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Terra Antartica2003, 10(3), 105-128

Metasedimentary Units of the Cambro-Ordovician Ross Orogen inNorthern Victoria Land and Oates Land:

Implications for Their Provenance and Geotectonic Setting fromGeochemical and Nd-Sr Isotope Data

F. HENJES-KUNST1* & U. SCHÜSSLER2

1Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655 Hannover - Germany 2Institut für Mineralogie, Universität Würzburg, Am Hubland, D-97074 Würzburg – Germany

Received 11 November 2002; accepted in revised form30 October 2003

Abstract – Metasediments in the three early Palaeozoic Ross orogenic terranes in northern Victoria Land andOates Land (Antarctica) are geochemically classified as immature litharenites to wackes and moderatelymature shales. Highly mature lithotypes with Chemical Index of Weathering values of >95 are typicallyabsent. Geochemical and Rb-Sr and Sm-Nd isotope results indicate that the turbiditic metasediments of theCambro-Ordovician Robertson Bay Group in the eastern Robertson Bay Terrane represent a veryhomogeneous series lacking significant compositional variations. Major variations are only found in chemicalparameters which reflect differences in degree of chemical weathering of their protoliths and in mechanicalsorting of the detritus. Geochemical data, 87Sr/ 86Sr t=490 Ma ratios of 0.7120 – 0.7174, εNd, t=490 Ma values of-7.6 to –10.3 and single-stage Nd-model ages of 1.7 – 1.9 Ga are indicative of an origin from a chemicallyevolved crustal source of on average late Palaeoproterozoic formation age. There is no evidence forsignificant sedimentary infill from primitive “ophiolitic” sources. Metasediments of the Middle CambrianMolar Formation (Bowers Terrane) are compositionally strongly heterogeneous. Their major and traceelement data and Sm-Nd isotope data (εNd, t=500 Ma values of -14.3 to -1.2 and single-stage Nd-model ages of1.7 – 2.1 Ga) can be explained by mixing of sedimentary input from an evolved crustal source of at leastearly Palaeoproterozoic formation age and from a primitive basaltic source. The chemical heterogeneity ofmetasediments from the Wilson Terrane is largely inherited from compositional variations of their precursorrocks as indicated by the Ni vs TiO2 diagram. Single-stage Nd-model ages of 1.6 -2.2 Ga for samples frommore western inboard areas of the Wilson Terrane (εNd, t=510 Ma -7.0 to -14.3) indicate a relatively highproportion of material derived from a crustal source with on average early Palaeoproterozoic formation age.Metasedimentary series in an eastern, more outboard position (εNd, t=510 Ma -5.4 to -10.0; single-stage Ndmodel ages 1.4 – 1.9) on the contrary document stronger influence of a more primitive source with youngerformation ages. The chemical and isotopic characteristics of metasediments from the Bowers and Wilsonterranes can be explained by variable contributions from two contrasting sources: a cratonic continental crustsimilar to the Antarctic Shield exposed in Georg V Land and Terre Adélie some hundred kilometers west ofthe study area and a primitive basaltic source probably represented by the Cambrian island-arc of the BowersTerrane. While the data for metasediments of the Robertson Bay Terrane are also compatible with an originfrom an Antarctic-Shield-type source, there is no direct evidence from their geochemistry or isotopegeochemistry for an island-arc component in these series.

*Corresponding author([email protected] )

INTRODUCTION

The Ross-Delamerian Orogenic Belt in Antarcticaand SE Australia marks the position of the palaeo-Pacific continental margin of East Gondwana whichwas initiated during Neoproterozoic breakup ofRodinia and subsequent rifting between Gondwanaand its potential former counterpart Laurentia(Moores, 1991; Dalziel, 1991; Powell et al., 1994;Unrug, 1996; Veevers et al., 1997; Foden et al.,2001). Sedimentary successions reflecting the different

geotectonic stages in the evolution of the palaeo-Pacific margin of East Gondwana in Neoproterozoicto early Palaeozoic times are well documented fromthe Delamerian Orogen in SE Australia. Here, theyaccommodate Neoproterozoic shallow watersedimentary sequences of the Adelaide Geosynclinewhich are overlain by Cambrian platform deposits ofthe Normanville Group and Cambrian flysch-typedeposits of the Kanmantoo Group (Preiss, 1987;Haines & Flöttmann, 1998; Flöttmann et al., 1998).This evolution is interpreted to reflect the

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development from a passive-margin type settingrelated to continental break-up to a sedimentaryenvironment close to the evolving DelamerianOrogen. To the east of the Adelaide Syncline in amore outboard position, sedimentary sequences ofCambrian age are found in close association withgreenstones of oceanic to island-arc affinity in thearea of Mt. Stavely (e.g. Moore et al., 1998).Sedimentation in the area of the Adelaide Synclineand the greenstone belt was terminated by/during theCambro-Ordovician Delamerian Orogeny.

The Delamerian Orogenic Belt in SE Australiahas traditionally been correlated with the Ross Orogenin northern Victoria Land and Oates Land at thePacific end of the Transantarctic Mountains (TAM) inEast Antarctica (Stump et al., 1986; Flöttmann et al.,1993). According to this model, the NeoproterozoicAdelaidian and Cambrian sequences of SE Australiaare to be correlated with the basement rocks of theinboard Wilson Terrane in Antarctica while the Mt.Stavely greenstone belt in SE Australia probably findsits counterpart in the volcanosedimentary BowersTerrane of northern Victoria Land. In both regions,these zones are fault-bound to the east by terranesbuilt up by turbiditic sediments of early Palaeozoicage (Robertson Bay Terrane in Antarctica; Stawellzone of the Lachlan Fold Belt in Australia).Sedimentological and palaeontological observations inthe Cambrian sedimentary sequences in the Bowersand Robertson Bay terranes provide convincingevidence in favor of a correlation with earlyPalaeozoic strata in outboard zones of the DelamerianOrogen. The age and origin of basement rocks in theWilson Terrane, however, are poorly constrained. Acorrelation with the Neoproterozoic to Cambrianrocks of the inboard Delamerian Orogen remainsthus speculative and is based on indirect evidence asfor instance Nd isotope signature or age pattern ofdetrital zircons (Turner et al., 1993; Ireland et al.,1995, 1999).

From the early geological investigations innorthern Victoria Land and Oates Land it becameclear that the basement rocks of the Wilson Terraneoriginated to a very large extent from clasticsedimentary precursors (see GANOVEX-Team 1987for review). Limitations to outcrop exposures due tosnow and ice cover as well as Ross-orogenicpolyphase deformation make it impossible toconstrain stratigraphic relationships within andbetween the different metasedimentary units.Comparison of the metasedimentary units in northernVictoria Land and Oates Land is further complicatedby the variable metamorphic recrystallization ofbasement rocks in the Wilson Terrane whichculminated in the formation of high- to very-high-grade migmatites. This precludes the classicalsediment-petrographical approach often used in thestudy of (meta-)sedimentary sequences. The chemicalcomposition of metasedimentary rocks, however, has

great potential for reconstructing their geologicalhistory. Geochemistry of clastic (meta-)sediments hasoften been used to define their protoliths (Pettijohn etal., 1972; Blatt et al., 1980), to constrain thegeotectonic setting of the sedimentary basin (Bhatia,1983, 1985; Roser & Korsch, 1986, 1988; McLennanet al., 1990) and to identify protolith components inthe sedimentary infills (Bjorlykke, 1974; Hiscott,1984; Warfter & Graham, 1989; Floyd et al., 1991;Fralick & Kronberg, 1997). In addition togeochemical studies, Nd isotope investigations ofmetasediments have proven as a potential tool todiscriminate between source components of differentorigin and crustal age (Turner et al., 1993; Farmer &Ball, 1997). Following this geochemical-isotopicapproach in the study of metasediments, the objectiveof the present study is:a) to investigate and compare the protolith

lithologies of metasedimentary units of the threetectonometamorphic terranes in northern VictoriaLand and Oates Land,

b) to evaluate the bearing of these data on theprovenance and plate tectonic setting of theinvestigated units, and

c) to discuss plate tectonic implications for theearly-Ross-orogenic evolution of the palaeo-Pacific margin of East Gondwana.

