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[The Journal of Geology, 2010, volume 118, p. 601–619] ! 2010 by The University of Chicago.All rights reserved. 0022-1376/2010/11806-0002$15.00. DOI: 10.1086/656385

601

Continuation of the Laurentian Grenville Province acrossthe Ross Sea Margin of East Antarctica

John W. Goodge, C. Mark Fanning,1 Devon M. Brecke,2 Kathy J. Licht,3 andEmerson F. Palmer3

Department of Geological Sciences, University of Minnesota, Duluth, Minnesota 55812, U.S.A.(e-mail: [email protected])

A B S T R A C T

Zircon U-Pb ages from glacial clasts in Quaternary tills of the central Transantarctic Mountains indicate the presenceof Grenville-age crust along the Ross Sea margin of East Antarctica. The polymict tills contain a variety of igneous,metaigneous, and metasedimentary clasts with Proterozoic ages not known from basement exposure. Four orthogneissclasts have igneous ages of !1065–1100 Ma and Ross Orogen metamorphic overgrowths of !500–550 Ma. The latterages indicate that these clasts are not glacially far traveled. Grenville-like signatures also come from a paragneisscontaining detrital zircons ranging from 925 to 1130 Ma, an early Ross granitoid (!563 Ma) containing inheritedzircons of !1020 Ma, and detrital zircons from Neogene and Quaternary glacial deposits with a composite age peakof !1045 Ma. Other igneous clasts with ages of !1460, !1580, and !1880 Ma provide further evidence of Proterozoiccrust and corroborate earlier finding of an !1440-Ma A-type granite clast with isotopic signatures matching similar-age granites in Laurentia. Together, the glacial clasts indicate that !1.1-Ga Grenville-age igneous crust lies beneaththe ice sheet along the Ross Sea margin of East Antarctica. The clast ages are similar to those of Mesoproterozoicrelicts in other parts of East Antarctica, and they resemble the ages of basement rocks in western Laurentia, includingigneous rocks in west Texas (1070–1120 Ma) where the Grenville Orogen (sensu stricto) terminates abruptly, or,alternatively, metamorphic assemblages within Proterozoic rift-margin strata of northern Idaho (1000–1150 Ma). Theglacial clasts provide new evidence that an !1.1-Ga-age belt extends from western Laurentia into central East Ant-arctica inboard of the present-day Pacific margin, supporting both the SWEAT (southwest U.S.–East Antarctic) fit ofRodinia cratons and the suggestion that a Mesoproterozoic orogen integral to Rodinia assembly crosses East Antarctica.

Online enhancement: appendix.

Introduction

East Antarctica is one of Earth’s largest intact Pre-cambrian shields, it contains some of the oldescrtcontinental crust, and it is a key piece in the Ro-dinia and Gondwana supercontinents (fig. 1; Fitz-simons 2003; Cawood and Buchan 2007; Harley andKelly 2007; Goodge et al. 2008). Due to near-com-

Manuscript received September 24, 2009; accepted June 7,2010.

1 Research School of Earth Sciences, Australian NationalUniversity, Mills Road, Canberra, Australian Capital Territory0200, Australia.

2 Natural Resources Research Institute, University of Min-nesota, 5013 Miller Trunk Highway, Duluth, Minnesota 55811,U.S.A.

3 Department of Earth Sciences, Indiana University–PurdueUniversity Indianapolis, Indianapolis, Indiana 46202, U.S.A.

plete coverage by the modern ice cap, however, theshield interior remains unknown. Geological ex-ploration of this last continental frontier is there-fore mainly limited to study of coastal outcrop, sed-iment provenance, and over-ice geophysics. Thelimitations imposed by ice cover are exemplifiedby our poor understanding of the Mesoproterozoicassembly history of East Antarctica and its neigh-bors during growth of Rodinia. Correlation ofcoastal geology with counterparts in India and Aus-tralia, for example, suggests that Grenville-age (0.9–1.3 Ga) orogenic belts extend into the interior ofEast Antarctica (Fitzsimons 2000, 2003), but icecover and Pan-African-age overprint stymie effortsto trace these belts inland from the coast. Likewise,in Dronning Maud Land (fig. 1), a Grenville-age oro-

602 J . W . G O O D G E E T A L .

Figure 1. Paleogeographic setting of East Antarctica in a Neoproterozoic SWEAT (southwest U.S.–East Antarctic)fit of Rodinia (Moores 1991; Dalziel 1991; Goodge et al. 2008), in present-day East Antarctic geographic referenceshowing major crustal age provinces. Cross-hatched areas show possible extension of Grenville Orogen from Laurentiatoward East Antarctica, connecting either with the Maud province or with the Nimrod province. Continuity of sucha belt across East Antarctica is unknown. Stars indicate locations of !1.4-Ga igneous clasts (white) and !1.1-Gametaigneous clasts (black). CL p Coats Land, DG p Denman Glacier; DML p Dronning Maud Land, GSM pGamburtsev Subglacial Mountains, PB p Prydz Bay, SR p Shackleton Range, T p Tasmania, and TA p Terre Adelie.Geologic features and provinces: B p Beardmore Group, D p Delamerian Orogen, G p Gawler Craton, G-R pGranite-Rhyolite province, Moj p Mojave province, Mz p Mazatzal province, N p Nimrod Group (Miller Range),PM p Pensacola Mountains, R p Ross Orogen, Wy p Wyoming Craton; and Yav p Yavapai province.

gen (Maud belt) separates rocks of the GrunehognaCraton from basement of the East Antarctic Cratonin the Shackleton Range, but the continuity of thisbelt is uncertain. Whether and where these beltscontinue across the shield are unknown because,

with one exception in the Nimrod Group ("1.7 Ga;Goodge et al. 2001; Goodge and Fanning 2002), noPrecambrian basement is exposed for at least one-third of the cratonic margin along the Transant-arctic Mountains (TAM; fig. 1). Complementary to

Journal of Geology C O N T I N U A T I O N O F L A U R E N T I A N G R E N V I L L E P R O V I N C E 603

the question of Mesoproterozoic orogens in EastAntarctica, a long-standing problem in Rodinia pa-leogeography concerns continuity of the GrenvilleOrogen outside Laurentia (e.g., Dalziel 1991, 1997;Moores 1991). Mobile belts with Grenville-likeages are common in the various Rodinia fragmentsand likely relate to late Mesoproterozoic supercon-tinent assembly. However, the actual Grenvilleprovince—a continental-scale collisional belt—ter-minates abruptly at the southwestern Laurentia riftmargin. The prerift continuation of this belt isunknown.

