the Mid- Upper Keweenawan contact here is a fault.What's yours?

Mellen Granite Brief optional stops along the road in both cases to observe the porphyriticgranite.

C-I 2

2) composed of cumulus plagioclase (An64-62) and olivine (Fo62-57) along with intercumulus clinopyroxene and Fe-Ti oxides.

STOP 4 Along gravel road west of Upson. Rhythmic layering in the Picritic Zone.

Continue about one mile N along the gravel road to an overgrown logging road to the left. There is active logging in the area and roads spring up from year to year. To distinguish the correct one there should not be a road to the right directly opposite this one as there are in other places. Park on the overgrown road and walk W up a slight hill to the top (50-100 m). At the top of the hill turn right (N) into the woods 100-200 m, Here is a low outcrop of layered troctolite of the upper part of the Picritic Zone. The distinctive feature of this rock is rhythmic modal layering. Plagioclase (An67.5) rich and olivine (Fo63.5) rich layers alternate on the scale of centimeters. Lath-shaped plagioclase grains are laminated in the leucocratic layers. Here the plagioclase and olivine are more calcic and magnesian respectively than in the adjacent Main Zone rocks but less so than in the massive troctolites. This part of the Picritic Zone is the only part of the intrusion that exhibits rhythmic layering. What do you make of that?

STOP 5 Along gravel road W of Upson. Main Zone II rocks.

Continue along gravel road about 1.5 miles. As the road turns left to round a hill stop when you are heading W and park. The hill is underlain by olivine gabbro of the Main Zone I1 (Tables 1 & 2). This is cut by a typical Keweenawan high Ti-P dike. This is about the highest occurrence of olivine gabbro in the intrusion. Plagioclase here is not strongly laminated, its composition is about An60 and olivine composition is about Fo55.

STOP 6 Along gravel road rest of Upson. Main Zone Ill rocks.

Continue on the gravel road for about 1 mile until there is another hill just to the left. If you reach a small lake on the right you have gone too far. This hill is composed of quartz bearing gabbro of the Main Zone Ill (Tables 1 & 2) also cut by a diabase dike. The rock here is composed of cumulus plagioclase, clinopyroxene and Fe-Ti oxides with interstitial quartz and alkali feldspar. This is the lower part of the Main Zone Ill; after the appearance of cumulus oxides, but before cumulus apatite.

Return to Hwy.77, turn right (W) toward Mellen which is about 13 miles. On reaching Mellen proceed N on Hwy.13 through town about 1 mile. Turn right on Hwy 169 to Gurney following the signs to Copper Falls State Park where we will eat lunch.

STOP A You are encouraged to walk the short distance along the trails to and just beyond

Brownstone Falls where the contact between the volcanics (reversed polarity) and the upper Keweenawan may be seen. Continuing along the path beyond the Brownstone Falls you are proceeding up section through the Copper Harbor Conglomerate, the Nonsuch Formation and into the lower part of the Freda Sandstone. As you will see the units are vertical here with strike about N60E. The uppermost volcanics at the Brownstone Falls are rhyolite flows overlying basalt. One of us at least (JFO) thinks the Mid- Upper Keweenawan contact here is a fault. What's yours?

Mellen Granite Brief optional stops along the road in both cases to observe the porphyritic granite.

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Figure 3 C-13

STOP B Intrusive Breccia along the W side of Hwy 13 about 1 mile N of Mellen.

Blocks of gabbro enclosed in coarse grained granite. Note how it is possible to mentally'fit many of the gabbro fragments back together. Also note lack of chilling of the granite.

Proceed N on Hwy 13 about 1 mile and turn left onto County Hwy C. Continue W about3 miles where road turns right (N), continue N 1/2 mile. Turn left on gravel road proceed Wabout 1/2 mile to top of rise.

STOP C Massive Mellen Granite cut by dikes.

Stop at top of hill where you see outcrops of granite on both sides of road. This isprivate land so please respect the owners fence. Walk south of the road to examine texturesand dike occurrences.

Continue along road, first W one mile then S one mile to a T in the road. Turn right (W)proceed W 1/2 mile to a 4-corners. Turn left (S) (This becomes National Forest Road 188 .)continuing for about 2 miles to a gravel road left to English Lake. Please be respectful here asthe natives are sometimes unfriendly. Turn left (roughly S) into the English Lake road,proceeding about 1/4 mile to an unimproved but passable road to the right. Look for a signstating the name E. Ceres. Proceed carefufly about 1/4 mile looking for a fork to the right.There is an inconspicuous 2-wire power line overhead that follows this fork of the road. It youarrive at a cottage on the lake you have gone too far. Return to the top of the hill. Park in thefork far enough so others can pass to the cottage.

STOP 7. Basal contact of the Mineral Lake intrusion.

Walk along under the power line (200 m) to the third power pole that is planted in theoutcrop. The hilt drops rapidly here into a small valley. Look for many drill core holes in therock. Most of the rock exposed here is fine grained gabbro but the rock on the SE corner ofthe outcrop is hornfels. Look for this contact and look for inclusions of hornfels within the finegrained gabbro. The texture of the gabbro here at the contact is intergranular with veryelongate plagioclase (An42-45). (6-5-65 & MG 55 etc. Tables 1 & 2). (Ol.-Fo40, Cpx. Wo40En34 Fs26) Note that back along the power line away from the contact the rock graduallybecomes more coarse and the texture subophitic. Proceeding southwesterly through the woodsacross a small valley to the top of the next hill where there are a number of outcrops of basalolivine rich gabbro (MG-4). Return via the power line.

STOP 8 Olivine Gabbro and "Pegmatitic" and Layered Zone.

Return to FR 188. Turn left, W, proceed about 2.5 miles to intersection with FR187.Turn right, continue westerly about 1/2 mile to a driveway left into the home of Harold Smith,proceed about 1/2 mile to the house. Park near the house and seek permission. Walk down tothe beach and left to the south end of the beach and into the trees to the first outcrop. This isan olivine gabbro about 600 m above the base of the intrusion. Return to the car and backalong the road about 3/8 mile to the base of a steep rise in the road. Small outcrop on W sideof the road of very coarse "pegmatitic" gabbro. This unit has been mapped over two milesalong the north side of Mineral Lake and beyond and represents an important horizon in theintrusion. If the water in the lake is very low we may cross the river where it exits the lake toexamine modal layering that occurs just above the pegmatite. Olivine continues as a minorphase above this level but is no longer abundant. The rock is exceptionally feldspathic above.

c-i 4

STOP B Intrusive Breccia along the W side of Hwy 13 about 1 mile N of Mellen. \

Blocks of gabbro enclosed in coarse grained granite. Note how it is possible to mentally 'fit many of the gabbro fragments back together. Also note lack of chilling of the granite.

Proceed N on Hwy 13 about 1 mile and turn left onto County Hwy C. Continue W about 3 miles where road turns right (N), continue N 112 mile. Turn left on gravel road proceed W about 112 mile to top of rise.

STOP C Massive Mellen Granite cut by dikes.

Stop at top of hill where you see outcrops of granite on both sides of road. This is private land so please respect the owners fence. Walk south of the road to examine textures and dike occurrences.

Continue along road, first W one mile then S one mile to a T in the road. Turn right (W) proceed W 112 mile to a 4-corners. Turn left (S) (This becomes National Forest Road 188 .) continuing for about 2 miles to a gravel road left to English Lake. Please be respectful here as the natives are sometimes unfriendly. Turn left (roughly S) into the English Lake road, proceeding about 114 mile to an unimproved but passable road to the right. Look for a sign stating the name E. Ceres. Proceed carefully about 114 mile looking for a fork to the right. There is an inconspicuous 2-wire power line overhead that follows this fork of the road. If you arrive at a cottage on the lake you have gone too far. Return to the top of the hill. Park in the fork far enough so others can pass to the cottage.

STOP 7. Basal contact of the Mineral Lake intrusion.

Walk along under the power line (200 m) to the third power pole that is planted in the outcrop. The hill drops rapidly here into a small valley. Look for many drill core holes in the rock. Most of the rock exposed here is fine grained gabbro but the rock on the SE corner of the outcrop is hornfels. Look for this contact and look for inclusions of hornfels within the fine grained gabbro. The texture of the gabbro here at the contact is intergranular with very elongate plagioclase (An42-45). (6-5-65 & MG 55 etc. Tables 1 & 2). (01.-Fo40, Cpx. Wo40 En34 Fs26) Note that back along the power line away from the contact the rock gradually becomes more coarse and the texture subophitic. Proceeding southwesterly through the woods across a small valley to the top of the next hill where there are a number of outcrops of basal olivine rich gabbro (MG-4). Return via the power line.

STOP 8 Olivine Gabbro and "Pegmatitic" and Layered Zone.

Return to FR 188. Turn left, W, proceed about 2.5 miles to intersection with FR187. Turn right, continue westerly about 112 mile to a driveway left into the home of Harold Smith, proceed about 112 mile to the house. Park near the house and seek permission. Walk down to the beach and left to the south end of the beach and into the trees to the first outcrop. This is an olivine gabbro about 600 m above the base of the intrusion. Return to the car and back along the road about 318 mile to the base of a steep rise in the road. Small outcrop on W side of the road of very coarse "pegmatitic" gabbro. This unit has been mapped over two miles along the north side of Mineral Lake and beyond and represents an important horizon in the intrusion. If the water in the lake is very low we may cross the river where it exits the lake to examine modal layering that occurs just above the pegmatite. Olivine continues as a minor phase above this level but is no longer abundant. The rock is exceptionally feldspathic above.

STOP 9 Briefly: Spotted Anorthositic Gabbro

Return to FR 187, turn left (W) about 1/2 mile to a large outcrop on the right. (Justbefore this exposure look for low blasted outcrops on either side of the road which are goodplaces to collect this rocktype.) Poikilitic Anorthositic Gabbro (MG-i 4, 11-9). An excellentexample showing a common texture. Plag. An60, 01. Fo65, Opx En66, Cpx Wo42,En39,Fs19.

STOP 10 Briefly Laminated Gabbroic Anorthosite

Continue Won FR187 about 1.5 miles then north at "Pine Stump Corner" and thenabout one mile past the Lake "3" Campgrounds, turn right NE on FR 189 go 5/8 mile stop atwhat was once a small gravel pit on the left side of the road, now quite overgrown. Largepolished outcrop of anorthositic rock in back wall of the pit (MG-9 ). Plag. An60, CpxWo37,En29,Fs34.

STOP ii Ferrodiorite, Monzogabbro etc. (Transition Zone)

Continue along FR 189 Cross Brunsweiler River at 2 miles, continue about 3/4 milebeyond the river to where the road turns to an E direction. Walk straight S into the woods about150 m where an irregular EW ridge exposes black rust stained Ferrodiorite (25-42; Plag. An45,01. Fo24, Opx. En37, Cpx.Wo38 En24 Fs38 ) note cumulate and laminated texture andconspicuously finer grain size. Retrace your path back crossing the road to the north sideworking your way north along the west side of the hill. Several outcrops here show graduallyincreasing reddish felsic content (MG-16, 17).

STOP 12 Rearing Pond Intrusion

Turn around and go back over the Brunsweiler River past a gravel pit on right and turnright on woods road a few meters beyond. Continue northerly about 1/2 mile, turn left continueanother 1/ 2 mile or less, then left again, and left again in about 1/4 mile. (If you encounter abridge over a brook go back.) After the last turn proceed cautiously about 1 OOm to a brookwhere the road ends. An outcrop on your left is peridotite (W-74, similar to 87-i 1 P)of the lowerpart of the Rearing Pond intrusion. Walk along what remains of the road (a path by now) crossthe brook jumping rocks, and pick up the remains of the road on the other side. A few metersbeyond the brook search to the right of the path for the remains of a picnic grounds (a coupleof fire places is all that's left) and several large cliff like outcrops beyond. We will work our wayaround to the left and circle the hill studying the rocks and their textures and features as weproceed (RPMag, 87-liT and W-163) . This is the remains of a State trout hatchery that wasabandoned at the beginning of WWll. Good Archaeology here.

STOP D Granophyre Time Permitting

Return to FR 189, turn right and return to FR 187. Proceed N on FR 187, 1.5 miles tothe inflection point of an "S" turn (Large Outcrop on right) park on left W side of road. A fewmeters SW into the woods is an outcrop ot a granitic rock. From this point proceed on aheading of 290 about 300 meters, eventually up a fairly tiring hill. Good exposures ofGranophyre here. (MG-18, 19)

STOP E Alternate Granophyre exposure. Morgan Falls.

From the intersection of FR 187 & 198 go N 1/2 mile, turn left on FR 193 following thesigns to Morgan Falls. Park in the lot provided and walk the trail to the falls. This is aspectacular 100 foot high waterfall over the brick red granophyre. Contact of granophyre andgabbro can be seen west of the falls along the upper part of the ridge.

