© Terra Antartica Publication

Terra Antartica 2002, 9(2), 57-72

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

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

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

Received 10 December 2001; accepted in revised form 2 July 2002

Abstract - Previous geological mapping of the inland Dry Valleys area of southern Victoria Land identified a number of 1-10 km-size granitoid plutons, and assigned them to calc-alkaline (DV1a), adakitic (DV1b) and monzonitic (DV2) suites. In the adjacent coastal Wilson Piedmont Glacier area, mapping hitherto consisted of regional reconnaissance or local structural and chemical studies. Our new mapping of this part of the Ross Orogen shows that most Wilson Piedmont granitoid rocks can be assigned to plutons and suites previously recognised in the inland Dry Valleys. We have mapped and clarified the boundaries of the previously-described Bonney, Denton, Avalanche Bay, Gonville and Caius, Swinford, Harker and Brownworth plutons. Some new plutons are recognised, namely Evans, Discovery, Flint and Coleman. DV1a and DV1b plutons 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 more extensive in the Wilson Piedmont than in the Dry Valleys, and their geochemistry suggests they are petrogenetically related to the DV1a granitoid suite.

INTRODUCTION & PREVIOUS WORK al., 1992; Turnbull et al., 1994; Isaac et al., 1996). Detailed work on the Dry Valleys basement granitoids The Wilson Piedmont Glacier lies west of and metasedimentary rocks has been published, using McMurdo Sound, between New Harbour and Granite a conceptual framework of pluton mapping, by Cox Harbour (Fig. 1), and separates the inland Dry Valleys & Allibone (1991), Allibone et al. (1992); Allibone et from the coast. In the 1997/98 Antarctic al. (1993a, 1993b); Cox 1993; and Cox et al. (2000). summer field season, we investigated the rocks in the These workers rationalised previously confused and Wilson Piedmont Glacier area. One of the aims of overlapping nomenclature within granitoid rocks of this work was to confirm or refute, using geological the Dry Valleys, recognised 15 major granitoid mapping, the presence of previously postulated major plutons and many smaller plugs and dikes, and faults which may have been the locus of clarified their intrusive relationships. Transantarctic Mountains uplift. Another aim was to Structural studies of the Wilson Piedmont area, prepare a geological map of basement units and the related to the uplift history of the Transantarctic present paper summarises the results of this basement Mountains, have been made by Fitzgerald et al. mapping. (1986), Fitzgerald (1992), and Wilson (1991, 1994). The fringes of the Wilson Piedmont were visited Results of our own uplift and faulting work are in the early 20th century by geological parties who reported by Mortimer et al. (2002). The Cape Roberts named Granite Harbour and identified several types of offshore drilling programme has also focussed granitoid rocks (e.g. Ferrar, 1907). The first (and attention on the Wilson Piedmont as a possible source only) published geological map of the Wilson area for basement clasts retrieved from Cape Roberts Piedmont area is that of Gunn & Warren (1962). The drillholes (e.g. Talarico & Sandroni, 1998; Smellie, adjacent ice-free Dry Valleys, in contrast, have been 2000; Talarico et al., 2000). visited by many workers, from early reconnaissance Our 1:50 000 geological field data are incorpo- mapping parties (e.g. Allen & Gibson, 1962; rated into the VALMAP digital database held by the McKelvey & Webb, 1962; Haskell et al., 1965) to University of New Hampshire (Prentice et al,. 1999). more detailed studies of individual plutons, dike Although a significant improvement on earlier swarms, metasediments or surficial deposits. Much of published maps of the Wilson Piedmont, the the inland geology has been summarised, and geological mapping is still incomplete and some areas supplemented by new work, in a more recent series have not been thoroughly investigated. Where we of 1:50 000 maps (Allibone et al., 1991; Pocknall et have no field observations, we have used air photo

*Corresponding author ([email protected]) 58 J. Forsyth et al.

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

interpretation, and data and samples collected by (2001), using the term Skelton Group to describe previous workers (where available) to fill gaps. undifferentiated metasediments of the Dry Valleys region. Skelton Group rocks in the Wilson Piedmont REGIONAL GEOLOGY area include marble, calc-silicate and psammite, which are invaded, in many places, by foliation- Basement rocks include late Precambrian parallel sheets of orthogneiss and amphibolite. In the metasediments and a variety of younger granitoid study area, Skelton Group rocks form several plutons. Metasedimentary rocks have not been a geographic belts, separated by granitoid plutons particular focus of our study and we follow the recent (Fig. 2). The dominant foliation almost always strikes revision of high-level terminology by Cook & Craw SE, with variable dips. Plutonic Rocks from the Cape Roberts Hinterland 59

Fig. 2 – Inferred basement geology of southern Victoria Land, updated from Allibone et al. (1993a,b) and Cox et al. (2000) with new information 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.

