© Terra Antartica Publication

Terra Antartica2002, 9(2), 57-72

Plutonic Rocks from the Cape Roberts Hinterland: Wilson Piedmont Glacier, Southern Victoria Land, Antarctica

P.J. FORSYTH, N. MORTIMER & I.M. TURNBULL

Institute of Geological & Nuclear Sciences, Private Bag 1930, Dunedin - New Zealand

Received10 December 2001; accepted in revised form2 July 2002

Abstract - Previous geological mapping of the inland Dry Valleys area of southern Victoria Land identifieda number of 1-10 km-size granitoid plutons, and assigned them to calc-alkaline (DV1a), adakitic (DV1b) andmonzonitic (DV2) suites. In the adjacent coastal Wilson Piedmont Glacier area, mapping hitherto consistedof regional reconnaissance or local structural and chemical studies. Our new mapping of this part of theRoss Orogen shows that most Wilson Piedmont granitoid rocks can be assigned to plutons and suitespreviously recognised in the inland Dry Valleys. We have mapped and clarified the boundaries of thepreviously-described Bonney, Denton, Avalanche Bay, Gonville and Caius, Swinford, Harker and Brownworthplutons. Some new plutons are recognised, namely Evans, Discovery, Flint and Coleman. DV1a and DV1bplutons are mostly foliated, while DV2 rocks are unfoliated. We outline three major swarms of Vanda dikes,eastward continuations of those mapped in the Dry Valleys. Mafic to intermediate intrusive bodies are moreextensive in the Wilson Piedmont than in the Dry Valleys, and their geochemistry suggests they arepetrogenetically related to the DV1a granitoid suite.

*Corresponding author(j.forsyth@gns.cri.nz)

INTRODUCTION & PREVIOUS WORK

The Wilson Piedmont Glacier lies west ofMcMurdo Sound, between New Harbour and GraniteHarbour (Fig. 1), and separates the inland Dry Valleysfrom the Ross Sea coast. In the 1997/98 Antarcticsummer field season, we investigated the rocks in theWilson Piedmont Glacier area. One of the aims ofthis work was to confirm or refute, using geologicalmapping, the presence of previously postulated majorfaults which may have been the locus ofTransantarctic Mountains uplift. Another aim was toprepare a geological map of basement units and thepresent paper summarises the results of this basementmapping.

The fringes of the Wilson Piedmont were visitedin the early 20th century by geological parties whonamed Granite Harbour and identified several types ofgranitoid rocks (e.g. Ferrar, 1907). The first (andonly) published geological map of the WilsonPiedmont area is that of Gunn & Warren (1962). Theadjacent ice-free Dry Valleys, in contrast, have beenvisited by many workers, from early reconnaissancemapping parties (e.g. Allen & Gibson, 1962;McKelvey & Webb, 1962; Haskell et al., 1965) tomore detailed studies of individual plutons, dikeswarms, metasediments or surficial deposits. Much ofthe inland geology has been summarised, andsupplemented by new work, in a more recent seriesof 1:50 000 maps (Allibone et al., 1991; Pocknall et

al., 1992; Turnbull et al., 1994; Isaac et al., 1996).Detailed work on the Dry Valleys basement granitoidsand metasedimentary rocks has been published, usinga conceptual framework of pluton mapping, by Cox& Allibone (1991), Allibone et al. (1992); Allibone etal. (1993a, 1993b); Cox 1993; and Cox et al. (2000).These workers rationalised previously confused andoverlapping nomenclature within granitoid rocks ofthe Dry Valleys, recognised 15 major granitoidplutons and many smaller plugs and dikes, andclarified their intrusive relationships.

Structural studies of the Wilson Piedmont area,related to the uplift history of the TransantarcticMountains, have been made by Fitzgerald et al.(1986), Fitzgerald (1992), and Wilson (1991, 1994).Results of our own uplift and faulting work arereported by Mortimer et al. (2002). The Cape Robertsoffshore dril l ing programme has also focussedattention on the Wilson Piedmont as a possible sourcearea for basement clasts retrieved from Cape Robertsdrillholes (e.g. Talarico & Sandroni, 1998; Smellie,2000; Talarico et al., 2000).

Our 1:50 000 geological field data are incorpo-rated into the VALMAP digital database held by theUniversity of New Hampshire (Prentice et al,. 1999).Although a significant improvement on earlierpublished maps of the Wilson Piedmont, thegeological mapping is still incomplete and some areashave not been thoroughly investigated. Where wehave no field observations, we have used air photo

J. Forsyth et al.58

interpretation, and data and samples collected byprevious workers (where available) to fill gaps.

REGIONAL GEOLOGY

Basement rocks include late Precambrianmetasediments and a variety of younger granitoidplutons. Metasedimentary rocks have not been aparticular focus of our study and we follow the recentrevision of high-level terminology by Cook & Craw

(2001), using the term Skelton Group to describeundifferentiated metasediments of the Dry Valleysregion. Skelton Group rocks in the Wilson Piedmontarea include marble, calc-silicate and psammite,which are invaded, in many places, by foliation-parallel sheets of orthogneiss and amphibolite. In thestudy area, Skelton Group rocks form severalgeographic belts, separated by granitoid plutons(Fig. 2). The dominant foliation almost always strikesSE, with variable dips.

Fig. 1 – The Wilson Piedmont Glacier area: location diagram and simplified geology. A = Avalanche Bay, F = Flint Ridge, H = HansonRidge, R = Robertson Ridge, S = Staeffler Ridge.

Plutonic Rocks from the Cape Roberts Hinterland 59

Plutonic and meta-plutonic rocks - GraniteHarbour Intrusive Complex of Gunn & Warren (1962)- make up at least two-thirds of the basement in thearea mapped. Orthogneiss bodies are the oldest rocksin this complex. Biotite orthogneiss and biotite-hornblende orthogneiss are recognised (Cox &Allibone, 1991; Allibone et al., 1993b), and inaddition, mafic orthogneisses are mapped in the StJohns Range (Turnbull et al., 1994). Foliated andunfoliated granitoid plutons dominate and arediscussed in more detail below. Gabbro and dioritebodies, such as Delta Diorite (Gunn & Warren, 1962)appear to be more common around the WilsonPiedmont than in the Dry Valleys. Mafic and felsicdikes of the Vanda dike swarms cut, and thereforepost-date, most basement units. Older plutons wereapparently intruded at greater depths than younger

bodies: pressures suggested by Allibone et al. (1993a)reduce from >5 to <2 kbar over the duration ofemplacement.

The basement rocks are truncated by theregionally extensive Kukri Erosion Surface, andoverlain by late Palaeozoic to Mesozoic BeaconSupergroup sediments which form the highest parts ofthe Transantarctic Mountains to the west of the DryValleys. Both Beacon and basement rocks areintruded by dolerite sills of the Ferrar Supergroup.Remnants of Beacon Supergroup overlie basementrocks in the Gonville and Caius Range and on theeast side of the Upper Newall Glacier – sections arepoorly exposed and generally <5 m thick, beneathFerrar Dolerite sills. Ferrar Dolerite occurs on thetops of most of the main ranges, and also occurs onthe Ross Sea coast at Kolich Point (Fitzgerald, 1992).

Fig. 2 – Inferred basement geology of southern Victoria Land, updated from Alliboneet al. (1993a,b) and Cox et al. (2000) with newinformation from this study and from Isaac et al. (1996). Small plutons are labelled as follows: C = Cavendish, Ca = Calkin, D = Dun, N= Nibelungen, MF = Mt Falconer, P = Packard, S = Swinford.

J. Forsyth et al.60

The Dry Valleys region, including the WilsonPiedmont, was uplifted to form part of theTransantarctic Mountains chain in the early Cenozoic,and later glacially eroded to form the presenttopography by the middle Miocene (Sugden et al.,1999). Large parts of the inland area, and many ofthe exposures in the Wilson Piedmont region, aremantled by surficial deposits (e.g. Haskell et al.,1965; Denton et al., 1970).

