Lithosphere - California State University, Northridge

Lithosphere

doi: 10.1130/L134.1 2011;3;247-260Lithosphere

M. Robinson Cecil, George Gehrels, Mihai N. Ducea and P. Jonathan Patchett Implications for petrogenesis and crustal architectureU-Pb-Hf characterization of the central Coast Mountains batholith:

Email alerting servicesarticles cite this article

to receive free e-mail alerts when newwww.gsapubs.org/cgi/alertsclick

Subscribe to subscribe to Lithospherewww.gsapubs.org/subscriptions/click

Permission request to contact GSAhttp://www.geosociety.org/pubs/copyrt.htm#gsaclick

official positions of the Society.citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflectpresentation of diverse opinions and positions by scientists worldwide, regardless of their race, includes a reference to the article's full citation. GSA provides this and other forums for thethe abstracts only of their articles on their own or their organization's Web site providing the posting to further education and science. This file may not be posted to any Web site, but authors may postworks and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequenttheir employment. Individual scientists are hereby granted permission, without fees or further Copyright not claimed on content prepared wholly by U.S. government employees within scope of

Notes

© 2011 Geological Society of America

on July 26, 2011lithosphere.gsapubs.orgDownloaded from

http://lithosphere.gsapubs.org/cgi/alerts

http://lithosphere.gsapubs.org/subscriptions/index.ac.dtl

http://www.geosociety.org/pubs/copyrt.htm#gsa

http://lithosphere.gsapubs.org/

LITHOSPHERE | Volume 3 | Number 4 | www.gsapubs.org 247

RESEARCH

INTRODUCTION

The Coast Mountains batholith is a 1700-km-long belt of Jurassic through Tertiary plutonic rocks that extends along the length of coastal British Columbia, southeast Alaska, and southwestern Yukon (Fig. 1). These plutonic rocks are emplaced along the suture zone between two large arc-type fragments: the Alexander and Wrangellia terranes to the west, and the Stikine and associated terranes to the east. Although clearly related to plate convergence along the western margin of North America (e.g., Engebretson et al., 1985), it has long been suspected that Jurassic–Cretaceous collision of terranes along this suture played a signifi cant role in the generation and exhumation of igneous rocks that make up the Coast Mountains batholith (e.g., Monger et al., 1982). This report uses Hf isotope data from plutonic rocks within and adjacent to the Coast Mountains batholith to investigate the petrogenesis of gran-itoids that make up this segment of the Coast Mountains batholith, and the architecture of the terranes that comprise the central-western Coast Mountains. The coupled Hf and geochronologic data shed new light on the nature of terranes at depth and the structural boundaries that separate them.

Much attention has been paid to the metasedimentary assemblages that occur as pendants within the widespread Jurassic through Eocene Coast Mountains batholith (e.g., Samson et al., 1991a; Jackson et al., 1991; Boghossian and Gehrels, 2000; Gareau and Woodsworth, 2000)(Fig. 2). Typically, these rocks are highly deformed and metamorphosed

to amphibolite or even granulite grade (Hollister and Andronicos, 2000). Because of the voluminous nature of the plutonic rocks and the high grade of metamorphism, contacts are commonly obscured, and protolith determinations are diffi cult to make. This in turn has caused diffi cul-ties in correlating pendant rocks with terranes described for other parts of the Cordillera, and it has led to a limited understanding of the tec-tonostratigraphic relationships between the various crustal assemblages making up the central Coast Mountains batholith. In various parts of the batholith, however, the Nd and Sr isotopes of the metamorphic country rocks have been studied (Samson et al., 1990, 1991a, 1991b; Jackson et al., 1991; Patchett et al., 1998; Boghossian and Gehrels, 2000; Gareau and Woodsworth, 2000). These isotopic data, together with published U-Pb detrital zircon data and geologic observations, allow us to defi ne the isotopic character of the various amalgamated terranes that make up the Coast Mountains. Hf isotopes of magmatic rocks that have inter-acted, even to a small degree, with a given terrane, should record signa-tures consistent with that region. Hf data from widely distributed plutons within the Coast Mountains batholith can therefore be used to identify the terranes with which the plutons interacted and/or partly assimilated.

Hafnium isotopes in magmatic zircons also act as probes for the chem-ical maturity of the lithosphere from which the melts were extracted. As such, the distribution of Hf isotopic signatures across the batholith can be used to glean important information about the tectonic construction of the terranes that make up the central Coast Mountains. Terranes are commonly conceptualized as discrete, coherent blocks of lithosphere that are accreted to, or transported laterally along, the margins of con-tinents. In such a conceptual framework, terranes form “side-by-side” panels separated from adjacent terranes or continents by through-going,

U-Pb-Hf characterization of the central Coast Mountains batholith: Implications for petrogenesis and crustal architecture

M. Robinson Cecil*, George Gehrels, Mihai N. Ducea, and P. Jonathan PatchettDEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, TUCSON, ARIZONA 85721, USA

ABSTRACT

We present U-Pb geochronologic and Hf isotopic data from 29 plutonic samples within the Coast Mountain batholith, north-coastal British Columbia and southeast Alaska. Hf isotopic values do not correlate with age or variation in magmatic fl ux, but rather they increase system-atically from west (εHf[t] = +2 to +5) to east (εHf[t] = +10 to +13) in response to changing country rock assemblages. By comparing our pluton Hf data with previously reported Nd-Sr and detrital zircon characteristics of associated country rocks, we identify three crustal domains in an area where crustal affi nity is largely obscured by metamorphism and voluminous pluton intrusion: (1) a western domain, emplaced into continental-margin strata of the Banks Island assemblage; (2) a central domain, emplaced into the Alexander terrane; and (3) an eastern domain, underlain by the Stikine terrane and its inferred metamorphic equivalents. Between the interpreted Alexander and Stikine terranes, there is a zone of variable εHf(t) (+2 to +13) that coincides with the suture zone separating inboard (Stikine and Yukon-Tanana) from outboard (Alexander and associated) terranes. This variation in εHf(t) values apparently results from the structural imbrication of juvenile (Alexander and Stikine) and evolved (Yukon-Tanana) terranes along mid-Cretaceous thrust faults and the latest Cretaceous–early Tertiary Coast shear zone. Shifts in the Hf values of plutons across inferred terranes imply that they are separated at lower- to midcrustal levels by steep boundar-ies. Correlation between these Hf values and the isotopic character of exposed country rocks further implies the presence of those or similar rocks at magma-generation depths.

LITHOSPHERE; v. 3; no. 4; p. 247–260. doi: 10.1130/L134.1

For permission to copy, contact [email protected] | © 2011 Geological Society of America

*Current address: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA; [email protected].



CECIL ET AL.

248 www.gsapubs.org | Volume 3 | Number 4 | LITHOSPHERE

vertical structures. Alternatively, during their accretion to or collision with an existing margin, terranes can become imbricated by large-scale thrusts, making the boundaries between them more diffuse and less steep, as is the case with the mid- to Late Cretaceous fold-and-thrust sys-tem that developed along the boundary between the Insular (Wrangellia and Alexander) and Intermontane (Stikine and Yukon-Tanana) superter-ranes (Rubin et al., 1990). The thrust belt thickened the crust and effec-tively stacked rocks of the various existing terrane assemblages, laterally smearing them along thrusts. We use the arc-perpendicular distribution of Hf isotopes to evaluate the nature of the terrane boundaries and the degree to which the terranes have been structurally interleaved.

GEOLOGIC AND TECTONIC FRAMEWORK OF THE COAST

MOUNTAINS BATHOLITH

Most of the igneous rocks that make up the Coast Mountains batholith are tonalitic and range in age from 160 Ma to 50 Ma (Gehrels et al., 2009). In general, the ages become younger progressively eastward, although there are also Jurassic ages in the easternmost part of the Coast Mountains batholith at the latitude of the study area. This is interpreted to result from sinistral strike-slip duplication of the Jurassic portion of the batholith dur-ing Early Cretaceous time (Gehrels et al., 2009).

Country rocks of the batholith are generally mid- to high-grade metasedimentary assemblages derived from marine strata (Wheeler and McFeely, 1991). Due to the interpreted geologic setting of these pro-toliths, and supported by Nd-Sr (e.g., Samson et al., 1989) and detrital zircon (e.g., Gehrels and Boghossian, 2000) data, the country rocks are interpreted to have formed in settings ranging from juvenile volcanic arcs to pericratonic passive margins.

The lithologic and isotopic character of the main plutonic suites and their country rocks are described next and shown in Figures 1 and 2.

Western Portion of the Coast Mountains Batholith

The western portion of the Coast Mountains batholith is underlain by three distinct belts of plutonic rocks of Late Jurassic, Early Cretaceous, and mid-Cretaceous age. The ages of these bodies decrease systematically eastward (van der Heyden, 1989, 1992; Butler et al., 2006). Their com-position also changes eastward, from predominantly quartz diorite on the west to mainly tonalite on the east. The emplacement depth of these bod-ies ranges from ~10 km on the west to ~25 km on the east (Butler et al., 2001, 2006), a change that is also refl ected in the increasing metamorphic grade of the metasedimentary host rocks. A magmatic fl ux curve for the western portion of the Coast Mountains batholith, based on ages from these plutons, suggests high-fl ux periods from 160 to 140 Ma and 120 to 80 Ma, with little magmatism between 140 and 120 Ma (Gehrels et al., 2009, their fi g. 9).

Country rocks to these plutons (Fig. 1), from west to east, include the Wrangellia terrane, Banks Island assemblage, Alexander terrane, Gravina belt, and Yukon-Tanana terrane, as described in the following.

The Wrangellia terrane in the study area consists of Upper Paleo-zoic arc-type metavolcanic and metasedimentary rocks and Triassic rift-related(?) pillow basalts (Monger et al., 1992). These rocks presumably formed in a marine volcanic arc setting on the basis of their primitive Nd-Sr signature of correlative rocks on Vancouver Island (Samson et al., 1990).

The Banks Island assemblage consists mainly of highly folded quartzite interlayered with marble and subordinate pelitic schist (Geh-rels and Boghossian, 2000). These rocks are intruded by a ca. 357 Ma orthogneiss and are crosscut by nondeformed dikes that are ca. 147 Ma in age (G. Gehrels, 2010, personal commun.). Detrital zircons in the quartz-rich strata yield dominant ages of 410–480 Ma, 1700–1850 Ma, 1940–2250 Ma, and 2620–2940 Ma. Metamorphic rocks of the Banks Island assemblage have evolved continental isotopic signatures, with ini-tial Nd values ranging between +0.5 and −9.9, and relatively radiogenic Sr values ranging from 0.71178 to 0.71934 (Boghossian and Gehrels, 2000). These rocks are interpreted to have formed in a continental-mar-gin environment because of the occurrence of interlayered metaclastic quartzite and marble and cratonal detrital zircon and Nd-Sr signatures. The occurrence of ca. 410–480 Ma detrital zircons, however, suggests possible connections with the Alexander terrane.

The Alexander terrane is composed of Neoproterozoic–Cambrian and Ordovician–Silurian meta-igneous and metasedimentary rocks that are

N

Coast Mountainsbatholith

Undifferentiatedmetamorphic rocks

(locally Yukon-Tanana terrane)

Chugachterrane

Gravina andTyaughton-

Methow Basins

Alexander terrane

Wrangellia terrane

Stikineterrane

Taku terrane

Yukon-Tananaterrane

Banks IslandAssemblage

0 150

km

Strike-slip fault

Thrust fault

Main trace of Coast shear zone

10050

PacificOcean

BowserBasin

Prince Rupert

Kitimat

Fig. 2

Bella Coola

Queen

Charlotte

Islands

Ketchikan

Petersburg

Juneau

Skagway

? 52ºN

52ºN

58ºN

58ºN

128ºW

124ºW

138ºW

Figure 1. Geologic framework of the Coast Mountains batholith (CMB),

modifi ed from Wheeler and McFeely (1991), Wheeler at al. (1991), and

Gehrels et al. (2009). Outline represents the study area shown in Figure 2.