GEOLOGICAL SETTING AND SAMPLING DETAILS

GENERAL REMARKS

Northern Victoria Land and Oates Land, located atthe Pacific end of the Transantarctic Mountains inEast Antarctica, are traditionally subdivided into threetectonostratigraphic units bounded by NNW-SSEtrending faults (Fig. 1). These are from east to westthe Robertson Bay Terrane (RBT), the BowersTerrane (BT) and the Wilson Terrane (WT). Thewestern “inboard” WT is interpreted as the activepalaeo-Pacific continental margin of the PrecambrianAntarctic Craton. The two “outboard” BT and RBTwere accreted to the WT by plate convergence andcraton-ward directed subduction of oceanic crust(Kleinschmidt & Tessensohn 1987, Tessensohn 1997,Flöttmann et al. 1998) and thus may be interpreted astrue allochthonous terranes. Formation of the activecontinental margin, accretion of the terranes as wellas metamorphism and deformation within the terranesoccurred during the Ross Orogeny in Cambro-Ordovician times (e.g., Stump 1995; Tessensohn1997; Bassett et al. 2002). A detailed description ofthe geology of northern Victoria Land and OatesLand as well as of the various geological units withinthe different terranes can be found elsewhere (e.g.,GANOVEX-Team, 1987; Stump, 1995). In thefollowing, only metasedimentary units investigated in

Metasedimentary Units of the Cambro-Ordovician Ross Orogen in NVL and Oates Land 107

this study will be described. The metasedimentaryrocks are arbitrarily divided into “fine-grained”lithologies (“meta-argill ites”) including slates,siltstones and their higher-grade metamorphicequivalents and “coarse-grained” lithologies(“metapsammites”) including sandstones, greywackesand their metamorphic equivalents. Samples werecollected during the 1988/89 GANOVEX V, the1990/91 GANOVEX VI, the 1992/93 GANOVEX VIIand the joint German-Italian 1999/2000 expeditions tonorthern Victoria Land and Oates Land.

Robertson Bay Terrane

The outboard RBT is mainly built up by theRobertson Bay Group (referred to as rb), amonotonous turbiditic sequence of alternatingmetapsammites and -argillites (Harrington et al., 1964;Field & Findlay, 1983). The protoliths of the serieswere deposited in distal fans to basin plains withNNW-ward directed transport (Wright, 1981, 1985;Wright et al., 1984). A maximum Late Cambrian to

early Ordovician sedimentation age is re-emphasizedby more recent dating of detrital zircon (Flöttmann,pers. communication; Ireland et al., 1998; Fioretti etal., 2003) and by 40Ar-39Ar laser dating of detritalmica (Henjes-Kunst, 2003) from samples of differentareas of the RBT. Since the series were affected bylarge-scale folding and very-low to low-grademetamorphism in the course of the Ross Orogeny(e.g., Buggisch & Kleinschmidt, 1991; Wright &Dallmeyer, 1991), an early Palaeozoic syn-Ross-orogenic sedimentation age of the Robertson BayGroup can be concluded. Lithologically, themetapsammites are greywackes with abundant clastsof quartz and lithics while the meta-argillites (slates,phyllites) represent silty mudstones rather than trueclaystones. Graded bedding is common. Volcanic ormajor calcareous intercalations are unknown. Basedon a preliminary study of the clastic components ofthe metasediments, Wright (1981) argued for aconsiderably uniform provenance of the RobertsonBay Group from a mainly mature continental source

Fig. 1 - Schematic map showing the Ross-orogenic geology of the three fault-bounded terranes of northern Victoria Land and Oates Land(Robertson Bay Terrane, Bowers Terrane, Wilson Terrane). Post-Cambro-Ordovician lithologies are undifferentiated. Location of the studyarea in Antarctica is indicated in the inset (upper right). Sample locations in the various metasedimentary units are depicted by differentsymbols as explained in the lower inset.Abbreviations: ArN, Archangel Nunataks; BeM, Berg Mountains; BoM, Bowers Mountains; DaR, Daniels Range; HeH, Helliwell Hills;KaH, Kavrayskiy Hills; LaM, Lazarev Mountains; LaR, Lanterman Range; MCB, McCain Bluff; MoR, Morozumi Range; OuN, OutbackNunataks, SaR, Salamander Range; WiH, Wilson Hills.


of medium to high-grade rocks. For this study, a totalof 36 samples of psammitic and argill it icmetasediments collected at 14 sites spread over thenorthern part of the RBT (Fig. 1) were considered.One sample (G8-27a; McKenzie Ntk.) was collectedwithin the Millen Schists, which is a strongerdeformed zone of metasediments of unclear terraneassignment at the boundary of the RBT against theBT. The sample from McKenzie Ntk., however, doesnot show evidence for pervasive polyphasedeformation and quartz veining typical of the MillenSchists. It is therefore structurally well comparable tosamples from other parts of the RBT. The samplingscheme for the Robertson Bay Group should enabledetection of any lateral variations in the compositionof the metasediments.

Typical Millen Schists from the westernmost partof RBT were not included in this study since they arecharacterized by abundant quartz veining. As SiO2appears to be the most mobile chemical componentduring metamorphism (Ague, 1994) this veiningstrongly indicates the influence of post-sedimentaryelement mobility.

Bowers TerraneIn the BT, three different metasedimentary units

having been formed in contrasting depositionalenvironments are known (e.g., GANOVEX-Team,1987). These units comprise stratigraphically frombottom to top: (i) the Cambrian slope- to shelf-facies Molar

Formation which is intercalated with primitiveisland-arc volcanics (Sledgers Group),

(ii) the shallow marine Cambrian to early OrdovicianMariner Formation which rests in partconformably (Wodzicki & Robert, 1986) on themetasediments and metavolcanics of the SledgersGroup, and

(iii) the fluviatile to deltaic early Ordovician LeapYear Group which rests unconformably on both,the Sledgers Group and the Mariner Formation. Since the amount of samples for both, the Mariner

Formation and the Leap Year Group is very small,they have not been considered in this study.

The Molar Formation (bmo) is a turbiditicsequence consisting of alternating greywackes andsilty mudstones with minor conglomerates andintercalated volcaniclastic rocks (Laird et al., 1982). AMiddle Cambrian depositional age has been re-emphazised by more recent fossil investigations(Wolfart 1994; Cooper et al. 1996) and is consistentwith the results of SHRIMP dating of zircon (Bassettet al., 2002) and 40Ar-39Ar laser dating of detritalmica (Henjes-Kunst, 2003). The transport directionwas to the southeast with palaeoslopes predominantlyoriented to the southwest (Laird et al., 1982;Wodzicki & Robert, 1986). All previous studies agreein that the clastic sediments of the Molar Formationwere fed mainly by two sources: a magmatic arcprobably represented by the intercalated island-arc

volcanics and a continental landmass. Theinvestigated 25 samples (13 metapsammites and 12meta-argillites) were collected in the area between thesoutheastern Solidarity Range (upper Sledger Glacier)in the central BT and the northern Explorer Range inthe northern BT (Fig. 1). They include specimensfrom both, the western slope and eastern shelf faciesof Wodzicki & Robert (1986).

Wilson TerraneThe basement of the western inboard WT is built

up by a collage of units consisting predominantly ofmetasedimentary rocks which were intruded in Rossorogenic times by large volumes of the GraniteHarbour Intrusives. Lithologically these units may begrouped in monotonous turbiditic series consistingmostly of clastic metasediments on the one hand andmore variegated shallow-water series containingclastic metasediments, metamorphosed limestones andmarls on the other hand (GANOVEX Team, 1987).Intercalations of metavolcanics of basic to acidiccompositions are present in some units of bothgroups. They are, however, found only in subordinateamounts. Regional metamorphic degree varies fromunit to unit from low grade via medium grade to highand very-high grade but is relatively constant within agiven unit. Steep metamorphic gradients in someareas may in part be related to the thermal influenceof nearby Granite Harbour Intrusives. Thestratigraphic relations between the different units arenot constrained for the central and northern part ofthe WT. Major thrust systems in the northwestern partof the WT (Flöttmann & Kleinschmidt, 1991) separateunits with contrasting metamorphic degrees and wereactivated during final tectonometamorphism of theRoss Orogeny (Schüssler & Henjes-Kunst, 1994;Henjes-Kunst, unpubl. results). These structures wereinterpreted to indicate that blocks of metasedimentaryrocks with different metamorphic degree representingdifferent crustal levels of the WT basement weretectonically exhumed to various degrees along thethrust zones during the late stage of the RossOrogeny (Flöttmann & Kleinschmidt, 1991; Schüssler,1996). In Late Cambrian to early Ordovician times,the metasedimentary units of the WT were intrudedby the late-Ross orogenic Granite Harbour Intrusives.In this study, only metasedimentary units occurringnorth of about 73°S in the WT were investigated. Thedescription of the units closely follows thenomenclature of GANOVEX Team (1987).