C-15

STOP 9 Briefly: Spotted Anorthositic Gabbro \

Return to FR 187, turn left (W) about 112 mile to a large outcrop on the right. (Just before this exposure look for low blasted outcrops on either side of the road which are good places to collect this rocktype.) Poikilitic Anorthositic Gabbro (MG-14, 11-9). An excellent example showing a common texture. Plag. An60, 01. Fo65, Opx En66, Cpx Wo42,En39,Fs19.

STOP 10 Briefly Laminated Gabbroic Anorthosite

Continue W on FR187 about 1.5 miles then north at "Pine Stump Corner" and then about one mile past the Lake "3" Campgrounds, turn right NE on FR 189 go 518 mile stop at what was once a small gravel pit on the left side of the road, now quite overgrown. Large polished outcrop of anorthositic rock in back wall of the pit (MG-9 ). Plag. An60, Cpx Wo37,En29,Fs34.

STOP 1 1 Ferrodiorite, Monzogabbro etc. (Transition Zone)

Continue along FR 189 Cross Brunsweiler River at 2 miles, continue about 314 mile beyond the river to where the road turns to an E direction. Walk straight S into the woods about 150 m where an irregular EW ridge exposes black rust stained Ferrodiorite (25-42; Plag, An45, 01. Fo24, Opx. En37, Cpx.Wo38 En24 Fs38 ) note cumulate and laminated texture and conspicuously finer grain size. Retrace your path back crossing the road to the north side working your way north along the west side of the hill. Several outcrops here show gradually increasing reddish felsic content (MG-16, 17).

STOP 12 Rearing Pond Intrusion

Turn around and go back over the Brunsweiler River past a gravel pit on right and turn right on woods road a few meters beyond. Continue northerly about 112 mile, turn left continue another 11 2 mile or less, then left again, and left again in about 114 mile. (If you encounter a bridge over a brook go back.) After the last turn proceed cautiously about 100m to a brook where the road ends. An outcrop on your left is peridotite (W-74, similar to 87-1 1 P)of the lower part of the Rearing Pond intrusion. Walk along what remains of the road (a path by now) cross the brook jumping rocks, and pick up the remains of the road on the other side. A few meters beyond the brook search to the right of the path for the remains of a picnic grounds (a couple of fire places is all that's left) and several large cliff like outcrops beyond. We will work our way around to the left and circle the hill studying the rocks and their textures and features as we proceed (RPMag, 87-1 I T and W-163) . This is the remains of a State trout hatchery that was abandoned at the beginning of WWII. Good Archaeology here.

STOP D Granophyre Time Permitting

Return to FR 189, turn right and return to FR 187. Proceed N on FR 187, 1.5 miles to the inflection point of an "S" turn (Large Outcrop on right) park on left W side of road. A few meters SW into the woods is an outcrop of a granitic rock. From this point proceed on a heading of 290 about 300 meters, eventually up a fairly tiring hill. Good exposures of Granophyre here. (MG-18, 19)

STOP E Alternate Granophyre exposure. Morgan Falls.

From the intersection of FR 187 & 198 go N 1/2 mile, turn left on FR 193 following the signs to Morgan Falls. Park in the lot provided and walk the trail to the falls. This is a spectacular 100 foot high waterfall over the brick red granophyre. Contact of granophyre and gabbro can be seen west of the falls along the upper part of the ridge.

GENERAL GEOLOGY AND STRUCTURE OF ARCHEAN ROCKS OF THE VIRGINIAHORN AREA, NORTHEASTERN MINNESOTA

J.L. Welsh, D.L. England, D.A. Groves, E. Levy

I nt roduct ion

The "Virginia Horn" refers to the prominent Z-shaped outcroppattern of Animikie Group rocks present in the Virginia-Eveletharea of the Mesabi Range (Fig. 1), and reflects the broadnortheasterly trending anticline and syncline into which theserocks have been folded. These Proterozoic rocks unconformablyoverlie Archean basement rocks, which are exposed north of theMesabi Range along the Laurentian Divide. Supracrustalmetavolcanic and rnetasedimentary rocks and assorted smallintrusive bodies of the basement complex occur along a narrowstrip which extends from Mountain Iron on the west to the Auroravicinity on the east, and which are bordered by the Giants RangeComplex on the north. These rocks are particularly well exposedin the anticlinal axis of the Virginia Horn, and have been therecent focus of gold exploration.

Previous Work

Until recently, Archean rocks in the Virginia Horn area havereceived little attention, and little has been published on theserocks. Brief descriptions of Archean rocks in the area werefirst provided by Leith (1903) as a result of his investigationsinto the geology of the Mesabi District. J.W. Gruner alsoinvestigated the Archean rocks during his studies of the MesabiDistrict, and reported an occurrence of visible gold in a felsicporphyry body (Grout, 1937) . Sutton (1963) developed a moredetailed geological map of the area, based largely on Gruner'swork, and provided petrographic descriptions for somemetasedimentary rocks and the felsic porphyry bodies.

The metavolcanic and metasedimentary rocks in the VirginiaHorn area have in the past been correlated with the ElyGreenstone and the Knife Lake Series, respectively, of theVermilion District (e.g. Grout et al., 1951). As these rocks arephysically separated from the Vermilion District by the GiantsRange Complex and were correlated only because of lithologicsimilarity, designations of Ely Greenstone and Knife Lake Seriesfor rocks in the Virginia Horn should no longer be used.

D- 1

GENERAL GEOLOGY AND STRUCTURE OF ARCHEAM ROCKS OF THE V I R G I N I A HORN AREA, NORTHEASTERN MINNESOTA

J.L. Welsh, D.L. England, D.A. Groves, E. Levy

Introduction

The "Virginia Horn" refers to the prominent 2-shaped outcrop pattern of Animikie Group rocks present in the Virginia-Eveleth area of the Mesabi Range (Fig. I ) , and reflects the broad northeasterly trending anticline and syncline into which these rocks have been folded. These Proterozoic rocks unconformably overlie Archean basement rocks, which are exposed north of the Mesabi Range along the Laurentian Divide. Supracrustal metavolcanic and metasedimentary rocks and assorted small intrusive bodies of the basement complex occur along a narrow strip which extends from Mountain Iron on the west to the Aurora vicinity on the east, and which are bordered by the Giants Range Complex on the north. These rocks are particularly well exposed in the anticlinal axis of the Virginia Horn, and have been the recent focus of gold exploration.

Previous Work

Until recently, Archean rocks in the Virginia Horn area have received little attention, and little has been published on these rocks. Brief descriptions of Archean rocks in the area were first provided by Leith (1903) as a result of his investigations into the geology of the Mesabi District. J.W. Gruner also investigated the Archean rocks during his studies of the Mesabi District, and reported an occurrence of visible gold in a felsic porphyry body (Grout, 1937). Sutton (1963) developed a more detailed geological map of the area, based largely on Grunerls work, and provided petrographic descriptions for some metasedimentary rocks and the felsic porphyry bodies.

The metavolcanic and metasedimentary rocks in the Virginia Horn area have in the past been correlated with the Ely Greenstone and the Knife Lake Series, respectively, of the Vermilion District ( e . g . Grout et al., 1951). As these rocks are physically separated from the Vermilion District by the Giants Range Complex and were correlated only because of lithologic similarity, designations of Ely Greenstone and Knife Lake Series for rocks in the Virginia Horn should no longer be used.

Figure 1. Regional map of the field trip area showing the geologic framework and themajor roads. The dotted contact is a major unconformity separating gently dippingProterozoic strata of the Animikie Group (on the south) from deformed Archeanrocks. The Animikie Group, consisting of the Pokegama Quartzite and BiwabikIron—Formation (open circles) and the Virginia Formation (diagonal rule), is

invaded by gabbroic rocks of Keweenawan age (1000 Ma) in the southeast corner ofthe map area. The field trip stops are all within Archean terrane.

D- 2

Figure 1. Regional map of t h e f i e l d t r i p a r e a showing the geologic framework and t h e major roads. The dot ted con tac t is a major unconformity separa t ing gen t ly dipping Pro te rozo ic s t r a t a of t h e Animikie Group (on t h e south) from deformed Archean rocks. The Animikie Group, c o n s i s t i n g of t h e Pokegama Q u a r t z i t e and Biwabik Iron-Formation ( open c i r c l e s 1 and t h e Virgin ia Formation (d iagonal r u l e ) , is invaded by gabbroic rocks of Keweenawan age (1000 Ma) i n t h e sou theas t corner of t h e map area. The f i e l d t r i p s t o p s a r e a l l within Archean t e r rane .

Lithologic Units

Supracrustal metavolcanic and metasedirnentary rocks comprisethe bulk of Archean rocks of the area. Also included in thissequence are a number of small felsic intrusive bodies, the mostimportant of these being a dacite(?) porphyry body, which is thetarget of current mineral exploration.

Metavolcanic rocks of the region occur in two separateareas, the north and northwest, and the southeast (Fig. 2); andare separated by a central band of metasedimentary rocks.Significant portions of the contacts between the majormetasedimentary and metavolcanic sequences, and probably allcontacts between these sequences, are faults. The northernmetavolcanic sequence is also in fault contact with the GiantsRange Complex between the Pike River and Highway 53 north ofVirginia.

1etavo1canjc Rocks

Metavolcanjc rocks consist primarily of grayish green todark green, fine- to medium-grained, massive to locally pillowedor brecciated flows. While chemical analyses of these rocks haveyet to be obtained, the rocks appear to be predominantly ofintermediate composition. Dacitic(?) fragmental rocks areinterlayered with the flows in the southernmost part of the areaand north of Biwabik. Coarser grained, probably intrusivemetadioritic or metagabbroic rocks also occur within thissequence, as do finer grained mafic dikes.

Foliation in the metavolcanic rocks is generally not welldeveloped. Typical greenschist facies assemblages of chlorite—albite+actinolite+epidote are present. Secondary mineralsinclude calcite' and sericite.

Metasedjmentary Rocks

Metasedimentary rocks of the area are subdivided into threefault-bounded units: a northern unit of moderately to highlyaltered, relatively quartzose metagraywackes, a centralconglomeratic unit, and a southern unit of metagraywackes.

Southern metagraywackes. This unit comprises the bulk ofthe metasedimentary rocks of the region. Numerous outcrops occursouth of Hwy. 135. These rocks consist predominantly ofinterbedded graywackes and slates. Quartz in these rocksaverages 5-10%, rarely exceeding 15%. When normalized to excludematrix, all rocks contain greater than 50% lithic fragments, with

D- 3

Lithologic U n i t s

Supracrustal metavolcanic and metasedimentary rocks comprise the bulk of Archean rocks of the area. Also included in this sequence are a number of small felsic intrusive bodies, the most important of these being a dacite(?) porphyry body, which is the target of current mineral exploration.

Metavolcanic rocks of the region occur in two separate areas, the north and northwest, and the southeast (Fig. 2); and are separated by a central band of metasedimentary rocks. Significant portions of the contacts between the major metasedimentary and metavolcanic sequences, and probably all contacts between these sequences, are faults. The northern metavolcanic sequence is also in fault contact with the Giants Range Complex between the Pike River and Highway 53 north of Virginia.

Metavolcanic Rocks

Metavolcanic rocks consist primarily of grayish green to dark green, fine- to medium-grained, massive to locally pillowed or brecciated flows. While chemical analyses of these rocks have yet to be obtained, the rocks appear to be predominantly of intermediate composition. Dacitic(?) fragmental rocks are interlayered with the flows in the southernmost part of the area and north of Biwabik. Coarser grained, probably intrusive metadioritic or metagabbroic rocks also occur within this sequence, as do finer grained mafic dikes.

Foliation in the metavolcanic rocks is generally not well developed. Typical greenschist facies assemblages of chlorite- albite+actinolite+epidote are present. Secondary minerals include calcite? and sericite.

Metasedimentarv Rocks

Metasedimentary rocks of the area are subdivided into three fault-bounded units: a northern unit of moderately to highly altered, relatively quartzose metagraywackes, a central conglomeratic unit, and a southern unit of metagraywackes.

Southern metaqrawackes. This unit comprises the bulk of the metasedimentary rocks of the region. Numerous outcrops occur south of Hwy. 135. These rocks consist predominantly of interbedded graywackes and slates. Quartz in these rocks averages 5-10% rarely exceeding 15%. When normalized to exclude matrix, all rocks contain greater than 5 0 % lithic fragments, with

felsic to felsic-intermediate clasts predominating. Intermediateto mafic clasts, though not abundant, are normally present.Detrital hornblende occurs in graywackes at one locality. Minoramounts of sericite are ubiquitous. Moderate to abundantsericite and carbonate are locally present, especially in morehighly strained zones.

These sandstones are turbidites, with Bouma A, AB, ABC, andE divisions discernable. Coarse clasts are rare, though somepebbly units do occur.