Hf isotopes of the Coast Mountains batholith | RESEARCH

Coastshear

zone

X

X′

126°

W12

8°W

130°

W

52°N

54°N

Terrace

Kitimat

Klemtu

BellaBella

Bella Coola

Prince Rupert

AlaskaCanadaCoast Mountainsbatholith

Banks Islandassemblage

Alexanderterrane

Gravina belt

Metamorphic rocks of the CGC (locallyYTT and Stikine)

Stikineterrane

Assemblageboundary

Coastshearzone

101 Ma5.4

414 Ma12.9

498 Ma9.8

445 Ma12.0

430 Ma8.6

394 Ma12.2

405 Ma10.4

58 Ma1.5

92 Ma6.0

68 Ma10.3

94 Ma6.8

89 Ma8.1

53 Ma2.0

119 Ma7.5

56 Ma9.2

60 Ma9.7

0 5025

km

124 Ma2.6

151 Ma12.3

82* Ma11.2

96* Ma9.8

76 Ma7.3

97 Ma8.1

95 Ma11.6

55 Ma*9.6

153 Ma4.3

142 Ma5.2

142 Ma5.4

142 Ma2.7

125 Ma12.0

82 Ma*11.2

206Pb/238U age(*denotes sampleswith zircon inheritance)

εHf

(t)

N

Figure 2. Locations, U-Pb ages, and εHf

(t) values of plutonic rocks sampled from the west-central Coast Moun-

tains batholith, British Columbia, and from the Alexander terrane, southeast Alaska. Generalized geology is

modifi ed from Wheeler and McFeely (1991), and Wheeler et al. (1991). Hf isotope and U-Pb age data are also

available in Table 2. CGC—central gneiss complex; YTT—Yukon-Tanana terrane.



CECIL ET AL.


interpreted to have formed in a marine volcanic arc (Gehrels and Saleeby, 1987). There is no sign of continental input in these assemblages in the geologic units (e.g., quartz-rich clastic strata; Gehrels and Saleeby, 1987), U-Pb geochronologic data (e.g., Precambrian inherited or detrital zir-cons; Gehrels et al., 1987, 1996), or Nd-Sr isotopes (Samson et al., 1989). Beginning in Early Devonian time, conglomeratic strata (referred to as the Karheen Formation) were shed from a source to the southwest (in pres-ent coordinates) that included rocks of 1120–2230 Ma age (Gehrels et al., 1996). Middle and Upper Paleozoic strata consist of shallow-marine clastic strata and carbonates with little sign of arc- or craton-derived detri-tus. The sequence is capped by a Triassic assemblage of rift-related(?) metavolcanic and metasedimentary rocks.

The Gravina belt consists of Upper Jurassic through Lower Cre-taceous volcaniclastic turbidites and subordinate mafi c and felsic metavolcanic rocks (Berg et al., 1972). These strata are interpreted to depositionally overlie the Alexander terrane to the west and the Yukon-Tanana terrane (described next) to the east (Gehrels, 2001). Detrital zircons in the metaclastic rocks record derivation from both the Alex-ander and Yukon-Tanana terranes, which suggests proximity of the two terranes by Late Jurassic time (Gehrels, 2001), as indicated on the basis of geologic relations examined by McClelland et al. (1992) and Saleeby (2000).

The Yukon-Tanana terrane consists of a Proterozoic–Lower Paleo-zoic assemblage of mainly quartz-rich metaclastic rocks (psammitic schist) and marble, a mid-Paleozoic assemblage of metavolcanic rocks, and Upper Paleozoic metasedimentary and metavolcanic rocks (Gehrels et al., 1992; Gehrels, 2001). In the study area, these rocks are locally referred to as the Scotia-Quaal assemblage, and they consist mainly of Devonian–Mississippian metavolcanic rocks and orthogneisses (Gareau, 1989). Although somewhat ambiguous, Nd-Sr data from the Scotia-Quaal are more evolved than those from the Wrangellia, Alexander, and Stikine terranes, and they indicate the presence of old continental crust (Gareau and Woodsworth, 2000).

Primary relations between these assemblages are diffi cult to document because of younger deformation, pluton intrusion, and/or lack of expo-sure. For example, the contact between Wrangellia and the Banks Island assemblage is everywhere under water or intruded by Mesozoic plutons, the contact between the Banks Island assemblage and Alexander terrane is a major sinistral strike-slip fault (Kitkatla shear zone) of Early Cretaceous age, and the Yukon-Tanana terrane and eastern Gravina belt are highly imbricated along west-vergent thrust faults of mid-Cretaceous age (Char-don et al., 1999; Gehrels et al., 2009).

Axial Portion of the Coast Mountains Batholith

Axial portions of the Coast Mountains are underlain primarily by Paleocene tonalitic sills, large bodies of Eocene granodiorite, and high-grade metasedimentary rocks. Emplacement and ductile deformation of the Paleocene tonalitic sills occurred during east-side-up motion along the Coast shear zone, which can be traced for most of the length of the Coast Mountains (Gehrels et al., 2009). Elsewhere, the Coast shear zone reveals evidence for older dextral motion (Gehrels, 2000) and younger east-side-down motion (Rusmore et al., 2001; Hollister and Andronicos, 2006). Barometric studies of tonalitic sills in southeast Alaska indicate emplace-ment depths of ~15–20 km (Hollister et al., 1987; Stowell and Crawford, 2000; Rusmore et al., 2005).

East of the sills and Coast shear zone, there are large plutons of homo-geneous granodiorite that are primarily of Eocene age. These plutons are generally nondeformed, but locally they were emplaced along an east-side-down normal fault and associated ductile shear zone referred to as

the Shames mylonite zone (Heah, 1990, 1991; Andronicos et al., 2003) or eastern boundary detachment (Rusmore et al., 2005).

Metasedimentary assemblages within axial portions of the Coast Mountains batholith, which are commonly referred to as the Central Gneiss complex, consist mainly of pelitic and psammitic schist with sub-ordinate quartzite, marble, and calc-silicate gneiss (undivided metamor-phic rocks in Figs. 1 and 2). These rocks are generally sillimanite grade, locally with sillimanite replacing kyanite and/or staurolite (Stowell and Crawford, 2000; Hollister and Andronicos, 2000; Rusmore et al., 2005). The tectonic affi nity of these metasedimentary rocks in the study area is uncertain. To the north, in southeast Alaska, detrital zircon and Nd-Sr anal-yses record derivation primarily from continental source regions (Samson et al., 1990; Gehrels et al., 1992). Regional correlations and northward continuity suggest that these rocks belong to the Yukon-Tanana terrane. In the study area, however, quartzites and marbles that are characteristic of the Yukon-Tanana terrane are rare, and some workers have suggested correlations with strata of the Stikine terrane (Hill, 1985).

Eastern Portion of the Coast Mountains Batholith

The eastern portion of the Coast Mountains batholith consists of Jurassic through Eocene plutons that intrude low-grade Upper Paleo-zoic through Tertiary sedimentary and volcanic rocks of the Stikine ter-rane and overlying strata of the Bowser Basin (Wheeler and McFeely, 1991; Haggart et al., 2006a, 2006b, 2007; Mahoney et al., 2007a, 2007b, 2007c, 2007d, 2007e, 2009). The Jurassic–Cretaceous history of this portion of the batholith is somewhat different from that of the western portion because magmatism did not migrate eastward, and it was appar-ently continuous through Early Cretaceous time (Gehrels et al., 2009; Mahoney et al., 2009).

The Stikine terrane consists largely of widespread Triassic and Juras-sic arc-type assemblages blanketed by Jurassic–Cretaceous marine strata of the Bowser Basin (Monger et al., 1992). Locally, arc-type volcanic and sedimentary assemblages as old as Devonian are exposed. Available Nd-Sr data from these rocks suggest formation in a juvenile arc setting with little continental infl uence (Samson et al., 1989). The only exception to this is the occurrence of inherited zircons of Precambrian age in Early Jurassic plutons, which may refl ect the presence of Precambrian basement in some portions of the terrane (Thorkelson et al., 1995). Alternatively, it has been suggested that the Stikine terrane rests depositionally on rocks of the Yukon-Tanana terrane (e.g., McClelland et al., 1992; Jackson et al., 1991), which carries predominantly Precambrian detrital zircons.

ANALYTICAL STRATEGY AND METHODS

Zircons from 29 individual plutonic samples were separated, picked, and mounted, along with appropriate U-Pb and Hf standards, in 2.5 cm epoxy mounts. Epoxy mounts were imaged in plain light with a binocu-lar microscope, and grain maps were produced. U-Pb analysis was fi rst performed on individual zircon grains from each sample via ablation of 40-μm-diameter pits using methodology described next. After all U-Pb analyses for a sample were completed, Hf isotope measurements were made via ablation on top of the preexisting U-Pb pits; the following sec-tions describe the details of these analyses. This analytical technique allows for the measurement of both U-Pb and Hf in the same part of the zircon crystal, which is important for two reasons: (1) Crystallization age of the analyzed zircons is required for calculating initial 176Hf/177Hf; and (2) zircons with complex growth histories may record multiple ages, such that it is necessary to collect Hf and U-Pb data from the same zircon domain. Because the data are acquired sequentially, and not simultaneously, it is





possible that the successive pits drilled for U-Pb and Hf measurement are sampling different crystal depth domains (Kemp et al., 2009). This is likely not the case with zircons from the Coast Mountains batholith sam-ples, however, given the characteristic homogeneity of analyzed crystals, as determined by cathodoluminescence imaging and age mapping of large grains, and the fact that we observed no marked changes in Hf isotopic ratios with time during data acquisition.

Instrumentation

Reported U-Pb and Hf isotope data were collected using a New Wave 193 nm ArF laser ablation system coupled to a Nu Plasma HR induc-tively coupled plasma–mass spectrometer (ICP-MS) at the University of Arizona (for additional information, see http://sites.google.com/a/laser-chron.org/laserchron/). Ablation was performed in a New Wave Super-Cell™, and sample aerosol was transported with He carrier gas through Tefl on-lined tubing, where it was mixed with Ar gas before introduction to the plasma torch. The multicollector (MC) ICP-MS utilizes 12 Faraday detectors equipped with 3 × 1011 Ω resistors and four discrete dynode ion counters, which remain fi xed as beams are directed into them via an elec-trostatic zoom lens. For U-Pb analyses, U, Th, and Pb isotopes were mea-sured simultaneously in Faraday collectors, with the exception of 204Pb, which was measured using an ion counter. For Hf analyses, masses 171 through 180 were all measured simultaneously in Faraday collectors. Pure Hf solutions and Hf solutions doped with various amounts of Yb and Lu were introduced in Ar carrier gas via a Nu DSN-100 desolvating nebulizer.