Plutonic and meta-plutonic rocks - Granite bodies: pressures suggested by Allibone et al. (1993a) Harbour Intrusive Complex of Gunn & Warren (1962) reduce from >5 to <2 kbar over the duration of - make up at least two-thirds of the basement in the emplacement. area mapped. Orthogneiss bodies are the oldest rocks The basement rocks are truncated by the in this complex. Biotite orthogneiss and biotite- regionally extensive Kukri Erosion Surface, and hornblende orthogneiss are recognised (Cox & overlain by late Palaeozoic to Mesozoic Beacon Allibone, 1991; Allibone et al., 1993b), and in Supergroup sediments which form the highest parts of addition, mafic orthogneisses are mapped in the St the Transantarctic Mountains to the west of the Dry Johns Range (Turnbull et al., 1994). Foliated and Valleys. Both Beacon and basement rocks are unfoliated granitoid plutons dominate and are intruded by dolerite sills of the Ferrar Supergroup. discussed in more detail below. Gabbro and diorite Remnants of Beacon Supergroup overlie basement bodies, such as Delta Diorite (Gunn & Warren, 1962) rocks in the Gonville and Caius Range and on the appear to be more common around the Wilson east side of the Upper Newall Glacier – sections are Piedmont than in the Dry Valleys. Mafic and felsic poorly exposed and generally <5 m thick, beneath dikes of the Vanda dike swarms cut, and therefore Ferrar Dolerite sills. Ferrar Dolerite occurs on the post-date, most basement units. Older plutons were tops of most of the main ranges, and also occurs on apparently intruded at greater depths than younger the Ross Sea coast at Kolich Point (Fitzgerald, 1992). 60 J. Forsyth et al.

The Dry Valleys region, including the Wilson 100 m - 1 km scale with Skelton Group meta- Piedmont, was uplifted to form part of the sediments, with which they have been metamorphosed Transantarctic Mountains chain in the early Cenozoic, to amphibolite facies. Contact relationships between and later glacially eroded to form the present orthogneisses and adjacent units are commonly topography by the middle Miocene (Sugden et al., obscured by ice and scree in the Wilson Piedmont 1999). Large parts of the inland area, and many of area. Our scale of mapping did not allow the the exposures in the Wilson Piedmont region, are orthogneiss bodies to be mapped in detail, but some mantled by surficial deposits (e.g. Haskell et al., general observations are made below. 1965; Denton et al., 1970). Biotite orthogneiss with minor hornblende-biotite orthogneiss forms most outcrops along the coast, from Gneiss Point (e.g. P62348) to Cape Roberts. PLUTONS AND FIELD RELATIONSHIPS Orthogneisses are less common inland but biotite Major intrusive bodies (plutons, sheets and dike orthogneiss also forms much of the north side of the swarms) are described in approximate older to Debenham Glacier (P62103), and small outcrops on younger age sequence, as inferred from intrusive the south side. Biotite orthogneiss is also associated relationships seen in the field and supplemented by with Skelton Group around the Clark Glacier (this U-Pb zircon radiometric ages where available. The study) and various gneisses occur on the north ridge relationship of each pluton to adjacent units is of Mt Newall (Gunn & Warren, 1962; Palmer, 1987) described. Granitoid rock names follow Streckeisen and within Skelton Group at Mt Falconer (Ghent & (1976); pluton and suite nomenclature follows Henderson, 1968). These outcrops form the SE Tulloch (1988), Allibone et al. (1993a), Cox & continuation of a metasediment/orthogneiss belt Allibone (1995) and Cox et al. (2000). In the field mapped in the St Johns Range (Turnbull et al., 1994). we were able to map the areal extent of Generally biotite-hornblende orthogneiss is much rarer mineralogically and texturally distinct granitoid than biotite orthogneiss in the Wilson Piedmont area. plutons (Fig. 2). Petrographic and geochemical data Mafic orthogneisses occur in bodies generally too supplement field observations and allow the plutonic small to show in figure 2; they are discussed rocks to be classified in terms of previously separately below under “minor gabbroic and dioritic established Dry Valleys I-type igneous suites: DV1a intrusions”. (calc-alkaline), DV1b (adakitic) and DV2 (monzonitic) (Allibone et al., 1993b, Cox et al., BONNEY, EVANS, DENTON AND DISCOVERY PLUTONS 2000). Geochemistry and suite interpretation of the plutons are mostly discussed in a separate section These four granodiorite to monzodiorite plutons though some reference is made to previous contain hornblende, biotite, ± clinopyroxene, and petrological work in the descriptive sections where characteristic K-feldspar megacrysts which are appropriate. Sample numbers prefixed “P” refer to commonly deformed into augen (Fig. 3). Com- specimens held in the National Petrology Reference positional banding of mafic and felsic schlieren, and Collection of the Institute of Geological & Nuclear aligned megacrysts, are variably developed and give a Sciences. streaky, swirly or foliated effect. Foliations tend to strike NW and SW. Rafts and inclusions of fine- FELSIC ORTHOGNEISSES grained mafic (amphibolitic) material, orthogneiss and Skelton Group metasediments are generally present Felsic orthogneiss bodies are strongly deformed, especially near pluton margins. Orbicular textures are disrupted granitoid plutons. They are intercalated on a developed locally.