Altered metacTraywackes. Outcrops of this unit are not asabundant as the graywackes to the south. These rocks consist ofmoderately to highly altered, felsic volcanic-rich metagraywacke.In contrast to the southern metagraywackes, these rocks contain10-25% quartz and generally do not contain clasts of intermediateto mafic composition. Secondary sericite and carbonate areabundant, in some places comprise as much as 60% of the rock.It is likely that this alteration is related to the quartzfeldspar porphyry bodies that have intruded this sequence.

Beds tend to be thick, with a fair development of grading.Where discernable, Bouma A, AB, and ABC divisions (after Walker,1984) predominate.

Conglomeratic Unit. This unit is typically a dark green,heterogeneous, conglomerate. it is a composite unit, comprisedof various subunits based on differences in clast lithology. Thesubunits are typically heterolithic, though one distinctivemonolithic unit and units of pyroclastic affinity occur withinthis sequence. The subunits are difficult to trace along strike,and at least two of these subunits are in fault contact with eachother.

Clasts are normally rounded and of volcanic origin, butconsist of various compositional, types, generally of felsic tointermediate affinity. Particularly interesting are clasts ofeuhedral brown hornblende. These hornblende clasts are notrounded and suggest a crystal tuff origin. One distinctivesubunit is monolithic, with angular to subangular andesiticclasts, and was probably deposited as a debris flow. It toocontains clasts of brown hornblende. With the exception of themonolithic unit, the conglomerates are typically clast-'supported.The matrix of the conglomerates is generally chloritic; quartz israre in the matrix.

0-4

felsic to felsic-intermediate clasts predominating. Intermediate to mafic clasts, though not abundant, are normally present. Detrital hornblende occurs in graywackes at one locality. Minor amounts of sericite are ubiquitous. Moderate to abundant sericite and carbonate are locally present, especially in more highly strained zones.

These sandstones are turbidites, with Bouma A, AB, ABC, and E divisions discernable. Coarse clasts are rare, though some pebbly units do occur.

Altered metaqravwackes. Outcrops of this unit are not as abundant as the graywackes to the south. These rocks consist of moderately to highly altered, felsic volcanic-rich metagraywacke. In contrast to the southern metagraywackes, these rocks contain 10-25% quartz and generally do not contain clasts of intermediate to mafic composition. Secondary sericite and carbonate are abundant, in some places comprise as much as 60% of the rock. It is likely that this alteration is related to the quartz feldspar porphyry bodies that have intruded this sequence.

Beds tend to be thick, with a fair development of grading. Where discernable, Bouma A, AB, and ABC divisions (after Walker, 1984) predominate.

Conqlomeratic Unit. This unit is typically a dark green, heterogeneous, conglomerate. It is a composite unit, comprised of various subunits based on differences in clast lithology. The subunits are typically heterolithic, though one distinctive monolithic unit and units of pyroclastic affinity occur within this sequence. The subunits are difficult to trace along strike, and at least two of these subunits are in fault contact with each other.

Clasts are normally rounded and of volcanic origin, but consist of various compositional types, generally of felsic to intermediate affinity. Particularly interesting are clasts of euhedral brown hornblende. These hornblende clasts are not rounded and suggest a crystal tuff origin. One distinctive subunit is monolithic, with angular to subangular andesitic clasts, and was probably deposited as a debris flow. It too contains clasts of brown hornblende. With the exception of the monolithic unit, the conglomerates are typically clast-supported. The matrix of the conglomerates is generally chloritic; quartz is rare in the matrix.

Origin of the Metasedimentary Rocks

The conglomerates are interpreted to be upper fan channeldeposits, deposted by mass flow mechanisms. The graywackes areinterpreted to be proximal to mid-fan deposits, deposited byturbidity currents. The two graywacke units are alsocompositionally different, and neither compositions arecompatible with those of the conglomeratic unit. Nor are theycompositionally compatible with the adjacent metavolcanic units.These relationships support the notion that all three sedimentaryunits are in fault juxtaposition, being parts of differentsubmarine fans. The fans were located on the flanks of volcanicedifices, for all the detritus is volcanogenic.

Felsic Intrusjveg

A series of quartz-feldspar (probably dacite) porphyrybodies intrude the altered metagraywackes along a narrow belttrending approximately N8OE, and also appear to intrude thesouthernmost exposures of metavolcanic rocks along the contactwith the altered graywackes. These bodies are recognizable forthe most part by their conspicuous ovoidal quartz phenocrysts.Although not present in all rocks, these phenocrysts typicallyvary in amount from 2-5% and range in size from .5 cm to 2 cm.Plagioclase phenocrysts comprise 20-30% of the rock, range insize from 2 mm to 1.5 cm, and are nearly pure albite incomposition (Sutton, 1963) . The phenocrysts are set in anaphanitic, white to greenish-gray matrix of quartz and feldspar.Secondary sericite and carbonate (siderite or ankerite) areabundant, giving the rock its white to greenish color. Finelydisseminated sulfides are associated with quartz-carbonate veinsand vary greatly in amount from one location to another. Commonsulfides are pyrite and arsenopyrite, with rare sphalerite andchalcophyrite.

At least one non-porphyritic phase (it is microporphyriticin thin section) cuts the main porphyry body. Numerous,generally aphanitic, felsic dikes also cut the southern group ofgraywackes. A few of these intrusives contain quartzphenocrysts, but most do not. In thin section these dikesContain 30—50% plagioclase microphenocrysts in a fine felsicmatrix.

Structure

Upon first examination, these rocks appear to be relativelyundeformed; beds typically strike N60-70E. However, it has beendetermined that these rocks have been subjected to at least three

D- 5

Oriqin of the Metasedimentary Rocks

The conglomerates are interpreted to be upper fan channel deposits, deposted by mass flow mechanisms. The graywackes are interpreted to be proximal to mid-fan deposits, deposited by turbidity currents. The two graywacke units are also compositionally different, and neither compositions are compatible with those of the conglomeratic unit. Nor are they compositionally compatible with the adjacent metavolcanic units. These relationships support the notion that all three sedimentary units are in fault juxtaposition, being parts of different submarine fans. The fans were located on the flanks of volcanic edifices, for all the detritus is volcanogenic.

A series of quartz-feldspar (probably dacite) porphyry bodies intrude the altered metagraywackes along a narrow belt trending approximately N80E, and also appear to intrude the southernmost exposures of metavolcanic rocks along the contact with the altered graywackes. These bodies are recognizable for the most part by their conspicuous ovoidal quartz phenocrysts. Although not present in all rocks, these phenocrysts typically vary in amount from 2-5% and range in size from . 5 cm to 2 cm. Plagioclase phenocrysts comprise 20-30% of the rock, range in size from 2 mm to 1.5 cm, and are nearly pure albite in composition (Sutton, 1963). The phenocrysts are set in an aphanitic, white to greenish-gray matrix of quartz and feldspar. Secondary sericite and carbonate (siderite or ankerite) are abundant, giving the rock its white to greenish color. Finely disseminated sulfides are associated with quartz-carbonate veins and vary greatly in amount from one location to another. Common sulfides are pyrite and arsenopyrite, with rare sphalerite and chalcophyrite.

At least one non-porphyritic phase (it is microporphyritic in thin section) cuts the main porphyry body. Numerous, generally aphanitic, felsic dikes also cut the southern group of graywackes. A few of these intrusives contain quartz phenocrysts, but most do not. In thin section these dikes contain 30-50% plagioclase microphenocrysts in a fine felsic matrix.

Structure

Upon first examination, these rocks appear to be relatively undeformed; beds typically strike -N60-70E. However, it has been determined that these rocks have been subjected to at least three

periods of folding, and have been cut by a significantnortheasterly trending fault/shear system (Welsh, 1988)

Folding

F1 folds have not been recognized in the field, but areidentified by reversals in structural facing. F2 folds are tightto isoclinal, trend northeasterly, and are steeply plunging.While a few F2 folds are visible in outcrop, most are identifiedby reversals in stratigraphic facing. Cleavage (S2) trends areconsistently parallel to the axial planes of the F2 folds. Minorfolds with gentle south-plunging axes have been recognized inthree localities, and are tentatively designated as F3.

Faulting

These rocks have been cut by a complex northeast trendingfault/shear system which roughly coincides with the axis of theVirginia Horn (Fig. 2) . In the northern part of the area mostdisplacement appears to have been taken up along the Pike RiverFault (Welsh, 1988) . Outcroppings adjacent to the fault exhibitthe effects of considerable ductile shear. In the southern partof the area, a number of subparallel, probably en echelon, faultstrands Cut throughmetasedimentary rocks and appear to link withthe Pike River Fault. The contact between the metasedimentaryrocks and rnetavolcanic rocks to the southeast is also a fault.Here north-topping metavolcanic rocks are juxtaposed againstsouth—topping metasedimentary rocks. These en echelon faults areinterpreted as being part of a strike-slip duplex (afterWoodhouse, 1986).

Strain in these rocks is concentrated into narrow zones ofductile shear associated with these faults. Clasts along themargins of the conglomerate are distinctly flattened in the planeof foliation. Clasts internal to the body are generally not asflattened, but appear to be elongated in the vertical direction.

Movement along this system was probably complex. Althoughoffset market units have not been identified, map patternssuggest sinistral drag, as beds are typically rotated to a N40-50E strike in the vicinity of the Pike River Fault. En echelonquartz—filled tension fractures in the dacite porphyry alsosupport a sinistral sense of shear. Other kinematic indicators,however, such as minor Z-folds, and asymmetric pressure shadowson porphyroclasts suggest that the (latest?) sense of shear wasdextral. In addition, high angle slickensides are associatedwith the en echelon strands of the duplex and indicate verticalmovement at some time in the structural history of the area.

D-6

periods of folding, and have been cut by a significant northeasterly trending fault/shear system (Welsh, 1988).

Foldinq

F, folds have not been recognized in the field, but are identified by reversals in structural facing. F, folds are tight to isoclinal, trend northeasterly, and are steeply plunging. While a few F, folds are visible in outcrop, most are identified by reversals in stratigraphic facing. Cleavage (S,) trends are consistently parallel to the axial planes of the F, folds. Minor folds with gentle south-plunging axes have been recognized in three localities, and are tentatively designated as F3.

Faultinq

These rocks have been cut by a complex northeast trending fault/shear system which roughly coincides with the axis of the Virginia Horn (Fig. 2). In the northern part of the area most displacement appears to have been taken up along the Pike River Fault (Welsh, 1988). Outcroppings adjacent to the fault exhibit the effects of considerable ductile shear. In the southern part of the area, a number of subparallel, probably en echelon, fault strands cut through,metasedimentary rocks and appear to link with the Pike River Fault. The contact between the metasedimentary rocks and metavolcanic rocks to the southeast is also a fault. Here north-topping metavolcanic rocks are juxtaposed against south-topping metasedimentary rocks. These en echelon faults are interpreted as being part of a strike-slip duplex (after Woodhouse, 1986).

Strain in these rocks is concentrated into narrow zones of ductile shear associated with these faults. Clasts along the margins of the conglomerate are distinctly flattened in the plane of foliation. Clasts internal to the body are generally not as flattened, but appear to be elongated in the vertical direction.

Movement along this system was probably complex. Although offset market units have not been identified, map patterns suggest sinistral drag, as beds are typically rotated to a N40- 50E strike in the vicinity of the Pike River Fault. En echelon quartz-filled tension fractures in the dacite porphyry also support a sinistral sense of shear. Other kinematic indicators, however, such as minor 2-folds, and asymmetric pressure shadows on porphyroclasts suggest that the (latest?) sense of shear was dextral. In addition, high angle slickensides are associated with the en echelon strands of the duplex and indicate vertical movement at some time in the structural history of the area.

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Economic Geology

The dacite porphyry body is a current target for goldexploration in the area. Gold was first reported in this unit byGruner in 1924 (Grout, 1937), and is associated with the quartz-carbonate veins and shear zones within the body.

Sericite—carbonate alteration has also been identified withthe fault/shear zones of Pike River System (Welsh, 1988)Although silicification and sulfidation along the shear zonesappears minimal where rocks are exposed, these zones meritfurther investigation.

References

Grout, F. F., 1937, Petrographic study of gold prospects ofMinnesota: Econ. Geol., vol. 32, pp. 56-68.

Grout, F.F., Gruner, J.W., Schwartz, G.M., and Thiel, G.A., 1951,Precambrian stratigraphy of Minnesota: Geol. Soc. Am. Bull.,v. 62, pp. 1017—78

Leith, C. K., 1903, The Mesabi iron-bearing district ofMinnesota: U.S.Geol. Surv. Monograph, vol. 43.

Sutton, T. C., 1963, Geology of the Virginia Horn area: M.S.thesis, University of Minnesota, Minneapolis.

Walker, R.G., 1984, Turbidites and associated coarse clasticdeposits: in Facies Models, R.G. Walker ed., GeoscienceCanada Reprint Series 1, pp. 171-188.