Low- to medium-grade units in the central tonorthern WT comprise the Morozumi Phyllites (wsm),Rennick Schists (wsr), metasediments of the BergGroup (ws/BM) and from the small outcrop ofMcCain Bluff (ws/MC). These units have in commonthat primary sedimentary features similar to thoseobserved in the Molar Formation of the BT and inthe Roberston Bay Group of the RBT are mostly wellpreserved in outcrop to thin section scale. However,


unlike most of the metasediments from the BT andRBT, all low- to medium-grade units in the WT haveexperienced to various degrees a static contactmetamorphism during the emplacement of the GraniteHarbour Intrusives. The Morozumi Phyllites (wsm)which crop out at Morozumi Range and at smallisolated nunataks north and south of it (Lonely One,Onlooker Ntk.) are an alternating sequence ofmetagreywackes to metasiltstones. Grading bedding iscommon. Characteristic primary sedimentarystructures and the absence of calcareous intercalationsindicate that the protolith of the Morozumi Phylliteshave been formed in a turbiditic environment.Volcanic intercalations are unknown. The age ofdeposition is not constrained by fossils. However,SHRIMP dating of detrital zircons (Flöttmann, pers.comm.) from the southern Morozumi Range indicatethat the Morozumi Phyllites contain clastic input fromsource rocks of Cambrian age thus argueing for amaximum Cambrian sedimentation age of theturbiditic sequence. Since the metasediments wereintruded by a late-Ross-orogenic pluton of the GraniteHarbour Intrusives, a Cambrian to early Ordoviciansedimentation age can be inferred. 12 specimen (9metapsammites, 3 meta-argill ites) from variousoutcrops of this unit were investigated (Fig. 1). Onesamples was taken from a metasedimentary xenolithin the Granite Harbour Intrusive at the northernMorozumi Range. Lithologically, petrographically andstructurally, this metasedimentary xenolith compareswell to the Morozumi Phyllites. Therefore, it isinterpreted to represent a small raft of the countryrocks incorporated into the roof zone of the intrusion.This is supported by the geochemical data.

Rennick Schists (wsr) according to the definitionof Sturm & Carryer (1970) occur in the area fromsouth of Sequence Hill (southwestern WT) to theDaniels Range (northwestern WT). Lithologically,Rennick Schists are fine- to medium-grained schistsand gneisses originating from argillitic to psammiticmetasediments which are often associated with layersand lenses of calc-sil icate rocks. Metavolcanicintercalations have so far not been described.Lamination is well preserved; even relict gradedbedding can be found in some places. Thestratigraphic age of the Rennick Schists is notconstrained by fossil findings. However, a Cambrianto early Ordovician sedimentation age of themetasediments is constrained by Cambrian ages ofdetrital minerals in the rocks (Henjes-Kunst 2003;Flöttmann, pers. comm.) on the one hand and thepresence of abundant late-Ross-orogenic crosscuttingGranite Harbour Intrusives on the other hand.Stronger deformation of the series in some areas asfor instance in the Welcome Mountains and thesouthern Daniels Range can probably be related to avery proximal position to branches of major Ross-orogenic thrust systems (Läufer et al., 2003; see alsoFlöttmann & Kleinschmidt, 1991). A total of 15

samples (14 metapsammites, 1 meta-argill ite)collected in the area of Outback Nunataks, HelliwellHills and southern Daniels Range were investigated(Fig. 1). Some of the specimens from the OutbackNunataks were sampled from large metasedimentaryrafts in roofs zones of Granite Harbour IntrusivesHere, granitic veins of variable width and densitycrosscut metasediments, which can be traced backinto typical Rennick Schist series.

Metasediments of the Berg Group (ws/BM) occurin the coastal area of Oates Land (“Oates Coast”) inthe northwesternmost part of the WT close to itssuspected boudary to the Antarctic Craton. Originallydescribed by Soviet workers (Klimov & Soloviev,1960; Ravich et al. 1965) the Berg Group has beenreinvestigated in detail more recently (Skinner et al.1996; Adams 1996). It is a turbiditic sequence ofpsammitic to pelitic rocks with intercalations ofnodules, lenses and layers of carbonate rocks. Asection of about 50 m on one of the exposed ridgesconsists predominately of carbonate beds. Contrary toa Riphean stratigraphic age which was proposed onthe basis of fossil data (Iltchenko 1972), more recentgeochronological data obtained on detrital minerals inthe metasediments (Henjes-Kunst 2003; Flöttmann,pers. comm.) and on the crosscutting Berg Granite(Adams 1996) constrain a Cambrian sedimentationage of the Berg Group. 13 psammitic and 1 argilliticsamples collected from the area of Berg Mountainsand Archangel Nunataks have been investigated.Included are two specimen of well-laminatedmetapsammites collected from large rafts in theGranite Harbour Intrusion at Outrider Nunatak(Archangel Nunataks). Lithologically andpetrographically, these metapsammites compare wellto the metasediments forming the country rocks of theintrusion and, therefore, are interpreted as rafts of theBerg Group incorporated into the roof zone of theintrusion.

Metasediments comparable in lithology andsedimentary inventory to the low- to medium-gradeseries of the WT described above are also found atMcCain Bluff (ws/MC) in the northeastern USARPMountains (northern WT; Fig. 1). Because of thesmall outcrop area only a very limited sedimentarysection can be observed. Since the metasediments atMcCain Bluff are located directly east of the easternbranch (Wilson Fault) of the late-Ross orogenic thrustsystems in the WT, they may be correlated with theMorozumi Phyllites and likely also with the RennickSchists further to the south. The depositional age isso far not constrained. Five metapsammites and fourmeta-argillites from McCain Bluff were investigated.

Medium- to very-high-grade metasedimentary unitsin the WT are characterized by complete regionalmetamorphic recrystallisation thus obliterating anyprimary sedimentary fabrics and minerals.Nevertheless, sedimentary protolith can well bededuced because of typical rock associations and


relict sedimentary layering and in part gradation. TheLanterman Metamorphics (wsl) which build up theLanterman and Salamander ranges close to thetectonic boundary of the WT against the BT (Fig. 1)were described in detail by Roland et al. (1984) andTalarico et al. (1998). They consist of psammitic topelitic schists and gneisses with some intercalations ofamphibolites suggesting that the rocks originated fromclastic metasediments with some minor volcanicintercalations. The sedimentation age is so far notconstrained. Preliminary investigations on theamphibolites indicate that they comparegeochemically and isotopically well to primitivebasaltic rocks of the BT volcanics (Henjes-Kunst,unpubl. results). In the western Lanterman Range,calc-silicate bands and pods become more abundant.Thirteen samples of overall psammitic compositionfrom the Lanterman Range and from the northern partof the Salamander Range were investigated. Inaddition one gneiss was included which was collectedat a small isolated nunatak in the northern EvansNévé in the southern prolongation of the SalamanderRange. Lithologically and petrographically, thegneisses from this small nunatak compare well to themetamorphic rocks from the Salamander andLanterman ranges.

Metasedimentary rocks of Kavrayskiy Hills(wg/KH) were considered as typical representativesfor the medium to high-grade in part migmatiticgneisses found in large areas in the northern part ofthe WT (GANOVEX Team 1987). According toSchubert et al. (1984) rock series at Kavrayskiy Hillscomprise fine-grained biotite-plagioclase gneissesintermediate in colour alternating with light-colouredgneisses with low biotite contents interpreted asformer pelitic to arenaceous sediments. Coarse-grainedgneisses containing feldspar megacrysts were assumedto derive from greywacke source rocks. Primarysedimentary and relict graded bedding can beobserved. Similar to the Lanterman Metamorphics theseries include some minor intercalations ofamphibolites. No age constraint can be given for thedeposition age of the likely clastic protoliths of thegneisses. For this study, 9 samples some of whichwere already studied by Schubert et al. (1984) wereinvestigated.

The high- to very-high-grade and mostlymigmatitic basement of the Wilson Hills area(wg/WH) consists of extensive series of psammiticmetasediments with local variations to more quartziticor more pelitic compositions (Schüssler 1996;Schüssler 2000; Schüssler et al. 1999). Differences inmetamorphic grade allow to distinguish a very-high-grade central zone from high-grade eastern andwestern zones (Schüssler 1996). Whereasmetasediments in the latter two zones are extremelymonotonous, calc-sil icate and amphiboliticintercalations are relatively abundant in the centralzone resulting in a more variegated series. According

to the results of recent SHRIMP dating (Henjes-Kunstet al. 2003, in prep.) detrital zircons of latestNeoproterozoic age are present in the very-high-graderocks thus argueing for a maximum Cambriansedimentation age of their protoliths. A minimum ageof deposition is constrained by a ~500 Ma age forhigh- to very-high-grade metamorphism of the rocks(Henjes-Kunst et al. 2003, in prep.). A total of 36samples including four argill it ic varieties wereinvestigated from the Wilson Hills area (Fig. 1).