Fig. 3 – Foliated megacrystic biotite granodiorite of the Evans Pluton, typical of the Bonney “family” of granitoids. West end of Staeffler Ridge, Greenwood Glacier. Plutonic Rocks from the Cape Roberts Hinterland 61

Each pluton intrudes the Skelton Group and 1996). Each of these bodies is separated from the associated orthogneisses. U-Pb zircon dating of main Bonney Pluton by a belt of older metasediment Bonney Pluton gives ages of 505±2 Ma (Encarnacion and orthogneiss, and is regarded as a separate & Grunow, 1996; Cox et al., 2000) and 499±6 Ma intrusion within the same calc-alkaline magmatic (Allibone & Wysoczanski, 2002). An Rb-Sr biotite event. age of 476± 15 Ma (Deutsch & Webb, 1964) for the Discovery Pluton, like other such mineral isochron FLINT PLUTON (new name) ages in southern Victoria Land, is probably a cooling age rather than an emplacement age (Allibone et al., Much of Flint Ridge, between the lower Newall 1993a). and Commonwealth glaciers, is formed of dioritic Bonney Pluton has previously been mapped in gneiss (e.g. P62321), here named Flint Pluton. The the Dry Valleys area (Allibone et al., 1991; Allibone probable northern margin of the pluton is located et al., 1993a; Turnbull et al., 1994) and extended SE along the southeastern side of the lower Newall across the Royal Society Range to the Skelton Glacier Glacier, but it was not well defined in this study due (Cox et al., 2000). The eastern edge of this pluton to lack of access. The western margin, probably lies in the Wright Valley, at the head of the Newall against Denton Pluton, was identified in only one Glacier and in the Taylor Valley. Rafts of Bonney outcrop. Pluton occur within a complex Ferrar Dolerite Flint Pluton intrudes Skelton Group metasediments intrusion at the upper Newall Glacier. Evans Pluton and accompanying biotite orthogneiss on the southeast (new name, from Mt Evans) extends from the end of Flint Ridge, where it is cut by garnetiferous southern end of Killer Ridge to the Greenwood granite dikes (probably from the nearby Coleman Glacier (e.g. P62302, 62306, 62307), and a deformed Pluton). Elsewhere it is intruded by variably foliated megacrystic granodiorite at the west end of King Pin but much less deformed diorite dikes, by biotite (P62346) is probably a part of Evans Pluton detached granite dikes and small stocks, and by some Vanda by the later intrusion of Brownworth Pluton. mafic and felsic dikes. Flint Pluton (including dikes Discovery Pluton (new name, from Discovery Bluff) of this material which intrude Skelton Group) is is exposed around Granite Harbour at Discovery variably foliated, with foliation striking generally SW, Bluff, Finger Point, Cuff Cape and the Kar Plateau. and in places strongly lineated with augen of feldspar Denton Pluton (Ellery, 1989; Allibone et al., 1993a; in a matrix of biotite hornblende gneiss (Fig. 4). P62324) extends from Robertson Ridge southwards In thin section, e.g. P62320 and 62321, the across Lower Wright Valley. There is an area of poor gneiss has up to 40% mafic minerals (biotite, exposure beneath the screes of the Lower Victoria hornblende and augite). Quartz stringers and Valley (unvisited by us) where Denton Pluton could plagioclase (oligoclase-andesine) megacrysts occur in merge with Evans Pluton along the eastern margin of many samples. Significant titanite and minor allanite Packard Pluton (see Fig. 2). occur as accessories, with secondary epidote. A garnet Bonney, Evans, Denton and Discovery plutons amphibolite from the lower Newall Glacier (P62322) belong to a suite of similar genesis, age, appearance may be an altered hornblende diorite or possibly a and deformation history, which includes the piece of relict Skelton Group material. Unlike the previously-mapped Wheeler and Cavendish plutons majority of the other mafic bodies examined in this (Allibone et al., 1993a; Turnbull et al., 1994) and un- study, most of the Flint Pluton does not show named foliated granodiorites in the Wright Valley significant alteration of hornblende to actinolite and/or North Fork and the Balham Valley (Isaac et al., chlorite.