To study basement beneath the ice sheet alongthe Ross Sea margin of East Antarctica, we sampledglaciogenic materials sourced from the westerncontinental interior (present-day coordinates) anddeposited in the central TAM (2A). The area of fo-cus is the paleogeographic equivalent of crust thatnow underlies the TAM and the adjacent inlandregion. Here we present U-Pb zircon age data fromlithic clasts entrained in Quaternary glacial tills inthe upper Byrd and Nimrod glacier drainages anddetrital zircon age spectra from tills and glaciogenicsediments of the Neogene Sirius Group. The formerallow us to directly analyze basement rocks fromotherwise ice-covered East Antarctic crust, and thelatter allow us to assess regional patterns of crustalage distribution (Goodge 2007b). These new agedata provide the first direct evidence of Grenville-age crust along the Ross Sea margin of East Ant-arctica and expand the known occurrence of olderProterozoic basement. We suggest that Grenville-age rocks of Laurentia and associated older Prote-rozoic belts continue into the central part of EastAntarctica, strengthening Rodinia linkages be-tween the formerly conjugate rift margins of west-ern Laurentia and East Antarctica.

Grenville Provinces in Rodinia

East Antarctica’s paleogeographic position in the!1-Ga supercontinent Rodinia remains controver-sial (e.g., Goodge 2002; Li et al. 2008) due to itsremoteness, poor rock exposure, and lack of Neo-proterozoic paleomagnetic poles. Basement con-nections to present-day India, Africa, and Australiaare well known, but identifying its conjugate Ro-dinia neighbor along the TAM rift margin is diffi-cult because Precambrian basement there ismasked by the late Neoproterozoic to early Paleo-zoic Ross Orogen and younger cover. A key aid inRodinia reconstructions are Grenville-age (0.9–1.3Ga) orogenic belts (e.g., Dalziel 1991; Hoffman1991; Moores 1991) whose geometry and age arerelated to Rodinia assembly. One prominent model,

the southwest United States–East Antarctica(SWEAT) connection between the southwesternUnited States (Laurentia) and East Antarctica(Moores 1991), predicts continuation of the Gren-ville Orogen proper across the Ross Sea margin ofEast Antarctica, yet no discrete Grenville-age base-ment has been documented in the modern TAM.Grenville-age basement occurs elsewhere in EastAntarctica (fig. 1), disrupted by Pan-African (ca.500–600 Ma) events related to assembly of Gond-wana. In the Gondwana sector of East Antarctica,Fitzsimons (2000) distinguished three age provincesof 1030–1090 (Maud), 900–990 (Rayner), and 1130–1330 Ma (Wilkes), which correlate with basementbelts, respectively, in Africa, India, and Australia(fig. 1). Within the Maud province, Grenville-agerelicts of 1060–1070 Ma are preserved within Pan-African belts in H.U. Sverdrupfjella (Board et al.2005) and the Shackleton Range (Will et al. 2009).Igneous and high-grade metamorphic rocks in theseseparate collisional belts are related to sequentialgrowth of the Mesoproterozoic supercontinent, yetno comparable provinces are known in the TAM.

Suspected Grenville-age basement along theTAM margin of East Antarctica is based on isotopicage provinces defined by !500-Ma Ross Orogengranites and detrital zircon provenance in Neopro-terozoic–Lower Cambrian rift-margin sandstones.Sr and Nd isotope data from Ross-age granites de-fine isotopic provinces with model ages of about2.0–2.4, 1.6–1.9, and 1.1–1.5 Ga (Borg et al. 1990;Borg and DePaolo 1994). The general trend fromolder evolved crust like that in the Nimrod Groupto more juvenile crust south of Scott Glacier re-sembles that in the Mojave-Yavapai-Mazatzal prov-inces of southwestern Laurentia (Bennett andDePaolo 1987). Although representing a mixture ofage components, model ages as young as !1.2 Gain the southern TAM granitoids (Borg and DePaolo1994) indicate hidden Mesoproterozoic crustalsources.

Upper Neoproterozoic to Lower Cambrian(!670–520 Ma) siliciclastic rift- and passive-marginstrata in the central TAM (Beardmore and lowerByrd groups; fig. 1) contain abundant detrital zir-cons in the range 1080–1175 Ma (Goodge et al.2002, 2004) in addition to populations of about 1.4,1.6–1.8, and "2.5 Ga. A Grenville-age derital-zirconsignature is also recognized in rift-margin strata ofsoutheast Australia, Victoria Land, and the Pen-sacola Mountains (Williams et al. 1992; Ireland etal. 1998; Goodge et al. 2004; Wysoczanski and Al-libone 2004; Squire et al. 2006; Fergusson et al.2007), suggesting widespread dispersal of clasticmaterial of this age following Rodinia assembly.

Figure 2. A, Modern ice catchments defined from ice-balance velocity field (Bamber et al. 2000), showing major ice-flow directions and ice divides superimposed on BEDMAP subice surface of East Antarctica (Lythe et al. 2000). Whitestars indicate sample sites. Drainages: Brd p Beardmore Glacier, Byd p Byrd Glacier, Dav p David Glacier, Fnd pFoundation Ice Stream, Mul p Mulock-Skelton Glacier, Nim p Nimrod Glacier, Sct p Scott-Reedy Glacier, Shk pShackleton Glacier, Whi p Whillans Ice Stream, and Wlk p Wilkes Land-Mertz ice stream. B, Low-contrast MODISimage of the central Transantarctic Mountains region, showing locations of dated glacial clasts and detrital zircon samplesobtained from glacial deposits (white stars). White arrows show modern ice flow. Proterozoic Nimrod igneous province,Nimrod Group basement complex, and Neoproterozoic rift margin inferred from subice magnetic anomalies (Goodgeand Finn, forthcoming). CM p The Cloudmaker, GR p Geologists Range, MR p Miller Range, and OB p Oliver Bluff.