PLUTONS AND FIELD RELATIONSHIPS

Major intrusive bodies (plutons, sheets and dikeswarms) are described in approximate older toyounger age sequence, as inferred from intrusiverelationships seen in the field and supplemented byU-Pb zircon radiometric ages where available. Therelationship of each pluton to adjacent units isdescribed. Granitoid rock names follow Streckeisen(1976); pluton and suite nomenclature followsTulloch (1988), Allibone et al. (1993a), Cox &Allibone (1995) and Cox et al. (2000). In the fieldwe were able to map the areal extent ofmineralogically and texturally distinct granitoidplutons (Fig. 2). Petrographic and geochemical datasupplement field observations and allow the plutonicrocks to be classified in terms of previouslyestablished Dry Valleys I-type igneous suites: DV1a(calc-alkaline), DV1b (adakitic) and DV2(monzonitic) (Allibone et al., 1993b, Cox et al.,2000). Geochemistry and suite interpretation of theplutons are mostly discussed in a separate sectionthough some reference is made to previouspetrological work in the descriptive sections whereappropriate. Sample numbers prefixed “P” refer tospecimens held in the National Petrology ReferenceCollection of the Institute of Geological & NuclearSciences.

FELSIC ORTHOGNEISSES

Felsic orthogneiss bodies are strongly deformed,disrupted granitoid plutons. They are intercalated on a

100 m - 1 km scale with Skelton Group meta-sediments, with which they have been metamorphosedto amphibolite facies. Contact relationships betweenorthogneisses and adjacent units are commonlyobscured by ice and scree in the Wilson Piedmontarea. Our scale of mapping did not allow theorthogneiss bodies to be mapped in detail, but somegeneral observations are made below.

Biotite orthogneiss with minor hornblende-biotiteorthogneiss forms most outcrops along the coast, fromGneiss Point (e.g. P62348) to Cape Roberts.Orthogneisses are less common inland but biotiteorthogneiss also forms much of the north side of theDebenham Glacier (P62103), and small outcrops onthe south side. Biotite orthogneiss is also associatedwith Skelton Group around the Clark Glacier (thisstudy) and various gneisses occur on the north ridgeof Mt Newall (Gunn & Warren, 1962; Palmer, 1987)and within Skelton Group at Mt Falconer (Ghent &Henderson, 1968). These outcrops form the SEcontinuation of a metasediment/orthogneiss beltmapped in the St Johns Range (Turnbull et al., 1994).Generally biotite-hornblende orthogneiss is much rarerthan biotite orthogneiss in the Wilson Piedmont area.Mafic orthogneisses occur in bodies generally toosmall to show in figure 2; they are discussedseparately below under “minor gabbroic and dioriticintrusions”.

BONNEY, EVANS, DENTON AND DISCOVERY PLUTONS

These four granodiorite to monzodiorite plutonscontain hornblende, biotite, ± clinopyroxene, andcharacteristic K-feldspar megacrysts which arecommonly deformed into augen (Fig. 3). Com-positional banding of mafic and felsic schlieren, andaligned megacrysts, are variably developed and give astreaky, swirly or foliated effect. Foliations tend tostrike NW and SW. Rafts and inclusions of fine-grained mafic (amphibolitic) material, orthogneiss andSkelton Group metasediments are generally presentespecially near pluton margins. Orbicular textures aredeveloped locally.

Fig. 3 – Foliated megacrystic biotite granodioriteof the Evans Pluton, typical of the Bonney“family” of granitoids. West end of StaefflerRidge, Greenwood Glacier.

Plutonic Rocks from the Cape Roberts Hinterland 61

Each pluton intrudes the Skelton Group andassociated orthogneisses. U-Pb zircon dating ofBonney Pluton gives ages of 505±2 Ma (Encarnacion& Grunow, 1996; Cox et al., 2000) and 499±6 Ma(Allibone & Wysoczanski, 2002). An Rb-Sr biotiteage of 476± 15 Ma (Deutsch & Webb, 1964) for theDiscovery Pluton, like other such mineral isochronages in southern Victoria Land, is probably a coolingage rather than an emplacement age (Allibone et al.,1993a).

Bonney Pluton has previously been mapped inthe Dry Valleys area (Allibone et al., 1991; Alliboneet al., 1993a; Turnbull et al., 1994) and extended SEacross the Royal Society Range to the Skelton Glacier(Cox et al., 2000). The eastern edge of this plutonlies in the Wright Valley, at the head of the NewallGlacier and in the Taylor Valley. Rafts of BonneyPluton occur within a complex Ferrar Doleriteintrusion at the upper Newall Glacier. Evans Pluton(new name, from Mt Evans) extends from thesouthern end of Killer Ridge to the GreenwoodGlacier (e.g. P62302, 62306, 62307), and a deformedmegacrystic granodiorite at the west end of King Pin(P62346) is probably a part of Evans Pluton detachedby the later intrusion of Brownworth Pluton.Discovery Pluton (new name, from Discovery Bluff)is exposed around Granite Harbour at DiscoveryBluff, Finger Point, Cuff Cape and the Kar Plateau.Denton Pluton (Ellery, 1989; Allibone et al., 1993a;P62324) extends from Robertson Ridge southwardsacross Lower Wright Valley. There is an area of poorexposure beneath the screes of the Lower VictoriaValley (unvisited by us) where Denton Pluton couldmerge with Evans Pluton along the eastern margin ofPackard Pluton (see Fig. 2).

Bonney, Evans, Denton and Discovery plutonsbelong to a suite of similar genesis, age, appearanceand deformation history, which includes thepreviously-mapped Wheeler and Cavendish plutons(Allibone et al., 1993a; Turnbull et al., 1994) and un-named foliated granodiorites in the Wright ValleyNorth Fork and the Balham Valley (Isaac et al.,

1996). Each of these bodies is separated from themain Bonney Pluton by a belt of older metasedimentand orthogneiss, and is regarded as a separateintrusion within the same calc-alkaline magmaticevent.

FLINT PLUTON (new name)

Much of Flint Ridge, between the lower Newalland Commonwealth glaciers, is formed of dioriticgneiss (e.g. P62321), here named Flint Pluton. Theprobable northern margin of the pluton is locatedalong the southeastern side of the lower NewallGlacier, but it was not well defined in this study dueto lack of access. The western margin, probablyagainst Denton Pluton, was identified in only oneoutcrop.

Flint Pluton intrudes Skelton Group metasedimentsand accompanying biotite orthogneiss on the southeastend of Flint Ridge, where it is cut by garnetiferousgranite dikes (probably from the nearby ColemanPluton). Elsewhere it is intruded by variably foliatedbut much less deformed diorite dikes, by biotitegranite dikes and small stocks, and by some Vandamafic and felsic dikes. Flint Pluton (including dikesof this material which intrude Skelton Group) isvariably foliated, with foliation striking generally SW,and in places strongly lineated with augen of feldsparin a matrix of biotite hornblende gneiss (Fig. 4).

In thin section,e.g. P62320 and 62321, thegneiss has up to 40% mafic minerals (biotite,hornblende and augite). Quartz stringers andplagioclase (oligoclase-andesine) megacrysts occur inmany samples. Significant titanite and minor allaniteoccur as accessories, with secondary epidote. A garnetamphibolite from the lower Newall Glacier (P62322)may be an altered hornblende diorite or possibly apiece of relict Skelton Group material. Unlike themajority of the other mafic bodies examined in thisstudy, most of the Flint Pluton does not showsignificant alteration of hornblende to actinolite and/orchlorite.

Fig. 4 – Flint Pluton dioritic gneiss with stronglinear fabric and stretched (and younger,undeformed) felsic veins. Southern end of FlintRidge.

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Near the probable northern margin in the lowerNewall Glacier, Flint Pluton dioritic gneiss appearsinterlayered with Skelton Group, and based on thisand its strongly deformed nature it is placed amongthe earliest intrusive rocks of the Wilson Piedmontarea. Smaller bodies of similar appearance andmineralogy occur within mafic orthogneisses on theSt Johns Range, and are mapped as early dioritic andgabbroic intrusives (Turnbull et al., 1994).