Welsh, J. L., 1988, Preliminary Structural Analysis of Archeanrocks in the Virginia Horn area, northeastern Minnesota(abs): 34th Ann. Inst.on Lake Superior Geology, v. 34,part 1, pp. 119—120.

Woodhouse, N. H., 1986, Strike-slip duplexes: Journal ofStructural Geology, V. 8, no. 7, pp. 725—735.

0-8

Economic Geology

The dacite porphyry body is a current target for gold exploration in the area. Gold was first reported in this unit by Gruner in 1924 (GroutI l937lI and is associated with the quartz- carbonate veins and shear zones within the body.

Sericite-carbonate alteration has also been identified with the faultlshear zones of Pike River System (WelshI 1988). Although silicification and suliidation along the shear zones appears minimal where rocks are exposedI these zones merit further investigation.

Grout, I?. E d I 1937, Petrographic study of gold prospects of Minnesota: Econ. Ge01.~ vol. 32# pp. 56-68.

Grout I E'. I?. I Gruner, J. W. I Schwartz, G. M. I and ThielI G.A. I 1951 I Precambrian stratigraphy of Minnesota: Geol. Soc. Am. Bul1.I v. 6ZI pp. 1017-78

LeithI C. K. I 1903# The Mesabi iron-bearing district of Minnesota: U.S.Geo1. Surv. MonographI vol. 43.

SuttonI T. C., 1963# Geology of the Virginia Horn area: M.S. thesisI University of Minnesota, Minneapolis.

WalkerI R.GeI 19841 Turbidites and associated coarse clastic deposits: in Facies ModelsI R.G. Walker edaI Geoscience Canada Reprint Series lI pp. 171-188.

WelshI J. L. 1988/ Preliminary Structural Analysis of Archean rocks in the Virginia Horn area, northeastern Minnesota (abs) : 34th Ann. Inst .on Lake Superior GeologyI v. 3dI part lI pp. 119-120.

WoodhouseI N. H e I 198GI Strike-slip duplexes: Journal of Structural GeologyI V. EI no. pp. 725-735.

6-0

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Road Log

From Duluth, travel north along U.S. 53 to Eveleth. Passthrough Eveleth, and turn right at the intersection of U.S. 53and Minn. 135, and proceed toward Gilbert for 1.5 miles, turningleft at dirt road (with gate) . Stop at gate.

STOP 1. Entrance to Viking Explosive property. At this localitythe contact between the southern graywacke-slates and theconglomeratic unit can be observed. Rhythmically beddedturbiditic graywackes are exposed on the east side of the accessroad. Though tops are difficult to discern here, tops are north.Careful examination of the outcrop reveals that the sedimentlayers are disrupted along faults at low angles to bedding.Small scale isoclinal folds are also present.

Rhyhmically bedded, more highly sericitized graywackes alsooccur on the west side of the access road north of the gate, butwith tops reversed. A prominent 'Z' kink fold is also displayedin these metasediment,s.

Outcrops on the west side of the access road occupying thehigher ground, and exposed along the highway are in thecoriglomeratic unit. Here the conglomerate is heterolithic,consisting of various felsic volcanic clasts in a chloriticmatrix. Evidence of grading occurs in outcrops on the south sideof the highway, and indicate south tops.

The contact between the graywackes and the conglomerate is aphyllonite, and is exposed along the west edge of the accessroad. Clasts in the conglomerate along the contact are flattenedas a result of the strain. Minor asymmetrical 'Z' folds in thephyllonite can be sernen in thin section, as are small Z—stylekinks. A secondary cleavage, probably the result of pressuresolution is also developed in the phyllonite.

The phyllonite is interpreted to represent a strand of thePike River shear zone, and appears to run along much (and perhapsalong the length) of the contact between the graywackes and theconglornerate.The contact between the conglomerate and the alteredgraywackes is also highly strained and altered, and probablyrepresents another shear zone. The "Z" structures associatedwith these shear zones indicate dextral shear.

0-10

Road Log

From Duluthf travel north along U.S. 53 to Eveleth. Pass through Eveleth, and turn right at the intersection of U.S. 53 and Minn. 135! and proceed toward Gilbert for 1.5 miles, turning left at dirt road (with gate). Stop at gate.

STOP 1. Entrance to Viking Explosive property. At this locality the contact between the southern graywacke-slates and the conglomeratic unit can be observed. Rhythmically bedded turbiditic graywackes are exposed on the east side of the access road. Though tops are difficult to discern here, tops are north. Careful examination of the outcrop reveals that the sediment layers are disrupted along faults at low angles to bedding. Small scale isoclinal folds are also present.

Rhyhmically beddedf more highly sericitized graywackes also occur on the west side of the access road north of the gate, but with tops reversed. A prominent 'Zf kink fold is also displayed in these metasediments.

Outcrops on the west side of the access road occupying the higher ground, and exposed along the highway are in the conglomeratic unit. Here the conglomerate is heterolithic, consisting of various felsic volcanic clasts in a chloritic matrix. Evidence of grading occurs in outcrops on the south side of the highway, and indicate south tops.

The contact between the graywackes and the conglomerate is a phyllonite, and is exposed along the west edge of the access road. Clasts in the conglomerate along the contact are flattened as a result of the strain. Minor asymmetrical #Zf folds in the phyllonite can be seen in thin sectionf as are small Z-style kinks. A secondary cleavage, probably the result of pressure solution is also developed in the phyllonite.

The phyllonite is interpreted to represent a strand of the Pike River shear zonef and appears to run along much (and perhaps along the length) of the contact between the graywackes and the conglomerate.The contact between the conglomerate and the altered graywackes is also highly strained and alteredf and probably represents another shear zone. The "2" structures associated with these shear zones indicate dextral shear.

Proceed back along Hwy. 135 toward Virginia for about 1/4 mileand turn right secondary road. Follow this road forapproximately one mile, and turn right on dirt road to landfill.Proceed to DM&IR Railroad Tracks and park. Walk east forapproximately 1/4 mile, until first outcrops are reached.

STOP 2. At this locality two feldspar porphyry "dikes",generally lacking quartz phenocrysts, are interfingered withgraywackes. Both the metasediments and porphyry are extensivelyaltered with sericite and iron carbonate. The porphyry containsabundant quartz-iron carbonate veins and abundant disseminatedpyrite. J.W. Gruner reported visible gold at this locality (seeGrout, 1937)

Continue eastward along the railroad tracks for approximately 1/3mile to the next set of outcroppings.

STOP 3. Metagraywacks of the altered graywacke unit.

These graywacke-slates are more thickly bedded than therhythmically bedded turbidites of Stop 1. The rocks aredeformed, and show good bedding-cleavage relationships. The highangle between bedding and cleavage suggests that these outcropsoccur in the nose of a fold, and top relationships from othergraywacke outcrops in the area support this interpretation.Cleavages are gently folded, possibly the result of intrusion ofthe dacite porphyry. The contact with the porphyry occursapproximately 50 feet south of the south exposure, and isrelatively sharp.

From these outcrops walk south to the outcrops near the abandonedbuildings which are in sight.

STOP 4. Rhude and Fryberger prospect.

Numerous outcrops of quartz-feldspar porphyry have been strippedat this location. In general the QFP consists of quartzphenocrysts (.5-2 cm/2-5%) and plagioclase phenocrysts (2 mm—1.5cm/25-30%) set in a greenish aphanitic groundmass of quartz,feldspar, and sericite. Fine-grained disseminated pyrite andarsenopyrite occur in variable amounts throughout the area,particularly in association with quartz-carbonate veins.

D-11

Proceed back along Hwy. 135 toward Virginia for about 114 mile and turn right secondary road. Follow this road for approximately one mile, and turn right on dirt road to landfill. Proceed to DM&IR Railroad Tracks and park. Walk east for approximately 114 mile, until first outcrops are reached.

STOP 2. At this locality two feldspar porphyry "dikes", generally lacking quartz phenocrysts, are interfingered with graywackes. Both the metasediments and porphyry are extensively altered with sericite and iron carbonate. The porphyry contains abundant quartz-iron carbonate veins and abundant disseminated pyrite. J.W. Gruner reported visible gold at this locality (see Groutf 1937).

Continue eastward along the railroad tracks for approximately 113 mile to the next set of outcroppings.

STOP 3. ~etagraywackks of the altered graywacke unit.

These graywacke-slates are more thickly bedded than the rhythmically bedded turbidites of Stop 1. The rocks are deformedf and show good bedding-cleavage relationships. The high angle between bedding and cleavage suggests that these outcrops occur in the nose of a fold, and top relationships from other graywacke outcrops in the area support this interpretation. Cleavages are gently folded, possibly the result of intrusion of the dacite porphyry. The contact with the porphyry occurs approximately 50 feet south of the south exposure, and is relatively sharp.

From these outcrops walk south to the outcrops near the abandoned buildings which are in sight.

STOP 4. Rhude and Fryberger prospect.

Numerous outcrops of quartz-feldspar porphyry have been stripped at this location. In general the QFP consists of quartz phenocrysts (-5-2 cm/2-5%) and plagioclase phenocrysts ( 2 mm-1.5 cm/25-30%) set in a greenish aphanitic groundmass of quartz, feldspar, and sericite. Fine-grained disseminated pyrite and arsenopyrite occur in variable amounts throughout the areaf particularly in association with quartz-carbonate veins.

The non-porphyritic phase of the intrusion is present atthis location. It can be seen cross-cutting the main porphyriticbody on an east-southeasterly trend. Located under the powerline to the west is a rotated, rafted sedimentary inclusionwithin QFP. (As you survey the outcrops take note of theattitudes of the quartz-carbonate veins, and how fracturingaffects these veins, and their relationship to foliation.

From these outcrops proceed east along the railroad tracks forapproximately 1/3 mile (about 100 yards before the large railroadcut), to the outcrop on the south side of the tracks.

STOP 5. Newmont Prospect

This historical occurrence first received attention at the turnof the century. C. K. Leith's (1903) exemplary study of thegeology of the Mesabi Range described the older volcanic andsedimentary rocks of the Virginia Horn area. He noted thepresence of "porphyritic granite" with secondary sericite,chlorite and quartz at this locality. In 1924, Dr. John Grunerled a field party through the Virginia Horn area and collectedseveral gold-bearing samples from "rhyolite porphyry".Petrographic descriptions of these samples were published by F.F. Grout (1937), and he noted grains of gold in several samples.

On the south side of the railroad grade approximately 1/4 mileeast of the old Hercules powder plant, outcrops of quartz,feldspar porphyry flank a stream channel within a narrowtopographic low. A flat, glaciated outcrop on the east side ofthe stream exhibits several quartz and quartz—carbonate vein setswithin variably sericitized and silicified QFP.

At this location, a prominent, steeply-dipping vein set strikesapproximately N—S. Adjacent walirock contains very fine-grained,disseminated pyrite and arsenopyrite within a gray-greenaphanitic groundmass. The N-S veins occupy en echelon tensionfractures which flank narrow shear zones striking N4OE. Theattitude of the tension veins to the shear planes indicates aleft-lateral sense of shear. Other vein sets within the QFPinclude 1) foliation-parallel quartz-carbonate veins withinshears, and 2) subhorizontal quartz-carbonate veins occupyingconjugate fractures. Gold mineralization is associated withseveral vein sets.

D-12

The non-porphyritic phase of the intrusion is present at this location. It can be seen cross-cutting the main porphyritic body on an east-southeasterly trend. Located under the power line to the west is a rotated, rafted sedimentary inclusion within QFP. (As you survey the outcrops take note of the attitudes of the quartz-carbonate veins, and how fracturing affects these veins, and their relationship to foliation.

From these outcrops proceed east along the railroad tracks for approximately 1/3 mile (about 100 yards before the large railroad cut), to the outcrop on the south side of the tracks,

STOP 5 . Newmont Prospect

This historical occurrence first received attention at the turn of the century. C. K. Leithts (1903) exemplary study of the geology of the Mesabi Range described the older volcanic and sedimentary rocks of the Virginia Horn area. He noted the presence of "porphyritic granite" with secondary sericite, chlorite and quartz at this locality. In 1924, Dr. John Gruner led a field party through the Virginia Horn area and collected several gold-bearing samples from "rhyolite porphyry". Petrographic descriptions of these samples were published by F. F. Grout (1937), and he noted grains of gold in several samples.

On the south side of the railroad grade approximately 1/4 mile east of the old Hercules powder plant, outcrops of quartz, feldspar porphyry flank a stream channel within a narrow topographic low. A flat, glaciated outcrop on the east side of the stream exhibits several quartz and quartz-carbonate vein sets within variably sericitized and silicified QFP.