Table 1. Summary of SHRIMP U-Pb Age Results for Pre-Ross Glacial Clasts

Sample Rock type Area U-Pb ages (Ma) Notes

LWE Granitoid Lonewolf Nunataks !1880, igneous crystallization Paleoproterozoic igneous crustLW-03 Deformed granitoid Lonewolf Nunataks !1580, igneous crystallization Mesoproterozoic igneous crustLW-18 Red granitoid Lonewolf Nunataks !1460, igneous crystallization Mesoproterozoic igneous crustTNQa Red A-type granite Turret Nunatak !1440, igneous crystallization Mesoproterozoic igneous crustMRH Sil-Grt-Bt gneiss Milan Ridge !1100, igneous cores; !550, metamorphic Grenville igneous protolithMRV Bt gneiss Milan Ridge !1090, igneous cores; !520, metamorphic Grenville igneous protolithAGD Sil-Crd-Grt-Bt gneiss Argo Glacier !1065, igneous grains; !500, metamorphic Grenville igneous protolithAGG Grt-Bt gneiss Argo Glacier !1065, igneous cores; !550 metamorphic Grenville igneous protolithKTW A-type granite Kon-Tiki Nunatak !565, granite; !1010–1020, inheritance Grenville inheritanceMRO Grt-Ms-Bt gneiss Milan Ridge Variety of Proterozoic igneous ages; detrital Metasedimentary precursor

a Goodge et al. (2008)

The autochthonous rift-margin succession in thecentral TAM has an East Antarctic provenance(Myrow et al. 2002), indicating that Mesoprotero-zoic crust is a significant component within theadjacent East Antarctic shield. Basement isotopicpatterns and detrital-zircon provenance thus pro-vide valuable signatures of Grenville-age basementin the Ross Sea sector of East Antarctica, but theyprovide only geologically imprecise constraints forcorrelation due to the processes of geochemical andphysical mixing.

U-Pb Age Results

We analyzed nine rock clasts taken from glacialmoraines in the upper Byrd and Nimrod glacier ar-eas (fig. 2; table 1), and detrital zircons separatedfrom seven samples of Quaternary and Neogeneglaciogenic sediment between Nimrod and Scottglaciers. U-Pb ages were determined using the sen-sitive high-resolution ion microprobe (SHRIMP) atthe Australian National University, followingmethods outlined by Williams (1998 and referencestherein). All plots and weighted mean 206Pb/238U or207Pb/206Pb age calculations were carried out usingISOPLOT/EX (Ludwig 2003). The “mixture mod-eling” algorithm of Sambridge and Compston(1994) was used for complex data sets to define sta-tistical age populations. The appendix, available inthe online edition or from the Journal of Geologyoffice, contains sample descriptions, details of theanalytical methods and age standards, and tabula-tion of all zircon U-Pb data by sample.

Lithic Clasts. Igneous and metamorphic rockclasts collected from polymict glacial tills in theupper Byrd and Nimrod glacier drainages representmaterial eroded directly from the adjacent shieldinterior (Palmer 2008; fig. 2B). Entrained basal de-bris is brought to the surface by upward flow as theoutlet glaciers encounter the TAM flank and de-posited at the surface as the surrounding ice ablates

(Scofield et al. 1991). Analysis of the upstream tillmatrix indicates a strong cratonic signature as com-pared to down-glacier sites, which show a higherproportion of locally derived cover and Ross Orogenmaterial (Licht and Palmer 2007). Lithic clasts inthe tills include abundant granitoids of diversecomposition and texture, gneisses, phyllites,schists, sedimentary diamictites (Beacon Super-group), and dolerite (Ferrar). A high ice velocityfield near the restricted entry to outlet glacierslikely leads to preferential erosion of proximal base-ment material. Indeed, among glacial clasts studiedfrom the upper Nimrod Glacier area (Brecke 2007),many are granitoids with geochemical signaturesmatching petrologically similar rocks from theGranite Harbour Intrusive suite and U-Pb zirconages of about 490–540 Ma that indicate local der-ivation from the Ross Orogen. However, severalclasts give new information about previously un-known Proterozoic basement in the region (table1).

Four clasts are high-grade orthogneisses frommoraines inboard of the Miller Range at Argo Gla-cier and Milan Ridge (fig. 2B). All are quartzofeld-spathic and banded to weakly layered, and theycontain variably strong L-S tectonite fabrics (fig.A1, available in the online edition or from the Jour-nal of Geology office). Metamorphic mineralogy inthe gneisses includes biotite, garnet, muscovite,cordierite, and/or sillimanite. Geothermobarome-try from three of the samples (AGD, AGG, andMRH) indicates cooling and decompression pathsduring garnet growth into the Sil stability field overa P interval of 10–4 kbar (Brecke 2007), consistentwith metamorphic P-T paths recorded in the Nim-rod Group (Goodge 2007a). Metamorphism of thesegneisses is evident in cathodoluminescence (CL)images as low-Th/U zircon rims overgrown onwell-preserved prismatic cores showing oscillatorygrowth zonation (fig. 3); the internal structure andmoderate Th/U ratios of the zircon cores and the

606 J . W . G O O D G E E T A L .

Figure 3. Cathodoluminescence images of representative zircon grains from metaigneous clast samples. Grain numbersin bold; spot ages (ellipses) in Ma; all are 207Pb/206Pb ages except for Ross Orogen zircons, for which 206Pb/238U ages areshown. Spots are !20 mm.

general rock compositions indicate these sampleshave igneous protoliths. Two of the orthogneissesfrom near Argo Glacier, AGD and AGG, have clearprismatic zircons that are variably discordant butyield upper-intercept ages of about 1065 Ma (fig.4A, 4B). Lower-intercept ages of !530–555 Ma re-flect radiogenic Pb loss during Ross metamor-phism. Eleven near-concordant spot analyses inAGG give a weighted mean 207Pb/206Pb age of