GONVILLE AND CAIUS PLUTON(Gunn & Warren, 1962)

The western part of the Gonville and Caius Rangeis underlain by a massive medium to coarse grainedequigranular hornblende/biotite granite (Gunn &Warren, 1962; P62288, 68016). This pluton has nowbeen mapped from Granite Harbour (at the Devil’sPunchbowl, Finger Point and Cuff Cape) in the northto the southern end of Killer Ridge. Along its easternmargin it intrudes Discovery Pluton and orthogneiss.Its NW margin, probably against Suess Pluton, wasnot mapped in this study. The SW margin lies againstSwinford Pluton – the contact occurs in cliffs at theNW corner of Killer Ridge opposite Queer Mountain,where Gonville and Caius Pluton forms the upper partof the face – but the contact relations are unknownbecause of inaccessibility. At the southern end,Gonville and Caius Pluton intrudes Evans Pluton.Marginal dikes, probably related to this pluton,intrude diorite at the south end of Killer Ridge.

Outcrops show characteristic mafic clots andmicrogranite and porphyritic inclusions in anotherwise homogeneous pink or grey rock. Thegranite becomes finer grained, and inclusions becomemore common, near its southern margin at the top ofSecond Facet. Petrographically the assemblage is K-feldspar, quartz, plagioclase (albite/oligoclase), biotite,amphibole (hornblende), zircon, apatite, with(secondary) magnetite and epidote. It is distinct fromneighbouring plutons chemically, petrographically andin outcrop, differing from the adjacent Suess andHarker plutons in containing significant hornblende aswell as biotite, and from the Swinford Pluton in beingequigranular.

Gonville and Caius Pluton lacks “Vanda-type”felsic porphyry enclaves and is intruded by manyVanda dikes: it predates emplacement of the majordike swarms. The lack of foliation and deformationsuggest that it post-dates the older, Bonney-likedeformed megacrystic granite suite.

PACKARD PLUTON(Turnbull et al., 1994)

The Packard Pluton, consisting of gabbro anddiorite, was first described from the lower PackardGlacier in the southern St Johns Range by Turnbull etal. (1994). We consider that outcrops of diorite at theeastern end of Robertson Ridge, north of the ClarkGlacier, are part of the Packard Pluton. Its eastern

limit, north of the Lower Victoria Glacier terminus,has not been seen; as its mapped position is based ondistant observations and air photo interpretation, itshould be considered approximate only.

Turnbull et al. (1994) and Cox et al. (2000)allocated the Packard Pluton to a very early intrusivephase, as it was known to intrude only Skelton Groupand orthogneiss. However, we have observed that it isalso younger than Denton and Evans plutons. PackardPluton and its contact zones are cut by Vanda felsicdikes, and less abundant mafic dikes.

Packard Pluton on Robertson Ridge consists ofmassive fine to medium grained hornblende biotitediorite. Plagioclase is highly altered, and extensivesecondary chlorite, actinolite, muscovite, epidote,calcite and sericite occur. Rare pods of coarsehornblende pegmatite occur near the western contact(cf. Turnbull et al,. 1994). The pluton is unfoliated,but in places exhibits primary igneous layering, withgraded felsic to mafic banding 5-10 cm across. Thistexture was also described from the type area of thePackard Pluton by Turnbull et al. (1994). In thecentral part of the Robertson Ridge exposure, slightlyyounger, paler, more felsic hornblende biotite dioritewith acicular hornblende sprays cuts the host diorite.Whether this is a late phase of the Packard, or aseparate intrusive phase, is unknown.

COLEMAN PLUTON (new name)

This heterogeneous pluton occurs around MtColeman, the type area, on the north side of thelower Taylor Valley. Despite lying less than 10 kmfrom the Mt Falconer Pluton, it has not beendescribed before. The western, southern andnortheastern contacts of the pluton intrude SkeltonGroup metasediments and associated orthogneisses;the other contacts are inferred to lie beneath the iceof the southern Wilson Piedmont Glacier. ColemanPluton is intruded by a Vanda Dike swarm sonumerous that dikes (mainly mafic) make up overhalf the outcrop in places. On Flint Ridge, dikes ofgarnetiferous granitoid with flow-banded texture,inferred to be from Coleman Pluton, cut SkeltonGroup, orthogneiss and Flint Pluton.

Notable outcrop features of this pluton are thebanded and folded texture (Fig. 5), and abundant palered garnets (e.g. P62325). The texture is inferred tobe a flow foliation, as it can be clearly seen inoutcrops where Coleman granite cuts across an older,unfoliated granodiorite, and also cuts across foliationin Skelton Group metasediments and associatedorthogneisses. No consistent orientation is discerniblefrom foliation measurements. The rock when fresh iswhite to pale grey, and weathers to a fawn colour.

Coleman Pluton is relatively sil iceous (>30volume % quartz in thin section) and has thecomposition of a monzogranite. Minor biotite is theonly mafic phase. Garnets (unanalysed) are up to7 mm long and anhedral in thin section. Garnetpegmatites parallel the swirled bands.

Plutonic Rocks from the Cape Roberts Hinterland 63

VANDA DIKE SWARMS (after McKelvey & Webb, 1962)

Several major swarms of mafic and felsic dikeshave been mapped previously in adjacent areas(Allibone et al., 1991; Turnbull et al., 1994), andeastward continuations of these were mapped in theWilson Piedmont area. The three main swarms, alloriented SW-NE, occur in the Gonville and CaiusRange and as far north as Granite Harbour; in theeastern Olympus Range and Lower Victoria Valley;and from the Lower Taylor Valley to Hogback Hill.From the eastern Asgard Range to Doorly Spur, (e.g.P62336) and near the Packard Glacier, lesser swarmshave the same orientation but fewer dikes. Dikes cutall the granitoid and gneiss bodies listed above, andlocally (e.g. Robertson Ridge; Hjorth-Hogback Hill-Mt Coleman area) about 30% of rock volume is dikematerial. Vanda dikes become rare or absent in theyounger plutons described below.

A field subdivision of “mafic” and “felsic” dikes,used during mapping, follows that of earlier authors(e.g. Allibone et al., 1993a). Detailed descriptions ofgeochemistry and associations at Mt Loke, in theeastern Asgard Range, are given by Keiller (1991). AtMt Falconer, Ghent & Henderson (1968) report 4types of dikes, of which 3 (“mafic”, “porphyriticgranophyre” and “camptonite”) conform to publisheddescriptions, and our own observations, of dikes inthe Vanda suite. The first two types clearly predateMt Falconer pluton, while the last type postdates it.

Vanda dikes have been dated in the eastern WrightValley (U-Pb zircon age 484± 7 Ma (Encarnacion &Grunow 1996) and Taylor Valley (K-Ar age 458±20Ma (Angino et al., 1962).

AVALANCHE BAY PLUTON(Graham & Palmer, 1987)

This pluton forms the coastal cliffs on the southside of Granite Harbour from the eastern end ofCouloir Cliffs west to Avalanche Bay, and also cropsout on the eastern side of Haystack Mountain. It

intrudes Discovery Pluton at Avalanche Bay, wherethe western margin is marked by a dramatic increasein enclaves and rafts of amphibolite, orthogneiss andSkelton Group metasediments. The contact withDiscovery Pluton on the east side of HaystackMountain is possibly a fault and is marked by brick-red, hydrothermally altered and shattered rock.However we infer that little offset has occurred onthis fault, as the pluton-margin facies, rich in rafts ofolder materials, is present nearby.

Avalanche Bay Pluton is dominated by mediumgrained, massive quartz monzodiorite, with slightlymegacrystic feldspars (Fig. 6). Scattered inclusions ofpartly-assimilated microgranitoid, and dark porphyryresembling Vanda dike material, occur along withpink aplitic dikes. No foliation is apparent in outcropor thin section. The assemblage is quartz, plagioclaseand K-feldspar, with hornblende and biotite aboutequal, and with accessory zircon, magnetite andallanite. Secondary chlorite and epidote replacebiotite.

The Avalanche Bay Pluton is undeformed and iscut by only a few mafic Vanda dikes near its westerncontact – features suggesting that it is a relativelyyoung rock in the intrusive sequence. It has a U-Pbzircon age of 498±4 Ma (Encarnacion & Grunow,1996) which, as expected, is significantly older thanRb-Sr mineral ages of 452±6Ma and 459± 4 Ma(Allibone et al., 1993a).