At this location, a prominent, steeply-dipping vein set strikes approximately N-S. Adjacent wallrock contains very fine-grained, disseminated pyrite and arsenopyrite within a gray-green aphanitic groundmass. The N-S veins occupy en echelon tension fractures which flank narrow shear zones striking N40E. The attitude of the tension veins to the shear planes indicates a left-lateral sense of shear. Other vein sets within the QFP include 1) foliation-parallel quartz-carbonate veins within shears, and 2) subhorizontal quartz-carbonate veins occupying conjugate fractures. Gold mineralization is associated with several vein sets.

Continue east to large railroad cut.

STOP 6. Conglomeratatic unit.

The most extensive exposures of the coarse volcaniclasticunit occur at this locality. Here the unit can be divided intotwo subunits which are separated by a fault, a monolithic unitand a heteroljthjc unit. The northwestern 250 feet of therailroad cut is marked by a remarkably monolithic, generallymatrix supported conglomerate. The clasts are quite irregular inboth size and shape, are angular to subrounded, and unsorted.Clasts are of porphyritc hornblende andesite. The matrix ischioritic, and contains hornblende crystal clasts. This unit isinterpreted to have been deposited by debris flow, probablysubaqueous lahars.

In the southeastern half of the cut, the rocks are marked bydistinct morphologic and lithologic differences from the north.The remainder of the conglomeratic unit is generally like theserocks at the southeastipart of the railroad cut. They aregenerally heterolithic and clast supported, and often have normaland inverse to normal grading. The matrix contains very littlequartz, but locally contains euhedral brown hornblende clasts?These conglomerates are thought to be upper fan channel deposits.

Upon returning to vehicles drive to Gilbert. From town, turnright and proceed uphill toward school.

STOP 6. (optional) Outcops behind Gilbert High School athleticfield.

These outcrops contain the best exposures of pillowedmetavolcanic rocks in the area. The pillows strike roughly east-west (note the northeasterly trend of the contact with themetasedimentary rocks to the west), and top north. (Themetasedimentary sequence to the west generally tops southeastalong the contact)

. The pillows at this locality are truncatedby massive volcanics along a N25E trend. Whether this contactrepresents an intrusive contact or shear has not been determined.However, foliation along this contact, minor quartz veining, andapparent drag of the pillows into the contact suggest rightlateral shear. Confusing the matter are the presence of pillowson the far northwest corner of the outcrop area.

0-13

Continue east to large railroad cut.

STOP 6. Conglomeratatic unit.

The most extensive exposures of the coarse volcaniclastic unit occur at this locality. Here the unit can be divided into two subunits which are separated by a fault, a monolithic unit and a heterolithic unit. The northwestern 250 feet of the railroad cut is marked by a remarkably monolithic, generally matrix supported conglomerate. The clasts are quite irregular in both size and shape, are angular to subrounded, and unsorted. Clasts are of porphyritc hornblende andesite. The matrix is chloritic, and contains hornblende crystal clasts. This unit is interpreted to have been deposited by debris flow, probably subaqueous lahars.

In the southeastern half of the cut, the rocks are marked by distinct morphologic and lithologic differences from the north. The remainder of the conglomeratic unit is generally like these rocks at the southeast~part of the railroad cut. They are generally heterolithic and clast supported, and often have normal and inverse to normal grading. The matrix contains very little quartz, but locally contains euhedral brown hornblende clasts? These conglomerates are thought to be upper fan channel deposits.

Upon returning to vehicles drive to Gilbert. From town, turn right and proceed uphill toward school.

STOP 6. (optional) Outcops behind Gilbert High School athletic field.

These outcrops contain the best exposures of pillowed metavolcanic rocks in the area. The pillows strike roughly east- west (note the northeasterly trend of the contact with the metasedimentary rocks to the west), and top north. (The metasedimentary sequence to the west generally tops southeast along the contact). The pillows at this locality are truncated by massive volcanics along a N25E trend. Whether this contact represents an intrusive contact or shear has not been determined. However, foliation along this contact, minor quartz veining, and apparent drag of the pillows into the contact suggest right lateral shear. Confusing the matter are the presence of pillows on the far northwest corner of the outcrop area.

FIELD TRIP 4, PART B

ARCHEAN GOLD OCCURRENCES AND THEIR STRUCTURAL SETTINGS: WESTERN AND CENTRALVERMILION DISTRICTLeaders: D.L. Southwick, P.J. Hudleston, R.L. Bauer, W. Ulland

INTRODUCTION

Several recent publications have highlighted the important role playedby ductile shear zones and faults in the later stages of transpressionaltectonism in the western Vermilion district of northeastern Minnesota(Sims, 1976; Bauer, 1985; Hudleston and others, 1987, 1988). These papersalso provide useful summaries of the regional structural geology, and werefer interested readers to them for background and further references onthe deformational history of the area.

Various lines of evidence indicate that the deformation responsible forthe regional ENE cleavage in the Vermilion district was the second toaffect the area, or D2. The regional cleavage therefore is labelled S2 andthe folds to which it is related are F2 structures. Finite strain studies(Hudleston, 1976; Schultz—Ela, 1988) and sense—of—shear observations implythat D2 was a transpression that involved regional north-south flattening,steeply to moderately pluiging extension, and dextral shear. The mostimportant structures to form in the later stages of D2 were zones ofintense ductile or brittle—ductile shear such as the Mud Creek shear zoneand the shear zones near Shagawa Lake. More brittle dextral faults, suchas the Vermilion fault and its subsidary breaks, may be still later mani-festations of the same transpressional regime.

Structural studies in gold-mining districts in the Superior Province ofCanada have demonstrated that shear zones were instrumental and perhapscritical in the localization of gold ores (Colvine and others, 1988, andreferences therein). The clear spatial association between shear zones andthe most important classes of greenstone—belt vein deposits of gold hasprompted much interest in shear zones as exploration targets. For thisreason exploration geologists have been working in the shear zones of theVermilion district for several years, and they have found shear phenomenaand even some major shear zones that were not recognized in publishedmapping by geological surveys.

Because of the widespread academic and applied interest in shear—zonephenomena, we here describe five outstanding and easily accessible shearzone outcrops in the Vermilion district where the features of the rocks canbe readily observed and debated.

ROAD LOG AND STOP DESCRIPTIONS

The log begins at the intersection of St. Louis County route 408 (MudCreek Road) and state highway 1-169 approximately 11.5 miles east ofSoudan (Fig. 1 and 2).

0.0 Intersection of county route 408 and highway 1—169. Proceed north on408 and drive cautiously; road is crooked and hilly and carries asurprising volume of traffic.

D-14

ARCHEAN GOLD OCCURRENCES VERMILION DISTRICT Leaders: D.L. Southwick,

FIELD TRIP 4, PART B

AND THEIR STRUCTURAL SETTINGS: WESTERN AND CENTRAL

P.J. Hudleston, R.L. Bauer, W. Ulland

INTRODUCTION

S e v e r a l r e c e n t pub l i ca t ions have h igh l igh ted t h e important r o l e played by d u c t i l e shea r zones and f a u l t s i n the l a t e r s t a g e s of t r a n s p r e s s i o n a l tec tonism i n the western Vermilion d i s t r i c t of no r theas t e rn Minnesota (Sims, 1976; Bauer, 1985; Hudleston and o t h e r s , 1987, 1988). These papers a l s o provide u s e f u l summaries of t he r e g i o n a l s t r u c t u r a l geology, and we r e f e r i n t e r e s t e d readers to them f o r background and f u r t h e r r e f e rences on t h e deformat iona l h i s t o r y of t he a rea .

Various l i n e s of evidence i n d i c a t e that the deformation r e spons ib l e f o r t h e r eg iona l ENE cleavage i n t he Vermilion d i s t r i c t was the second to a f f e c t the a r e a , o r D2. The r eg iona l c leavage t h e r e f o r e is l a b e l l e d S2 and t h e f o l d s t o which it is r e l a t e d a r e F2 s t r u c t u r e s . F i n i t e s t r a i n s t u d i e s (Hudleston, 1976; Schultz-Ela, 1988) and sense-of-shear observa t ions imply t h a t D2 was a t r ansp res s ion t h a t involved r e g i o n a l north-south f l a t t e n i n g , s t e e p l y t o moderately pludging ex tens ion , and d e x t r a l shear . The most impor tan t s t r u c t u r e s to form i n the l a t e r s t a g e s of Dz were zones of i n t e n s e d u c t i l e o r b r i t t l e - d u c t i l e s h e a r such a s t h e Mud Creek shea r zone and the shea r zones near Shaqawa Lake. More b r i t t l e d e x t r a l f a u l t s , such a s the Vermilion f a u l t and i ts subs ida ry breaks , may be s t i l l l a t e r mani- f e s t a t i o n s of the same t r a n s p r e s s i o n a l regime.

S t r u c t u r a l s t u d i e s i n gold-mining d i s t r i c t s i n t h e Super ior Province of Canada have demonstrated t h a t shea r zones were in s t rumen ta l and perhaps c r i t i c a l i n the l o c a l i z a t i o n of gold o r e s (Colvine and o t h e r s , 1988, and r e f e r e n c e s t h e r e i n ) . The c l e a r s p a t i a l a s s o c i a t i o n between shea r zones and t h e most important c l a s s e s of greens tone-be l t ve in d e p o s i t s of gold has prompted much i n t e r e s t i n shear zones a s e x p l o r a t i o n t a r g e t s . For this reason exp lo ra t ion geo log i s t s have been working i n the shea r zones of the Vermilion d i s t r i c t f o r s e v e r a l yea r s , and they have found shear phenomena and even some major shear zones that were no t recognized i n publ ished mapping by geo log ica l surveys.

Because of the widespread academic and app l i ed i n t e r e s t i n shear-zone phenomena, w e here desc r ibe f i v e ou t s t and ing and e a s i l y a c c e s s i b l e shea r zone outcrops i n the Vermilion d i s t r i c t where t h e f e a t u r e s of the rocks can

I be r e a d i l y observed and debated.

I ROAD LOG AND STOP DESCRIPTIONS

The l o g begins a t the i n t e r s e c t i o n of S t . Louis County rou te 408 (Mud I Creek Road) and s t a t e highway 1-169 approximately 11.5 mi les e a s t of ~ Soudan (Fig. 1 and 2 ) .

0.0 I n t e r s e c t i o n of county rou te 408 and highway 1-169. Proceed n o r t h on 408 and d r i v e caut ious ly ; road is crooked and h i l l y and c a r r i e s a s u r p r i s i n g volume of t r a f f i c .

D-14

3.7 Mud Creek; park vehicles on accessible high ground and disembark; walkto outcrops.

Stop 1. The Mud Creek shear zone; outcrops near the crossing of MudCreek Road over Mud Creek: SE1/4SE1/4 sec. 5, T. 62 N., R. 14 W.

Several small outcrops in the valley of Mud Creek illustrate varioussmall-scale structures characteristic of rocks that have undergone intenseshear strain, all attributed to D2.

The best exposures are in scrub just north of the creek and withinabout 100 m of the road on the east side. The best example of local S2'and F2 developed in a lens of otherwise uniform S2 is here (see Fig. 3).In general S2 is subparallel to the margins of the Mud Creek shear zone (N.700 E.). Locally, it has been perturbed and rotated clockwise about 40°,to form folds with a secondary crenulation cleavage (S21) developedparallel to the axial plane. Both cleavages can be traced from within theperturbed zone outward to merge into a single planar fabric, S2, in thesurrounding rock. Good examples of en echelon tension veins can also befound in nearby outcrops.

On the outside of the first bend in the road north of the creek is aroadcut in a pinkish quartz sericite schist, a rock produced by intenseshear. Nice shear bands (or C' planes; see Fig. 4) are developed in thisrock, which is rendered friable by the closely spaced and intersecting Sand C' planes.

A number of features of these outcrops provide indicators of sense ofshear, and these are consistently dextral. They include shear bands (or C'planes); local development of Sf where S2 has been perturbed and rotatedclockwise; and formation of Z folds in S2 foliation (most commonly in asso-ciation with Sf) and in quartz veins. Although well developed in highlysheared rocks such as at Mud Creek, similar features can be found throughmuch of the Vermilion district, increasing in the intensity of developmentas the Vermilion fault is approached.

The Mud Creek shear zone is flanked on the north by pillowed greenstone(upper member of the Ely Greenstone) that is moderately deformed except innarrow shear zones, and on the south by assorted felsic tuff, tuff-breccia,block breccia, and the reworked sedimentary equivalents of these(tuffaceous member of the Lake Vermilion Formation). Sims and Southwick(1980, 1985) interpreted the highly schistose material within the shearzone to have been derived chiefly from fine—grained felsic to intermediatetuff belonging to the Lake Vermilion sequence. It is now recognized thatshear zones of this magnitude commonly contain the sheared equivalent ofmany different rock types, all reduced to a more or less common "faultrock" composed chiefly of quartz, sericite, and chlorite. The phylloniticrocks of the Mud Creek shear zone are similar to the "fault rocks" alongthe Rainy Lake—Seine River fault zone in southern Ontario (Poulsen, 1983,1986) and to those associated with many other strike—slip fault zoneselsewhere in the Superior Province. The westward extent of the Mud Creekshear zone beneath Lake Vermilion has not been established.