Ma, within error of the upper-intercept1057 " 12age. Two other samples from a moraine near MilanRidge, MRH and MRV, likewise have variably dis-cordant igneous zircons that define upper inter-cepts at !1105 and !1090 Ma, respectively (fig. 4C,

4D). Seven near-concordant analyses in MRV givea weighted mean 207Pb/206Pb age of Ma.1070 " 9Rims with low Th/U ratios (!0.04) have 206Pb/238Uages of !550–500 Ma, indicating they are meta-morphic overgrowths. We consider this group ofclasts to have a common igneous origin with pri-mary ages of !1060–1100 Ma and a metamorphicoverprint at 500–550 Ma. A pattern of Pb loss in-dicates that the basement from which they werederived was affected by Ross metamorphism. Fromthis we interpret that the clasts are not far traveledand were derived from Grenville-age metaigneousbasement inboard of the TAM margin of EastAntarctica.


Figure 4. Concordia plots of SHRIMP zircon U-Pb data for Grenville-age orthogneiss clasts. All samples show significantdiscordance of some individual analyses, indicating Pb loss at !500 Ma. A, Sample AGD. Igneous protolith age definedby upper intercept at !1065 Ma. B, Sample AGG. Igneous protolith age defined by upper intercept at !1065 Ma. Insetshows distribution of 11 concordant or near-concordant analyses that define a weighted mean age of Ma. C,1057 " 12Sample MRH. Igneous protolith age defined by upper intercept at !1100 Ma. D, Sample MRV. Igneous protolith agedefined by upper intercept at !1090 Ma. Inset shows distribution of seven concordant or near-concordant analyses thatdefine a weighted mean age of Ma.1070 " 9

Three granitoid clasts from moraines at LonewolfNunataks give other Proterozoic crystallizationages (fig. 2B). The granitoids exhibit variable de-formation fabrics but are unmetamorphosed (fig.A2); oscillatory and sector zoning in their zirconsindicates magmatic crystallization (fig. 5A–5C).The oldest granitoid, LWE, is a biotite monzonitewith clear, elliptical to prismatic zircons givingmostly concordant analyses; seven near-concordantanalyses give a weighted mean 207Pb/206Pb age of

Ma (fig. 6A). A deformed muscovite-1878.1 " 7.0biotite granite, LW2-03, contains stubby clear zir-cons that cluster on a near-concordant array givinga weighted mean 207Pb/206Pb age of Ma1578.0 " 5.2( ) that is indistinguishable from a well-n p 11defined upper intercept (fig. 6B). Clast LW2-18 is acoarse-grained, red biotite leucogranite with gem-like, pink euhedral zircons giving a weighted mean207Pb/206Pb age of Ma ( ) that is1458.6 " 5.7 n p 17indistinguishable from an upper-intercept age (fig.

608 J . W . G O O D G E E T A L .

Figure 5. Cathodoluminescence images of representative zircon grains from igneous clast samples. Grain numbers inbold; spot ages (ellipses) in Ma; all are 207Pb/206Pb ages except for Ross Orogen zircons, for which 206Pb/238U ages areshown. Spots are !20 mm.

6C). This age is close to a petrographically similarred rapakivi A-type granite from the upper NimrodGlacier drainage at Turret Nunatak with an age of

Ma (Goodge et al. 2008). Together the1441.1 " 6.4igneous clasts from the upper Byrd Glacier area in-dicate that the interior catchment is underlain byPaleo- and Mesoproterozoic igneous crust. None ofthe clast ages are known from dated rocks of theTAM, although !1590–1600-Ma felsic volcanicrock clasts with geochemical affinity to GawlerRange Volcanics were dated from moraines at theTerre Adelie coast (Peucat et al. 2002), and !1850-Ma granitoids comprise the Donington Suite in theGawler Craton of South Australia (Parker et al.1988).

A red, coarse-grained A-type muscovite-biotitegranite from Kon-Tiki Nunatak in Nimrod Glacier,KTW (fig. A2D), has high SiO2 (76.8 wt%),K2O!Na2O (8.34 wt%), (peralu-A/CNK p 1.04minous), and immobile elements Ce (155 ppm) andZr (328 ppm). This clast contains highly complexigneous zircons, many of which have wide outerrims with moderate to bright CL appearance thatshow both multistage magmatic growth and in-heritance of older age components (figs. 5D, 6D).Fourteen near-concordant analyses cluster at !575Ma with ages ranging from !555 to 600 Ma; thevariable but low Th/U ratios and CL textures ofthese zircons indicate an interplay of magmatic andmetamorphic crystallization processes during this


Figure 6. Concordia plots of SHRIMP zircon U-Pb data for igneous clasts. A, Granite sample LWE from LonewolfNunataks moraine near upper Byrd Glacier. Igneous crystallization age defined by weighted mean 207Pb/206Pb age of

Ma. B, Granitoid sample LW2-03. Igneous crystallization age defined by upper intercept and weighted mean1878.1 " 7.0207Pb/206Pb age of Ma. C, Granitoid sample LW2-18. Igneous crystallization age defined by upper intercept1578.0 " 5.2and weighted mean 207Pb/206Pb age of Ma. D, Sample KTW from Kon-Tiki Nunatak, midstream in Nimrod1458.6 " 5.7Glacier. A range of Ross crystallization ages (inset) includes a young magmatic population with a weighted mean 206Pb/238U age of Ma. Inherited zircons define a chord with an upper intercept of !1025 Ma.563.0 " 5.3

time period. Mixture modeling of this group definesthe youngest magmatic population ( ) with an p 6weighted mean 206Pb/238U age of Ma,563.0 " 5.3which we interpret as the best estimate of finalcrystallization. Nine inherited zircons in the sam-ple fall on a chord with an upper intercept of !1025Ma and a lower intercept at !570 Ma, indicatingmelt incorporation of older xenocrystic zircon andsubsequent radiogenic Pb loss. The three oldestcores from this group with !2% discordance have

207Pb/206Pb ages between !1010 and !1015 Ma. Sam-ple KTW thus represents one of the oldest Rossmagmatic rocks yet dated from the TAM, and itappears to have interacted with and/or melted from!1.0-Ga igneous basement.