GEOCHEMISTRY

ANALYTICAL METHODS

Whole-rock preparation and geochemical analyseswere performed at the Bundesanstalt fürGeowissenschaften und Rohstoffe(BGR), Hannoverand at the Institut für Mineralogie, Würzburg. Whole-rock powders were prepared using steel jaw crushersand an agate mortar (BGR Hannover) or a tungsten-carbide mortar (Würzburg). About 2 kg of samplematerial free of weathering crusts for coarser grainedspecimens (e.g. metagreywackes and medium to high-grade gneisses) and about 0.5 to 1 kg for finergrained samples (e.g. meta-argillites to metapelites)were used. Concentrations of major and traceelements were determined by X-Ray FluorescenceSpectrometry (XRF) on lithium borate glass fusiondisks (Burhke et al., 1998) using automated PhilipsPW 1480 (Hannover, Würzburg) and PW 2400(Hannover). Sample powders were mixed with lithiumborate in the ratio of 1:5 at the BGR Hannover and1:6 at the Institut für Mineralogie, Würzburg. For thetrace elements, detection limits (and used tube/line)are 20 ppm for Ce (Rh/Lβ1) and La (Rh/Lα), 5 ppmfor Ba (Cr/Lα) and Th (Rh/Lα), and 2-3 ppm for Co(Rh/Kα), Cr (Rh/Kα), Ga (Rh/Kα), Nb (Rh/Kα), Ni(Rh/Kα), Rb (Rh/Kα), Sc (Cr/Kα), Sr (Rh/Kα), V(Rh/Kα), Y (Rh/Kα), Zn (Rh/Kα) and Zr (Rh/Kα) atthe BGR Hannover and 20 ppm for Ba, 15 ppm forZr, 10 ppm for Rb, Sc, Sr, V, Cr, 8 ppm for Y, and5 ppm for Ga, Ni, Th, and Zn (all but Ba and Th:Rh/Kα ; Ba and Th: Rh//Lα) at the Institut fürMineralogie, Würzburg.

RESULTS

General remarks

The geochemical data base used in this studycomprises major and trace element compositions of atotal of 175 samples from northern Victoria Land andOates Land. In table 1 only the average compositionsand the relevant standard deviations of metasedimentsfrom the investigated units are listed. The completegeochemical data set, including sample localities andcoordinates, is available online at http://www.mna.it/english/Publications/TAP/terranta.html. The loss-on-ignition (LOI) values of the samples are variable. All


but two specimens from the WT have low LOI valuesof less than 2 wt.%. The RBT samples show variableLOI between 2 and 8 wt.%. No correlation of LOIwith CaO or MgO is evident for metasediments fromthe RBT or the WT. A correction for modal carbonatecontent has therefore not been applied. The LOI valuefor BT metasediments ranges from 2 – 15 wt.% andis positively correlated with CaO and MgOabundances. Inspection of thin sections, however,reveals that clastic components in addition tointerstitial carbonate may be responsible for thiscorrelation. Therefore, a correction of the chemical

data for biogenic carbonate has not been applied.Only one BT sample has been disregarded because ofits very high LOI content of approximately 15 wt.%.The remaining specimen have values less than 7wt.%. For graphical presentation, the chemical data ofall samples have been recalculated on a LOI-freebasis to 100 %.

Chemical classification of the metasedimentaryrocks

A general relationship between the originalmineralogy and grain size of sandstones to mudstones

Tab. 1 - Major and selected trace element values (mean values and respective 1 standard deviations) of psammitic and argilliticmetasediments from the Robertson Bay, Bowers and Wilson terranes of the Ross Orogenic Belt in East Antarctica (rb: Robertson BayGroup; bmo: Molar Formation; wsl: Lanterman Metamorphics; wsm: Morozumi Phyllites; wsr: Rennick Schists; ws/BM: Berg Group;ws/MC: metasediments at McCain Bluff; wg/KH: Wilson Gneisses at Kavrayskiy Hills; wg/WH: Wilson Gneisses at Wilson Hills; comparetext for details).


and the chemical composition of the rock has not yetbeen found. Therefore, a classification of clasticsediments similar to conventional mineralogicalcriteria, as for instance the relative amount offragments of quartz, feldspar, and lithics cannot beobtained from chemical discrimination diagrams.Instead, the latter diagrams make use of chemicalparameters which mimic the maturity of (meta-)sedimentary rocks in terms of their relative amountsof quartz vs. feldspar and clay (e.g. their SiO2/Al 2O3ratio) on the one hand and of clay minerals vs.feldspar (e.g. Na2O/K2O and Fe2O3/K2O ratios) on theother hand. The chemical data of metasediments fromnorthern Victoria Land and Oates Land are plotted ina classification diagram using log(Fe2O3/K2O) vslog(SiO2/Al2O3) after Herron (1988) (Figs. 2 a-d).

Psammitic metasediments from the RBT (rb) showonly very restricted variations in their Fe2O3/K2O andSiO2/Al 2O3 ratios resulting in a tight cluster of datapoints on both sides of the boundary line between thewacke and litharenite fields (Fig. 2a). Argill iticmetasediments from the RBT range in compositionfrom wackes to typical shales. Noticeable is that both

metapsammites and meta-argill ites exhibit onlyrestricted variations in the Fe2O3/K2O ratio comparedto samples from all other units. Psammitic varieties ofsamples from the BT (bmo; Fig. 2b) and from thedifferent units in the WT (Fig. 2 c,d) show verysimilar variation fields. They all are characterized by,on average, slightly lower maturity in terms of theirSiO2/Al 2O3 ratio. While most of the BT and WTmetapsammites plot in the wacke field there is aconsiderable amount of data points plotting in theneighbouring fields of arkose, litharenite, Fe-sand orshale close to the wacke field. The position of onewsr sample in the upper part of the Fe-sand fieldresults from its very low potassium content. Argilliticvarieties of samples from the BT and the WT aredistinguished from metapsammites of these terranesmainly by lower SiO2/Al 2O3 ratios and plot in thefields of shale to Fe-shale (Fig. 2 b,c,d). Most of themeta-argillites from the WT (e.g., wg/WH, wsm,ws/MC, wg/KH) are however relatively immature andplot close to the boundary of shale to wacke.

The maturity of clastic sediments is mainlycontrolled by the degree of chemical weathering

Fig. 2 - Protolith classification diagrams for metapsammites and —argillites from the 3 terranes of northern Victoria Land and Oates Land

using log(Fe2O

3/K

2O) vs log(SiO

2/Al

2O

3) after Herron (1988); a) Robertson Bay Group (rb) (Robertson Bay Terrane), b) Molar Formation

(bmo) (Bowers Terrane), c) Morozumi Phyllites (wsm), Rennick Schists (wsr), Berg Group (ws/BM) and from McCain Bluff (ws/MC) (all

Wilson Terrane), d) Lanterman Metamorphics (wsl) and from Kavrayskiy Hills (wg/KH) and Wilson Hills (wg/WH) (all Wilson Terrane).

Field boundaries and rock names according to Herron (1988); psam., metapsammitic varieties; argill., meta-argillitic varieties.


either directly of the source rocks or during transportof the detritus. The Chemical Index of Weathering(CIW = 100*Al2O3/[Al 2O3 + CaO + Na2O]; molecularproportions) which was introduced by Harnois (1988)is plotted for the RBT, BT, and WT metasedimentsagainst the K2O/ Na2O ratio in figure 3. In thisdiagram, samples from the different metasedimentaryunits roughly follow a trend from low to high CIWvalues at increasing K2O/ Na2O ratios which mimicsthe increase of clay minerals at the expense offeldspar due to chemical disintegration of theframework minerals. During this process, selectiveleaching and removal of Na and Ca results in arelative increase of Al2O3. Because of the absorptionof K+ on clay minerals, K is regarded here as animmobile element. Almost all psammitic samplesfrom the investigated units of the RBT, BT, and WThave CIW values typical of unweathered to onlymoderately weathered igneous rocks (Fig. 3) (Harnois1988). Very low CIW values of < 50 obtained forsome samples from the BT and WT (Fig. 3 b-d) can

be explained by elevated CaO concentrations due touncorrected biogenic carbonate contents. Meta-argill ites from all 3 terranes are indicative ofmoderately to strongly weathered source rocks.Metapelites derived from highly mature pelagic shaleswith CIW values of >90 and K2O/ Na2O > 10 (e.g.,Mingram 1998) are however not present in all 3terranes. This is in line with field evidence that fine-grained rocks in the low-grade units of the 3 terraneswere derived from silty sandstones rather than shales.Interestingly, a considerable amount of samples ofbmo meta-argillites cannot be distinguished by theirCIW values and K2O/ Na2O ratios from psammiticvarieties of the formation (Fig. 3b).