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3.7 Mud Creek; park veh ic l e s on a c c e s s i b l e h igh ground and disembark; walk t o outcrops.

s t o p 1. The Mud Creek shear zone; ou tcrops nea r the c r o s s i n g of ~ u d Creek Road over Mud Creek: SE1/4SE1/4 sec . 5, T. 62 N., R. 14 W.

Seve ra l sma l l outcrops i n t h e v a l l e y of Mud Creek i l l u s t r a t e var ious smal l - sca le s t r u c t u r e s c h a r a c t e r i s t i c of rocks that have undergone i n t e n s e s h e a r s t r a i n , a l l a t t r i b u t e d t o D2.

The b e s t exposures a r e i n scrub j u s t no r th of the creek and wi th in about 100 m of the road on the e a s t s ide . The b e s t example of l o c a l S21 and Fy developed i n a l e n s of otherwise uniform S2 is he re ( s e e Fig. 3 ) . I n genera l S2 i s s u b p a r a l l e l t o the margins of the Mud Creek shear zone (N. 70Â E.). Local ly , it has been perturbed and r o t a t e d clockwise about 40Â° t o form f o l d s wi th a secondary c renu la t ion c leavage ( S 2 @ ) developed p a r a l l e l t o the a x i a l plane. Both cleavages can be t r a c e d from wi th in the per turbed zone outward t o merge i n t o a s i n g l e p l a n a r f a b r i c , S2, i n t he surrounding rock. Good examples of en echelon t e n s i o n ve ins can a l s o be found i n nearby outcrops.

On the o u t s i d e of the f i r s t bend i n t h e road no r th of t he c reek is a roadcut i n a p ink i sh qua r t z s e r i c i t e s c h i s t , a rock produced by i n t e n s e shear . Nice s h e a r bands ( o r C ' p lanes ; s e e Fig. 4 ) a r e developed i n t h i s rock, which is rendered f r i a b l e by the c l o s e l y spaced and i n t e r s e c t i n g S and C 1 planes.

A number of f e a t u r e s of t hese outcrops provide i n d i c a t o r s of sense of s h e a r , and these a r e c o n s i s t e n t l y d e x t r a l . They inc lude s h e a r bands ( o r C' p l a n e s ) ; l o c a l development of 82' where S5 has been per turbed and r o t a t e d clockwise; and formation of Z f o l d s i n 83 f o l i a t i o n (most commonly i n asso- c i a t i o n w i t h S$ ) and i n qua r t z veins. Although we l l developed i n h ighly sheared rocks such a s a t Mud Creek, s i m i l a r f e a t u r e s can be found through much of the Vermilion d i s t r i c t , i nc reas ing i n the i n t e n s i t y of development a s the Vermilion f a u l t is approached.

The Mud Creek shea r zone is f lanked on the nor th by pi l lowed greenstone (upper member of the Ely Greenstone) t h a t is moderately deformed except i n narrow shear zones, and on the south by a s s o r t e d f e l s i c t u f f , t u f f -b recc i a , block b recc i a , and the reworked sedimentary equ iva l en t s of t hese ( tu f f aceous member of t he Lake Vermilion Formation). Sims and Southwick (1980, 1985) i n t e r p r e t e d the h ighly s c h i s t o s e m a t e r i a l w i t h i n t h e shea r zone t o have been der ived c h i e f l y from f ine-grained f e l s i c t o i n t e rmed ia t e t u f f belonging t o the Lake Vermilion sequence. I t is now recognized that s h e a r zones of this magnitude commonly con ta in the sheared equ iva l en t of many d i f f e r e n t rock types , a l l reduced t o a more o r less common " f a u l t rock" composed c h i e f l y of qua r t z , s e r i c i t e , and c h l o r i t e . The p h y l l o n i t i c rocks of the Mud Creek shear zone a r e s i m i l a r t o the " f a u l t rocks" along t h e Rainy Lake-Seine River f a u l t zone i n southern O n t a r i o (Poulsen, 1983, 1986) and t o t hose a s s o c i a t e d with many o t h e r s t r i k e - s l i p f a u l t zones elsewhere i n the Supe r io r Province. The westward e x t e n t of the Mud Creek s h e a r zone beneath Lake Vermilion has no t been e s t a b l i s h e d .

3.7 Return to vehicles; continue northwest on Mud Creek road

5.0 Low, rusty outcrop of quartz vein and enclosing quartz—sericite—chlorite schist at edge of road on east (right) side. Park vehicles onshoulder where room permits; walk to roadside outcrop.

STOP 2A. Several outcrops of sheared, locally altered and veined meta—basalt near Mud Creek Road at the southern boundary of SuperiorNational Forest: SE 1/4 SE 1/4 sec. 31, T. 63 N., R. 14 W.

Many of the small—scale manifestations of intense dexra]. shear thatwere seen at the previous stop are seen to somewhat better advantage inthese exposures. The most interesting and informative outcrop is locatedabout 500 ft. into the woods east of the road (follow well beaten path),where phyllonitic quartz—sericite—chlorite schist displays excellent C'shear bands and 2' crenulations. Brown quartz—carbonate alterationzones, disseminated pyrite, and narrow, tectonized quartz veins indicateformer hydrothermal activity in this shear zone segment. Grab samples ofthe altered rock reportedly yield high gold assays, but the occurrence hasnot been explored further because of proximity to protected lands of theBoundary Waters Canoe Area.

5.0 Return to vehicles; Continue north and west on Mud Creek roadfollowing for about a mile a fault 'scarp' (ridge of higher grade rocksimmediately N. of the Vermilion fault and the road) westward.

6.2 Park - fairly straight stretch of road with ridge immediately tothe north.

STOP 2B. The outcrop is about 500 ft. from the road on the loggedridge running WSW from the road: NW 1/4 SE 1/4 sec. 36, T. 63 N., R. 14 W.

The Rice Creek gold prospect is located in a zone of highly deformedand altered rock here called the Vermilion deformation zone. The VDZ isbounded on the north by the Vermilion fault and on the south by the Mudcreek shear zone. The VDZ features widespread carbonate and aericitealteration as well as several gold showings. Gold is usually associatedwith ankerite and pyrite and occasionally with green mica and tourmaline.

The Rice Creek showing is located approximately 1,000 feet south of theVermilion fault at the south contact of a chert iron formation. Rocks withelevated gold values together with sericite-green mica—carbonate rocks werefound on a small dump created by early explorers for iron ore. The outcropsource of this material can also be seen.

Gold up to 2 ppm is associated with pyrite and ankerite in a dark brec-ciated chert. This chert is in contact with sheared sericite carbonaterocks. Subsequent drilling has found that similar gold values exist at thenorth contact of the iron formation and in shears in the iron formation aswell as in shears in a chiorite—sericite—carbonate schist lying between thenorth contact of the iron formation and the Vermilion fault.

6.2 Return to vehicles; turn around and drive south on Mud Creek Roadback to highway 1-169.

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3 . 7 Return t o v e h i c l e s ; c o n t i n u e nor thwes t on Mud Creek road

5.0 LOW, r u s t y o u t c r o p of q u a r t z ve in and e n c l o s i n g q u a r t z - s e r i c i t e - c h l o r i t e s c h i s t a t edge of road on e a s t ( r i g h t ) s i d e . Park v e h i c l e s on s h o u l d e r where room p e r m i t s ; walk t o r o a d s i d e o u t c r o p .

STOP 2A. S e v e r a l o u t c r o p s of sheared , l o c a l l y a l t e r e d and ve ined meta- b a s a l t n e a r Mud Creek Road a t the s o u t h e r n boundary of S u p e r i o r N a t i o n a l F o r e s t : SE 1/4 SE 1/4 sec. 31, T. 63 N., R. 1 4 W.

Many of the s m a l l - s c a l e m a n i f e s t a t i o n s of i n t e n s e d e x r a l s h e a r t h a t were s e e n a t the p r e v i o u s s t o p a r e s e e n t o somewhat b e t t e r advan tage i n t h e s e exposures . The most i n t e r e s t i n g and i n f o r m a t i v e o u t c r o p is l o c a t e d a b o u t 500 f t . i n t o the woods e a s t of the road ( f o l l o w w e l l b e a t e n p a t h ) , where p h y l l o n i t i c quartz-sericite-chlorite s c h i s t d i s p l a y s e x c e l l e n t C1 s h e a r bands and S y ' c r e n u l a t i o n s . Brown q u a r t z - c a r b o n a t e a l t e r a t i o n z o n e s , d i s s e m i n a t e d p y r i t e , and narrow, t e c t o n i z e d q u a r t z v e i n s i n d i c a t e fo rmer hydrothermal a c t i v i t y i n this s h e a r zone segment. Grab samples of t h e a l t e r e d rock r e p o r t e d l y y i e l d h igh gold a s s a y s , b u t the o c c u r r e n c e has n o t been exp lored f u r t h e r because of p rox imi ty t o p r o t e c t e d l a n d s of the Boundary Waters Canoe Area.

5.0 Return t o v e h i c l e s ; Cont inue n o r t h and w e s t on Mud Creek road f o l l o w i n g f o r a b o u t a m i l e a f a u l t ' s c a r p ' ( r i d g e o f h i g h e r g r a d e rocks immediate ly N. o f the Vermil ion f a u l t and the r o a d ) westward.

6.2 Park - f a i r l y s t r a i g h t s t r e t c h of road w i t h r i d g e immediate ly t o t h e nor th .

STOP 2B. The o u t c r o p is about 500 f t . from the road on the logged r i d g e running WSW from the road: NW 1/4 SE 1/4 sec. 36, T. 6 3 N., R. 14 W.

The Rice Creek g o l d p r o s p e c t i s l o c a t e d i n a zone o f h i g h l y deformed and a l t e r e d rock h e r e c a l l e d the Vermilion deformat ion zone. The VDZ i s bounded on the n o r t h by the Vermil ion f a u l t and on the s o u t h by the Mud c r e e k s h e a r zone. The VDZ f e a t u r e s widespread c a r b o n a t e and sericite a l t e r a t i o n a s w e l l a s s e v e r a l go ld showings. Gold i s u s u a l l y a s s o c i a t e d w i t h a n k e r i t e and p y r i t e and o c c a s i o n a l l y w i t h g r e e n mica and tourmal ine .

The R i c e Creek showing is l o c a t e d approx imate ly 1 ,000 f e e t s o u t h of the Vermil ion f a u l t a t the s o u t h c o n t a c t of a c h e r t i r o n fo rmat ion . Rocks w i t h e l e v a t e d go ld v a l u e s t o g e t h e r w i t h s e r i c i t e - g r e e n mica-carbonate rocks were found on a s m a l l dump c r e a t e d by e a r l y e x p l o r e r s f o r i r o n o r e . The o u t c r o p s o u r c e of this m a t e r i a l can a l s o be seen.

Gold up to 2 ppm is a s s o c i a t e d w i t h p y r i t e and a n k e r i t e i n a d a r k brec- c i a t e d c h e r t . T h i s c h e r t is i n c o n t a c t w i t h s h e a r e d sericite c a r b o n a t e r o c k s . Subsequent d r i l l i n g h a s found t h a t similar g o l d v a l u e s e x i s t a t the n o r t h c o n t a c t of the i r o n fo rmat ion and i n s h e a r s i n the i r o n fo rmat ion as w e l l a s i n s h e a r s i n a chlorite-sericite-carbonate s c h i s t l y i n g between the n o r t h c o n t a c t of the i r o n fo rmat ion and the v e r m i l i o n f a u l t .

6.2 Return t o v e h i c l e s ; t u r n around and d r i v e s o u t h on Mud Creek Road back t o highway 1-169.

12.4 Junction with 1—169; turn left (east) toward Ely.

21.0 County route 88 enters highway from left (north). Turn left, crossabandoned railroad grade, and turn right onto dirt track. Disembarkfrom vehicles and walk east to abandoned quarry.

STOP 3. Longstorff Bay shear zone; quarry and natural exposures in theSEI/4 sec. 36, T. 63 N., R. 13 W., west of Ely.

Major shear zones form a bifurcating, wishbone-shaped trace thatroughly corresponds to the outline of Shagawa Lake (Fig. 5). These zonesare collectively referred to here as the Shagawa Lake shear zones. Thethree arms of this shear zone system are informally referred to by therespective bays of Shagawa Lake that they transect: The Olson Bay shearzone and the Longstorff Bay shear zone in western Shagawa Lake and theSpaulding Bay shear zone in eastern Shagawa Lake. The Olson Bay andSpaulding Bay shear zones follow the trace of the Shagawa Lake fault, whichis inferred to be a tectonically long—lived structural feature. Thisfamily of shear zones is equivalent to the D2 structures, such as the MudCreek and Tower—Soudan shear zones of the western Vermilion district.