A sample of pelitic paragneiss from the MilanRidge moraine in the western Miller Range, MRO,contains zircons with a wide variety of ages, CLcharacter, and Th/U ratios (figs. 7, 8). We obtainednear-concordant analyses of !925 and !1130 Ma,

610 J . W . G O O D G E E T A L .

Figure 7. Cathodoluminescence images of representativezircon grains from sample MRO. Grain numbers in bold;spot ages (ellipses) in Ma; all are 207Pb/206Pb ages except forRoss Orogen zircons, for which 206Pb/238U ages are shown.

and older discordant analyses indicating Mesopro-terozoic sources (207Pb/206Pb ages of !1475 and!1550 Ma). Six CL-dark, wide zircon overgrowthswith low Th/U (!0.04) give a weighted mean 206Pb/238U age of Ma; their appearance and563.0 " 4.7age suggest they formed during Ross metamor-phism. The variety of zircon types and ages in thissample indicate it is probably a metasedimentarygneiss containing detrital zircons with Grenville-age and older sources.

Detrital Zircons in Glaciogenic Sediment. To ob-tain a broad sampling of material from the ice-covered area adjacent to the TAM, we analyzed de-trital zircon populations from glaciogenic sedi-ments considered to have a provenance beneath themodern ice sheet. Six samples are from the Neo-gene Sirius Group in the Beardmore and Scott-Reedy glaciers areas (fig. 2B; table 2; McKelvey etal. 1991; Wilson et al. 1998), collected from strat-igraphic sections representing subglacial and fjor-dal-marine settings. One sample of till matrix (AG-Ct) was collected from Quaternary moraine nearArgo Glacier. Each of the samples yields a uniquezircon age spectrum containing dominant popula-tions of either !250 or !500 Ma, and variable num-bers and ages of older zircons (tables A10–A16,available online or from the Journal of Geology of-fice). The two younger populations represent de-

trital input from Permian and Cambro-Ordovicianconvergent-margin arc systems, respectively, thatevolved along the Gondwana margin of East Ant-arctica, which are also the dominant populationsin Beacon Supergroup sandstones from the Shack-leton Glacier area (Elliot and Fanning 2008).

For our purposes, we grouped the analyses fromthese seven samples with near-concordant ages1800 Ma into a composite probability distribution(fig. 9) totaling 121 individual grain analyses (28%of the total). The resulting age distribution showsa dominant age peak at !1045 Ma, ranging from900 to 1200 Ma, and other peaks at about 1780,2250, 2755, and 3285 Ma. For comparison, we alsoshow representative ages of the North AmericanGrenville province (980–1190 Ma; Rivers 1997),detrital-zircon ages in Neoproterozoic and LowerCambrian sandstones (Goodge et al. 2004), andknown basement ages from the nearby NimrodGroup (1720, 2500, and 3100 Ma; Goodge and Fan-ning 1999, 2002; Goodge et al. 2001). The distri-bution of Precambrian detrital zircon ages in theseglaciogenic sediments thus shows a dominantsource comprised by Grenville-age igneous crust,as well as a mixed Paleoproterozoic, Neoarchean,and Mesoarchean provenance. The main detritalage peak, although representing a larger sedimentcatchment area, and possibly including some re-cycled zircon from Beacon Supergroup and olderstrata, is very similar to the glacial clast ages re-ported here (1065–1100 Ma). It is also similar to asubpopulation of detrital zircon ages between !950and 1270 Ma found in subglacial till matrix at Lone-wolf Nunataks (Licht and Palmer 2007). AlthoughPermian and Cambro-Ordovician detrital zirconpopulations dominate the glaciogenic sediments, itis unlikely that they contain a large fraction of re-cycled grains from Beacon Supergroup sandstones.First, only two of our seven glacial samples containany Permian zircons, yet all of them contain largeRoss-age grain populations; this is the opposite pat-tern in Beacon sandstones, which are dominated byPermian grains due to cratonward fluvial transportfrom the Gondwana-margin Permo-Triassic arc(Elliot and Fanning 2008). Second, the Beacon sand-stones contain #5% of grains older than 800 Ma,compared with 28% of the glacial grain popula-tions, and recycling of grains from the Beaconwould not enrich the glacial sediments in older zir-cons. The different Ross Orogen and Precambrianage distributions in samples of the Beacon Super-group as compared with those of the younger glacialdeposits preclude significant input of cratonic sig-natures by glacial erosion of existing Beacon fore-land-basin sediments. We conclude, therefore, that


Figure 8. Tera-Wasserburg plot of SHRIMP zircon U-Pb data for metamorphic paragneiss clast MRO. Distribution ofProterozoic ages represents a detrital component in a sedimentary precursor, overprinted by Ross-age rims (inset) witha weighted mean 206Pb/238U age of Ma. Note concordant ages at !925 and !1130 Ma and two discordant563.0 " 4.7207Pb/206Pb ages of !1475 and !1550 Ma.

the glaciogenic sediment detrital zircon popula-tions are representative of glacially eroded crystal-line basement.

Discussion

The catchment area of the Byrd and Nimrod gla-ciers covers a substantial part of the Ross Sea sectorof the East Antarctic Craton margin, and lithicclasts in glacial tills impounded behind the TAMprovide samples of the ice-covered craton notknown from basement exposure (fig. 2A). The RossOrogen metamorphic overprint at !500–550 Ma in-dicates the clasts have a proximal source contain-ing significant Mesoproterozoic and Paleoprotero-zoic igneous crust. Furthermore, petrographic andgeochemical signatures of many of our glacial clastsmatch closely the granitoids of the !500-Ma Gran-ite Harbour Intrusive suite exposed in the centralTAM (Brecke 2007), as would be expected from lo-cal derivation. Alternatively, it is also possible thatmetamorphic clasts of this age were derived fromPan-African-age inliers within the Maud province(fig. 1), but we consider this less likely because thepositions of the Last Glacial Maximum and recent

drainage divides would prevent long-distance trans-port toward the present-day central TAM area fromDronning Maud Land. Although metaigneousclasts from the Argo Glacier and Milan Ridge siteshave ages (1060–1100 Ma) that resemble those ofgranitic rocks in H.U. Sverdrupfjella and the Shack-leton Range (Board et al. 2005; Will et al. 2009), asource closer to the present-day central TAM ismost likely.