SWINFORD PLUTON (Waters, 1993)

Initially named “Swindon Granite” by Palmer(1990), this pluton was further mapped in the easternSt Johns Range by Waters (1993) and Turnbull et al.(1994). Our study has confirmed that this rock alsooccurs further east, on Killer Ridge (P68014, 68015).The two occurrences are separated by the HarkerPluton (see below), from which the Swinfordgranitoid is texturally and geochemically distinct.

The Swinford Pluton is typically coarse grainedwith alkali feldspar megacrysts, up to 2 cm long, in amatrix of plagioclase, quartz, alkali feldspar and

Fig. 5 – Coleman Pluton garnet-bearing granite,with deformed flow foliation outlined by biotiteconcentrations. East face of Mt Coleman.

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minor biotite with accessory hornblende. Analysesshow granitic to quartz monzodioritic compositions. Itis poorer in silica, and more mafic and calcic, thanthe neighbouring Harker Pluton, and field relationssuggest that it is older (Turnbull et al., 1994).

HARKER PLUTON (Palmer, 1990; Waters, 1993)

Following studies by Palmer (1990), this plutonwas further mapped in the eastern St Johns Range byWaters (1993) and Turnbull et al. (1994). The WilsonPiedmont study has extended this pluton as far southas Pond Peak. Harker Pluton intrudes Skelton Groupmetasediments, orthogneisses and Wheeler Pluton(Turnbull et al., 1994), and Evans Pluton (this study).It post-dates the majority of Vanda dikes, but bothmafic and felsic dikes intrude in places.

The pluton comprises mainly coarse grained,homogeneous equigranular granite. In colour the rockis generally grey but orthoclase is cloudy and pink inmany places. Biotite is the main mafic mineral, withvery minor amphibole, accessory allanite, magnetite,zircon and apatite, and secondary muscovite. Samplesfrom our study (P62101, 62329), and from Gunn &Warren (1962), are syenogranites and monzogranites.

BROWNWORTH PLUTON (Ellery, 1989)

Brownworth Pluton was mapped by Ellery (1989)in two small areas on either side of the lower WrightValley. Our mapping extends this pluton fromBlessing Bluff and Doorly Spur (e.g. P62293) in thenorth, to King Pin and the lower Newall Glacier inthe south.

This pluton is readily identified by its distinctive,abundant pink K-feldspar megacrysts, up to 4 cmlong; characteristic xenoliths of porphyry (resemblingVanda mafic dike material); and lack of foliation(Fig.7). Although a weak flow fabric of alignedmegacrysts and inclusions was noted at BlessingBluff, at most outcrops the only measurable structuresare widely-spaced joints. Some metamorphic (SkeltonGroup) xenoliths were recorded by Ellery (1989) inthe Lower Wright Valley, and inclusions ofmicrogranite and biotite-rich mafic clots also occur.

Brownworth Pluton intrudes Skelton Group at itswestern edge, Evans Pluton at its northern margin,and un-named hornblende biotite diorite at itssouthern margin. On the southeastern margin, at thewestern end of King Pin, megacrystic dikes from theBrownworth Pluton intrude a deformed megacrystic

Fig. 6 – Avalanche Bay Pluton monzo-diorite at thetype locality, with minor microgranite enclaves(lower left). Lens cap for scale (upper right).

Fig. 7 – Megacrystic Brownworth quartzmonzonite, with an enclave of “Vanda”-like felsicporphyry. Eastern end of Staeffler Ridge; pencilfor scale.

Plutonic Rocks from the Cape Roberts Hinterland 65

granite gneiss which is most likely a detachedfragment of Evans Pluton. A few Vanda mafic dikesintrude Brownworth Pluton (e.g. P62336).

Brownworth Pluton consists predominantly ofquartz monzonite with the assemblage K-feldspar-plagioclase-quartz-biotite-hornblende, and accessorytitanite, allanite and zircon. Secondary chlorite andepidote replace biotite. Allibone et al., (1993b) placeit in DV2 suite on the basis of field relations, texturesand the characteristic porphyritic xenoliths.

MT FALCONER PLUTON(Ghent & Henderson, 1968; Ghent, 1970)

This small (10 km2) pluton crops out only at MtFalconer on the north side of the Lower TaylorValley. It is surrounded by and intrudes SkeltonGroup metasediments, orthogneiss and diorite. Thepluton is strikingly discordant to the structural trendsof the metasediments and gneiss, and truncates maficand granophyric Vanda dikes. It shows little evidenceof post-emplacement deformation or metamorphism.

Most of the pluton consists of homogeneousmedium to coarse grained biotite-hornblende quartzmonzonite with the assemblage K-feldspar>plagioclase, biotite>hornblende, accessory allanite,titanite, apatite, zircon and ilmenite (Ghent &Henderson, 1968). Near the margins, grain sizedecreases in places, and elongate, oriented xenolithsand schlieren occur. Breccia zones mark the contactwith an adjacent dioritic body. Pink siliceous dikes,thought to be late-stage features, intrude Mt FalconerPluton and country rock only near the pluton margins.A few Vanda mafic dikes intrude Mt Falconer Pluton.

K-Ar biotite ages of 451-461 Ma are a minimum,but because of the shallow emplacement (at notgreater than 10 km), cooling was probably swift andthe actual intrusion age may not be much older(McDougall & Ghent, 1970). These ages, the“discordant” field relations and the lack ofdeformation are consistent with other young DV2plutons in the Dry Valleys area.

MINOR GABBROIC-DIORITIC INTRUSIONS

Gunn & Warren (1962) described “all intrusions ofpost-tectonic hornblende-andesine diorite found inVictoria Land” as Delta Diorite (from Delta Bluff onthe Skelton Glacier). Diorite bodies in the lowerWright Valley have been described by Keiller (1991)and Ellery (1989), and at Mt Falconer “dioritehybrid” rocks are described by Ghent & Henderson(1968). Aslund (1990) and Simpson & Aslund (1996)have summarised the petrology of much moreextensive mafic bodies at Mt Dromedary in the RoyalSociety Range, 90 km south of the Wilson Piedmont.

We revisited diorite outcrops at Gneiss Point, MtNewall and Killer Ridge which were previouslydescribed by Gunn & Warren (1962), and discoverednew diorites on King Pin and Hanson Ridge, and a

gabbro east of Staeffler Ridge. In the WilsonPiedmont area, dioritic and gabbroic bodies aregenerally small plugs or bosses; apart from the Flintand Packard bodies they are too small to be termedplutons. From their intrusive relations and degree offoliation, we conclude that most of these mafic rockswere emplaced relatively early in the plutonic historyof the area, but none have been dated. It is notuncommon for igneous pyroxene and hornblende tobe replaced by actinolite and/or chlorite in theserocks.

The Killer Ridge outcrops consist of mediumgrained, unfoliated, hornblende biotite diorite whichforms the southernmost tip of Killer Ridge, up to aheight of 200 m above the junction of the Miller andCrisp glaciers (see also Gunn & Warren, 1962, p. 97).The diorite intrudes megacrystic Evans Plutongranodiorite, and younger undeformed Harker Plutongranite, both of which occur as rafts within thediorite. Rare pegmatitic phases of diorite havehornblende laths up to 3 cm long. This diorite, alongwith the host granitoids, is cut by Vanda mafic dikes.

The western end of Hanson Ridge, including thesummit trig point, is composed of up to 75%undeformed diorite, clearly intruding interlayeredSkelton Group metasediments and biotite orthogneisswhich form the bulk of the ridge. The diorite(P62313) consists of pyroxene (e.g. augite), biotite,hornblende and plagioclase, with minor quartz and K-feldspar. It is weakly foliated near contacts, generallyfine to medium grained, and cut by pale grey biotitegranodiorite dikes. Further east on Hanson Ridge,Skelton Group encloses foliation-parallel dioritic togabbroic orthogneisses, which are texturally distinctfrom the undeformed diorite.