Provenance and tectonic setting of themetasedimentary rocks

Major and trace element compositions ofmetasedimentary rocks are routinely used to infer thecomposition and type of source rocks involved, thedepositional environment and therewith the plate

Fig. 3 - Relation between CIW and K2O/Na

2O ratio for metapsammites and —argillites from metasedimentary units of northern Victoria

Land and Oates Land. Boundary for unweathered igneous rocks (CIW < 60) and strongly weathered igneous rocks (CIW > 80) according

to Harnois (1988); a) Robertson Bay Group (rb) (Robertson Bay Terrane), b) Molar Formation (bmo) (Bowers Terrane), c) Morozumi

Phyllites (wsm), Rennick Schists (wsr), Berg Group (ws/BM) and from McCain Bluff (ws/MC) (all Wilson Terrane), d) Lanterman

Metamorphics (wsl) and from Kavrayskiy Hills (wg/KH) and Wilson Hills (wg/WH) (all Wilson Terrane). Field boundaries and rock names

according to Herron (1988); psam., metapsammitic varieties; argill., meta-argillitic varieties. CIW = 100*Al2O

3/[Al

2O

3+ CaO + Na

2O];

molecular proportions.


tectonic setting of the sedimentary basins (seeRollinson 1993 for references). Element characteristicsof lithologies in the source rocks may however beobliterated by secondary processes like weathering(see above) and/or mechanical sorting processesduring transport of the detritus. Floyd et al. (1989)used a plot of the concentrations of the relativelyimmobile elements Ti and Ni in the metasediments todistinguish between primary (e.g., source controlled)and secondary (e.g., sedimentary reworking) controlson the composition of metasediments. Using this plotdifferent trends become evident for metasedimentsfrom the RBT, BT, and WT (Fig. 4). Psammitic andargillitic metasediments of the RBT plot on both sidesand close to the boundary line between thecompositional fields of immature sediments controlledby magmatic precursor rocks and of mature sedimentscontrolled by sedimentary processes. The relativelylow degree of scatter again suggests that these rockswere derived from a very homogeneous source or thatsource-controlled variations were effectivelyhomogenized during transport and sedimentation ofthe clastic components. Slightly elevatedconcentrations of Ni and other “basaltophile”

elements (Cr, V; Tab. 1) in meta-argillitic varietiescompared to the metapsammites suggest that theseelements were preferentially ad-/absorbed by clayminerals and thus concentrated in the fine-grainedsediments. The elevated Ni, Cr and V contents canthus not be interpreted to indicate an elevated clasticinput from an “ophiolitic” source. This is furthersupported by, on average, low Cr/V ratios of < 2.Metasediments from the Molar Formation in the BTalso exhibit a horizontal spread across the boundaryline which is very similar to the sandstone trendaccording to Floyd et al. (1989). However,comparison with the chemical composition of theassociated Glasgow Volcanics (Henjes-Kunst unpubl.results) suggests that the trend of the bmometasediments in the Ni vs. TiO2 diagram is mainlycontrolled by the influence of the local volcanogenicsource rather than by sedimentary reworking(Fig. 4b). For instance, two meta-argillites sampled inthe northern Bowers Mountains have anomalouslylow Ni contents of < 10 ppm very similar to Niconcentrations of chemically strongly evolvedvolcanics also present in the northern Bowers Terrane.Most of the BT metavolcanics, however, have

Fig. 4 - Plot of Ni vs TiO2 for metapsammites and -argillites from northern Victoria Land and Oates Land according to Floyd et al. (1989);a) Robertson Bay Group (rb) (Robertson Bay Terrane), b) Molar Formation (bmo) (Bowers Terrane; variation field of associatedmetavolcanics is shown in grey), c) Morozumi Phyllites (wsm), Rennick Schists (wsr), Berg Group (ws/BM) and from McCain Bluff(ws/MC) (all Wilson Terrane), d) Lanterman Metamorphics (wsl) and from Kavrayskiy Hills (wg/KH) and Wilson Hills (wg/WH) (all WilsonTerrane).


relatively primitive basaltic compositions with high Nicontents. Clastic input from these igneous rocks offersa likely explanation for the relatively high Ni valuesof bmo metapsammites and -argillites. Metasedimentsfrom all investigated units in the WT can collectivelybe classified as immature “magmatogenic” sedimentsaccording to the Ni-TiO2 diagram (Fig. 4 c,d). Theirdata points clearly spread along the trend ofintermediate to acidic magmatic rocks (Floyd et al.1989) indicating that their compositional variationsare largely inherited from chemically evolved igneousprecursor rocks.

Roser & Korsch (1988) have proposed adiscriminant function diagram which is based on theconcentrations of all major elements exept Si in orderto distinguish between four different types ofprovenance of sedimentary rocks. This discriminantfunction diagram is shown in figure 5 for themetasediments from the RBT, BT, and WT. The datapoints of therb metasediments again exhibit a veryrestricted scatter and collectively plot in the field of“quartzose sedimentary provenance” (Fig. 5 a). A verysimilar position in the discriminant function diagram

is found for most of thebmo metasediments(Fig. 5b). There are, however, some significantoutliers. While a group of argillitic samples display atrend into the field of “mafic igneous provenance”, 2psammitic and 2 argillitic specimens plot in the fieldof “felsic igneous provenance”. A relatively localprovenance from felsic igneous rocks has alreadybeen indicated for the 2 argillitic samples on the basisof their very low Ni concentrations (Fig. 4b).Metapsammites and meta-argillites from the WTcannot be distinguished on the basis of the twodiscrimant functions. Data points of samples from theinvestigated units display a relatively large scatter andmostly plot on both sides of the boundary lineseparating the fields of felsic-igneous and quartzose-sedimentary provenances (Fig. 5 c,d). One samplefrom the Rennick Schists which plots in the mafic-igneous-provenance field may contain a significantinput from more primitive magmatic source.

Distinct provenance characteristics of (meta-)sediments can be used to infer the geotectonicenvironment in which the sedimentary basins wasformed. In the K2O/Na2O vs SiO2 diagram after Roser

Fig. 5 - Discriminant function diagram for the provenance signature of metapsammites and -argillites from northern Victoria Land andOates Land according to Roser and Korsch (1988); a) Robertson Bay Group (rb) (Robertson Bay Terrane), b) Molar Formation (bmo)(Bowers Terrane), c) Morozumi Phyllites (wsm), Rennick Schists (wsr), Berg Group (ws/BM) and from McCain Bluff (ws/MC) (all WilsonTerrane), d) Lanterman Metamorphics (wsl) and from Kavrayskiy Hills (wg/KH) and Wilson Hills (wg/WH) (all Wilson Terrane). DF1 = -1.773TiO2 + 0.607Al2O3 + 0.76Fe2O3(total) – 1.5 MgO + 0.616CaO + 0.50Na2O – 1.224K2O – 9.09DF2 = 0.445TiO2 + 0.07Al2O3 – 0.25Fe2O3(total) – 1.142 MgO + 0.438CaO + 1.475Na2O – 1.426K2O – 6.861


& Korsch (1986) three settings are distinguished:passive-continental margin, active-continental marginand oceanic-island arc. Psammitic and argill it icmetasediments from the RBT plot along the boundaryof the passive-continental-margin field to the active-continental-margin field in this diagram (Fig. 6 a).Within the BT Molar Formation, psammitic andargill it ic varieties again display differentcharcateristics. While all psammitic specimens plotinto the active-continental-margin field, 4 argilliticsamples are offset to lower SiO2 concentrations thusplotting in the oceanic-island-arc field and 3 meta-argillites plot in or close to the passive-continental-margin field. Roser & Korsch (1986) showed that theK2O/Na2O vs. SiO2 diagram yields consistent resultsfor sand-mud couplets derived from a common source(compare results for the RBT metasediments).Therefore, this plot can be regarded to behave robustagainst chemical influences caused by mechanicalsorting of detrital material and thus different grainsizes of the sediments. Consequently, it can beinferred that the different positions of the bmometasediments in this diagram are geologicallysignificant and indicate that their sedimentary basinwas fed from active-continental-margin and oceanic-

island-arc sources with minor contributions from apassive continental margin. Metasediments of the WTscatter mostly along the boundary of the fields ofpassive to active-continental margins. No significantdifferences between various units of the WT areapparent. As compared to metapsammites of the RBT,metapsammitic varieties of the WT may indicate aslightly higher sedimentary input from an active-continental-margin source.