U-Pb Geochronology

Geochronologic analyses presented here were performed by laser ablation (LA) ICP-MS at the Arizona Laserchron Center using meth-ods described by Gehrels et al. (2008). Laser ablation was done using a 40-μm-diameter spot and a pulse rate of 7 Hz. The laser was run in con-stant energy mode with output energy of 8 mJ/pulse, which corresponds to an energy density of ~2 J/cm2 and an estimated excavation rate of 0.7 μm/s. The analytical routine consisted of a 15 s on-peak background mea-surement with the laser off, followed by 15 s of peak measurement, per-formed at 1 s integration times, with the laser fi ring. This results in an analysis pit of ~15 μm depth.

The samples we used are a subset of those analyzed by Gehrels et al. (2009), and they generally can be characterized by simple, prismatic zircons with internal oscillatory zoning and rare inherited components or younger growth rims. For each sample, single pits were ablated on 20–30 individual zircons. In the case of samples 04GJP-09, 04GJP-13, and 05MT-135, which had zircons with rims and cores distinguishable in cath-odoluminescence images and of variable age, multiple analyses (2–5 spots at 40 μm per spot) were performed on single crystals. Weighted mean ages were then determined for each component, and a magmatic age was assigned based on the interpreted igneous domain. Fractionation between U and Pb was accounted for by bracketing every fi ve measurements with analysis of a Sri Lankan zircon standard of known age (see Gehrels et al., 2008). Corrections for the interference of mercury were made by moni-toring 202Hg and using the natural ratio of 202Hg/204Hg to subtract the Hg contribution from mass 204. Corrections for common Pb were made by measuring 204Pb and assuming an initial Pb composition based on the Pb evolution model of Stacey and Kramers (1975). Uncertainties for reported 238U-206Pb ages are ~1%–2% (2σ) and include both a systematic error (typ-ically ~1%–2%), and an error associated with the scatter and precision of a set of measurements for a given sample (~1%, 2σ) (for details of error analysis, see Gehrels et al., 2009).

Hf Isotope Measurement

Interference Correction

Accurate in situ measurement of Hf isotopes in zircon is made diffi cult by the isobaric interferences of 176Yb and 176Lu on 176Hf, the correction of which has been discussed in detail (e.g., Griffi n et al., 2002; Woodhead et al., 2004; Iizuka and Hirata, 2005; Hawkesworth and Kemp, 2006; Gerdes and Zeh, 2009; Wu et al., 2006; Kemp et al., 2009). Properly correcting for 176Yb (and to a lesser degree 176Lu) is critical given that 176Yb/176Hf of typical zircons is commonly between 10% and 30% and can be as much as 70%. The ratio of stable isotopes 179Hf/177Hf is used for mass bias cor-rections, and an exponential mass bias function is used in all calculations. Interference-free 173Yb and 171Yb were monitored during the Hf analysis to calculate Yb mass bias (β

Yb) and the contribution of Yb to the measure-

ment of 176(Hf + Lu + Yb). Because the magnitude of the Yb correction is so great, small inaccuracies in the Yb mass bias can lead to large ana-lytical errors (Woodhead et al., 2004). Unlike 179Hf/177Hf, the precision of the 173Yb/171Yb measurement, and consequently the accuracy of β

Yb,

is dependent upon the Yb signal intensity (Fig. 3). At 171Yb intensities of less than 0.015 V, it becomes very diffi cult to reliably estimate β

Yb, and for

those analyses, Hf mass bias (βHf

) was used to correct 176Yb/171Yb. Unfor-tunately, Chu et al. (2002) and Woodhead et al. (2004) have shown that Hf and Yb exhibit slightly different fractionation behavior, which we also observed to be true (Fig. 3C). So, although it is not ideal to use Hf fraction-ation factors to correct for Yb mass bias, low-Yb zircons require relatively minor correction, and as such it is possible to use β

Hf without introducing

large errors to the corrected 176Hf/177Hf ratio. The scatter introduced by the interference correction is not included in the fi nal error attached to 176Hf/177Hf values, but it is believed to be a relatively minor contribution to the quoted uncertainty. If there were a source of signifi cant, unaccounted error, we would expect a given set of measurements to be overly dispersed. This is not the case, as evidenced by a mean square weighted deviation (MSWD) of less than one for all reported sample data (see GSA Supple-mentary Data1).

The Lu correction was done by monitoring 175Lu and using 176Lu/175Lu = 0.02653 (Patchett, 1983) and β

Yb, assuming that Lu behaves similarly to

Yb. All corrections are performed on a line-by-line basis, and in all cases, Hf and Yb isotope data were normalized to 179Hf/177Hf = 0.72350 (Patchett and Tatsumoto, 1980) and 173Yb/171Yb = 1.132338 (Vervoort et al., 2004), respectively. A 176Lu decay constant of 1.876 × 10−11 (Scherer et al., 2001; Söderlund et al., 2004) was used in all calculations. Chondritic values of Bouvier et al. (2008) were adopted for the calculation of ε

Hf values.

Hf Solution Analysis

Analyses of pure Hf solutions, as well as Hf solutions doped with vari-able amounts of Yb and Lu, were performed to test our ability to reliably correct for Yb and Lu interferences. Solution analyses were run in three blocks of 20 measurements, with additional background measurements being automatically performed between blocks. Backgrounds were mea-sured using electrostatic analyzer defl ection for 60 s at the start of the run, and measurements were integrated over 5 s. For 10 ppb solutions, total Hf beams of ~5 V were achieved (this is the maximum possible with our 3 × 1011 Ω resistors). Solution data were collected during many analytical sessions over the course of this study, and Hf standard solution measure-ments were always made after instrument tuning and before acquisition

1GSA Data Repository Item 2011234, Weighted mean and concordia plots for all U-Pb and Hf data presented, is available at www.geosociety.org/pubs/ft2011.htm,

or on request from [email protected], Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.



CECIL ET AL.


of laser data. Repeated analysis of JMC 475 (n = 71) over the course of this study yielded a weighted mean of 176Hf/177Hf = 0.282159 ± 15, which was nearly identical to the accepted JMC 475 value of 0.282160 (Vervoort et al., 2004). No normalization of the data to JMC 475 was performed. Hafnium Spex solution, although not an ultrapure interlaboratory stan-dard, was also analyzed (n = 50) and found to be isotopically the same as JMC 475 (Hf Spex 176Hf/177Hf = 0. 282159 ± 12). Hafnium Spex solu-tions doped with natural Yb and Lu produced corrected 176Hf/177Hf values similar to that of the pure solution, although scatter in the high (Yb + Lu)/Hf data was greater (Fig. 4). No statistical correlation was found between

176Hf/177Hf and 176(Lu + Yb)/176Hf, indicating that Yb and Lu interferences were adequately removed.

Hf Laser-Ablation Analysis

In situ Hf isotope data were acquired using a 40 μm beam centered directly on top of the pit previously excavated for U-Pb analysis. Laser run conditions were the same as those described for U-Pb geochronology. Under those conditions, total Hf beams ranged from 2 to 7 V for standard zircons. The in situ analytical routine began with a 40 s on-peak back-ground measurement, followed by 60 s of laser ablation with a 1 s data integration time. This resulted in a laser pit that was ~50 μm in depth, with ~15 μm for the U-Pb analysis and 35 μm for the Hf analysis. All correc-tions were automatically calculated during the run on a line-by-line basis, and a 2σ fi lter was applied to each 60 measurement data block offl ine to remove outliers.

The zircon standards Mud Tank, Temora-2, FC-52 (compositionally similar to FC-1, from an anorthosite of the Duluth complex), 91500 (all described in Woodhead and Hergt, 2005), and Plesovice (Sláma et al., 2008) were analyzed. The results of repeated in situ analysis of these zir-cons during many analytical sessions over the course of roughly 6 mo are given in Figure 5 and Table 1. All zircon standards were added to each sample mount (3–4 samples per mount) and were analyzed between each set of unknowns in order to monitor laser stability and Hf ratio accuracy. In most cases, the long-term measured laser-ablation averages overlap (within error) the long-term solution values for those zircons, indicating that the previously described Lu and Yb interference correction method is also successful for laser analyses. A small discrepancy exists between the long-term 176Hf/177Hf laser average of FC-52 (0.282169 ± 10; 95% con-fi dence; n = 74) and the long-term solution average of FC-1 (0.282184 ± 16; 2 standard error [S.E.]; n = 42) (Woodhead and Hergt, 2005). The cause of this difference is not clear, although it is likely not a function of the interference correction, as discussed later.

The fi ve standards chosen have rare earth element (REE) concentra-tions ranging from REE-poor (Mud Tank) to REE-rich concentrations (Temora-2 and FC-52). Because of their high and variable REE content, Temora-2 and FC-52 are the most useful for testing the reliability of in situ Hf isotope data. Both have 176(Yb + Lu)/176Hf values that range from only a few percent to almost 50% (similar to the range that we observe in rocks from the Coast Mountains), and they are not correlated with 176Hf/177Hf.

1:1

Best fit

(linear)

-1.7 -1-1.1-1.2-1.4-1.5-1.6 -1.3

-1.1

-1.2

-1.3

-1.4

-1.5

-1.7

-1.6

-1

β Yb(173/171)

βHf(179/177)

Temora-2FC-52

C

0 0.100.080.060.040.02-20

20

10

0

-10

171Yb (V)

B

β Yb(173/171)

177Hf (V)

-2.0

-0.4

-0.8

-1.2

-1.6

0

0 0.4 0.8 1.2 1.6 2.0

A

β Hf(179/177)

Figure 3. Plots showing the relationship between Hf and Yb fractionation factors and signal intensity. βHf

and βYb

were measured simultaneously during

laser-ablation analysis of zircon standards. All laser standard data (line by line data, not analytical averages) are included in plots A and B, whereas

only a subset of averages from Temora-2 and FC-52 standard runs is included in plot C, because those standard zircons have higher concentrations of

Yb. The Hf fractionation factor does not vary with Hf signal intensity, making for a reliable Hf mass bias correction (A). On the other hand, 173Yb/171Yb

can only be used to correct for Yb mass bias when 171Yb signal intensity is at least 0.015 V (B). Although broadly correlated, βHf

and βYb

do not have a

simple 1:1 relationship, making it important to use βYb

for Yb mass bias corrections when possible (C).

0.28204

0.28208

0.28212

0.28216

0.28220

0.28224

0.28228

0 20 40 60 80 100(176Yb + 176Lu)/176Hf (%)

176 H

f/177 H

f

Weighted mean 176Hf/177Hf = 0.282161 ± 0.000013MSWD = 3.5; n = 293

0.28200

Figure 4. 176Hf/177Hf plotted against relative proportion of 176(Yb +

Lu)/176Hf (shown as a percentage), for all solution analyses per-

formed over the course of this study. Data were normalized to 179Hf/177Hf = 0.72350 (Patchett and Tatsumoto, 1980; Patchett, 1983),

and 173Yb/171Yb = 1.132338 (Vervoort et al., 2004). Error bars repre-

sent the internal precision of individual measurements (1 standard

error). The weighted mean of all solution analyses (0.282161

± 0.000013; solid line) is identical to the weighted mean of pure Hf

solution analyses. MSWD—mean square of the weighted deviates,

a statistical measure of scatter compared to analytical precision.