Multiple glacial clasts in the vicinity of NimrodGlacier therefore indicate that !1.1-Ga Grenville-age igneous crust lies beneath the East Antarcticice sheet adjacent to the central TAM. Igneousclasts from the upper Byrd Glacier indicate thepresence of previously unknown Paleoproterozoiccrust (!1.6–1.9 Ga), and corroborate earlier findingof an !1.4-Ga A-type rapakivi granite clast that hasgeochemical and isotopic signatures indistinguish-able from the large A-type granite province thatterminates in southwestern Laurentia (fig. 1; An-derson and Morrison 2005; Goodge and Vervoort2006; Goodge et al. 2008). The presence of a Me-soproterozoic igneous province explains the occur-rence in Antarctic rift-margin strata of !1.4-Ga de-trital zircons that have Hf-isotope compositions

612 J . W . G O O D G E E T A L .

Table 2. Summary of Glaciogenic Samples Used for Detrital-Zircon Analysis

Age/area/sample Locality Formation Section Lithotype Setting Reference

Neogene Sirius Group:Reedy Glacier:

94QH-10 Quartz Hills Quartz Hills ... Sandstone/siltstone Glaciomarine Wilson et al. 199894TS-23 Tillite Spur Tillite Spur Unit 8 Diamict Subglacial Wilson et al. 1998

Beardmore Glacier:95DMH-35 Oliver Bluffs Meyer Desert 5, OB unit 3 Diamict Subglacial/fluvioglacial McKelvey et al. 199191-OB 5-11 Oliver Bluffs Meyer Desert 5, OB unit 4 Stratified diamict Subglacial/fluvioglacial McKelvey et al. 1991CM-G The Cloudmaker The Cloudmaker CM-G Diamict Fjordal-marine McKelvey et al. 1991CM-top The Cloudmaker The Cloudmaker CM-top Diamict Fjordal-marine McKelvey et al. 1991

Quaternary till:Nimrod Glacier:

AG-Ct Argo Glacier Unnamed morainedeposit

Matrix sand Matrix sand Subglacial This article

also matching the A-type granite province (Goodgeet al. 2004, 2008). Proterozoic igneous crust is alsointerpreted to be the source of large-amplitude pos-itive magnetic anomalies observed in aeromagneticand satellite magnetic data (Goodge and Finn, forth-coming), outlined as the Nimrod igneous provincein figure 2B.

New U-Pb age data from glacial clasts thus in-dicate that !1.1-Ga Grenville-age igneous crust isan important element of the East Antarctic shieldadjacent to the Ross Sea (fig. 10). The clast agesoverlap that of the Grenville Orogen in Laurentia(980–1190 Ma; Rivers 1997), including igneousrocks in west Texas and the Llano uplift (fig. 1;1070–1120 Ma; Walker 1992; Reese et al. 2000; Liet al. 2007). The source of the glacial clasts in in-terior East Antarctica may correlate directly withthe Grenville province (sensu stricto) in Laurentia,thereby strengthening Rodinia ties of East Antarc-tica to Laurentia in a SWEAT-like geometry (Dal-ziel 1991; Moores 1991; Borg and DePaolo 1994;Goodge et al. 2008). In this case, the older Meso-and Paleoproterozoic granitoid clasts may correlatewith igneous rocks of the Mazatzal-Yavapai beltsin Laurentia, and the !1.4-Ga A-type granitoidsthat intrude them. If this is true, it implies that thecratonic core of Rodinia included Laurentia, south-eastern Australia, and the Mawson Continent partof East Antarctica, which were united before !1.1Ga and to which other elements were added during“Grenvillian” time. These include the crustalblocks of western and northern Australia, whichlie outboard of the late Mesoproterozoic Albany-Fraser belt (Cawood and Korsch 2008).

Alternatively, the assemblage of Grenville-agemetaigneous glacial clasts might correlate withother occurrences of this age in parts of DronningMaud Land and the Shackleton Range (fig. 10).These include a gneissic granite of Ma1072 " 10age in H.U. Sverdrupfjella showing Pan-African

overprint (Board et al. 2005) and a metagranitoid ofMa age in the eastern Shackleton Range1059 " 9

that was also overprinted by Pan-African meta-morphism at about 600 Ma (Will et al. 2009). Al-though there is little to tie these rocks togetherother than primary and overprint ages, it is possiblethey represent broader Grenville-age crustal blocks,perhaps originally coherent, that were finally joinedduring collisional shortening along the Mozam-bique belt at !600 Ma (Boger and Miller 2004), fol-lowed by reactivation at !500 Ma along the Kuungasuture that united East Antarctica, Australia, India,and southern Africa in East Gondwanaland.

Surprisingly similar Grenvillian ages are recentlyreported from inliers of Proterozoic metamorphicrocks in northern Idaho, just outboard of the Ar-chean Wyoming Craton (fig. 10). These rocks,within the Clearwater metamorphic complex nearBoehls Butte (Vervoort et al. 2005), are consideredto be metamorphosed Belt Supergroup protolithswith Lu-Hf isochron ages of , ,1149 " 4 1056 " 57and Ma. Belt-Purcell strata in this region1006 " 5also contain zircons with metamorphic overgrowthages of !1130 Ma (Doughty and Chamberlain 2008).Occurring in the same area are the !1.58-Ga La-clede gneiss (Doughty et al. 1998) hosted by theHauser Lake gneiss and a !1.86-Ga orthogneiss as-sociated with the Boehls Butte anorthosite innorthern Idaho (Vervoort et al. 2007). The 1.58-GaLaclede orthogneiss falls within a known magmatic“gap” in western Laurentia of 1580–1620 Ma andtherefore is of a distinctive age. The latter ortho-gneiss has a U-Pb zircon age of 1.86 Ga, and con-tains garnet giving successive Lu-Hf isochron agesof !1.47, 1.38, and 1.10–1.00 Ga. Calc-alkalinemetaigneous rocks of 1.86 Ga age also occur in theLittle Belt Mountains of Montana (Mueller et al.2002), constraining the age of the Great Falls tec-tonic zone. Together, the co-occurrence of igneousand metamorphic events at !1.88–1.86, 1.58, 1.44–