The Longstorff Bay shear zone, the best exposed of the three, deformsfelsic tuff, agglomerate, and graywacke of the Knife Lake Group and alamprophyre sill along the contact between the Knife Lake Group and theNewton Lake Formation (this stop). Lens—shaped islands of unsheared,mildly deformed lamprophyre occur locally within the shear zone. The zoneterminates to the west against the Wolf Lake fault.

Although no continuous outcrop was found across any of the Shagawa Lakezones, indirect evidence suggests that the Longstorff Bay zone may be aswide as 600 m and the Spaulding Bay zone (next stop) may be as wide as 1.2km. The absence of physical markers that could be correlated across thezones, together with the incomplete exposure of the zones, has inhibitedestimates of the amount of shear displacement.

Locality 3a: Sheared lamprophyre in quarry pit.

Intense shearing has produced a strong mylonite foliation along thesouthern margin of the lamprophyre sill exposed in this quarry. Formerpyroxene and hornblende megacrysts, now dark-green spots of actinolite ±chlorite, occur in the less deformed lamprophyre to the north of this expo-sure, The spots are highly flattened in the foliation and have a weak tomoderate linear aspect. Their shape is a typical product of the high flat-tening strains displayed within the Shagawa Lake shear zones. Small—scaleshear bands and actinolite foliation fish from this outcrop area indicate adextral sense of shear; however, these features were observed only in cuthand samples and are not readily distinguished in outcrop. Small(centimeter scale) symmetric crenulations and chevron folds deform thefoliation locally and are believed to have formed during the later stagesof shearing. Kink bands interpreted to be younger features unrelated tothe development of the shear zones are well developed locally.

Locality 3b: Sheared tuff of the Knife Lake Group.

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12.4 � unction with 1-169; t u r n l e f t ( e a s t ) toward Ely.

21.0 County rou te 88 e n t e r s highway from l e f t ( n o r t h ) . Turn l e f t , c r o s s abandoned r a i l r o a d grade, and t u r n r i g h t on to d i r t t rack . Disembark from v e h i c l e s and walk e a s t to abandoned quarry.

STOP 3. Longstorff Bay shea r zone; qua r ry and n a t u r a l exposures i n the SE1/4 sec. 36, T. 63 N., R. 13 W., west of Ely.

Major shea r zones form a b i f u r c a t i n g , wishbone-shaped t r a c e t h a t roughly corresponds t o t he o u t l i n e of Shagawa Lake (Fig. 5 ) . These zones are c o l l e c t i v e l y r e f e r r e d t o h e r e a s the Shagawa Lake shea r zones. The t h r e e arms of this shear zone system a r e informal ly r e f e r r e d t o by the r e s p e c t i v e bays of Shagawa Lake that they t r a n s e c t : The Olson Bay s h e a r zone and t h e Lonqstorff Bay shea r zone i n western Shagawa Lake and the Spaulding Bay shea r zone i n e a s t e r n Shagawa Lake. The Olson Bay and Spaulding Bay shea r zones fo l low the t r a c e of t he Shagawa Lake f a u l t , which i s i n f e r r e d t o be a t e c t o n i c a l l y long-l ived s t r u c t u r a l f ea tu re . Th i s f ami ly of shea r zones is equ iva l en t t o t h e D2 s t r u c t u r e s , such a s t h e Mud Creek and Tower-Soudan shear zones of t h e western Vermilion d i s t r i c t .

The Longstorff Bay shear zone, the b e s t exposed of t h e t h r e e , deforms f e l s i c t u f f , agglomerate, and graywacke of t he Knife Lake Group and a lamprophyre s i l l along the c o n t a c t between the Knife Lake Group and the Newton Lake Formation ( t h i s s t o p ) . Lens-shaped i s l a n d s of unsheared, m i l d l y deformed lamprophyre occur l o c a l l y w i th in t h e shea r zone. The zone t e r m i n a t e s to the west a g a i n s t the Wolf Lake f a u l t .

Although no continuous outcrop w a s found ac ros s any of the Shagawa Lake zones, i n d i r e c t evidence sugges t s that the Longstorff Bay zone may be a s wide a s 600 m and the Spaulding Bay zone ( n e x t s t o p ) may be as wide a s 1.2 km. The absence of phys ica l markers t h a t could be c o r r e l a t e d a c r o s s the zones , t oge the r with the incomplete exposure of the zones, has i n h i b i t e d e s t i m a t e s of the amount of s h e a r displacement.

. L o c a l i t y 3a: Sheared lamprophyre i n qua r ry p i t .

I n t e n s e shea r ing has produced a s t r o n g mylonite f o l i a t i o n a long the sou the rn margin of t he lamprophyre s i l l exposed i n t h i s quar ry . Former pyroxene and hornblende meqacrysts, now dark-green s p o t s of a c t i n o l i t e + c h l o r i t e , occur i n t h e less deformed lamprophyre t o t h e no r th of this e K p - s u r e . The s p o t s a r e h ighly f l a t t e n e d i n t h e f o l i a t i o n and have a weak to moderate l i n e a r aspec t . Thei r shape is a t y p i c a l product of the h igh f l a t - t e n i n g s t r a i n s d isp layed wi th in the Shagawa Lake shear zones. Small-scale s h e a r bands and a c t i n o l i t e f o l i a t i o n f i s h from this outcrop a r e a i n d i c a t e a d e x t r a l sense of shea r ; however, t h e s e f e a t u r e s were observed on ly i n c u t hand samples and a r e no t r e a d i l y d i s t i ngu i shed i n outcrop. Small ( cen t ime te r s c a l e ) symmetric c r e n u l a t i o n s and chevron f o l d s deform the f o l i a t i o n l o c a l l y and a r e be l ieved t o have formed dur ing the later s t a g e s o f shear ing . Kink bands i n t e r p r e t e d t o be younger f e a t u r e s un re l a t ed to t h e development of t he shear zones a r e w e l l developed l o c a l l y .

L o c a l i t y 3b: Sheared t u f f of t h e Knife Lake Group.

A narrow dirt road southeast of the quarry and the abandoned railroadgrade leads to art abandoned section of highway. A small outcrop of highlysheared Knife Lake tuff that contains numerous kink bands crops along thenorth side of the abandoned highway to the east of the dirt road intersec-tion.

21.0 Return to vehicles; retrace route to 1 and 169 and then to left(east) back through Ely.

25.1 Minnesota route 1 turns south toward Illgen CIty; continue east onhighway 169 toward Winton.

26.1 County route 88 (signs for Echo Trail) enters from left (north);turn left onto route 88.

28.1 Prominent roadcuts on either side of road; park on shoulder anddisembark.

STOP 4. Spaulding Bay shear zone exposed in cuts along County route88, NW1/4 sec. 23, T. 63 N., R. 12 W.

The Spaulding Bay shear zone occurs primarily in the Knife Lake Group,although it affects adjacent variolitic pillow basalts of the Newton LakeFormation to the north and units of the Ely Greertstorie to the south. Thisshear zone presumably extends toward Fall Lake, farther to the east, butthis has not been verified by publicly available mapping.

Locality 4a: Sheared tuff of the Knife Lake Group.

Highly sheared tuffaceous rocks of the Knife Lake Group near its con-tact with the Newton Lake Formation are exposed here. The rocks containlocal concentrations of sulfide mineralization and abundant kink bands.

Locality 4b: Sheared Newton Lake basalt.

Variolitic pillow basalt and more massive flows of the Newton LakeFormation are cut by discontinuous shear zones at this outcrop. Strainanalyses using varioles from this unit indicate high flattening strains(k=0.06 and 0.10) with X plunging moderately to the southwest. Weak shearbands with spacings on the order of 5 cm are visible on some of the ver-tical outcrop faces at a high angle to the shear zones.

This is the end of Field Trip 4. Return to vehicles, turn around, andretrace route to Ely.

REFERENCES

Bauer, R.L., 1985, Correlation of early recumbent and younger uprightfolding across the boundary between an Archean gneiss belt andgreenstone terrane, northeastern Minnesota: Geology, v. 13, p.6 57—660.

Colvine, A.C., Fyon, J.A., Heather, K.B., Marmont, S., Smith, P.M., andTroop, D.G., 1988, Archean lode gold deposits in Ontario: OntarioGeological Survey Misecl].aneous Paper 139, 136 p.

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A narrow d i r t road s o u t h e a s t o f the q u a r r y and the abandoned r a i l r o a d g r a d e l e a d s t o an abandoned s e c t i o n o f highway. A small o u t c r o p o f h i g h l y s h e a r e d Kni fe Lake t u f f t h a t c o n t a i n s numerous k ink bands c r o p s a l o n g the n o r t h s i d e o f the abandoned highway t o the e a s t of the d i r t road i n t e r s e c - t i o n .

21 .O Re turn t o v e h i c l e s ; retrace r o u t e t o 1 and 169 and t h e n t o l e f t (east ) back through Ely.

25.1 Minnesota r o u t e 1 t u r n s s o u t h toward I l l g e n C i t y ; c o n t i n u e east on highway 169 toward Winton.

26.1 County r o u t e 8 8 ( s i g n s f o r Echo T r a i l ) e n t e r s from l e f t ( n o r t h ) ; t u r n l e f t o n t o r o u t e 88.

28.1 prominent r o a d c u t s on e i t h e r s i d e of road; p a r k on s h o u l d e r and disembark.

STOP 4. Spau ld inq Bay s h e a r zone exposed i n c u t s a l o n g County r o u t e 8 8 , NW1/4 sec. 23, T. 6 3 N., R. 12 W.

The Spau ld inq Bay s h e a r zone o c c u r s p r i m a r i l y i n the Kni fe Lake Group, a l t h o u g h it a f f e c t s a d j a c e n t v a r i o l i t i c p i l l o w b a s a l t s of the Newton Lake Format ion t o the n o r t h and u n i t s of the E l y Greenstone t o t h e s o u t h . T h i s s h e a r zone presumably e x t e n d s toward F a l l Lake, f a r t h e r t o t h e east , b u t t h i s h a s n o t been v e r i f i e d by p u b l i c l y a v a i l a b l e mapping.

L o c a l i t y 4a: Sheared t u f f of the K n i f e Lake Group.

High ly s h e a r e d t u f f a c e o u s r o c k s o f the Kni fe Lake Group n e a r i ts con- t a c t w i t h the Newton Lake Formation are exposed here . The r o c k s c o n t a i n l o c a l c o n c e n t r a t i o n s of s u l f i d e m i n e r a l i z a t i o n and abundant k i n k bands.

L o c a l i t y 4b: Sheared Newton Lake b a s a l t . - V a r i o l i t i c p i l l o w b a s a l t and more massive f lows of the Newton Lake

Format ion are c u t by d i s c o n t i n u o u s s h e a r zones a t this ou tc rop . S t r a i n a n a l y s e s u s i n g v a r i o l a s from this u n i t i n d i c a t e h i g h f l a t t e n i n g s t r a i n s (k=0.06 and 0.10) w i t h X p lung ing modera te ly to the southwest . Weak s h e a r bands w i t h s p a c i n g s on the o r d e r o f 5 cm a r e v i s i b l e on some of the ver - t i c a l o u t c r o p f a c e s a t a h i g h a n g l e t o the s h e a r zones.

T h i s is the end of F i e l d T r i p 4. Re turn t o v e h i c l e s , t u r n around, and retrace r o u t e t o Ely.

REFERENCES

Bauer, R0L.t 1985, C o r r e l a t i o n o f e a r l y recumbent and younger u p r i g h t f o l d i n g a c r o s s the boundary between a n Archean g n e i s s b e l t and g r e e n s t o n e t e r r a n e , n o r t h e a s t e r n Minnesota: Geology, v. 1 3, p. 6 57-660.

C o l v i n e , A.C., Fyon, J .A . , Heather , K.B., Marmont, S., Smith , P.M., and Troop, D.G., 1988, Archean l o d e g o l d d e p o s i t s i n Onta r io : O n t a r i o G e o l o g i c a l Survey Misce l l aneous P a p e r 139, 136 p.

Green, J.C., arid Schulz, K.H., 1982, Geologic map of the Ely quadrangle,St. Louis and Lake Counties, Minnesota: Minnesota Geological SurveyMiscellaneous Map Series M—50, scale 1:24,000.

Hudleston, P.J., 1976, Early deformational history of Archean rocks in theVermilion district, northeastern Minnesota: Canadian Journal of EarthSciences, v. 13, P. 579—592.

Hudleston, P.J., Bauer, R.L., Southwick, D.L., Schultz—Ej.a, D.D., andBidwell, M.E., 1987, Structural geology of the boundary between Archeanterranes of low—grade and high-grade rocks, northern Minnesota, inBalaban, N.H., ed., Field trip guidebook for selected areas inPrecambrian geology of northeastern Minnesota: Minnesota GeologicalSurvey Guidebook Series no. 17, p. 1-42.