10 807060504020 30

FC-52 Plesovice

10 807060504020 30

Average = 0.282169 ± 10 (MSWD = 0.68)

Temora-2

0.28245

0.28255

0.28265

0.28275

0.28285

0.28295

10 807060504020 30

Mud Tank

0.28225

0.28235

0.28245

0.28255

0.28265

0.28275

20 160140120100806040

Analysis number

10 807060504020 30

176 H

f/177 H

f

Average = 0.282315 ± 13 (MSWD = 0.77)

0.28250

0.28220

0.28230

0.28240

0.28260

0.28270

0.28195

0.28205

0.28215

0.28225

0.28235

0.28245

0.28210

0.28220

0.28230

0.28240

0.28250

0.28260

176 H

f/177 H

f17

6 Hf/17

7 Hf

Average = 0.282476 ± 11 (MSWD = 0.33)

Average = 0.282508 ± 6 (MSWD = 0.60)

Average = 0.282684 ± 11 (MSWD = 0.51)

Analysis number

91500Figure 5. Laser-ablation Hf data from fi ve zircon stan-

dards measured over the course of this study. Error bars

on individual analyses represent within-run precision (1

standard error), and weighted mean averages are quoted

at 95% confi dence. Horizontal solid lines represent the

weighted mean of measured laser ablation–inductively

coupled plasma–mass spectrometry (LA-ICP-MS) 176Hf/177Hf

values; the dashed horizontal line represents average 176Hf/177Hf values measured from solutions after chemical

purifi cation from Woodhead and Hergt (2005; Temora-2,

Mud Tank, FC = 52, and 91500), and Sláma et al. (2008;

Plesovice). Where only one line is visible, the mean in situ

value reported here matches solution averages previously

measured. Shaded boxes represent the envelope of analyti-

cal error from the laser 176Hf/177Hf averages. MSWD—mean

square of the weighted deviates.



CECIL ET AL.


This provides further evidence that Yb and Lu are being properly sub-tracted from the laser 176Hf/177Hf calculations, as well as suggesting that any discrepancy in FC-52 Hf isotopic values is not related to REE interfer-ence correction. It is likely that the observed difference between the FC-52 long-term solution and laser averages is the result of isotopic heterogene-ity in the Duluth anorthosite, given that the solution Hf data were gener-ated from a different batch of zircon crystals than we analyzed.

U-Pb GEOCHRONOLOGY RESULTS

We present 29 new U-Pb zircon ages from two suites of plutonic rocks: (1) Jurassic–Eocene plutons of the central Coast Mountains batholith (n = 23), and (2) Ordovician–Early Devonian plutons clearly identifi ed as part of the Alexander terrane in southeast Alaska (n = 6). Detailed geochronol-ogy data, including concordia and weighted mean plots, can be found in the GSA Data Repository (see footnote 1).

Coast Mountains batholith pluton ages range from ca. 151 to 53 Ma and represent nearly the entire time span of magmatism in the Coast Moun-tains. West of the Coast shear zone, ages decrease systematically from west to east, indicating eastward migration of magmatism at ~1 km/m.y. across this area between 150 and 80 Ma. Jurassic to Early Cretaceous plutons are also found east of the Coast shear zone, which is ascribed to duplication of the Jurassic arc by sinistral displacement along strike-slip faults in the Early Cretaceous (Gehrels et al., 2009). Most samples from plutons east of the Coast shear zone record younger (Late Cretaceous to Eocene) ages and show no apparent migratory trends.

In addition to intrusive rocks of the central Coast Mountains batholith, Alexander terrane plutons from southeast Alaska were analyzed for the sake of comparing ε

Hf values of the Alexander terrane with those of the

plutons intruding Coast Mountains batholith crust of unknown affi nity, as discussed in later sections. U-Pb ages of the Alexander plutons range from ca. 480 to 390 Ma and are consistent with the range of ages reported by Gehrels and Saleeby (1987).

Hf ISOTOPIC RESULTS

Hafnium isotopic compositions were measured in situ via LA-MC-ICP-MS directly on top of the spot previously excavated for U-Pb analy-sis, such that each Hf isotopic measurement is directly tied to a cor-responding U-Pb age. Hf data, and related ages, are reported in Table 2 for the 29 plutonic samples discussed in the previous section. For each sample, between 15 and 55 individual spot measurements were made, and mean values are reported. Individual measurements and weighted mean plots of all Hf sample data can be found in the GSA Data Reposi-tory (see footnote 1).

Measurements from a given sample were highly reproducible, and uncertainties associated with the precision and scatter of a set of analy-ses are low (≤1 unit of ε

Hf at the 2σ level). Measured 176Hf/177Hf values

were corrected for the radiogenic in-growth of 176Hf, although that correc-tion is small, given that zircon incorporates relatively little Lu (measured 176Lu/177Hf values range from 0.0005 to 0.0015). Corrections for the iso-baric interference of 176Lu and 176Yb ranged from minor (~5% change in the 176Hf/177Hf ratio) to large (~35% change in the 176Hf/177Hf ratio). Good reproducibility of corrected 176Hf/177Hf values, however, inspires confi -dence that even major changes in isotope ratios were accurately accounted for. For example, sample 80JA11 yielded zircons that have up to 70% of their total mass 176 contributed from Yb and Lu, and yet the reproducibil-ity of corrected 176Hf/177Hf values from those sample grains was excellent (Fig. 6). Plots showing reproducibility of the corrected 176Hf/177Hf as a function of Yb and Lu interference can be found in the GSA Data Reposi-tory (see footnote 1).

Initial εHf

values from plutonic rocks of the Coast Mountains batho-lith range widely from +1 to +13, and values cluster between +9 and +13 for Paleozoic plutonic rocks of the Alaskan Alexander terrane. A general increase in ε

Hf(t) is observed from west to east across the central Coast

Mountains batholith, although nearly the entire range of Hf values is found in rocks located within and along the periphery of the Coast shear zone (see Fig. 2; Table 2).

INTERPRETATION OF U-Pb-Hf DATA AND IMPLICATIONS FOR

THE CRUSTAL ARCHITECTURE OF THE COAST MOUNTAINS

BATHOLITH

Hafnium isotopic signatures from intrusive rocks across the west-central Coast Mountains batholith are relatively juvenile, suggesting derivation of Coast Mountains batholith plutons from similarly juvenile mantle or crustal sources. In this respect, the Hf data presented here are consistent with the notion that the Coast Mountains batholith represents the growth of new crust in a continental arc system (e.g., Samson et al., 1989; Friedman et al., 1995). However, the range of measured ε

Hf(t) val-

ues is great (+1 to +13), and in all cases, those values are lower than the depleted mantle array (ε

Hf[500–0 Ma] values of +14 to +18; Vervoort and

Blichert-Toft, 1999), indicating heterogeneity in magma source regions and/or interaction with more evolved crustal materials. Although absent in most samples, traces of inherited zircon were present in three of the plutons analyzed in this study, also indicating the incorporation of older, recycled crust into melts.

Because a considerable amount of age control exists for this portion of the Coast Mountains batholith, our data can be used to evaluate rela-tions between petrogenesis and magmatic fl ux. Magmatism in the Coast Mountains batholith is interpreted to be strongly episodic, with distinct fl are-up events at 160–140 Ma, 120–78 Ma, and 55–48 Ma, and a long-lived period of relative magmatic inactivity between 140 and 120 Ma (Gehrels et al., 2009). Our geochronologic data do not refl ect that peri-odicity because we intentionally chose samples that were either known or inferred to have ages corresponding to both high- and low-fl ux events. Isotope pull-downs, or negative excursions in whole-rock initial ε

Nd val-

ues, have been temporally correlated with magmatic fl are-ups (Ducea and Barton, 2007; DeCelles et al., 2009), suggesting a link between isotopic signatures and periods of lithospheric thickening and crustal melt pro-duction. Our data show no clear negative excursions or any correlation between ε

Hf(t) and U-Pb age and/or the timing of magmatic fl ux events

(Fig. 7). This is probably attributable to the relatively small (n = 23) size of the Coast Mountains batholith U-Pb-Hf data set presented here. The range of isotopic values presented here likely records normal variation in

TABLE 1. AVERAGE Lu-Yb-Hf RATIOS OF STANDARD ZIRCONS

Zircon n 176(Yb + Lu)/176Hf(%)

176Hf/177Hf* 176Lu/177Hf Long-term solution

176Hf/177Hf†

Mud Tank 152 0.3 0.282508 (6) 0.00002 0.28250791500 46 6.1 0.282315 (13) 0.003 0.282306Plesovice 77 3.0 0.282476 (11) 0.001 0.282484Temora-2 73 18.8 0.282684 (11) 0.011 0.282686FC-52 73 17.6 0.282169 (10) 0.001 0.282184

*Numbers in parentheses are analytical uncertainties, based on precision and scatter of all individual measurements, quoted at 2 standard error.

†Solution values from Mud Tank, 91500, Temora-2, and FC-52 (FC-1) are from Woodhead and Hergt (2005); Plesovice is from Sláma et al. (2008).





20 8070605030 400.2824

0.2826

0.2828

0.2830

0.2832

0.283480JA11

176Hf/177Hf = 0.282884 ± 0.00002MSWD = 0.7; n = 34

176 H

f/177 H

f

(176Yb+176Lu)/176Hf (%)

Figure 6. Individual 176Hf/177Hf ratio measurements of sample

80JA11 as a function of 176(Yb + Lu) contribution to the total mass

176 signal. (176Yb + 176Lu)/176Hf is both high and variable in this

sample, ranging from 35% to 70%. MSWD—mean square of the

weighted deviates.

Avg

. mag

mat

ic fl

ux (

km3 /

m.y

./km

)

70

0

10

20

30

40

50

60

40 60 80 100 120 140 160 180 2000

2

4

6

8

10

12

14

16

U-Pb age Ma**

ε Hf(t

)

40 60 80 100 120 140 160 180 200

16

14

12

10

8

6

4

2

0

206Pb/238U age (Ma)

Figure 7. Plot showing the εHf

(t) values of Coast Mountains batholith

plutons as a function of age and magmatic fl ux (magmatic fl ux curve

from Gehrels et al., 2009). Data from Jurassic and Late Cretaceous plu-

tons in the eastern part of the study area are excluded, because those

intrusive bodies were not used in the magmatic fl ux calculations. Corre-

lation between Hf values and age or the timing of magmatic fl ux events

is weak, suggesting that either our sample set is not large enough to

capture Hf excursions, or that Hf is relatively insensitive to tectonic pro-

cesses (e.g., orogenic thickening, backarc extension, delamination, etc.)

that may be controlling episodes of magmatic lulls and fl are-ups.