Figure 9. Composite histogram and relative probability distribution (black line) of all detrital zircons 1800 Ma fromseven glaciogenic sediment samples, including subglacial diamict, fjordal-marine, and glaciolacustrine deposits of theNeogene Sirius Group (samples from Beardmore and Reedy glacier areas) and Quaternary till from Argo Glacier nearthe Miller Range. Samples contain large dominant populations of !250- and !500-Ma zircons (not shown). Also shownfor comparison: age distribution of detrital zircons between 900 and 1300 Ma from 12 Neoproterozoic-Cambrian sand-stones from the Transantarctic Mountains (gray line; Goodge et al. 2002, 2004), age ranges of North American Grenvilleprovince (dark gray shading; Rivers 1997), and Nimrod Group basement (light gray shading; Goodge and Fanning 1999,2002; Goodge et al. 2001).

1.46, and !1.10 Ga in the northern Rocky Moun-tains and the central TAM provides a set of uniqueages that may indicate geologic correlation be-tween the two areas.

Altogether, the newly reported occurrences ofGrenville-age metaigneous rocks in various partsof East Antarctica and western Laurentia provide ahost of intriguingly similar ages but as of yet pro-vide no spatially unique geologic correlations. Al-though it is premature to determine the trace andorientation of Grenville-age belts in the interior ofthe East Antarctic shield, it is clear that igneousand/or deformation belts of this age are presentwithin the reach of the Byrd and Nimrod glaciercatchments.

We show a modified SWEAT fit in figure 10B forpart of Rodinia based on the paleomagnetically con-strained model of Dalziel (1997), modified afterGoodge et al. (2008). The geometric relationshipsbetween Laurentia, Siberia, and Baltica are main-tained as in the Dalziel model, and we translated

the combined cratons of Australia, India, and Maw-son part of East Antarctica as a unit to the southrelative to Laurentia in order to align three prom-inent geologic tracers: (1) the common Gawlerprovince (Australia) source for Pandurra Formationand Belt Supergroup (Prichard Formation) detritus(Fanning and Link 2003); (2) the occurrence of !1.4-Ga A-type rapakivi granite clasts in glacial tills ofEast Antarctica that geochemically and isotopicallymatch similar age granites in southwestern Lau-rentia (Goodge et al. 2008; this study); and (3) theoccurrence of !1.1-Ga igneous clasts in East Ant-arctica that match Grenville-age basement rocks inwestern Laurentia (this study). Other geologic cor-relations reviewed by Goodge et al. (2008) are alsoindicated in figure 10A. This reconstruction doesnot resolve which of the Grenville-age areas in Lau-rentia—either the western end of the Grenvilleprovince proper in the Franklin Mountains of Texasor Grenville-age remnants in the northern RockyMountains—best correlate with similar-age glacial

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glacial clasts in East Antarctica and Laurentian basement are discussed in the text. Grenville-age metaigneous clasts incentral Transantarctic Mountains moraines may correlate with the southwestern termination of the Grenville Orogen,with inliers of Grenville-age metamorphic rocks in Belt-equivalent strata of northern Idaho or with other basementterrains in East Antarctica. Byrd Glacier catchment area shown by diagonal dotted-line pattern. Age sources for Pro-terozoic provinces cited in text. Inset B shows broader reconstruction of central Rodinia centered on Laurentia. Generaloutline based on paleomagnetic fit of Dalziel (1997). Coats Land position defined by paleomagnetic pole at 1100 Ma(Gose et al. 1997). The position of the Kalahari Craton at !1 Ga is consistent with paleomagnetic data of Gose et al.(2006). Geologic features: B p Beardmore Group; CL p Coats Land; DML p Dronning Maud Land;DV p Death Valley Group; G p Gawler Craton; GSM p Gamburtsev Subglacial Mountains; M p Musgrave Range;Mo p Mojave province; N p Nimrod Group; NN p Namaqua-Natal belt; Pa p Pandurra Formation; Pj p PinjarraOrogen; PM p Pensacola Mountains; Pr p Pritchard Formation; R p Ross Orogen; SR p Shackleton Range; T pTasmania; TA p Terre Adelie; U p Uinta Mountains; W p Wyoming Craton.

clasts in the central TAM. The primary igneousages of the clasts collected in the Nimrod Glaciercatchment match well the ages of igneous activityin western Texas, suggesting that the buried sourcein East Antarctica may correlate with the GrenvilleOrogen proper. The join along the Austral-Antarc-tic and Laurentian rift margins is refined from thatshown by Goodge et al. (2008) to emphasize exten-sion of the Grenville province into East Antarctica.Continuation of the Grenville province into theRoss Sea sector of East Antarctica explains isotopicpatterns and model ages of Ross Orogen granites inthe southern TAM (Borg et al. 1990; Borg andDePaolo 1994), which indicate the presence of lateMesoproterozoic crust, as well as the common oc-currence of large detrital zircon populations be-tween 900 and 1200 Ma in Neoproterozoic andLower Cambrian rift-margin sandstones (Goodge etal. 2002, 2004; Wysoczanski and Allibone 2004) andNeogene glacial deposits (this study). Discovery of!1.1-Ga igneous clasts in glacial tills of the centralTAM provides a direct sampling of rock materialeroded from ice-covered Grenville-age basement.