Hudleston, P.J., Schultz-Ela, D.D., and Southwick, D.L., 1988,Transpreession in an Archean greenstone belt, northern Minnesota:Canadian Journal of Earth Sciences, v. 25, p. 1060—1068.

Malavieille, J., 1987, Kinematics of compressional and extensional ductileshearing deformation in a metamorphic core complex of the northernBasin and Range: Journal of Structural Geology, v. 9, p. 541-554.

Poulsen, K.H., 1983, Structural setting of vein—type gold mineralization inthe Mine Centre—Fort Frances area: Implication for the WabigoonSubprovince, in Colvine, A.C., ed., The geology of gold in Ontario:Ontario Geological Survey Miscellaneous Paper 110, p. 174—180.

1986, Rainy Lake Wrench Zone: An example of an Archean subprovinceboundary in northwestern Ontario (extended abs.], in deWit, M.J., andAshwal, L.D., eds., Workshop on the tectonic evolution of greenstonebelts: LPI Technical Report 86-10; Houston, Lunar and PlanetaryInstitute, p. 177—179.

Schultz-Ela, D.D., 1988, Application of a three—dimensional finite—elementmethod to strain field analyses: Journal of Structural Geology, v. 10,263—272.

Sims, P.K., 1976, Early Precambrian tectonic—igneous evolution in theVermilion district, northeastern Minnesota: Geological Society ofAmerica Bulletin, v. 87, p. 379—389.

Sims, P.K., and Mudrey, M.G., Jr., 1978, Geologic map of the Shagawa Lakequadrangle, St. Louis County, Minnesota: U.S. Geological SurveyGeologic Quadrangle Map GQ—1423, scale 1:24,000.

Sims, P.K., and Southwick, D.L., 1980, Geologic map of the Soudanquadrangle, St. Louis County, Minnesota: U.S. Geological SurveyGeologic Quadrangle Map GQ—1540, scale 1:24,000.

1985, Geologic map of Archean rocks, western Vermilion district,northern Minnesota: U.S. Geological Survey MiscellaneousInvestigations Series Map 1—1527, scale 1:48,000.

D-19

Green, J.C., and S c h u l z , K.H., 1982, Geologic map of the E l y quadrang le , S t . Louis and Lake C o u n t i e s , Minnesota: Minnesota G e o l o g i c a l Survey Misce l l aneous Map S e r i e s M-50, scale 1:24,000.

Hudles ton, P.J., 1976, E a r l y d e f o r m a t i o n a l h i s t o r y o f Archean rocks i n the Vermil ion d i s t r i c t , n o r t h e a s t e r n Minnesota: Canadian J o u r n a l of E a r t h S c i e n c e s , v. 13 , p. 579-592.

Hudles ton, P.J., Bauer, R.L., Southwick, D.L., Schu l tz -E la , D.D., and Bidwel l , M.E., 1987, S t r u c t u r a l geology of the boundary between Archean t e r r a n e s of low-grade and high-grade r o c k s , n o r t h e r n Minnesota, i n - Balaban, N.H., ed. , F i e l d t r i p guidebook f o r s e l e c t e d a r e a s i n Precambrian geology of n o r t h e a s t e r n Minnesota: Minnesota Geolog ica l Survey Guidebook S e r i e s no. 17, p. 1-42.

Hudles ton, P.J., Schu l tz -E la , D.D., and Southwick, D.L., 1988, ~ r a n s p r e e s s i o n i n an Archean g r e e n s t o n e b e l t , n o r t h e r n Minnesota: Canadian J o u r n a l o f E a r t h S c i e n c e s , v. 25, p. 1060-1068.

M a l a v i e i l l e , J., 1987, Kinemat ics of compress iona l and e x t e n s i o n a l d u c t i l e s h e a r i n g d e f o r m a t i o n i n a metamorphic c o r e complex of the n o r t h e r n Basin and Range: J o u r n a l of S t r u c t u r a l Geology, v. 9 , p. 541-554.

Poulsen, K.H., 1983, S t r u c t u r a l s e t t i n g of ve in - type g o l d m i n e r a l i z a t i o n i n t h e Mine C e n t r e - F o r t F rances area: I m p l i c a t i o n f o r the Wabigoon Subprovince, i n Colv ine , A.C., ed., The geo logy of g o l d i n Onta r io : O n t a r i o ~ e o l o g i c a l Survey Misce l l aneous P a p e r 1 10 , p. 174-1 80.

1986, Rainy Lake Wrench Zone: An example o f a n Archean subprovince boundary i n n o r t h w e s t e r n O n t a r i o [extended a b s . ] , i n d e w i t , M.J . , and Ashwal, L.D., e d s . , Workshop on the t e c t o n i c e v o l u t i o n of g reens tone b e l t s : LPI T e c h n i c a l R e p o r t 86-10; Houston, Lunar and P l a n e t a r y I n s t i t u t e , p. 177-1 79.

Schu l tz -E la , D.D., 1988, A p p l i c a t i o n o f a th ree -d imens iona l f i n i t e - e l e m e n t method t o s t r a i n f i e l d a n a l y s e s : J o u r n a l o f S t r u c t u r a l Geology, v. 10,

/

263-272.

Sims, P.K., 1976, E a r l y Precambrian t e c t o n i c - i g n e o u s e v o l u t i o n i n the Vermil ion d i s t r i c t , n o r t h e a s t e r n Minnesota: G e o l o g i c a l S o c i e t y of ~merica B u l l e t i n , v. 87 , p. 379-389.

Sims, P.K., and Mudrey, M.G., Jr., 1978, Geolog ic map of the Shagawa Lake q u a d r a n g l e , S t . Lou is County, Minnesota: U.S. G e o l o g i c a l Survey Geologic Quadrangle Map GQ-1423, scale 1:24,000.

Sims, P.K., and Southwick, D.L., 1980, Geolog ic map of the Soudan q u a d r a n g l e , S t . Lou is County, Minnesota: U.S. G e o l o g i c a l Survey Geologic Quadrangle Map GQ-1540, scale 1:24,000.

1985, Geolog ic map o f Archean r o c k s , w e s t e r n Vermil ion d i s t r i c t , n o r t h e r n Minnesota: U.S. G e o l o g i c a l Survey Misce l l aneous I n v e s t i g a t i o n s S e r i e s Map 1-1527, scale 1:48,000.

Figure 1. Regional map of the field trip area showing the geologic framework and themajor roads. The dotted contact is a major unconformity separating gently dippingProterozoic strata of the Animikie Group (on the south) from deformed Archeanrocks. The Animikie Group, consisting of the Pokegama Quartzite and BiwabikIron—Formation (open circles) and the Virginia Formation (diagonal rule), is

invaded by gabbroic rocks of Keweenawan age (1000 Ma) in the southeast corner ofthe map area. The field trip stops are all within Archean terrane.

D-20

Figure 1. Regional map of t h e f i e l d t r i p a r e a showing t h e geologic framework and t h e major roads. The d o t t e d c o n t a c t is a major unconformity s e p a r a t i n g gen t ly d ipping P ro te rozo ic s t r a t a of t h e Animikie Group (on t h e sou th ) from deformed Archean rocks. The Animikie Group, c o n s i s t i n g of t h e Pokegama Q u a r t z i t e and Biwabik iron- orm mat ion (open c i r c l e s ) and t h e Vi rg in i a Formation (d iagonal r u l e ) , is invaded by gabbroic rocks of Keweenawan age ( 1000 Ma) i n t h e sou theas t corner of t h e map area. The f i e l d t r i p s t o p s a r e a l l w i th in Archean t e r r a n e .

Figure 2. Geologic sketch map of the field trip areashowing approximate stop locations. Modified fromGreen and Schulz (1982) and Sims and Southwjck(1985). Map explanation is on the facing page.

D-21

920 30 92° 15

I.92°OO

GRA N ITI cCvc.....'

94

V

-

V

Figure 2. Geologic sketch map of the field trip area showing approximate stop locations. Modified from Green and Schulz (1982) and Sims and Southwick (1985) . Map explanation is on the facing page.

EXPLANATION FOR FIGURE 2

Pdc Duluth Complex; various gabbroic rocks Middle Proterozoic(ca. 1100 Ma)

INTRUSIVE CONTACT

Pv)

Virginia Formation; turbidite

: Ppb] Pokegama Quartzite (tidal deposits) Early Proterozoicoverlain by Biwabik Iron—Formation (ca 2000 Ma)

MAJOR UNCONFORMITY

Giants Range batholith; granitoid rocks

xvgo Vermilion Granitic Complex, granitoid rocks,paragneiss, migmatite

INTRUSIVE OR FAULT CONTACT

Newton Lake Formation; tholeiitic andkomatiitic metabasalt; numerous sills

K Knife Lake Group; sedimentary rocks ofmixed volcanic provenance Late Archean

(ca. 2700 Ma)V

j

Lake Vermilion Formation; volcanic—derivedsedimentary rocks, mainly of daciticprovenance

EuJ

Ely Greenstone, upper member; chieflytholeiitic metabasalt

I

jJEsJ Ely Greenstone, Soudan Iron—formationlilil

Member; cherty iron—formation interbeddedwith felsic to mafic volcanic rocks

El Ely Greenstone, lower member; chieflycaic—alkaline metabasalt

m1 Metabasalt (unnamed); probably Elyequivalent

kti Tonalite gneiss, paragneiss, amphibo—lite; stratigraphic position uncertain

D-22

EXPLANATION FOR FIGURE 2

[y] Duluth Complex; v a r i o u s g a b b r o i c r o c k s

INTRUSIVE CONTACT

1 Pv 1 V i r g i n i a Formation; t u r b i d i t e

Pokegama Q u a r t z i t e ( t i d a l d e p o s i t s 1 o v e r l a i n by Biwabik Iron-Formation

- MAJOR UNCONFORMITY - El G i a n t s Range b a t h o l i t h ; g r a n i t o i d r o c k s

E] Vermil ion G r a n i t i c Complex, g r a n i t o i d r o c k s , p a r a g n e i s s , migmat i te

INTRUSIW OR FAULT CONTACT

Newton Lake Formation; t h o l e i i t i c and k o m a t i i t i c metabasa l t ; numerous s i l l s

FJ Knife Lake Group; sed imenta ry r o c k s o f mixed v o l c a n i c provenance

i y l Lake Vermil ion Formation; v o l c a n i c - d e r i v e d s e d i m e n t a r y rocks , mainly of d a c i t i c provenance

E ly Greenstone, upper member; c h i e f l y t h o l e i i t i c m e t a b a s a l t

Ely Greenstone, Soudan I ron- fo rmat ion Member; c h e r t y i ron-format ion i n t e r b e d d e d

- w i t h f e l s i c t o mafic v o l c a n i c r o c k s

Ely Greenstone, lower member; c h i e f l y c a l c - a l k a l i n e m e t a b a s a l t

[TI M e t a b a s a l t (unnamed 1 ; probab ly E ~ Y e q u i v a l e n t

ml T o n a l i t e g n e i s s , p a r a g n e i s s , amphibo- l i t e ; s t r a t i g r a p h i c p o s i t i o n u n c e r t a i n

Middle P r o t e r o z o i c ( c a . 1100 Ma)

Ear ly P r o t e r o z o i c ( c a . 2000 Ma)

L a t e Archean ( c a . 2700 Ma)

Figure 3. Schematic illustration of the developnent of F2'cleavage during a simple deformation that also producedCS2) being folded.

folds and S2'

the foliation

Figure 4. C' surfaces or shear bands. These develop in rock that has pre-viously acquired a strong planar foliation due to very high shearstrain. The C' surfaces are new surfaces of slip that cross the mainfoliation at moderate angles; slip on them is in the same sense as theoverall sense of shear in the shear zone. Modified from Malavieille(1987).

D- 23

CrenulationCleavage S2.

(a)Single Cleavage

S2 S0

P Crenulat~on

Cleavage S2. \A

Single Cleavage - s2=' s o

Figure 3. Schematic i l l u s t r a t i o n of t h e developnent of F2' f o l d s and S 2 '

c leavage du r ing a s imple deformation t h a t a l s o produced t h e f o l i a t i o n ( s 2 ) being folded.

Figure 4. C t s u r f a c e s or shea r bands. These develop i n rock t h a t has pre- v ious ly acqui red a s t r o n g p lanar f o l i a t i o n due t o very h igh shea r s t r a i n . The C' s u r f a c e s a r e new s u r f a c e s of s l i p t h a t c r o s s t he main f o l i a t i o n a t moderate angles ; s l i p on them is i n t h e same sense a s t he o v e r a l l s ense of shear i n t he shear zone. Modified from Malav ie i l l e (1987) .

N)

Figure 5.

Generalized geologic map of the Shagawa Lake

area

showing locations of stops

3-h

.G

eolo

gymodified from Sims and Mudrey (1978) and Green and

Schulz (1982).

9 1°

46'

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