TABLE 2. Hf ISOTOPIC DATA

Samplename

n 176(Yb + Lu)/176Hf (%)

176Hf/177Hf* 176Lu/177Hf* 176Hf/177Hf(t) εHf(t)*206Pb/238U age†

(Ma)Distance E of the CSZ§

(km)

Data from Jurassic through Eocene plutons of the Coast Mountains batholith

Otter-W 24 9.5 0.282846 (16) 0.0006 (2) 0.282844 5.4 (0.6) 141.6 (2.2) –88.904GJP-55 27 33.8 0.282766 (26) 0.0019 (15) 0.282761 2.7 (0.9) 141.5 (2.9) –7904GJP-58 27 36.6 0.282843 (36) 0.0018 (15) 0.282838 5.2 (1.3) 142.1 (2.7) –76.505MT-111 24 18.9 0.282797 (36) 0.0012 (6) 0.282794 2.6 (1.2) 124.1 (1.4) –74.104GJP-60 33 8.1 0.282816 (17) 0.0006 (3) 0.282814 4.3 (0.6) 153.0 (2.4) –64.6Stephens 38 14.9 0.282891 (13) 0.0012 (4) 0.282889 5.4 (0.4) 101.0 (1.1) –49.9McCauley-W 35 20.4 0.282915 (15) 0.0013 (6) 0.282912 7.5 (0.6) 118.8 (2.1) –66.104GJP-43 55 18.3 0.282954 (14) 0.0014 (5) 0.282952 8.1 (0.5) 97.4 (2) –47.104GJP-68 23 10.7 0.282915 (17) 0.0009 (4) 0.282913 6.8 (0.7) 94.0 (2.3) –36.804GJP-69 35 9.4 0.282959 (14) 0.0008 (3) 0.282958 8.1 (0.5) 88.5 (1.2) –27.204GJP-29 34 10.1 0.282944 (15) 0.0007 (3) 0.282943 7.3 (0.5) 75.8 (1) –10.905MT-106 25 11.5 0.283054 (25) 0.0009 (9) 0.283052 11.6 (0.9) 94.5 (2.3) –7.7Ecstall East 22 9.6 0.282899 (25) 0.0008 (4) 0.282898 6.0 (0.9) 91.8 (1.3) –6.604GJP-13 17 26.4 0.282988 (24) 0.002 (19) 0.282984 9.8 (0.9) 96.1 (1.7) –0.504GJP-84 32 10 0.283029 (14) 0.0008 (7) 0.283028 9.7 (0.5) 59.6 (1.2) 3.283GJ53 22 6.7 0.282792 (18) 0.0005 (3) 0.282791 1.5 (0.7) 58.2 (1.1) 4.604GJP-09 16 13.1 0.283043 (13) 0.0011 (16) 0.283041 11.2 (0.5) 82.2 (1.3) 12.404GJP-77 34 6.1 0.283025 (15) 0.0006 (2) 0.283024 9.6 (0.5) 55.2 (1.3) 14.405MT-135 29 14.8 0.283017 (16) 0.0012 (7) 0.283014 9.2 (1.6) 55.9 (3.0) 17.704GJP-89 54 15.1 0.283047 (21) 0.0007 (2) 0.283045 12.0 (1.0) 125.1 (1.8) 20.704GJP-24 23 11.2 0.283036 (16) 0.0008 (5) 0.283034 12.3 (0.6) 150.8 (1.8) 36.605MT-155 25 18.4 0.283035 (16) 0.0015 (6) 0.283033 10.3 (0.6) 67.6 (1.6) 74.805MT-145 36 17.2 0.282811 (26) 0.0009 (4) 0.282810 2.0 (1.0) 52.5 (0.8) 90.3

Data from Paleozoic plutons of the Alexander terrane, SE Alaska

79JD975 26 7.2 0.282893 (23) 0.0006 (4) 0.282889 12.9 (0.8) 414.4 (7) –4480JA11 34 47.7 0.282848 (21) 0.0032 (9) 0.282824 10.4 (0.7) 404.6 (9) –4680JA3 26 8.6 0.282887 (27) 0.0007 (6) 0.282882 12.2 (1.0) 393.9 (8) –4582GP702 24 23.9 0.282774 (24) 0.0017 (9) 0.282758 9.8 (0.9) 497.7 (8) –9683GP335 26 28.8 0.282858 (29) 0.0020 (7) 0.282841 12 (1.0) 445.3 (9) –8082GP626 28 3.8 0.282764 (25) 0.0003 (5) 0.282762 8.6 (0.9) 429.9 (9) –104

*Numbers in parentheses are analytical uncertainties, quoted at 2 standard error (S.E.).†Numbers in parentheses are analytical and systematic uncertainties, quoted at 2 S.E.§CSZ—Coast shear zone (see text for discussion).



CECIL ET AL.


the background arc–mantle wedge fl ux (Ducea and Barton, 2007), but it is nonetheless signifi cant because these variations can be used as tracers of input from distinctive sources.

Linking Hf Plutonic Signatures and Country Rock Assemblages

A fundamental observation of our data is that εHf

(t) increases from west to east; the lowest values were obtained for plutons along the western coast, and the highest values were obtained from plutons in the easternmost, inland areas (Fig. 2). Based on the distribution of Hf signatures, we discriminate between three distinct crustal domains into which plutons of the Coast Mountains batholith were emplaced: a western domain, characterized by relatively evolved Hf isotopic compositions, a central domain, with inter-mediate Hf compositions, and an eastern domain, characterized by juvenile Hf signatures. Because these variations coincide with variations in country rock assemblages, as well as previously reported Nd-Sr and detrital zircon data, we interpret those domains to be part of the Banks Island assemblage, the Alexander terrane, and the Stikine terrane, respectively (Fig. 8).

Samples from the Banks Island domain have the most evolved εHf

(t) values (+2.6 to +5.2), which are consistent with evolved continental Nd-Sr values from metamorphic country rocks of the west-central Coast Mountains batholith (Boghossian and Gehrels, 2000). Epsilon Nd values of those rocks are more negative (~+0.5 to −9.9) than our reported zircon Hf values for younger intrusive rocks of the same area. This appears to be typical of Coast Mountains batholith plutons, which likely originated from juvenile mantle, but which are sensitive to small amounts of more evolved crustal input, such that recorded ε

Hf(t) values represent mixing

between arc-type melts and older, preexisting crust. Old continental crust has high Hf concentrations and highly negative ε

Hf. For example,

Middle Proterozoic crust of the western United States has modern εHf

values ranging from −10 to −30; Early Proterozoic and Archean crust is even more negative (Vervoort and Patchett, 1996). Only small addi-tions (a few percent) of isotopically depleted continental material would therefore be necessary to drive down the ε

Hf of magmas, as has been

pointed out for Nd-Sr systematics (e.g., Patchett and Bridgwater, 1984; Samson et al., 1990, 1991a).

The central domain, which is the southern continuation of the Alex-ander terrane, consists of a north-south–trending belt of rocks located between the Banks Island domain to the west and the Gravina belt and mid-Cretaceous thrust system to the east. Plutons making up this belt have ε

Hf(t) values that range from +5.9 and +8.1, and they are more juvenile than

those of the Banks Island terrane (Fig. 8). They are interpreted to belong to the Alexander terrane because of: (1) the occurrence of distinctive Ordovi-cian–Silurian magmatic arc assemblage overlain by Devonian conglom-eratic strata; (2) the juvenile nature of the Alexander terrane understood from Nd-Sr isotopes (Samson et al., 1989), and the lack of continental input in geologic units and U-Pb zircon populations (Gehrels and Saleeby, 1987; Gehrels et al., 1987, 1996); and (3) geographic position; the crust into which this part of the batholith was built forms the southern continua-tion of the main Alexander terrane to the north (see Fig. 1).

To test the assignment of these rocks to the Alexander terrane, we ana-lyzed Hf isotopes from igneous rocks that are known to belong to the terrane in southern SE Alaska. These Paleozoic (ca. 480–390 Ma) intru-sive rocks have juvenile ε

Hf(t) values ranging from +8.6 to +12.9. The ε

Hf

values of the same plutons at 100 Ma, an age which approximates those of Alexander plutons in the central Coast Mountains batholith, yield highly consistent values of +2.5 to +7. Hafnium signatures of Cretaceous plutons intruding the inferred Alexander terrane (+5.9 to +8.1) are therefore likely recording either (1) direct melting of Paleozoic Alexander basement; or (2) melting of the mantle wedge followed by partial melting and assimi-lation of Alexander and/or Banks Island assemblages. It is also possible that the interpreted Alexander terrane of British Columbia represents a transitional zone between primitive Alexander of southeast Alaska and the more evolved Banks Island assemblage to the south. Connections between the northern Alexander terrane and Banks Island are consistent with the presence of ca. 480–410 Ma detrital zircons in Banks Island strata (Geh-rels and Boghossian, 2000). Furthermore, Hf isotopic signatures within plutons of the southeast Alaskan Alexander terrane become more evolved to the southwest, suggesting a gradational relationship between the Alex-ander and Banks Island terranes (Fig. 8).

The eastern domain is characterized by juvenile εHf

(t) values rang-ing from +10.2 to +15.1, and it is interpreted as Stikine terrane based on proximity with Stikine to the east, lack of a major tectonic boundary separating the two, the recognition by Hill (1985) that the central gneiss complex (eastern domain) contains fossiliferous marbles potentially cor-relative with Stikine strata, and primitive Nd-Sr values reported for Stikine rocks by Samson et al. (1989). There is no evidence for the input of older, reworked continental crust in any Stikine rocks in the study area, sug-gesting that, much like in the case of the Alexander terrane, they were produced either by the wholesale melting of juvenile Stikine lower crust or by direct melting of the mantle with variable contributions from terrane components.

Between the outboard Banks Island and Alexander terranes and the inboard Stikine terrane, there is a zone of structural deformation delin-eated by a regionally extensive belt of mid-Cretaceous thrust faults (e.g., Rubin et al., 1990) and by the Coast shear zone (e.g., Rusmore et al., 2005; Hollister and Andronicos, 2006), which is characterized by marked heterogeneity in Hf signatures (Fig. 8). Initial ε

Hf values within

this zone range from +1.5 to +11.6, and these are interpreted to rep-resent the imbrication of juvenile Alexander and Stikine terranes with

-120 -80 -40 0 40 800

2

4

6

8

10

12

14

16

Dist. E of the CSZDistance east of the CSZ (km)

ε Hf(t

)

K - Eocene plutons

Jurassic plutons

Paleozoic intrusives (SE Alaska)

Decreasing SW-ward trend Stikine

YTT (?)

Alexander

Banks Island

?

Figure 8. The εHf

(t) values of all Coast Mountains batholith and Paleozoic

southeast Alaska Alexander terrane plutons as a function of distance

from the Coast shear zone (CSZ). Hf values increase from west to east

across the main central Coast Mountains batholiths, and shifts in isoto-

pic values are used to infer boundaries between crustal terranes. A broad

zone of heterogeneous Hf signatures, including the lowest recorded εHf

(t)

value, is observed near the Coast shear zone (gray rectangle in center of

fi gure). This is interpreted to be a region of structural imbrication of Alex-

ander, Yukon-Tanana (YTT), and Stikine terranes along mid-Cretaceous (K)

thrust faults and the early Tertiary Coast shear zone. Question marks are

meant to indicate those samples for which we are less confi dent about

their tectonic affi nity, due to uncertainty about the geologic context or

isotopic character of the rocks.





continental-margin rocks of the Yukon-Tanana terrane. This is consistent with the geologic evidence for protracted and large-scale displacement along these structures.

It is important to note that there is very little interaction of plutons with country rocks at the present level of exposure, which represents emplacement depths of ~10–25 km (Butler et al., 2001, 2006). Further-more, new geochemical data from the Coast Mountains batholith indi-cate that plutons were generated and emplaced at depths greater than 35 km (Girardi et al., 2008). The fact that the Hf isotopic signatures of the plutons appear to be tracking with the country rock assemblages therefore raises interesting questions about the nature of the terranes at depth and the structural boundaries that separate them. Lateral changes in Hf isotopes imply marked heterogeneity in the magma source regions; the correlation between Hf signatures and the known isotopic character of country rocks indicates the presence of those or similar assemblages at depth. The assertion that presently exposed metamorphic rocks could be present at melt-generation depths is corroborated by high δ18O values in quartz from the same plutons (Wetmore and Ducea, 2011).