On the ridge east of the main mass of StaefflerRidge is a spectacular very coarse grained gabbrowith interlocking pyroxene crystals up to 2 cm long(P62292). The gabbro is finer grained and more felsicon the margins, and is cut by microgranite and Vandamafic dikes. Dikes of diorite off the gabbroic bodyintrude Skelton Group metasediments, andmegacrystic gabbro also forms a dike on the easternend of Staeffler Ridge. The Staeffler Ridge gabbroclosely resembles an elongated concordant gabbrobody mapped in the Clare Range by Turnbull et al.(1994).

Diorite dikes, a boss, and possible sills intrudeDenton Pluton on the south side of the lower WrightValley (Ellery, 1989; Keiller, 1991). The diorite dikestrend east-west, compared with the average NE strikeof Vanda dikes, and pre-date the Vanda Dike swarm(Keiller, 1991). These diorites are similar to dioritesat Gneiss Point, where hornblende-biotite quartzmonzodiorite forms dikes up to 20 m thick, intrudingorthogneisses and Skelton Group metasediments. Incoastal cliffs 4 km west of Gneiss Point, massivehornblende diorite (P62295) forms a sill up to 50 mthick within marble and amphibolite schist (Gunn &

J. Forsyth et al.66

Warren, 1962). Although appearing massive in handspecimen, in thin section the diorite is weaklyfoliated.

Much of King Pin nunatak is composed ofvariably foliated biotite-hornblende-pyroxene diorite(P62347). Pyroxene was formerly the dominant maficphase but has been partly replaced by hornblende.The diorite contains rafts and inclusions of SkeltonGroup, gneissic megacrystic granite, and deformedand strained amphibolite, in places forming a complexintrusion/injection breccia within the diorite. At thewestern end of the King Pin, the diorite clearlyintrudes mafic megacrystic orthogneiss, probablyEvans Pluton. The diorite is cut by undeformedgranitoid dikes, some megacrystic, which mayoriginate from the adjacent Brownworth Pluton.

In the Decker Glacier area, diorite occurs as raftswithin Brownworth Pluton. Nearby, around the lowerNewall Glacier (possibly the “east ridge of MtNewall” locality of Gunn & Warren, 1962), variablyfoliated monzodiorite containing hornblende, pyroxeneand biotite may be part of the Flint Pluton or ayounger intrusion. A diorite body is intruded by theMt Falconer Pluton (Ghent & Henderson, 1968).Diorite and related “hybrid” rocks range incomposition from hornblende-biotite (± pyroxene)diorite, through quartz monzonite to pyroxene-bearingquartz syenite. The amounts of quartz and alkalifeldspar vary irregularly over short distances, acharacteristic also noted in most of the dioritic rockswe have examined.

MINOR GRANITIC INTRUSIONS

During this study we observed, sampled andanalysed several small granitoid bodies. Their extentwas not mapped because of poor exposure and/orsmall size, and they are not shown on figure 2.Among these are two small (1-2 km) bosses ofbiotite-augite quartz monzodiorite, one unfoliated andintruded by (possibly part of) the Coleman Plutonwest of Mt Coleman (P62326), the other weaklyfoliated and intruding Flint Pluton in the central partof Flint Ridge. Another small, fine grained biotite-bearing granitoid intrusion in the upper NewallGlacier is not seen in contact with any other units(contacts are concealed under Ferrar dolerite or ice),but must intrude Bonney Pluton. It may be related tothe nearby Valhalla Pluton.

GEOCHEMISTRY & SUITE INTERPRETATION

New major and trace element analyses of WilsonPiedmont rocks are presented in table 1 andillustrated in figures 8 and 9. These supplement thealready large published database of Southern VictoriaLand intrusive rocks (Palmer 1987, 1990; Allibone etal., 1993b; Cox et al., 2000).

For convenience of discussion we have divided

the plutonic rocks into two broad compositionalgroups: granitic (SiO2 >60wt%) and gabbroic (SiO2<60wt%), but acknowledge that there is probably apetrologic continuum between them (see below).Ferrar Dolerite samples, and a mafic Vanda Dikesample, are included for comparison with thegabbroic group. Vanda dike chemistry is notconsidered in detail here, except to note that a varietyof compositions is recorded (Keiller, 1991; Wu &Berg, 1992); the suite is characterised in generalterms by high-K calc-alkaline chemistry, probablyrelated to the monzonitic DV2 suite of Allibone et al.(1993b). Skelton Group paragneisses are included intable 1 for comparison with orthogneiss samples.

Plutonic rocks in the study area have a wide rangeof sil ica contents (Fig. 8A, B). Ferrar Doleritesamples have expectedly low total alkali contents anda Vanda dike is significantly more alkaline (Fig. 8A).Apart from these rocks, no extremes of Na2O andK 2O content are seen in the dataset, and mostsamples straddle the alkaline/subalkaline dividing lineon the total alkalies-silica diagram (Fig. 8A).

GRANITIC ROCKS

We have attempted to classify the granitic rocks interms of the three I-type Dry Valley granitoid suitesproposed by Allibone et al. (1993b). Rocks from thecalc-alkaline DV1a, adakitic DV1b and monzoniticDV2 suites tend to converge in composition at highsilica contents, but the binary diagrams used todiscriminate these suites (Figs. 8A-E) can be usedwith some success. In most cases our geochemicalresults are consistent with field and petrographicobservations. It is clear that none of our sampledgranitoids have the distinctively high Nb+Y contentsof the Glee Intrusives and Skelton Glacier AlkalineProvince of the Royal Society Range (Read et al.,2001) (Fig. 8F).

In the Wilson Piedmont area the Denton andEvans plutons are grouped within the DV1a suite.DV1b suite rocks include the undifferentiated “early”biotite orthogneisses that intrude Skelton Group, andthe heterogeneous Mt Coleman garnet granite. Thesmall monzodiorite boss near Mt Coleman (P62326)has high Sr, Al2O3, Na2O and low K2O, Rb, Y, which

are characteristic of the DV1b suite (Cox & Allibone1991; Allibone et al., 1993b). The biotite ortho-gneisses and samples from near Mt Coleman are theonly DV1b suite rocks recorded in the WilsonPiedmont area. In contrast, several large plutons(Valhalla, St Johns, Suess) of this composition occurfurther inland.

DV2 suite plutons include Brownworth, Harkerand Swinford. Analyses by Palmer (1990), Waters(1993) and this study reveal metaluminous andsometimes strongly fractionated rocks with highlyevolved syenogranite compositions. Waters (1993)assigned both Harker and Swinford plutons to theDV2 suite, noting that minor differences between

Plutonic Rocks from the Cape Roberts Hinterland 67

Tab

. 1

– X

RF

ma

jor

an

d t

race

ele

me

nt

da

ta.

An

aly

st J

oh

n H

un

t, S

pe

ctra

che

m A

na

lytic

al,

We

llin

gto

n.

Th

ird

co

lum

n f

rom

le

ft s

ho

ws

wh

eth

er

sa

mp

le w

as

in s

itu (

Y),

no

t in

situ

(N

), a

lmo

st i

n s

itu (

A)

or

un

kno

wn

(U).

J. Forsyth et al.68

these rocks and the suite definition of Allibone et al.,(1993b) are the result of the highly evolvedchemistry.

Two plutons give ambiguous and/or problematicsuite correlations in terms of field relations,petrography and geochemistry. As mentioned above,the field evidence from the Gonville and Caius Plutonsuggests that it pre-dates Vanda dike swarms, seemingto rule out a DV2 correlation. The geochemistry ofour two and Palmer’s (1987) five samples of Gonville& Caius Pluton suggest either a DV1a or DV2

affinity, a result that is also supported by the presenceof hornblende. We provisionally regard the Gonville& Caius Pluton as a late, unfoliated, evolved memberof the DV1a suite, similar to the Catspaw Pluton ofAllibone et al. (1993b) (Fig. 2).