Rb-Sr AND Sm-Nd ISOTOPE COMPOSITIONS

ANALYTICAL METHODS

Rb-Sr and Sm-Nd isotope analyses wereperformed at the BGR using the same whole-rockpowders as for the geochemical analyses. About 0.1to 0.2 g of sample material were decomposed withHF-HNO3 in Teflon vessels after addition of theisotopic spikes. For Sm-Nd isotope analysis, sampleswere disolved at temperatures close to 190 °C in thePicotrace™ pressure-digestion system DAS™. Rb, Sr,and the lanthanides were separated on cation-exchange columns. Sm and Nd were isolated from

Fig. 6 - K2O/Na2O vs SiO2 discrimination diagram for the provenance signature of metapsammites and -argillites from northern VictoriaLand and Oates Land according to Roser and Korsch (1986); a) metasediments of the Robertson Bay Group (rb) (Robertson Bay Terrane),b) metasediments of the Molar Formation (bmo) (Bowers Terrane), c) metasediments of the Morozumi Phyllites (wsm), Rennick Schists(wsr), Berg Group (ws/BM) and from McCain Bluff (ws/MC) (all Wilson Terrane), d) paragneisses of the Lanterman Metamorphics (wsl)and from Kavrayskiy Hills (wg/KH) and from Wilson Hills (wg/WH) (all Wilson Terrane).


each other and adjacent lanthanides in separatecolumns using HDEHP-coated Teflon powder (Cerrai& Testa 1963). Sr, Sm, and Nd were loaded on Refilaments and run on a double-filament assemblyusing a Finnigan MAT 261 multicollector massspectrometer in static mode. Rb isotopes weremeasured on Re filaments using a VG MicromassMM30 mass spectrometer. The isotopic ratios werenormalized to 146Nd/144Nd = 0.7219 for Nd and to86Sr/88Sr = 0.1194 for Sr. 143Nd/144Nd isotope ratioswere further normalized to a LaJolla Nd standardvalue of 0.511860. Rb, Sr, Nd, and Sm concentrationswere determined by isotope-dilution techniques.Procedural blanks for Rb, Sr, Nd, and Sm are lessthan 0.1 % of the relevant sample concentration andare therefore negligible. Uncertainties at the 95 %confidence level in 87Sr/86Sr and 87Rb/86Sr are 0.01 %and 1 %, respectively and in 143Nd/144Nd and147Sm/144Nd, they are 0.005 % and 1.4 %,respectively. Calculation of the Nd model parameterεNd is based on 143Nd/144Nd = 0.512638 and147Sm/144Nd = 0.1967 for a CHUR reference(“chondritic uniform reservoir”, Jacobsen &Wasserburg 1980). Single-stage depleted-mantle modelages (TDM) were calculated assuming 143Nd/144Nd =0.513151 and 147Sm/144Nd = 0.219 for the present-daydepleted-mantle reservoir. In all calculations, theIUGS-recommended constants (Steiger & Jäger 1977)were used. Initial 87Sr/ 86Sr isotope ratios and εNdvalues of samples are calculated according to theavailable geochronological data base and geologicalconstraints (see above) at ages of 510 Ma, 500 Ma,and 490 Ma for metasediments of the Wilson, Bowersand Robertson Bay terranes, respectively. Whiledifferences in the assumed ages of 10-20 Ma have nosignificant effect on the calculated initial εNd valuesof all samples and of the initial 87Sr/ 86Sr isotoperatios of metapsammites with low Rb/Sr ratios, theywill result in significantly different initial 87Sr/ 86Srisotope ratios for meta-argillites with elevated Rb/Srratios. However, these age-dependant variations doonly extend the range of calculated ratios of two units(wsm; wsr) to somewhat lower initial 87Sr/ 86Srisotope compositions. Based on the errors quotedabove, initial Sr isotope ratios have absoluteanalytical uncertainties varying from 0.0001 forsamples with low 87Rb/86Sr ratios of 1 to 0.0008 forsamples with 87Rb/86Sr ratios of 10. Initial εNd valueshave absolute analytical uncertainties ofapproximately 0.6. The Rb-Sr and Sm-Nd analyticaldata are reported in table 2.

RESULTS

Sr isotope composition A consequent geological interpretation of the Sr

isotope composition of (meta-)sediments towardslikely source rocks, from which their clastic inputwas derived, is hampered by the mobility of the

elements Rb and Sr during for instance chemicalalteration or by biogenic carbonate contributingvariable amounts of oceanic Sr to the Sr budget ofthe sediment. The amount of carbonate can beinferred semiquantitatively from thin section or bygeochemical considerations (see above) at least of(meta-)sediments, which did not experience significantmetamorphic recrystallisation. However, the influenceof alteration processes having affected the protolith,the detrital material during sedimentary transportand/or the (meta-)sediment itself cannot be controlled.Therefore, metasediments showing evidence ofalteration or containing clastic components withstrong evidence of alteration (e.g. many meta-sediments of the BT Molar Formation) were notinvestigated for their Rb-Sr isotope composition.

Initial 87Sr/ 86Sr isotope ratios of samples of all 3terranes are listed in table 2 and graphically depictedin histogram plots in figure 7. Included are the Srisotope data of 3 high-grade migmatitic gneisses oflikely metasedimentary origin from the westernWilson Hills (US 380, 495, 501; Schüssler et al.1999). The data base for Wilson schists from McCainBluff and for the BT Molar Formation is only veryrestricted. Disregarding a geologically unrealistic lowratio of < 0.7 calculated for sample G7-StP3 from theBowers Terrane (Tab. 2), the samples from all 3terranes cover a range from approximately 0.708 to0.726 which is compatible with an origin of the rocksfrom isotopically moderately evolved continentalsource rocks (cf. Adams 1996, 1997). It becomesobvious that the Sr initial ratios of metasedimentsfrom the Robertson Bay Group (n= 11; Fig. 7a) showless scatter than those of the Morozumi Phyllites (n=10; Fig. 7c), the Rennick Schists (n= 10; Fig. 7d) orthe high-grade gneisses from the Wilson Hills (n= 7;Fig. 7h). This is in line with geochemical evidencesindicating relatively homogeneous composition for therb metasediments as compared to metapsammites andmeta-argillites from all other units. High initial Srisotope ratios of >0.72 as obtained for part of thesamples from the Wilson Terrane (Figs. 7 d, h) andBowers Terrane (Figs. 7b) may indicate a locallypredominant influence of an isotopically moreevolved continental protolith while low ratios of< 0.71 (e.g. Fig. 7 d, g, i) are probably related toclastic input from more juvenile sources. The initialSr isotope ratios for argillitic varieties from two WTmetasedimentary units (wsm: Fig. 7c;wsr: Fig. 7f) areon average lower than the ratios for metapsammiticsamples of the respective units. This suggests a higherproportion of material derived from more juvenilesources in the finer-grained metasediments which issupported by on average somewhat less evolved Ndisotope compositions in these samples (see below).

Nd isotope compositionThe Sm-Nd isotope system of rocks or minerals is

generally assumed not to be affected in the course of


Tab

. 2

- R

b-S

r a

nd

Sm

-Nd

ele

me

nt

con

cen

tra

tion

s a

nd

iso

top

e r

atio

s o

f m

eta

sed

ime

nts

fro

m t

he

th

ree

te

cto

no

stra

tigra

ph

ic t

err

an

es

of

no

rth

ern

Vic

tori

a L

an

d a

nd

Oa

tes

La

nd

(A

nta

rctic

a).

Ele

me

nt

con

cen

tra

tio

ns

are

de

term

ine

d b

y is

oto

pe

-dilu

tio

n t

ech

niq

ue

s.

87 S

r/ 8

6 Sr t

an

d ε N

d.

ta

re c

alc

ula

ted

fo

r a

ge

s o

f 5

10

Ma

(W

Tm

eta

sed

ime

nts

), 5

00

Ma

(B

Tm

eta

sed

ime

nts

) a

nd

49

0 M

a (

RB

T

met

ased

imen

ts).

Com

pare

tex

t fo

r de

tails

.


Tab

. 2

- C

ontin

ued.


hydrothermal alteration or weathering processes.Furthermore, addition of biogenic carbonates will notsignificantly contribute to the Sm-Nd budget of clasticsediments. The Sm-Nd isotope signature of (meta-)sediments thus directly mirrors the relative amountsof different types of source rocks involved andtherefore is an important tool to unravel theirprovenance.

Initial εNd values of samples of all 3 terranes arereported in table 2 and plotted in histograms infigure 8. Included in this figure are the Nd isotopedata of 3 high-grade paragneisses (US 380, 495, 501)from the Wilson Hills area (Schüssler et al. 1999).Samples from the 3 terranes span a range of εNd, t

values from – 1 to -14. Very similar to the Rb-Srisotope evidence,rb samples from the RBT showrelatively small variations between εNd, t of -7.6 and -10.3 (n= 12; Fig. 8a). Their single-stage Nd-modelages vary between 1.66 Ga (G7-Dyo1) and 1.91 (G7-MMu1) (Tab. 2) and thus are indicative of a wellmixed sedimentary input from an continental crustwith on average late Palaeoproterozoic formation age.Metapsammitic and meta-argilliticrb varieties areindistinguishable with respect to their Nd isotope

compositions. Metasediments of the BT cover thecomplete range of εNd, t values denoted above (– 14 to-1; Fig. 8b) and can be interpreted as mixturesbeween an early Palaeoproterozoic or oldercontinental source and a primitive source with mantle-like εNd, t values. The Sledgers Group metavolcanicswhich are associated with thebmo metasediments inthe northern Bowers Terrane and which have εNd, tvalues in the range of +2 - +7 (Henjes-Kunst, unpubl.results) represent the likely protolith for the latterprimitive component in thebmo metasediments. Thecoarser-grainedbmo varieties have on average morenegative εNd, t values and therefore likely contain ahigher proportion of the evolved continental source ascompared to the fine-grained samples of the unit.Samples fromws/MC and wg/WH metasedimentaryunits in the WT display εNd, t values in the range ofapproximately -7 to – 11 that is similar to those ofthe RBT samples (Fig. 8 e, h). Their single-stage Nd-model ages range from approximately 1.7 Ga to1.9 Ga. Thewsm, wsr and ws/MC series additionallycontain samples with low εNd, t values of -12 to -14(Fig. 8 c, d, f) and single-stage Nd-model ages of 2.0– 2.2 Ga documenting a higher proportion of clastic