Relationships between Crustal Terranes of the Coast Mountains

Batholith

Discernible shifts in Hf isotopic values in plutons are interpreted to refl ect boundaries between discrete crustal terranes (Fig. 9). This implies

that structures controlling those boundaries act as through-going crustal barriers restricting magma migration and/or that the crustal boundaries have remained steeply dipping through time. For example, the thrusting and crustal thickening of mid-Cretaceous age along a regional belt in the axial Coast Mountains batholith (Rubin et al., 1990) have not affected the magma source regions in adjacent parts of the batholith. This is evi-denced in pluton ages in the eastern domain, which range from 151 to 55 Ma, but which have no associated change in Hf behavior. This is also observed in individual samples with zircon inheritance. For example, sam-ple MT05-135, an Eocene pluton located on the eastern periphery of the Coast shear zone, has inherited Jurassic and Early Cretaceous cores and magmatic rims with measured ages of 55 Ma (Fig. 10). There is very little change, however, in ε

Hf(t) values across those zircon domains, despite the

fact that a major period of deformation and shortening between 101 and 85 Ma occurred in the zone immediately adjacent to where this pluton was intruded (Rubin et al., 1990) (Fig. 10). Although thrusting has imbricated various terranes along the axial belt, the zone of imbrication is apparently thick skinned and restricted to a narrow region (~20-km-wide swath). Thrusting may have been thin skinned and low angle along the margins of the belt at shallow crustal levels, e.g., above the present 10–25 km levels of exposure.

Many of the structural and/or stratigraphic relationships between the crustal terranes discussed here are unclear because the original contacts between those terranes have been commonly overprinted by more recent

Subducting slab

Banks Island

Alexander

Stikine

Yukon-Tanana

Moho

Underplating of primitivemantle magmas

??

To Wrangellia/trench

X X′

Coastshearzone

Kitkatlafault

Shamesnormal fault

εHf (t) of plutons

+10 to +13

+6 to +9

+2 to +6

<+2

Crustal terranes

Juvenile, mantle-derived basalts

Zones of melting and assimilationof preexisting crust of varying age and isotopic composition

elacs

ot to

N

elacs

ot to

N

10

40

30

20

Dep

th (

km)

0 25 50

km

Vertically exaggerated; surface and Moho topography estimated

Central CMB at ca. 50 Ma

Figure 9. Schematic cross section of the Coast Mountains batholiths (CMB) at the latitude of the Douglas channel (line X–X′ in Fig. 2) at ca. 50 Ma, modifi ed after Gehrels et al. (2009). Crustal terranes are inferred on the basis of Hf data (presented

here) and existing Nd-Sr and detrital zircon data (see text). Patterned section of the lower crust is intended to represent

regions of mingling of juvenile, mantle-derived melts (dark-gray areas) and partial melts of the different crustal terranes,

which impart distinctive Hf signatures to the plutons emplaced above. Although there is little evidence to suggest that

evolved, continental-margin rocks of the Yukon-Tanana terrane underlie the crust east of the Coast shear zone, one pluton

from the easternmost part of the Stikine terrane yields a uniquely low εHf

(t) value, which could possibly be attributed to

Yukon-Tanana terrane contribution, although the nature of the contact between Yukon-Tanana terrane and Stikine terrane

is ambiguous. Crustal thickness interpreted from Morozov et al. (1998, 2001). Geology and structure are adapted in part

from Wheeler and McFeely (1991) and Gehrels et al. (2009).



CECIL ET AL.


deformation. It remains unclear, for example, why the Banks Island con-tinental margin–type assemblage is located outboard of the primitive Alexander terrane. Relationships among Alexander, Stikine, and Yukon-Tanana terranes are likewise enigmatic. East of the Coast shear zone, plutonic rocks record uniformly juvenile, Stikine-like ε

Hf(t) signatures.

This is different than relationships inferred to the north, where isotopi-cally evolved rocks of the Yukon-Tanana terrane and associated metamor-phic assemblages are observed wedged between primitive Alexander and Stikine terranes (Samson et al., 1991a). There appears to be an along-strike change, therefore, in the tectonostratigraphic relationship between the Yukon-Tanana terrane and Stikine terrane. One notable exception to this comes from an Eocene pluton (53 Ma) located at the eastern margin of outcropping batholithic rocks, which has an ε

Hf(t) value of +2.0, i.e.,

distinctly more evolved than those from any other portion of the Coast Mountains batholith. It is possible that this signature refl ects a compo-nent of evolved continental-margin strata of the Yukon-Tanana terrane that extends beneath the western Stikine terrane. Although the nature of the links between Stikine and Yukon-Tanana is unclear, they have been proposed on the basis of inherited Precambrian zircons in Jurassic Stikine plutons (Thorkelson et al., 1995), and evolved Nd isotopic characteristics of Upper Triassic Stikine strata (Jackson et al., 1991).

CONCLUSIONS

Plutons making up the west-central Coast Mountains batholith rep-resent ~100 m.y. of continental arc magmatism. In general, the juvenile character of Hf isotopic data from those plutonic rocks suggests the pro-duction of new continental crust derived primarily from mantle sources, with little recycling of Precambrian continental crust into arc-type melts. Substantial variation in ε

Hf(t) values of plutons (+1 to +13), and the sys-

tematic spatial distribution of those values, however, suggests that Hf isotopes are tracing heterogeneities in source regions and that those het-erogeneities are a function of the infl uence of different crustal terranes. From outboard to inboard, discrete crustal panels appear to be composed of the Banks Island assemblage, the Alexander terrane, and the Stikine terrane, with the imbrication of thin fragments of Yukon-Tanana terrane along mid-Cretaceous thrust faults and along the Coast shear zone, and the possibility of Yukon-Tanana terrane basement beneath Stikine strata in the easternmost part of the study area.

The juxtaposition of these crustal terranes requires complicated struc-tural and/or stratigraphic relationships between the various terranes, particularly in the case of the Yukon-Tanana and Stikine terranes. If the Yukon-Tanana terrane to the north was indeed emplaced outboard of the Stikine terrane during a transpressive regime, as has been suggested by Samson et al. (1991a), the inferred structural imbrication of the Yukon-Tanana terrane near the Coast shear zone in our study area could perhaps represent the pinching out of the Yukon-Tanana terrane to the south. Struc-tural emplacement of Yukon-Tanana terrane rocks outboard of the Stikine terrane along Cretaceous thrusts and left-lateral faults could explain an enigmatic quartzite cobble conglomerate located in the south-central part of our study area, west of the Coast shear zone, which, unlike other local lithologies, has Archean detrital zircon populations (Boghossian and Geh-rels, 2000; Gehrels and Boghossian, 2000). The presence of an isotopi-cally evolved pluton to the east, however, suggests a potential stratigraphic tie between the Stikine and the Yukon-Tanana terranes. That relationship remains cryptic due to the uniformity of juvenile Hf signatures in plutonic rocks presented here and juvenile Nd-Sr signatures of Stikine country rocks (Samson et al., 1989). Further isotopic work to the east and to the south of the study could help resolve the extent of Yukon-Tanana terrane infl uence in Coast Mountains batholith plutons.

ACKNOWLEDGMENTS

This work was sponsored by National Science Foundation (NSF) award EAR-0309885 for support of the BATHOLITHS projects, and by EAR-0732436 for support of the Arizona LaserChron Center. The authors thank two anonymous reviewers, whose thoughtful and critical comments greatly improved the manuscript.

REFERENCES CITED

Andronicos, C.L., Chardon, D.H., and Hollister, L.S., 2003, Strain partitioning in an obliquely convergent orogen, plutonism, and synorogenic collapse: Coast Mountains Batholith, British Columbia, Canada: Tectonics, v. 22, 24 p., doi:10.1029/2001TC001312.

20 40 60 80 100 120 140 160 180 200 0

2

4

6

8

10

12

14

16

18

20

206Pb/238U age (Ma)

ε Hf(t

)

Mag

mat

ic

age

= 5

5 M

a

MT05-135

Xenocrystic coresMixing of U-Pb/Hf domains?

Depleted mantleB

300

260

220

180

140

100

60

20

0.00

0.01

0.02

0.03

0.04

0.05

0.0 0.1 0.2 0.30 0.1 0.2 0.3

0.05

0.04

0.03

0.02

0.01

0

207Pb/235U

206 P

b/23

5 U

A

Analyses used for magmaticage determination

Thrust belt development and crustal thickening

Figure 10. U-Pb geochronometric and Hf isotopic data for zircons

from sample MT05-135, an Eocene pluton intruded ~15 km east of

the Coast shear zone. (A) U-Pb ages range from 180 to 55 Ma, with

systematically older cores and younger rims, which were used to

interpret the magmatic age. (B) Hf isotope values as a function of

age. Hf values vary between +8.5 and +12.5 and show little change

with zircon age. The string of zircon ages intermediate between

the 125 Ma cores and the 55 Ma rims likely represents mixing of

the two age domains as a result of the 40 μm laser spot size used.

The gray vertical bar represents the proposed timing of major

deformation and thickening along an axial fold-and-thrust belt in

the mid-Cretaceous (Rubin et al., 1990).





Berg, H.C., Jones, D.L., and Richter, D.H., 1972, Gravina-Nutzotin belt—Tectonic signifi cance of an Upper Mesozoic sedimentary and volcanic sequence in southern and southeast-ern Alaska: U.S. Geological Survey Professional Paper 800-D, p. D-1–D-24.

Boghossian, N.D., and Gehrels, G.E., 2000, Nd isotopic signature of metasedimentary pen-dants in the Coast Mountains between Prince Rupert and Bella Coola, British Columbia, in Stowell, H.H., and McClelland, W.C., eds., Tectonics of the Coast Mountains, South-eastern Alaska and British Columbia: Geological Society of America Special Paper 343, p. 77–87.

Bouvier, A., Vervoort, J.D., and Patchett, P.J., 2008, The Lu-Hf and Sm-Nd isotopic composi-tion of CHUR: Constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets: Earth and Planetary Science Letters, v. 273, p. 48–57, doi:10.1016/j.epsl.2008.06.010.

Butler, R.F., Gehrels, G.E., Crawford, M.L., and Crawford, W.A., 2001, Paleomagnetism of the Quottoon plutonic complex in the Coast Mountains of British Columbia and southeast-ern Alaska: Evidence for tilting during uplift: Canadian Journal of Earth Sciences, v. 38, p. 1367–1384, doi:10.1139/e01-021.

Butler, R.F., Gehrels, G.E., Hart, W., Davidson, C., and Crawford, M.L., 2006, Paleomagne-tism of Late Jurassic to mid-Cretaceous plutons near Prince Rupert, British Columbia, in Haggart, J.W., Enkin, R.J., and Monger, J.W.H., eds., Paleogeography of the North American Cordillera: Evidence For and Against Large-Scale Displacements: Geological Association of Canada Special Paper 46, p. 171–200.

Chardon, D., Andronicos, C.L., and Hollister, L.S., 1999, Large-scale transpressive shear zone patterns and displacements within magmatic arcs: The Coast plutonic complex, British Columbia: Tectonics, v. 18, p. 278–292, doi:10.1029/1998TC900035.

Chu, N.C., Taylor, R.N., Chavagnac, V., Nesbitt, R.W., Boella, R.M., Milton, J.A., German, C.R., Bayon, G., and Burton, K., 2002, Hf isotope ratio analysis using multi-collector inductively coupled plasma mass spectrometry: An evaluation of isobaric interference corrections: Journal of Analytical Atomic Spectrometry, v. 17, p. 1567–1574, doi:10.1039/b206707b.

DeCelles, P.G., Ducea, M.N., Kapp, P., and Zandt, G., 2009, Cyclicity in Cordilleran orogenic systems: Nature Geoscience, v. 2, p. 251–257, doi:10.1038/ngeo469.

Ducea, M.N., and Barton, M.D., 2007, Igniting fl are-up events in Cordilleran arcs: Geology, v. 35, p. 1047–1050, doi:10.1130/G23898A.1.