Taken at face value, the 498±4 Ma U-Pb zirconage for the Avalanche Bay Pluton is closer to theDV1a suite (c. 500 Ma) than the younger Vanda dikes(484±7 Ma U-Pb zircon age of a dike from theeastern Taylor Valley; Encarnacion & Grunow 1996).This would suggest that the Avalanche Bay could not

Fig. 8 – Binary diagrams showing the range in chemical composition of Wilson Piedmont intrusive rocks, and correlation with previouslyproposed suites of Allibone et al. (1993b). A. normative proportional quartz vs normative proportional anorthite/orthoclase (Streckeisen &Le Maitre, 1979) B. SiO2 vs total alkalies diagram, rock names after Middlemost (1994). C. SiO2 vs MgO. D. SiO2 vs Al 2O3. E. SiO2 vsSr. F. Nb+Y vs Rb (after Pearce et al., 1984). KGAP=Skelton Glacier Alkaline Province (Read et al., 2001).

Plutonic Rocks from the Cape Roberts Hinterland 69

be a DV2 pluton. However the four major and traceelement analyses of this pluton (Palmer, 1987; Fig. 8)reveal monzonitic trends and element ratios andconcentrations much more suggestive of a DV2affinity than DV1. We provisionally propose a DV2correlation of this pluton, noting that the intrusive agerange of Vanda dikes is extremely poorly constrained,and that there is no petrogenetic reason why DV1 andDV2 suites cannot have overlapped in time.

These problems with the Gonville & Caius andAvalanche Bay Plutons may be due to several causes.Our sampling may have been inadequate to properlycharacterise the plutons; the plutons may have hybrid

compositions (such as reported for the Swinford,Brownworth, Orestes and Harker Plutons by Alliboneet al., 1993b); or a modification of the suite definitionand/or approach to SVL granitoid classification maybe required. Based on available evidence, figure 10summarises the inferred age and compositionrelationships between the granitic plutons of theWilson Piedmont area.

GABBROIC ROCKS

Seven analyses of variably foliated andmetamorphosed gabbroic and dioritic rocks (exclusiveof Ferrar sill and Vanda dike samples) were made.Some scatter of points is present on binary diagrams,but it is clear from the MgO and Sr contents that themafic plutonic rocks are neither coarser grainedequivalents of the Ferrar Dolerite, nor of the Vandadikes. On a multi-element plot, appropriate for rocksof basaltic composition (Fig. 9A), they arguably forma coherent suite although P62295 and P62300 havesomewhat higher Th, La, Ce and Zr than the othermafic rocks. The Wilson Piedmont mafic rocks do notobviously resemble Glee tholeiitic or alkaline maficsuites reported from the Royal Society Range to thesouth (Fig. 9B); specifically our rocks have higherRb, Th, Ba, K and Zr contents than the tholeiiticsuites reported by Aslund (1990) and lower Ti andNb and higher K and Sr than Dromedary alkalinerocks.

A comparison of multi-element plots of the WilsonPiedmont gabbroic-dioritic rocks with Dry Valleysgranitoid suites (Fig. 9C) reveals a possiblepetrogenetic relationship. Compared with DV1a rocks,the mafic samples have somewhat lower Rb, Ba, Th,K, La and Ce and slightly higher to overlapping P,Zr, Ti and Y. These features are what would beexpected if the DV1a granitoids were morefractionated and/or evolved compositions related tothe gabbroids. A rigorous petrogenetic treatment isbeyond the scope of this field-based paper, but bothAllibone et al. (1993a) and Cox et al. (2000) reportrare mafic compositions (e.g. from Bonney Pluton) ofDV1a suite rocks. We raise this correlation betweenmafic and DV1a felsic suites as a tentative hypothesisthat should be more rigorously tested by radiometricdating and trace element and isotopic data andmodelling.

STRUCTURE

The Wilson Piedmont Glacier region can beregarded as the eastern side of the Dry Valleysstructural block, contiguous with previously-mappedareas to the west. Skelton Group metasediments andassociated orthogneisses form several NNW-trendingbelts which, although interrupted by younger plutons,can be traced from the Ferrar Glacier as far north asthe Convoy Range (Fig. 2). The oldest DV1a mega-

Fig. 9 – Multi-element diagrams normalised to primitive mantlevalues of Sun & McDonough (1989). A. seven gabbroic-dioriticplutonic rocks (<60wt% SiO2) from the Wilson Piedmont area (newdata, this study). B. fields of gabbroic suites of southern VictoriaLand (data of present study, Aslund, 1990). C. fields of graniticsuites of Dry Valleys area (data of Allibone et al., 1993b; Cox etal., 2000).

J. Forsyth et al.70

crystic plutons such as Bonney, Wheeler, Denton andEvans also tend to be elongated NNW or NW.Included in this group is an un-named deformedgranodiorite in the Balham Valley (Isaac et al., 1996).Of the DV1b plutons, some (e.g. Hedley, St Johns)share the NW trend but others (e.g. Valhalla,Coleman) appear to be more structurally discordant.The NNW-trending fabric is generally cross-cut bythe discordant DV2 plutons, which includePearse/South Fork (Allibone et al., 1991; Isaac et al.,1996), Orestes, Brownworth, Harker and Mt Falconer.

The basement is cut by numerous brittle faults ona predominant NE trend (Mortimer et al., 2002 andreferences therein) which, until recently, have beeninferred to result from Cenozoic dextral transtensionalstress (e.g. Wilson, 1995). However, Mortimer et al.(2002) suggested that many of these faults are likelyto be Palaeozoic in age – including some of thosedescribed at Doorly Spur (Fitzgerald, 1992; Wilson,1999). The prominent Vanda dike swarms share thisNE strike, and dip subvertically.

Based on our mapping of the basement granitoidsof the Wilson Piedmont, and on previous mapping ofthe remainder of the Mackay Glacier to Ferrar Glacierstructural block, we are confident that there are noshear or high-strain zones of any significant extentaffecting the basement – beyond those which predateyounger pluton injection. The later Palaeozoicmarker, the Kukri Erosion Surface, and JurassicFerrar Dolerite sills also preclude significant verticalfault movement, at least to within 10 km of theWilson Piedmont coastline (Mortimer et al., 2002).

RELATION TO CAPE ROBERTS DRILLINGPROGRAMME RESULTS

The Wilson Piedmont Glacier region liesimmediately onshore from the recently completed

series of dril lholes off Cape Roberts (CRP-1,CRP-2/2A and CRP-3), and is considered to be alikely source for sediments cored in the Cape Robertsholes. Petrographic comparisons between clasts fromthe drillholes and rocks exposed inland support alocal provenance for basement clasts, and inferencesfrom sand detrital modes generally support thisconclusion (Talarico et al., 2000; Smellie, 2000).

Few rock types reported from Cape Robertsdrillholes can be matched to specific intrusions withinthe Transantarctic Mountains, although such a matchwould be desirable in order to constrain the sourcearea for the sediment. The commonest clastlithologies reported from all the Cape Robertsdrillholes (Talarico & Sandroni, 1998; Talarico et al.,2000; Cape Roberts Project Team 2000; Sandroni &Talarico, 2001) are grey and pink, undeformed,biotite ± hornblende monzogranites, which could bederived from many different granitoid bodies withinthe Dry Valleys region. Clasts with large K-feldsparphenocrysts resemble rock types such as Swinfordand Brownworth plutons, while more equigranularmaterial could be derived, for example, from Gonvilleand Caius Pluton. Some of the clast l ithologiesreported - calc-silicate rocks and schist - are clearlyderived from the widespread Skelton (Koettlitz)Group metasediments which host the granitoids.Biotite orthogneiss (commonly intercalated with meta-sediments) is the most widespread type of gneissexposed onshore, and is the only type reported fromthe drillholes, while the rarer hornblende-biotiteorthogneiss was not reported. Foliated biotite-hornblende granodiorites are most likely derived fromdeformed plutons such as Bonney and Evans.Monzogranitic porphyry probably originated in theVanda dike swarms of which several are known inthe Dry Valleys/Wilson Piedmont area. Pink-greybiotite haplogranites are most likely derived from theubiquitous aplite dikes that cut most basement rocks.

Fig. 10 – Schematic drawing of theinferred relative timing of igneousintrusions in the Wilson Piedmont area,from field relationships, U-Pbgeochronology, and suite correlations.Age is constrained by only six U-Pbzircon dates: Vanda dike 484±7 Ma,Avalanche Bay 498±4 Ma, Bonney505±2 Ma (Encarnacion & Grunow1996), Valhalla 488±2 Ma (Cox et al.,2000) and two orthogneisses 516±10Ma and 531±10 Ma (Allibone &Wysoczanski, 2002).