Fig. 7 - Histogram plot of initial 87Sr/ 86Sr isotope ratios of metapsammites and meta-argillites from northern Victoria Land and OatesLand calculated for ages of 510 Ma, 500 Ma, and 490 Ma for the Wilson Terrane, Bowers Terrane and Robertson Bay Terrane,respectively. Bars for metapsammites are shown in light grey, those for meta-argillites in dark grey; a) Robertson Bay Group (rb)(Robertson Bay Terrane), b) Molar Formation (bmo) (Bowers Terrane), c) Morozumi Phyllites (wsm), d) Rennick Schists (wsr), e) McCainBluff (ws/MC), f) Berg Group (ws/BM), g) Kavrayskiy Hills (wg/KH), h) Wilson Hills (wg/WH), i) Lanterman Metamorphics (wsl) (allWilson Terrane).


infill from source rocks of early Palaeproterozoic orolder formation age. On the opposite, εNd, t values ofapproximately -6 (Fig. 8 g, i) and single-stage Nd-model ages of 1.7 – 1.4 Ga in thewg/KH and wslseries provide evidence for somewhat higher amountsof input from a more primitive source compared tothe other WT series.

DISCUSSION AND INTERPRETATION

IMPLICATIONS FOR GEOTECTONIC SETTINGAND PROVENANCE

From the geochemical evidence it becomesobvious that the turbiditic metasediments of theRoberson Bay Group which cover most of the largeoutcrop area of the Robertson Bay Terrane innorthern Victoria Land with a thickness of at leastseveral thousand meters (e.g. GANOVEX Team 1987)represent a very homogeneous series of alternatingimmature litharenites and wackes to moderatelymature shales. There are no significant variations inthe composition of coarse-grained varieties between

samples from the easternmost RBT compared to thearea close to the boundary against the Bowers Terrane(Millen Schist Zone) approximately 200 km furtherwest. Major variations are only found in chemicalparameters which reflect differences in degree ofchemical weathering of the source rocks or theircomponents and in mechanical sorting of the detritusand which are well documented in the gradationalevolution from coarse- to fine-grained sedimentstypical of turbiditic series. Metasedimentary series ofin part turbiditic character in the western WilsonTerrane show very similar gradations betweenimmature litharenites and wackes and moderatelyimmature shales but are characterized by strongercompositional variations. Metasediments of the BTMolar Formation display a variability which for somechemical elements exceeds that of the WT series.However, they compare well to metasediments of theWT and RBT in that the protoliths of thebmosamples were immature wackes to moderatelyimmature shales. In all three terranes, very maturesedimentary rock types are typically absent. Thisdemonstrates that protracted and deep weatheringeither of the source rocks or during transport of the

Fig. 8 - Histogram plot of initial εNd values of metapsammites and -argillites from northern Victoria Land and Oates Land calculated forages of 510 Ma, 500 Ma, and 490 Ma for the Wilson Terrane, Bowers Terrane and Robertson Bay Terrane, respectively. Bars formetapsammites are shown in light grey, those for meta-argillites in dark grey; a) Robertson Bay Group (rb) (Robertson Bay Terrane), b)Molar Formation (bmo) (Bowers Terrane), c) Morozumi Phyllites (wsm), d) Rennick Schists (wsr), e) McCain Bluff (ws/MC), f) BergGroup (ws/BM), g) Kavrayskiy Hills (wg/KH), h) Wilson Hills (wg/WH), i) Lanterman Metamorphics (wsl) (all Wilson Terrane).


detritus has not taken place. It is therefore not likelythat the protoliths of the respective metasedimentaryseries formed during periods of tectonic stagnationwith sea-level lowstands characteric of passive marginsettings. Instead, it is plausible that the sedimentarybasins evolved in front of tectonically activehinterland experiencing rapid exhumation. Theavailable age data which are summarized abovedemonstrate that the protoliths of therb and bmometasediments and of most series in the WilsonTerrane incorporate strata of Cambrian sedimentationage. Only for the Lanterman Metamorphics (wsl), theWilson gneisses from Kavrayskiy Hills (wg/KH) andthe small outcrop of low-grade metasediments atMcCain Bluff (ws/MC) no age constraints areavailable so far. It can therefore be inferred that theprotoliths of most of the metasedimentary series inthe three terranes of northern Victoria Land and OatesLand were formed in early Palaeozoic time byerosion of mountain belts that were exhumed in thelate Neoproterozoic to early Cambrian.

The chemical heterogeneity of metasedimentaryunits in the WT demonstrates that compositionalvariations in the sedimentary infil l were noteliminated by sedimentary mixing processes. It cantherefore be inferred that the protoliths of these serieswere deposited in relatively small basins. Thecovariation of the Ni and TiO2 abundances of the WTmetasediments suggests that their chemicalheterogeneity reflects compositional variations in theeroded hinterland (“magmatic trend” in Fig. 4 c,d)and not mechanical sorting for instance duringtransport. Local source-rock dependent variations arealso obvious from the Rb-Sr and Sm-Nd isotoperecord of the WT metasediments. Single-stage Nd-model ages of 2.1-2.2 Ga for samples from moreinboard westernmost areas of the Wilson Terrane(wsr, ws/BM) indicate higher proportion of materialderived from source rocks with significantly oldermantle-separation ages as compared to the majority ofthe WT samples which have model ages close to1.8 Ga. Metasedimentary series in a more outboardeasternmost position (wg/KH; wsl) on the contrarydocument somewhat stronger local influence of moreprimitive source rocks with younger model ages.These differences in the nature of the source rocks ofWT metasedimentary series are not obvious from theprovenance and tectonic discrimination diagrams(Figs. 5 c,d & 6 c,d, respectively). Only for theWilson gneisses from Kavrayskiy Hills, the positionof the data points in the felsic- and intermediate-igneous provenance fields may be correlated with theisotope evidence in order to argue for a greatercontribution from magmatic rocks with less evolvedisotope compositions. Although the geochemical andisotopic characteristics of all WT metasediments canbe explained by a binary mixing model using acratonic source of at least early Palaeoproterozoic

formation age on the one hand and a relatively youngand primitive source on the other hand, it is alsopossible that more than two different sources wereinvolved. Another discrete source could have beenformed by a continental crust similar in averagecomposition to the old cratonic source envisagedabove but with on average late Palaeoproterozoicmantle-separation age. In summary, the differentmetasedimentary series of the northern Wilson Terraneshow almost identical provenances indicating acommon origin of their protoliths from predominantlyactive-continental-margin rocks with on average latePalaeoproterozoic mantle-separation ages. While thereis evidence for a stronger influence of an oldercraton-type continental source in the western WilsonTerrane close to its suspected boundary to the EastAntarctic Shield, series from the eastern “outboard”terrane record a higher proportion of material derivedfrom a more juvenile magmatic-arc-type source.

The strong chemical heterogeneity ofmetasediments from the Molar Formation (BT) canreadily be explained by variable amounts ofsedimentary input from two constrasting sources intheir protolith. One potential end member similar tothe old cratonic source already identified inmetasedimentary series from the western WT is wellrepresented by samples showing low εNd, t values of-13 to -14 (Fig. 8b) and compositional affinities to anactive-margin-type continental crust. The second endmember is most likely the volcanogenic SledgersGroup of the Bowers Terrane representing a Ross-orogenic juvenile island-arc of Cambrian age (Weaveret al. 1984; Wodzicki & Robert 1986). Sedimentaryinput from primitive to locally evolved igneous rocksof this island-arc source is identified by thegeochemical oceanic-island-arc affinity of some meta-argillites (Fig. 6b), by elevated concentrations ofbasaltophile elements (Ni, Cr, V; Fig. 4b) in mostsamples and by a relatively primitive Nd isotopecomposition in one sample (Fig. 8b). Thecompositional variations of the Molar Formation (BT)can thus be interpreted to indicate sedimentarymixture of erosional material derived from an oldcratonic-type continental crust on the one hand and anjuvenile island-arc on the other hand. Similar to theWT metasedimentary series, however, it is alsopossible that additional sources as for instancecontinental crust of late Palaeoproterozoic formationage have contributed to the protolith of the bmometasediments.

The remarkable compositional homogeneity ofmetapsammites and meta-argillites of the RobertsonBay Group can be interpreted to indicate that thesedimentary basin was constantly fed by onehomogenous source. In this model, the source can bedescribed as a continental crust built up by highamounts of chemically evolved rocks having onaverage late Proterozoic mantle-separation ages. This


is very similar to what can be deduced for themajority of samples from the WT metasedimentaryseries. It is also possible, however, that repeatedtransport and deposition in large turbiditic currentsmay have led to almost perfect mixing of sedimentaryinfill derived from two or more sources with differentchemical and isotopic characteristics as envisaged forthe BT and WT metasedimentary series. Geochemicaland Rb-Sr and Sm-Nd isotope investigations are notcapable of discriminating between these two models.