Engebretson, D.C., Cox, A., and Gordon, R.G., 1985, Relative Motions between Oceanic and Continental Plates in the Pacifi c Basin: Geological Society of America Special Paper 206, 59 p.

Friedman, R.M., Mahoney, J.B., and Cui, Y., 1995, Magmatic evolution of the southern Coast belt: Constraints from Nd-Sr isotopic systematics and geochronology of the south-ern Coast plutonic complex: Canadian Journal of Earth Sciences, v. 32, p. 1681–1698, doi:10.1139/e95-133.

Gareau, S.A., 1989, Metamorphism, deformation, and geochronology of the Ecstall-Quaal Rivers area, Coast plutonic complex, British Columbia: Geological Survey of Canada Paper 89-1E, p. 155–162.

Gareau, S.A., and Woodsworth, G.J., 2000, Yukon-Tanana terrane in the Scotia-Quaal belt, Coast plutonic complex, central-western British Columbia, in Stowell, H.H., and McClelland, W.C., eds., Tectonics of the Coast Mountains, Southeastern Alaska and Brit-ish Columbia: Geological Society of America Special Paper 343, p. 23–44.

Gehrels, G.E., 2000, Reconnaissance geology and U-Pb geochronology of the west fl ank of the Coast Mountains between Juneau and Skagway, southeastern Alaska, in Stowell, H.H., and McClelland, W.C., eds., Tectonics of the Coast Mountains, Southeastern Alaska and British Columbia: Geological Society of America Special Paper 343, p. 213–234.

Gehrels, G.E., 2001, Geology of the Chatham Sound region, southeast Alaska and coastal British Columbia: Canadian Journal of Earth Sciences, v. 38, p. 1579–1599, doi:10.1139/e01-040.

Gehrels, G.E., and Boghossian, N.D., 2000, Reconnaissance geology and U-Pb geochronol-ogy of the west fl ank of the Coast Mountains between Bella Coola and Prince Rupert, coastal British Columbia, in Stowell, H.H., and McClelland, W.C., eds., Tectonics of the Coast Mountains, Southeastern Alaska and British Columbia: Geological Society of America Special Paper 343, p. 61–76.

Gehrels, G.E., and Saleeby, J.B., 1987, Geology of the southern Prince of Wales Island, southeastern Alaska: Geological Society of America Bulletin, v. 98, p. 123–137, doi:10.1130/0016-7606(1987)98<123:GOSPOW>2.0.CO;2.

Gehrels, G.E., Saleeby, J.B., and Berg, H.C., 1987, Geology of Annette, Gravina, and Duke Islands, southeastern Alaska: Canadian Journal of Earth Sciences, v. 24, p. 866–881, doi:10.1139/e87-086.

Gehrels, G.E., McClelland, W.C., Samson, S.D., Patchett, P.J., and Orchard, M.J., 1992, Geol-ogy of the western fl ank of the Coast Mountains between Cape Fanshaw and Taku Inlet, southeastern Alaska: Tectonics, v. 11, p. 567–585, doi:10.1029/92TC00482.

Gehrels, G.E., Butler, R.F., and Bazard, D.R., 1996, Detrital zircon geochronology of the Alex-ander terrane, southeastern Alaska: Geological Society of America Bulletin, v. 108, p. 722–734, doi:10.1130/0016-7606(1996)108<0722:DZGOTA>2.3.CO;2.

Gehrels, G.E., Valencia, V.A., and Ruiz, J., 2008, Enhanced precision, accuracy, effi ciency, and spatial resolution of U-Pb ages by laser ablation–multicollector–inductively cou-pled plasma–mass spectrometry: Geochemistry Geophysics Geosystems, v. 9, 13 p., doi:10.1029/2007GC001805.

Gehrels, G., Rusmore, M., Woodsworth, G., Crawford, M., Andronicos, C., Hollister, L., Patchett, J., Ducea, M., Butler, R., Klepeis, K., Davidson, C., Friedman, R., Haggart, J., Mahoney, B., Crawford, W., Pearson, D., and Girardi, J., 2009, U-Th-Pb geochronology of the Coast Mountains batholith in north-coastal British Columbia: Constraints on age and tectonic evolution: Geological Society of America Bulletin, v. 121, p. 1341–1361, doi:10.1130/B26404.1.

Gerdes, A., and Zeh, A., 2009, Zircon formation versus zircon alteration—New insights from combined U-Pb and Lu-Hf in-situ LA-ICP-MS analyses, and consequences for the inter-

pretation of Archean zircon from the Central zone of the Limpopo belt: Chemical Geol-ogy, v. 261, p. 230–243, doi:10.1016/j.chemgeo.2008.03.005.

Girardi, J.D., Patchett, P.J., Ducea, M.N., Gehrels, G.E., Manthei, C.D., Pearson, D.M., Rusmore, M.E., Woodsworth, G.J., Fan, J., Kerrich, R.W., Thole, J.T., and Wirth, K.R., 2008, Elemental and isotopic evidence for positive and negative feedback mechanisms governing mag-matic fl ux in the Coast Mountains batholith, British Columbia: Eos (Transactions, Ameri-can Geophysical Union), v. 89, Fall Meeting supplement, abstract V33A-2204.

Griffi n, W.L., Wang, X., Jackson, S.E., Pearson, N.J., O’Reilly, S.Y., Xu, X., and Zhou, X., 2002, Zircon chemistry and magma mixing, SE China: In situ analysis of Hf isotopes, Tonglu and Pingtan igneous complexes: Lithos, v. 61, p. 237–269, doi:10.1016/S0024-4937(02)00082-8.

Haggart, J.W., Diakow, L.J., Mahoney, J.B., Woodsworth, G.J., Struik, L.S., Gordee, S.M., and Rusmore, M., 2006a, Geology, Bella Coola Region (NTS 93D/01, /07, /08, /10, /15, and Parts of 93D/02, /03, /06, /09, /11, /14, /16, and 92M/15 and /16), British Columbia: Geological Survey of Canada Open-File 5385, and British Columbia Geological Survey Geoscience Map 2006–7, 3 sheets, scale 1:100,000.

Haggart, J.W., Woodsworth, G.J., and McNicoll, V.J., 2006b, Uranium-Lead Geochronology of Two Intrusions in the Southern Bowser Basin, British Columbia: Geological Survey of Canada Current Research 2006-F2, 6 p.

Haggart, J.W., Diakow, L.J., Mahonry, J.B., Woodsworth, G.J., Struik, L.C., Gordee, S.M., Israel, S., Hooper, R.L., van der Heyden, P., Rusmore, M., Lett, R., Ceh, M., Hastings, N.L., and Wagner, C., 2007, Bedrock Geology, Bella Coola Region (93C, 93D, 93E, 92M, and 92N), British Columbia: Geological Survey of Canada Open-File 5410, CD-ROM.

Hawkesworth, C.J., and Kemp, A.I.S., 2006, Using hafnium and oxygen isotopes in zir-cons to unravel the record of crustal evolution: Chemical Geology, v. 226, p. 144–162, doi:10.1016/j.chemgeo.2005.09.018.

Heah, T.S.T., 1990, Eastern margin of the Central gneiss complex in the Shames River area, Terrace, British Columbia, in Current Research, Part E: Geological Survey of Canada Paper 90-1E, p. 159–169.

Heah, T.S.T., 1991, Mesozoic Ductile Shear and Paleogene Extension along the Eastern Mar-gin of the Central Gneiss Complex, Coast Belt, Shames River Area, near Terrace, Brit-ish Columbia [M.S. thesis]: Vancouver, British Columbia, Canada, University of British Columbia, 155 p.

Hill, M.L., 1985, Remarkable fossil locality: Crinoid stems from migmatite of the Coast plu-tonic complex, British Columbia: Geology, v. 13, p. 825–826, doi:10.1130/0091-7613(1985)13<825:RFLCSF>2.0.CO;2.

Hollister, L.S., and Andronicos, C., 2000, The Central gneiss complex, Coast Mountains, British Columbia, in Stowell, H.H., and McClelland, W.C., eds., Tectonics of the Coast Mountains, Southeastern Alaska and British Columbia: Geological Society of America Special Paper 343, p. 45–60.

Hollister, L.S., and Andronicos, C.L., 2006, Formation of new continental crust in western British Columbia during transpression and transtension: Earth and Planetary Science Letters, v. 249, p. 29–38, doi:10.1016/j.epsl.2006.06.042.

Hollister, L.S., Grissom, G.C., Peters, E.K., Stowell, H.H., and Sisson, V.B., 1987, Confi rmation of the empirical correlation of aluminum in hornblende with pressure of solidifi cation of calc-alkaline plutons: The American Mineralogist, v. 72, p. 231–239.

Iizuka, T., and Hirata, T., 2005, Improvements of precision and accuracy in in situ Hf isotope microanalysis of zircon using the laser ablation MC-ICPMS technique: Chemical Geol-ogy, v. 220, p. 121–137, doi:10.1016/j.chemgeo.2005.03.010.

Jackson, J.L., Gehrels, G.E., Patchett, P.J., and Mihalynuk, M.G., 1991, Stratigraphic and isotopic link between the northern Stikine terrane and an ancient continental-margin assemblage, Canadian Cordillera: Geology, v. 19, p. 1177–1180, doi:10.1130/0091-7613(1991)019<1177:SAILBT>2.3.CO;2.

Kemp, A.I.S., Foster, G.L., Schersten, A., Whitehouse, M.J., Darling, J., and Storey, C., 2009, Concurrent Pb-Hf isotope analysis of zircon by laser ablation multi-collector ICP-MS, with implications for the crustal evolution of Greenland and the Himalayas: Chemical Geology, v. 261, p. 244–260, doi:10.1016/j.chemgeo.2008.06.019.

Mahoney, J.B., Haggart, J.W., Woodsworth, G.J., Hooper, R.L., and Snyder, L.S., 2007a, Geology, Kitlope Lake (East Part) (93E/04), British Columbia: Geological Survey of Can-ada Open-File 5588, and Geoscience British Columbia Map 2007-11-4, 1 sheet, scale 1:50,000.

Mahoney, J.B., Haggart, J.W., Hooper, R.L., Snyder, L.S., and Woodsworth, G.J., 2007b, Geol-ogy, Tsaytis River (93E/05), British Columbia: Geological Survey of Canada Open-File 5587, and Geoscience British Columbia Map 2007-11-3, 1 sheet, scale 1:50,000.

Mahoney, J.B., Haggart, J.W., Hooper, R.L., Snyder, L.S., and Woodsworth, G.J., 2007c, Geology, Parts of Chikamin Mountain and Troista Lake (93E/06, 11), British Columbia: Geological Survey of Canada Open-File 5586, and Geoscience British Columbia Map 2007-11-2, 1 sheet, scale 1:50,000.

Mahoney, J.B., Haggart, J.W., Hooper, R.L., Snyder, L.S., and Woodsworth, G.J., 2007d, Geol-ogy, Tahtsa Peak (93E/12), British Columbia: Geological Survey of Canada Open-File 5585, and Geoscience British Columbia Map 2007-11-1, 1 sheet, scale 1:50,000.

Mahoney, J.B., Hooper, R.L., Gordee, S.M., and Haggart, J.W., 2007e, Geology, Foresight Mountain (93E/03), British Columbia: Geological Survey of Canada Open-File 5386 (revised), and Geoscience British Columbia Map 2006-2, 1 sheet, scale 1:50,000.

Mahoney, J.B., Gordee, S.M., Haggart, J.W., Friedman, R.M., Diakow, L.J., and Woodsworth, G.J., 2009, Magmatic evolution of the eastern Coast plutonic complex, Bella Coola region, west-central British Columbia: Geological Society of America Bulletin, v. 121, p. 1362–1380, doi:10.1130/B26325.1.