Plutonic Rocks from the Cape Roberts Hinterland 71

All the rock types above are so widespread along thissector of the Transantarctic Mountains that noparticular point sources and/or palaeoglacialcatchments can be nominated.

However, the uncommon grey biotite syenogranite(two clasts; Talarico et al., 2000) is so unusual incomposition that it matches only Harker and Orestesplutons. Garnet-bearing biotite syenogranite (Talarico& Sandroni 1998) and garnetiferous biotite monzo-granite (Talarico et al., 2000) are clast types with fewpotential sources within the Dry Valleys, matchingonly Coleman Pluton and some ortho-gneisses.Smellie (2000) and others have pointed to KirkpatrickBasalt as a significant sand source in CRP 2/2A;major occurrences of this unit are currently restrictedto the catchment of the Mackay Glacier. Significantly,mafic plutonic rocks (gabbro, diorite) which formnumerous bodies in the Skelton Glacier region havenot been reported from the dril lholes, perhapsindicating that the southerly-derived ice componentsuggested by Powell et al. (1998) may have bypassedCape Roberts – or that such rocks do not travel well.

CONCLUSIONS

Granitoid intrusions, and their host rocks, havenow been mapped from the furthest inland DryValleys to the Ross Sea coast. The oldest rocks of theRoss Orogen in the Wilson Piedmont area, as furtherwest in the Dry Valleys, are Skelton Groupmetasediments. Associated orthogneisses areextensive, particularly along the coast, and biotiteorthogneiss predominates over the hornblende-biotitevariety. Granitoid plutons previously described in theDry Valleys can be extended eastwards to the WilsonPiedmont, and the system of mapping plutons andgrouping analyses into I-type suites generally workswell. Rocks of the calc-alkaline DV1a suite, adakiticDV1b suite and monzonitic DV2 suite occur in theWilson Piedmont area. Mafic bodies, particularlydiorite, are more common in the Wilson Piedmontthan in the Dry Valleys, and from field relations anddeformation they appear to span a wide range ofages. The deformation and metamorphism of theSkelton Group and associated orthogneisses, and theintrusion of DV1a, DV1b, DV2 and mafic suites canall be considered as episodes within the broader RossOrogeny. Recent interpretations of the precise timingand overlap of Ross events in the Dry Valleys aregiven by Cox et al. (2000) and Allibone &Wysoczanski (2002). Alkaline plutonic rocks, such asthose reported from the Royal Society Range to thesouth, have not been found in the Dry Valleys-WilsonPiedmont area.

Acknowledgements- We are grateful to the 1997/98 staff atScott Base, and to the pilots and crew of the USAF,RNZAF 3 Squadron and PHI Helicopters. We thank DavieRobinson for his excellent field assistance, Neville Orr forthin sections, John Hunt for XRF analyses and Belinda

Smith Lyttle for figure preparation. For discussions, reviewsand improvements we thank A. Allibone, S. Cox, A.Tulloch, N. Roland and F. Talarico. The study was fundedby the N.Z. Foundation for Research, Science &Technology contract CO5523. GNS contribution number2260.

Editorial handling: C.A. Ricci

REFERENCES

Allen A.D. & Gibson G.W., 1962. Geological investigations insouthern Victoria Land, Antarctica. Part 6 - outline of thegeology of the Victoria Valley region. New Zealand Journal ofGeology and Geophysics,5, 234-242.

Allibone A.H., Cox S.C., Graham I.J., Smillie R.W., JohnstoneR.D., Ellery S.G. & Palmer K., 1993a. Granitoids of the DryValleys area of southern Victoria Land, Antarctica: plutons,field relations and isotopic dating. New Zealand Journal ofGeology and Geophysics,36, 281-297.

Allibone A.H., Cox S.C. & Smillie R.W., 1993b. Granitoids of theDry Valleys area, southern Victoria Land, Antarctica:Geochemistry and evolution along the early Paleozoic Antarcticcraton margin. New Zealand Journal of Geology andGeophysics,36, 299-317.

Allibone A.H., Forsyth P.J., Sewell R.J., Turnbull I.M.& BradshawM.A., 1991. Geology of the Thundergut area, Southern VictoriaLand, Antarctica, scale 1:50 000. New Zealand GeologicalSurvey miscellaneous map series21, Wellington, Department ofScientific and Industrial Research.

Allibone A.H., Smillie R.W., Cox S.C. & Johnstone R.D., 1992.The intrusive history of granitoid plutons and suites, DryValleys area, south Victoria Land. In: Sixth InternationalSymposium on Antarctic Earth Sciences, Tokyo.

Allibone A.H & Wysoczanski R., 2002. Initiation of magmatismduring the Cambro-Ordovician Ross Orogeny in southernVictoria Land, Antarctica. Geological Society of AmericaBulletin, 114, 1007-1018.

Angino E.E., Turner M.D. & Zeller E.J., 1962. Reconnaissancegeology of Lower Taylor Valley, Victoria Land, Antarctica.Geological Society of America Bulletin,73, 1553-1562.

Aslund T., 1990. Metamorphism and magmatism of maficintrusives at Dromedary massif, Antarctica. Unpublished MScthesis lodged in the library, University of Otago, Dunedin.

Cape Roberts Project Team, 2000. Studies from the Cape RobertsProject, Ross Sea, Antarctica – Initial Report on CRP-3.Chapter 4 – Petrology. Terra Antartica,7(1/2), 107-131.

Cook Y.A. & Craw D., 2001. Amalgamation of disparate crustalfragments in the Walcott Bay - Foster Glacier area, SouthVictoria Land, Antarctica. New Zealand Journal of Geologyand Geophysics, 44, 403-416.

Cox S.C., 1993. Inter-related plutonism and deformation in SouthVictoria Land, Antarctica. Geological Magazine,130, 1-14.

Cox S.C. & Allibone A.H., 1991. Petrogenesis of orthogneisses inthe Dry Valleys region, South Victoria Land. Antarctic Science,3, 405-417.

Cox S.C. & Allibone A.H., 1995. Naming of igneous andmetamorphic rock units in Antarctica: recommendation by theSCAR Working Group on Geology: discussion. AntarcticScience, 7, 303-306.

Cox S.C., Parkinson D. ., Allibone A.H. & Cooper A.F., 2000.Isotopic character of Cambro-Ordovician plutonism, southernVictoria Land, Antarctica. New Zealand Journal of Geologyand Geophysics, 43, 501-520.

Denton G.H., Armstrong R.L. & Stuiver M., 1970. Late Cenozoicglaciation in Antarctica: the record in the McMurdo Soundregion. Antarctic Journal of the United States, 5, 15-21.

Deutsch S. & Webb P. N., 1964. Sr/Rb dating of basement rockfrom Victoria Land. In: R.J. Adie (ed.),Antarctic Geology.Amsterdam: North-Holland Publishing Co., 557-562.

Ellery S.C., 1989. Lower Wright geology. Unpublished MSc thesislodged in the library, Geology Department, University ofOtago, Dunedin.

J. Forsyth et al.72

Encarnacion J. & Grunow A.M., 1996. Changing magmatic andtectonic styles along the paleo-Pacific margin of Gondwanaand the onset of early Paleozoic magmatism in Antarctica.Tectonics,15, 1325-1341.

Ferrar H.T. 1907. Report on the field geology of the regionexplored during the Discovery Antarctic Expedition, 1901-1904. National Antarctic Expedition 1901-1904. NaturalHistory, 1, 1-100.

Fitzgerald P.G., 1992. The Transantarctic Mountains of southernVictoria land: the application of apatite fission track analysis toa rift shoulder uplift. Tectonics, 11, 634-662.

Fitzgerald P.G., Sandiford M., Barrett P.J. & Gleadow A.J.W., 1986.Asymmetric extension associated with uplift and subsidence inthe Transantarctic Mountains and Ross embayment. Earth andPlanetary Science Letters, 81, 67-78.