PLATE TECTONIC IMPLICATIONS

The constraints on the provenance of themetasedimentary series of the three terranes innorthern Victoria Land and Oates Land can be usedto test plate tectonic models which were put forwardin the past for the geological evolution of the Rossorogenic belt. Most models agree in that the outboardBowers Terrane, which formed as a primitive island-arc, and the Robertson Bay Terrane were accreted byplate convergence and subduction processes to theactive continental margin of the East Antarctic Shield(Wilson Terrane) in late Cambrian to early Ordoviciantimes (Weaver et al., 1984; Gibson & Wright, 1985;Kleinschmidt & Tessensohn, 1987; Flöttmann et al.,1993; Matzer, 1995; Goodge & Dallmeyer, 1996;Flöttmann et al., 1998). In this scenario, themetasedimentary series in the Wilson Terrane can beexplained by early Ross-orogenic formation ofdifferent basins which were situated at the margin ofthe East Antarctic Shield and which were fed bysedimentary material from the western Antarcticcraton and the eastern BT island arc (cf. Kleinschmidt& Tessensohn, 1987). Cratonic continental crustwhich fits the geochemical and isotopic specificationsof a potential sedimentary source is known fromGeorg V Land and Terre Adélie some hundredkilometers further west (Peucat et al., 1999) and fromother areas west of the Ross-Delamerian orogenicBelt of East Gondwana (e.g. Fitzsimons, 2000).Lithological differences in the metasedimentary seriesof the WT may then be related to different positionsof the depositional environments: more variegatedseries were formed as shallow-water continental-shelfsediments while monotonous turbiditic series weredeposited at the continental slope.

For the Molar Formation of the Bowers Terrane,palaeocurrent investigations (Laird et al., 1982;Wodzicki & Robert, 1986) suggest that the clasticmaterial was derived from a landmass to the east tonortheast of the terrane (present-day position).According to the data provided in this study thelandmass should have been built up by old “cratonic”continental crust similar to what is envisaged for theWilson Terrane. Additionally, conglomerate horizonsin the Molar Formation containing fist-size cobbles ofgranitic rocks strongly argue for a proximal positionof the landmass to the Bowers Terrane. Therefore, a

position east of the very wide Robertson Bay Groupdepositional environment can be excluded. In none ofthe geotectonic models cited above, however, alandmass consisting of cratonic continental crust isfound located between the Bowers Terrane to thewest and the Robertson Bay Terrane to the east. Ifthere has originally been a continental landmassbetween these two terranes plate tectonic processeshave erased any direct evidence of it. Similarities ingeochemical and isotopic characteristics of thesources are compatible with a common origin of“cratonic” materials in metasediments of the Bowersand Wilson terranes from the continental crust of theEast Antarctic Shield. Results of recentgeochronological investigations on detrital componentsin the Molar Formation (Bassett et al., 2002; Henjes-Kunst, 2003) indicate that deposition of sediments inthe Bowers Terrane has occurred almostcontemporaneously with the onset of the Ross-orogenic collision event by which the Bowers Terranewas accreted onto the Wilson Terrane. Collision ofplates will result in thickening of crust and tectonicexhumation which enhances erosion. It is thereforelikely that the “cratonic” component in the BT MolarFormation was derived from the continental margin ofthe East Antarctic Shield which was re-activated bythe Ross Orogeny.

The depositional environment of the RobertsonBay Group has been modelled either in a veryproximal position to the east of the evolving Ross-orogenic belt (Kleinschmidt & Tessensohn, 1987) orin a very distal position close to a suspect continentallandmass in the east and separated from thePalaeopacific margin of East Antarctica by oceaniccrust (Gibson & Wright, 1985; Matzer, 1995). In thefirst model which is in accordance with the results ofsedimentological investigations by Wright (1981,1985) and Wright et al. (1984), sedimentary materialis derived from the evolving Ross Orogen includingthe already accreted Bowers Terrane island arc(“dissected arc”). In the second model, the RobertsonBay basin is mainly fed from the suspect landmass tothe east. Similarities in the geochemical and isotopicsource characteristics between metasediments of theRobertson Bay Group and from the WT are inaccordance with the first model but cannot prove it.Problems arise from the Bowers Terrane squeezedbetween the Robertson Bay and Wilson terranes.According to the available geochronological data, theBT island arc was stil l active during turbiditicsedimentation of the Robertson Bay Group. Therbmetasedimentary sequence, however, lacks indicationof an intervening juvenile island-arc activity.Furthermore, the very remarkable compositional andisotopic homogeniety of therb metasediments acrossthe RBT including the area close to the boundaryagainst the BT contrasts with the heterogeneity of theBT metasediments. The obvious hiatus in lithologicalinventory and geochemical and isotopic characteristics


of the metasedimentary sequences between theBowers and Robertson Bay terranes may beinterpreted in favour of the terrane model of Gibson& Wright (1985) and Matzer (1995). Alternatively, itis possible that a western part of therb sedimentarybasin which evolved proximal to the BT island arcwas subducted beneath the two western terranesduring the Ross Orogeny. Origin of the sedimentarymaterial of the Robertson Bay Group from a suspectlandmass in the east cannot be evaluated since thereis only very little evidence for basement rocks in anoutboard position of the Ross-Delamerian Belt of SEAustralia or East Antarctica. The S-type metagraniteof Surgeon Island which is located at the northeasterncoast of northern Victoria Land was discussed byBorg & DePaolo (1991) and Fioretti et al. (2002) asevidence for a continent landmass east of the RossOrogenic Belt. The metagranite meets some of thegeochemical and isotopic requirements. Furthermore,recent age determinations (Fioretti et al., 2001)demonstrate a Cambrian formation age for the granitewhich would also be in line with the required active-continental-margin setting of the Robertson BayGroup. Since Surgeon Island is almost completelybuilt up by a homogenous granite and because thesmall island is in a very isolated position, a moredetailed discussion is not meaningful.

REGIONAL COMPARISON

Turner et al. (1993) performed a detailedgeochemical and Rb-Sr and Sm-Nd isotope study onNeoproterozoic to Ordovician sedimentary sequencesof the Delamerian orogenic Belt and on Archaean toMid-Proterozoic basement as their potential sourcerocks in SE Australia. On the basis of the Nd isotopecompositions of two samples from the Wilson Terranewhich were investigated for comparison, they arguedthat the WT metasediments correlate with passive-margin sedimentary strata of the NeoproterozoicAdelaidean sequence but not with the CambrianNormanville and Kanmantoo sequences of SEAustralia. This is not supported by the results of thepresent study. The Nd isotope composition of the WTmetasediments is quite variable. Their range of εNd, tvalues (-14 to -5; n = 36) overlaps with the valuesobtained for sediments of Neoproterozoic (-10.6 to -4.4; n = 15) and of Cambrian ages (-13.4 to -8.5; n =11) of the Delamerian orogenic Belt (Turner et al.,1993). Lithological and age constraints clearly supporta correlation of the WT metasedimentary units in EastAntarctica with the Cambrian Normanville andKanmantoo groups of SE Australia. Age spectra ofdetrital zircons additionally argue in favour ofcommon provenances for metasediments fromnorthern Victoria Land and Oates Land and Cambriansediments from SE Australia. Zircons from WTsamples and from the turbiditic Kanmantoo Group ofSE Australia show a very prominent late-Pan-African

age component for which there is only minorevidence in the Neoproterozoic Adelaidean and theCambrian Normanville sediments (Ireland et al., 1995,1998, 1999; Berry et al., 2001; Henjes-Kunst et al.,2003, in prep.). The greater variability in Nd isotopecomposition of metasediments from East Antarctica ascompared to Cambrian strata in SE Australia thenindicates more variable provenances and a greaterinfluence from primitive sources in Antarctica.

Acknowledgements- We are grateful to SchorseKleinschmidt, Thomas Flöttmann, Michael Schmidt-Thoméand Heinz Jordan for companionship during field trips andfor providing additional samples. Don Henry, JulianLodziak, Detlef Requard, Monika Bockrath, Ilse Deneke,Beate Eichmann and Peter Macaj are thanked for laboratoryassistance at BGR Hannover, Peter Späthe and RosemarieBaur for thin section preparation and laboratory assistanceat the Institut für Mineralogie, Würzburg, and AxelHöhndorf for discussions of the analytical results. Twoconstructive reviews by Anna M. Fioretti and Sergio Rocchihelped to improve the manuscript. US gratefullyacknowledges financial support by the DeutscheForschungsgemeinschaft(Schu 243/9-1; Schu 873/1-1).

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