McClelland, W.C., Gehrels, G.E., Samson, S.D., and Patchett, P.J., 1992, Structural and geo-chronologic relations along the western fl ank of the Coast Mountains batholith—Stikine River to Cape Fanshaw, central southeastern Alaska: Journal of Structural Geol-ogy, v. 14, p. 475–489, doi:10.1016/0191-8141(92)90107-8.



CECIL ET AL.


Monger, J.W.H., Price, R.A., and Tempelman-Kluit, D.J., 1982, Tectonic accretion and the ori-gin of the two major metamorphic and plutonic welts in the Canadian Cordillera: Geol-ogy, v. 10, p. 70–75, doi:10.1130/0091-7613(1982)10<70:TAATOO>2.0.CO;2.

Monger, J.W.H., Wheeler, J.O., Tipper, H.W., Gabrielse, H., Harms, T., Struik, L.C., Campbell, R.B., Dodds, C.J., Gehrels, G.E., and O’Brien, J., 1992, Upper Devonian to Middle Trias-sic assemblages, Part B: Cordilleran terranes, in Gabrielse, H., and Yorath, C.J., eds., Geology of the Cordilleran Orogen in Canada: Geological Society of Canada, v. G-2, p. 281–327.

Morozov, I.B., Smithson, S.B., Hollister, L.S., and Diebold, J.B., 1998, Wide-angle seismic imaging across accreted terranes, southeastern Alaska and western British Columbia: Tectonophysics, v. 299, p. 281–296, doi:10.1016/S0040-1951(98)00208-X.

Morozov, I.B., Smithson, S.B., Chen, J.R., and Hollister, L.S., 2001, Generation of new conti-nental crust and terrane accretion in southeastern Alaska and western British Colum-bia: Constraints from P- and S-wave wide-angle seismic data (ACCRETE): Tectono-physics, v. 341, p. 49–67, doi:10.1016/S0040-1951(01)00190-1.

Patchett, P.J., 1983, Importance of the Lu-Hf isotopic system in studies of planetary chro-nology and chemical evolution: Geochimica et Cosmochimica Acta, v. 47, p. 81–91, doi:10.1016/0016-7037(83)90092-3.

Patchett, P.J., and Bridgwater, D., 1984, Origin of continental crust of 1.9–1.7 Ga defi ned by Nd isotopes in the Ketilidian terrane of Greenland: Contributions to Mineralogy and Petrology, v. 87, p. 311–318, doi:10.1007/BF00381287.

Patchett, P.J., and Tatsumoto, M., 1980, A routine high-precision method for Lu-Hf isotope geochemistry and chronology: Contributions to Mineralogy and Petrology, v. 75, p. 263–267, doi:10.1007/BF01166766.

Patchett, P.J., Gehrels, G.E., and Isachsen, C.E., 1998, Nd isotopic characteristics of metamor-phic and plutonic rocks of the Coast Mountains near Prince Rupert, British Columbia: Canadian Journal of Earth Sciences, v. 35, p. 556–561, doi:10.1139/e98-007.

Rubin, C.M., Saleeby, J.B., Cowan, D.S., Brandon, M.T., and McGroder, M.F., 1990, Region-ally extensive mid-Cretaceous west-vergent thrust system in the northwestern Cor-dillera—Implications for continent-margin tectonism: Geology, v. 18, p. 276–280, doi:10.1130/0091-7613(1990)018<0276:REMCWV>2.3.CO;2.

Rusmore, M.E., Woodsworth, G.J., and Gehrels, G.E., 2000, Late Cretaceous evolution of the eastern Coast Mountains, Bella Coola, British Columbia, in Stowell, H.H., and McClelland, W.C., eds., Tectonics of the Coast Mountains, Southeastern Alaska and Brit-ish Columbia: Geological Society of America Special Paper 343, p. 89–106.

Rusmore, M.E., Gehrels, G., and Woodsworth, G.J., 2001, Southern continuation of the Coast shear zone and Paleocene strain partitioning in British Columbia–southeast Alaska: Geological Society of America Bulletin, v. 113, p. 961–975, doi:10.1130/0016-7606(2001)113<0961:SCOTCS>2.0.CO;2.

Rusmore, M.E., Woodsworth, G.J., and Gehrels, G.E., 2005, Two-stage exhumation of mid-crustal arc rocks, Coast Mountains, British Columbia: Tectonics, v. 24, 25 p., doi:10.1029/2004TC001750.

Saleeby, J.B., 2000, Geochronologic investigations along the Alexander-Taku terrane bound-ary, southern Revillagigedo Island to Cape Fox areas, southeast Alaska, in Stowell, H.H., and McClelland, W.C., eds., Tectonics of the Coast Mountains, Southeastern Alaska and British Columbia: Geological Society of America Special Paper 343, p. 107–143.

Samson, S.D., McClelland, W.C., Patchett, P.J., Gehrels, G.E., and Anderson, R.G., 1989, Evi-dence from neodymium isotopes for the mantle contributions to Phanerozoic crustal genesis in the Canadian Cordillera: Nature, v. 337, p. 705–709, doi:10.1038/337705a0.

Samson, S.D., Patchett, P.J., Gehrels, G.E., and Anderson, R.G., 1990, Nd and Sr isotopic characterization of the Wrangellia terrane and implications for crustal growth of the Canadian Cordillera: The Journal of Geology, v. 98, p. 749–762, doi:10.1086/629438.

Samson, S.D., Patchett, P.J., McClelland, W.C., and Gehrels, G.E., 1991a, Nd isotopic char-acterization of metamorphic rocks in the Coast Mountains, Alaskan and Canadian Cordillera—Ancient crust bounded by juvenile terranes: Tectonics, v. 10, p. 770–780, doi:10.1029/90TC02732.

Samson, S.D., Patchett, P.J., McClelland, W.C., and Gehrels, G.E., 1991b, Nd and Sr isoto-pic constraints on the petrogenesis of the west side of the northern Coast Mountains batholith, Alaskan and Canadian Cordillera: Canadian Journal of Earth Sciences, v. 28, p. 939–946, doi:10.1139/e91-085.

Scherer, E., Munker, C., and Mezger, K., 2001, Calibration of the lutetium-hafnium clock: Sci-ence, v. 293, p. 683–687, doi:10.1126/science.1061372.

Sláma, J., Kosler, J., Condon, D.J., Crowley, J.L., Gerdes, A., Hanchar, J.M., Horstwood, M.S.A., Morris, G.A., Nasdala, L., Norberg, N., Schaltegger, U., Schoene, B., Tubrett, M.N., and Whitehouse, M.J., 2008, Plesovice zircon—A new natural reference material for U-Pb and Hf isotopic microanalysis: Chemical Geology, v. 249, p. 1–35, doi:10.1016/j.chemgeo.2007.11.005.

Söderlund, U., Patchett, J.P., Vervoort, J.D., and Isachsen, C.E., 2004, The 176Lu decay con-stant determined by Lu-Hf and U-Pb isotope systematics of Precambrian mafi c intrusions: Earth and Planetary Science Letters, v. 219, p. 311–324, doi:10.1016/S0012-821X(04)00012-3.

Stacey, J.S., and Kramers, J.D., 1975, Approximation of terrestrial lead isotope evolution by a two-stage model: Earth and Planetary Science Letters, v. 26, p. 207–221, doi:10.1016/0012-821X(75)90088-6.

Stowell, H.H., and Crawford, M.L., 2000, Metamorphic history of the Coast Mountains oro-gen, western British Columbia and southeast Alaska, in Stowell, H.H., and McClelland, W.C., eds., Tectonics of the Coast Mountains, Southeastern Alaska and British Colum-bia: Geological Society of America Special Paper 343, p. 257–284.

Thorkelson, D.J., Mortensen, J.K., Marsden, H., and Taylor, R.P., 1995, Age and tectonic set-ting of Early Jurassic episodic volcanism along the northeastern margin of the Hazel-ton Trough, northern British Columbia, in Miller, D.M., and Busby, C., eds., Jurassic Magmatism and Tectonics of the North American Cordillera: Geological Society of America Special Paper 299, p. 83–94.

van der Heyden, P., 1989, U-Pb and K-Ar Geochronometry of the Coast Plutonic Complex, 53N to 54N, British Columbia, and Implications for the Insular-Intermontane Superter-rane Boundary: Vancouver, British Columbia, Canada, University of British Columbia, 392 p.

van der Heyden, P., 1992, A Middle Jurassic to early Tertiary Andean-Sierran arc model for the Coast belt of British Columbia: Tectonics, v. 11, p. 82–97, doi:10.1029/91TC02183.

Vervoort, J.D., and Blichert-Toft, J., 1999, Evolution of the depleted mantle: Hf isotope evi-dence from juvenile rocks through time: Geochimica et Cosmochimica Acta, v. 63, p. 533–556, doi:10.1016/S0016-7037(98)00274-9.

Vervoort, J.D., and Patchett, P.J., 1996, Behavior of hafnium and neodymium isotopes in the crust: Constraints from Precambrian crustally derived granites: Geochimica et Cosmo-chimica Acta, v. 60, p. 3717–3733, doi:10.1016/0016-7037(96)00201-3.

Vervoort, J.D., Patchett, P.J., Soderlund, U., and Baker, M., 2004, Isotopic composition of Yb and the determination of Lu concentrations and Lu/Hf ratios by isotope dilution using MC-ICPMS: Geochemistry Geophysics Geosystems, v. 5, 15 p., doi:10.1029/2004GC000721.

Wetmore, P., and Ducea, M.N., 2011, Geochemical evidence of a near-surface history for source rocks of the central Coast Mountain batholith, British Columbia: International Geology Review, v. 53, p. 230–260, doi:10.1080/00206810903028219.

Wheeler, J.O., and McFeely, P., 1991, Tectonic Assemblage Map of the Canadian Cordillera: Geologic Survey of Canada Map 1712A, scale 1:2,000,000.

Wheeler, J.O., Brookfi eld, A.J., Gabrielse, H., Monger, J.W.H., Tipper, H.W., and Woodsworth, G.J., 1991, Terrane Map of the Canadian Cordillera: Geological Survey of Canada Map 1713A, scale 1:2,000,000.

Woodhead, J.D., and Hergt, J.M., 2005, A preliminary appraisal of seven natural zircon refer-ence materials for in situ Hf isotope determination: Geostandards and Geoanalytical Research, v. 29, p. 183–195, doi:10.1111/j.1751-908X.2005.tb00891.x.

Woodhead, J., Hergt, J., Shelley, M., Eggins, S., and Kemp, R., 2004, Zircon Hf-isotope analysis with an excimer laser, depth profi ling, ablation of complex geometries, and concomitant age estimation: Chemical Geology, v. 209, p. 121–135, doi:10.1016/j.chem-geo.2004.04.026.

Wu, F.-Y., Yang, Y.-H., Xie, L.-W., Yang, J.-H., and Xu, P., 2006, Hf isotopic compositions of the standard zircons and baddeleyites used in U-Pb geochronology: Chemical Geology, v. 234, p. 105–126, doi:10.1016/j.chemgeo.2006.05.003.

MANUSCRIPT RECEIVED 2 JANUARY 2011REVISED MANUSCRIPT RECEIVED 3 MAY 2011MANUSCRIPT ACCEPTED 4 MAY 2010

Printed in the USA



Lithosphere - California State University, Northridge

Documents

Transcript of Lithosphere - California State University, Northridge