Fig. 4 – Flint Pluton dioritic gneiss with strong linear fabric and stretched (and younger, undeformed) felsic veins. Southern end of Flint Ridge. 62 J. Forsyth et al.

Near the probable northern margin in the lower limit, north of the Lower Victoria Glacier terminus, Newall Glacier, Flint Pluton dioritic gneiss appears has not been seen; as its mapped position is based on interlayered with Skelton Group, and based on this distant observations and air photo interpretation, it and its strongly deformed nature it is placed among should be considered approximate only. the earliest intrusive rocks of the Wilson Piedmont Turnbull et al. (1994) and Cox et al. (2000) area. Smaller bodies of similar appearance and allocated the Packard Pluton to a very early intrusive mineralogy occur within mafic orthogneisses on the phase, as it was known to intrude only Skelton Group St Johns Range, and are mapped as early dioritic and and orthogneiss. However, we have observed that it is gabbroic intrusives (Turnbull et al., 1994). also younger than Denton and Evans plutons. Packard Pluton and its contact zones are cut by Vanda felsic GONVILLE AND CAIUS PLUTON dikes, and less abundant mafic dikes. (Gunn & Warren, 1962) Packard Pluton on Robertson Ridge consists of massive fine to medium grained hornblende biotite The western part of the Gonville and Caius Range diorite. Plagioclase is highly altered, and extensive is underlain by a massive medium to coarse grained secondary chlorite, actinolite, muscovite, epidote, equigranular hornblende/biotite granite (Gunn & calcite and sericite occur. Rare pods of coarse Warren, 1962; P62288, 68016). This pluton has now hornblende pegmatite occur near the western contact been mapped from Granite Harbour (at the Devil’s (cf. Turnbull et al,. 1994). The pluton is unfoliated, Punchbowl, Finger Point and Cuff Cape) in the north but in places exhibits primary igneous layering, with to the southern end of Killer Ridge. Along its eastern graded felsic to mafic banding 5-10 cm across. This margin it intrudes Discovery Pluton and orthogneiss. texture was also described from the type area of the Its NW margin, probably against Suess Pluton, was Packard Pluton by Turnbull et al. (1994). In the not mapped in this study. The SW margin lies against central part of the Robertson Ridge exposure, slightly Swinford Pluton – the contact occurs in cliffs at the younger, paler, more felsic hornblende biotite diorite NW corner of Killer Ridge opposite Queer Mountain, with acicular hornblende sprays cuts the host diorite. where Gonville and Caius Pluton forms the upper part Whether this is a late phase of the Packard, or a of the face – but the contact relations are unknown separate intrusive phase, is unknown. because of inaccessibility. At the southern end, Gonville and Caius Pluton intrudes Evans Pluton. COLEMAN PLUTON (new name) Marginal dikes, probably related to this pluton, intrude diorite at the south end of Killer Ridge. This heterogeneous pluton occurs around Mt Outcrops show characteristic mafic clots and Coleman, the type area, on the north side of the microgranite and porphyritic inclusions in an lower Taylor Valley. Despite lying less than 10 km otherwise homogeneous pink or grey rock. The from the Mt Falconer Pluton, it has not been granite becomes finer grained, and inclusions become described before. The western, southern and more common, near its southern margin at the top of northeastern contacts of the pluton intrude Skelton Second Facet. Petrographically the assemblage is K- Group metasediments and associated orthogneisses; feldspar, quartz, plagioclase (albite/oligoclase), biotite, the other contacts are inferred to lie beneath the ice amphibole (hornblende), zircon, apatite, with of the southern Wilson Piedmont Glacier. Coleman (secondary) magnetite and epidote. It is distinct from Pluton is intruded by a Vanda Dike swarm so neighbouring plutons chemically, petrographically and numerous that dikes (mainly mafic) make up over in outcrop, differing from the adjacent Suess and half the outcrop in places. On Flint Ridge, dikes of Harker plutons in containing significant hornblende as garnetiferous granitoid with flow-banded texture, well as biotite, and from the Swinford Pluton in being inferred to be from Coleman Pluton, cut Skelton equigranular. Group, orthogneiss and Flint Pluton. Gonville and Caius Pluton lacks “Vanda-type” Notable outcrop features of this pluton are the felsic porphyry enclaves and is intruded by many banded and folded texture (Fig. 5), and abundant pale Vanda dikes: it predates emplacement of the major red garnets (e.g. P62325). The texture is inferred to dike swarms. The lack of foliation and deformation be a flow foliation, as it can be clearly seen in suggest that it post-dates the older, Bonney-like outcrops where Coleman granite cuts across an older, deformed megacrystic granite suite. unfoliated granodiorite, and also cuts across foliation in Skelton Group metasediments and associated PACKARD PLUTON orthogneisses. No consistent orientation is discernible (Turnbull et al., 1994) from foliation measurements. The rock when fresh is white to pale grey, and weathers to a fawn colour. The Packard Pluton, consisting of gabbro and Coleman Pluton is relatively siliceous (>30 diorite, was first described from the lower Packard volume % quartz in thin section) and has the Glacier in the southern St Johns Range by Turnbull et composition of a monzogranite. Minor biotite is the al. (1994). We consider that outcrops of diorite at the only mafic phase. Garnets (unanalysed) are up to eastern end of Robertson Ridge, north of the Clark 7 mm long and anhedral in thin section. Garnet Glacier, are part of the Packard Pluton. Its eastern pegmatites parallel the swirled bands. Plutonic Rocks from the Cape Roberts Hinterland 63