The source of Grenville-age clasts in the Byrd andNimrod glacier catchments might also be repre-sented by an extension of the !1100–1070-Ma Pin-jarra Orogen in western Australia (Bruguier et al.1999; Collins 2003), a cryptic suture zone that prob-ably played an important role in East Gondwanaamalgamation and which Fitzsimons (2003) sug-gested may continue as a major crustal structureacross central East Antarctica. An orogenic belt ofthis age may underlie the Gamburtsev SubglacialMountains (Veevers et al. 2008), although crustalthickness estimates of Block et al. (2009) indicatethat the Gamburtsev Mountains represent a high-standing massif lacking a significant crustal root,suggesting that their present-day relief is due toyounger reactivation of an older, possibly Gren-ville-age structure. Regardless of the uncertaintiesregarding correlation with western Laurentia, the

relicts of Grenville-age igneous rocks within Pan-African terrains in Dronning Maud Land and theShackleton Range may reflect the presence of a net-work of !1.1-Ga suture belts within central EastAntarctica, a part of which may have provided thesource of the glacial clasts studied here.

Moores (1991) speculated that the Grenville Oro-gen of Laurentia was continuous with rocks of sim-ilar age in Dronning Maud Land of East Antarctica(fig. 10), representing a key line of evidence in sup-port of the SWEAT hypothesis. Paleomagnetic data(Dalziel 1997; Gose et al. 1997) indicate that rocksof !1100-Ma age exposed in a series of small nun-ataks in present-day Coats Land lay south of Lau-rentia and were not continuous with the Grenvillebelt, yet these are distinct from the Namaqua-Na-tal-Maud belt that extends from the southern mar-gin of the Kalahari Craton into Dronning MaudLand of Antarctica (fig. 10). Thus, both the paleo-geographic positions and relationship of thesecrustal elements remain highly uncertain (Gose etal. 1997, 2006; Dalziel 1997; Jacobs et al. 2008;Kleinschmidt and Boger 2009). While it is beyondthe scope of this article to resolve the paleogeo-graphic relation of the Coats Land and Kalahariblocks to Laurentia, our data indicate that Gren-ville-age basement may extend from one of twopossible occurrences in western Laurentia into cen-tral East Antarctica.

The presence of Grenville-age and older glacialclasts in the central TAM may also help to con-strain the age of Rodinia crustal assembly. Dalziel(2010) reasons that because there is no knownGrenvillian suture along the boundary juxtaposingthe reconstructed Laurentia and Austral–East Ant-arctic cratons, the combined craton existed beforethe global Mesoproterozoic–early Neoproterozoicamalgamation of Rodinia. Our data correlating!1.4-Ga granitoids between East Antarctica andwestern Laurentia confirms that these cratons in-deed were juxtaposed and pierced by the crustal-

616 J . W . G O O D G E E T A L .

derived Mesoproterozoic granite province by atleast 1.4 Ga. The finding of Grenville-age metaig-neous rocks in East Antarctica, however, raises thepossibility that the already assembled Mesoprote-rozoic supercraton was further modified by oro-genic displacements, possibly including intracra-tonic collision, at about 1.1 Ga.

Examples of Grenville-age remnants occur else-where along the paleo-Pacific margin of Austral-Antarctica. Fioretti et al. (2005) cited an !1120-Masyenite clast obtained by offshore dredge haul asevidence that the Grenville province extends intoTasmania, but it is not clear whether the polymictcoarse sediment of the dredge sample is represen-tative of submerged basement on the South Tasmancontinental rise, or if the basement has been offsetduring subsequent Mesozoic Pacific-rim transla-tion. Other offshore samples include an !1050-Maorthogneiss (Berry et al. 2008) indicating the intru-sion of Mesoproterozoic granitoids in western Tas-mania before and after episodes of metamorphismat !1290 and !920 Ma. Although this tectonic his-tory is similar to that in the Musgrave belt in Aus-tralia, these workers argue that the Grenville-agemetaigneous rocks represent Mesoproterozoicbasement underlying western Tasmania and mayextend into northern Victoria Land of the TAM,thereby strengthening ties between Australia, EastAntarctica, and western Laurentia.

No other Grenville-age crust is known in EastAntarctica between the Shackleton Range and theDenman Glacier–Windmill Islands area (fig. 1;Sheraton et al. 1992; Moeller et al. 2002; Harleyand Kelly 2007; Will et al. 2009), covering an arcof about 230# around the present-day margin oncelinked to Australia and Laurentia. Our findings donot negate the suggestion that Grenville-age base-ment continues from the Pinjarra Orogen in theDenman Glacier area of Wilkes Land across to cen-tral East Antarctica in the Nimrod Glacier area (fig.

1; Fitzsimons 2003), but a positive correlation isuncertain. Such a continuation, however, mightalso explain the occurrence of Grenville-age (900–1200 Ma) detrital zircons in Cretaceous and youn-ger deposits in Prydz Bay, whose source may be inthe Gamburtsev Subglacial Mountains (Veevers etal. 2008). Continuity of a Mesoproterozoic orogencrossing East Antarctica may clarify our under-standing of Rodinia assembly and interpretation ofsubice geophysical data from the Gamburtsev Sub-glacial Mountains. That the Byrd Glacier catch-ment overlaps the flank of the Gamburtsevs raisesthe possibility that this massif may be one sourceof the Grenville-age debris now found in the centralTAM. Because subglacial transport leads to an ex-ponential decline in concentration of clasts withincreasing distance from their source (e.g., Clark1987), it is unlikely that the Grenville-age clasts inour study were transported directly from the Gam-burtsev Subglacial Mountains during a single gla-cial stage; however, if glacial flow directions haveremained relatively stable over time, the consistentflow might be capable of transporting a populationof clasts over the greater distances required.

A C K N O W L E D G M E N T S

This work was supported by grants from the Na-tional Science Foundation (0440160 to J. W. Goodgeand 0440885 to K. J. Licht). Fieldwork was madepossible by the expert help provided by P. Braddockand A. Barth, along with Twin Otter flight crewsof Kenn Borek Air; we are grateful for their manyspectacular and safe field landings. We thank A.Ashworth and D. Harwood for providing sedimentsamples of the Sirius Group. We are grateful to V.Hansen for review of an earlier manuscript, and weappreciate the constructive suggestions provided byjournal reviewers P. Cawood and I. Dalziel.

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