Ghent E.D., 1970. Chemistry and mineralogy of the Mt FalconerPluton and associated rocks, lower Taylor Valley, south VictoriaLand, Antarctica. Transactions of the Royal Society of NewZealand, 8, 117-132.

Ghent E.D. & Henderson R.A., 1968. Geology of the Mt FalconerPluton, lower Taylor Valley, south Victoria Land, Antarctica.New Zealand Journal of Geology and Geophysics, 11, 851-880.

Graham I.J. & Palmer K., 1987. New precise Rb-Sr mineral andwhole-rock dates for I-type granitoids from Granite Harbour,South Victoria Land, Antarctica. New Zealand AntarcticRecord, 8, 72-80.

Gunn B.M. & Warren G., 1962. Geology of Victoria Land betweenthe Mawson and Mulock Glaciers, Antarctica. New ZealandGeological Survey Bulletin71.

Haskell T.R., Kennett J.P., Prebble M.W., Smith G. & Willis I.A.G., 1965. The geology of the middle and lower Taylor Valleyof South Victoria Land, Antarctica. Transactions of the RoyalSociety of New Zealand,2, 169-186.

Isaac M.J., Chinn T.J., Edbrooke S.W.& Forsyth P.J., 1996.Geology of the Olympus Range area, southern Victoria Land,Antarctica, scale 1:50 000 Institute of Geological & NuclearSciences geological map 20. Lower Hutt, Institute ofGeological & Nuclear Sciences. 1 sheet + 60p.

Keiller I., 1991. Wright Dikes. Unpublished MSc thesis lodged inthe library, University of Otago, Dunedin.

McDougall I.& Ghent E.D., 1970. K/Ar dates on minerals from theMt. Falconer area, lower Taylor Valley, south Victoria Land,Antarctica. New Zealand Journal of Geology and Geophysics,13, 1026-1029.

McKelvey B.C. & Webb P. N., 1962. Geological investigations insouthern Victoria Land, Antarctica. New Zealand Journal ofGeology and Geophysics, 5, 143-162.

Middlemost, E.A.K., 1994. Naming materials in the magma/igneousrock system. Earth Science Reviews, 37, 215-224.

Mortimer N., Forsyth P.J. & Turnbull I.M., 2002. Reassessment offaults in the Wilson Piedmont Glacier area: implications forage and style of Transantarctic Mountains uplift. In: GambleJ.A., Skinner D.N.B, Henrys S. & Lynch R. (eds). Antarcticaat the close of a Millennium. Proceedings Volume 8thInternational Symposium of Antarctic Earth Sciences. RoyalSociety of New Zealand Bulletin 35. Wellington.

Palmer K. 1987. XRF analyses of granitoids and associated rocks,South Victoria Land, Antarctica. Victoria University ofWellington Antarctic Data Series, 13.

Palmer K., 1990. XRF analyses of granitoids and associated rocks,St Johns Range, south Victoria Land, Antarctica . VictoriaUniversity of Wellington Antarctic Data Series, 15.

Pearce J.A., Harris N.B.W. & Tindle A.G., 1984. Trace elementdiscrimination diagrams for the interpretation of granitic rocks.Journal of Petrology, 25, 956-983.

Pocknall D.T., Chinn T., Sykes R.O. & Skinner D.N.B., 1992.Geology of the Convoy Range area, southern Victoria Land,Antarctica New Zealand. Geological Survey miscellaneous mapseries 19. Wellington, Department of Scientific and IndustrialResearch. 1 sheet and notes.

Powell R.D., Hambrey M.J. & Krissek L.A., 1998. Quaternary andMiocene Glacial and Climatic History of the Cape RobertsDrillsite Region, Antarctica. Terra Antartica,5, 341-351.

Prentice M., Mullins J., Thomas J.-C., Isaac M., Smith S., ChinnT., Bockheim J., Campbell I., Turnbull I., Claridge G., Glidden,S., Routhier M., Heron D., Sidenbladh A., Kleman J., HallamC., 1999. VALMAP: A GIS for Physical Features in theMcMurdo Dry Valleys, Antarctica (abstract). Pp 250. In: D.N.B. Skinner (ed.), 8th International Symposium of AntarcticEarth Sciences, Wellington.

Read S.E., Cooper A.F.& Walker N.W., 2001. Geochemistry and U-Pb geochronology of the Neoproterozoic-Cambrian SkeltonGlacier Alkaline Province, Royal Society Range, TransantarcticMountains, Antarctica. In: D.N.B. Skinner, J. Gamble (ed.), 8thInternational Symposium of Antarctic Earth Sciences,Wellington.

Sandroni S., Talarico F., 2001. Petrography and Provenance ofBasement Clasts and Clast Variability in CRP-3 Drillcore(Victoria Land Basin, Antarctica). Terra Antartica,8, 449-467.

Simpson G. & Aslund T., 1996. Diorite and gabbro of theDromedary mafic complex, South Victoria Land, Antarctica.New Zealand Journal of Geology and Geophysics, 39, 403-414.

Smellie J.L., 2000. Erosional History of the TransantarcticMountains Deduced from Sand Grain Detrital Modes in CRP-2/2A, Victoria Land Basin, Antarctica. Terra Antartica,7, 545-552.

Streckeisen A.L., 1976. To each plutonic rock its proper name.Earth Science Reviews, 12, 1-33.

Streckeisen A.L., LeMaitre R.W., 1979. A chemical approximationto the modal QAPF classification of the igneous rocks. NeuesJahrbuch für Mineralogie Abhardlungen 136,169-206.

Sugden D.E., Summerfield M.A., Denton G.H., Wilch T.I.,McIntosh W.C., Marchant D.R. & Rutford R.H., 1999.Landscape development in the Royal Society Range, southernVictoria Land, Antarctica: stability since the mid-Miocene.Geomorphology, 28, 181-200.

Sun S.S. & McDonough W.F., 1989. Chemical and isotopicsystematics of oceanic basalts: implications for mantlecompositions and processes. Geological Society of LondonSpecial Publication 42, 313-345.

Talarico F. & Sandroni S., 1998. Petrography, Mineral Chemistryand Provenance of Basement Clasts in the CRP-1 Drillcore(Victoria Land Basin, Antarctica). Terra Antartica, 5, 601-610.

Talarico F., Sandroni S., Fielding C. & Atkins C., 2000. Variability,petrography and Provenance of Basement Clasts in Core fromCRP-2/2A, Victoria Land Basin, Antarctica. Terra Antartica, 7,529-544.

Tulloch A.J., 1988. Batholiths, plutons and suites: nomenclature forgranitoid rocks of Westland-Nelson, New Zealand. NewZealand Journal of Geology and Geophysics, 31, 505-509.

Turnbull I.M., Allibone A.H., Forsyth P.J. & Heron D.W., 1994.Geology of the Bull Pass - St Johns Range area, SouthernVictoria Land, Antarctica 1:50 000 Institute of Geological &Nuclear Sciences geological map 14. Wellington, Institute ofGeological & Nuclear Sciences Ltd., 1 sheet + 52p.

Waters A.S., 1993. The Swinford and Harker Plutons: a study inlate-orogenic magmatism in southern Victoria Land, Antarctica.Unpublished MSc thesis lodged in the library, Department ofGeology, University of Otago, Dunedin.

Wilson T.J., 1991. Mesozoic and Cenozoic structural patterns in theTransantarctic Mountains, southern Victoria Land. AntarcticJournal of the United States 1991,31-35.

Wilson T.J., 1994. Structural kinematic studies in the TransantarcticMountains, Southern Victoria Land. Terra Antartica, 1, 537-538.

Wilson T.J., 1995. Cenozoic transtension along the TransantarcticMountains - West Antarctic rift boundary, southern VictoriaLand, Antarctica. Tectonics, 14, 531-545.

Wilson, T. J., 1999. Cenozoic structural segmentation of theTransantarctic Mountains rift flank in southern Victoria Land.Global and Planetary Change, 23, 105-127.

Wu B. & Berg J. H., 1992. Early Paleozoic lamprophyre dikes ofsouthern Victoria Land: geology, petrology and geochemistry.In: Y. Yoshida et al. (ed.), Recent progress in Antarctic EarthScience. Tokyo, TERRAPUB