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

VANDA DIKE SWARMS intrudes Discovery Pluton at Avalanche Bay, where (after McKelvey & Webb, 1962) the western margin is marked by a dramatic increase in enclaves and rafts of amphibolite, orthogneiss and Several major swarms of mafic and felsic dikes Skelton Group metasediments. The contact with have been mapped previously in adjacent areas Discovery Pluton on the east side of Haystack (Allibone et al., 1991; Turnbull et al., 1994), and Mountain is possibly a fault and is marked by brick- eastward continuations of these were mapped in the red, hydrothermally altered and shattered rock. Wilson Piedmont area. The three main swarms, all However we infer that little offset has occurred on oriented SW-NE, occur in the Gonville and Caius this fault, as the pluton-margin facies, rich in rafts of Range and as far north as Granite Harbour; in the older materials, is present nearby. eastern Olympus Range and Lower Victoria Valley; Avalanche Bay Pluton is dominated by medium and from the Lower Taylor Valley to Hogback Hill. grained, massive quartz monzodiorite, with slightly From the eastern Asgard Range to Doorly Spur, (e.g. megacrystic feldspars (Fig. 6). Scattered inclusions of P62336) and near the Packard Glacier, lesser swarms partly-assimilated microgranitoid, and dark porphyry have the same orientation but fewer dikes. Dikes cut resembling Vanda dike material, occur along with all the granitoid and gneiss bodies listed above, and pink aplitic dikes. No foliation is apparent in outcrop locally (e.g. Robertson Ridge; Hjorth-Hogback Hill- or thin section. The assemblage is quartz, plagioclase Mt Coleman area) about 30% of rock volume is dike and K-feldspar, with hornblende and biotite about material. Vanda dikes become rare or absent in the equal, and with accessory zircon, magnetite and younger plutons described below. allanite. Secondary chlorite and epidote replace A field subdivision of “mafic” and “felsic” dikes, biotite. used during mapping, follows that of earlier authors The Avalanche Bay Pluton is undeformed and is (e.g. Allibone et al., 1993a). Detailed descriptions of cut by only a few mafic Vanda dikes near its western geochemistry and associations at Mt Loke, in the contact – features suggesting that it is a relatively eastern Asgard Range, are given by Keiller (1991). At young rock in the intrusive sequence. It has a U-Pb Mt Falconer, Ghent & Henderson (1968) report 4 zircon age of 498±4 Ma (Encarnacion & Grunow, types of dikes, of which 3 (“mafic”, “porphyritic 1996) which, as expected, is significantly older than granophyre” and “camptonite”) conform to published Rb-Sr mineral ages of 452±6Ma and 459± 4 Ma descriptions, and our own observations, of dikes in (Allibone et al., 1993a). the Vanda suite. The first two types clearly predate Mt Falconer pluton, while the last type postdates it. SWINFORD PLUTON (Waters, 1993) Vanda dikes have been dated in the eastern Wright Valley (U-Pb zircon age 484± 7 Ma (Encarnacion & Initially named “Swindon Granite” by Palmer Grunow 1996) and Taylor Valley (K-Ar age 458 (1990), this pluton was further mapped in the eastern ±20Ma (Angino et al., 1962). St Johns Range by Waters (1993) and Turnbull et al. (1994). Our study has confirmed that this rock also AVALANCHE BAY PLUTON occurs further east, on Killer Ridge (P68014, 68015). (Graham & Palmer, 1987) The two occurrences are separated by the Harker Pluton (see below), from which the Swinford This pluton forms the coastal cliffs on the south granitoid is texturally and geochemically distinct. side of Granite Harbour from the eastern end of The Swinford Pluton is typically coarse grained Couloir Cliffs west to Avalanche Bay, and also crops with alkali feldspar megacrysts, up to 2 cm long, in a out on the eastern side of Haystack Mountain. It matrix of plagioclase, quartz, alkali feldspar and 64 J. Forsyth et al.

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

minor biotite with accessory hornblende. Analyses BROWNWORTH PLUTON (Ellery, 1989) show granitic to quartz monzodioritic compositions. It is poorer in silica, and more mafic and calcic, than Brownworth Pluton was mapped by Ellery (1989) the neighbouring Harker Pluton, and field relations in two small areas on either side of the lower Wright suggest that it is older (Turnbull et al., 1994). Valley. Our mapping extends this pluton from Blessing Bluff and Doorly Spur (e.g. P62293) in the HARKER PLUTON (Palmer, 1990; Waters, 1993) north, to King Pin and the lower Newall Glacier in the south. Following studies by Palmer (1990), this pluton This pluton is readily identified by its distinctive, was further mapped in the eastern St Johns Range by abundant pink K-feldspar megacrysts, up to 4 cm Waters (1993) and Turnbull et al. (1994). The Wilson long; characteristic xenoliths of porphyry (resembling Piedmont study has extended this pluton as far south Vanda mafic dike material); and lack of foliation as Pond Peak. Harker Pluton intrudes Skelton Group (Fig.7). Although a weak flow fabric of aligned metasediments, orthogneisses and Wheeler Pluton megacrysts and inclusions was noted at Blessing (Turnbull et al., 1994), and Evans Pluton (this study). Bluff, at most outcrops the only measurable structures It post-dates the majority of Vanda dikes, but both are widely-spaced joints. Some metamorphic (Skelton mafic and felsic dikes intrude in places. Group) xenoliths were recorded by Ellery (1989) in The pluton comprises mainly coarse grained, the Lower Wright Valley, and inclusions of homogeneous equigranular granite. In colour the rock microgranite and biotite-rich mafic clots also occur. is generally grey but orthoclase is cloudy and pink in Brownworth Pluton intrudes Skelton Group at its many places. Biotite is the main mafic mineral, with western edge, Evans Pluton at its northern margin, very minor amphibole, accessory allanite, magnetite, and un-named hornblende biotite diorite at its zircon and apatite, and secondary muscovite. Samples southern margin. On the southeastern margin, at the from our study (P62101, 62329), and from Gunn & western end of King Pin, megacrystic dikes from the Warren (1962), are syenogranites and monzogranites. Brownworth Pluton intrude a deformed megacrystic

Fig. 7 – Megacrystic Brownworth quartz monzonite, with an enclave of “Vanda”-like felsic porphyry. Eastern end of Staeffler Ridge; pencil for scale. Plutonic Rocks from the Cape Roberts Hinterland 65 granite gneiss which is most likely a detached gabbro east of Staeffler Ridge. In the Wilson fragment of Evans Pluton. A few Vanda mafic dikes Piedmont area, dioritic and gabbroic bodies are intrude Brownworth Pluton (e.g. P62336). generally small plugs or bosses; apart from the Flint Brownworth Pluton consists predominantly of and Packard bodies they are too small to be termed quartz monzonite with the assemblage K-feldspar- plutons. From their intrusive relations and degree of plagioclase-quartz-biotite-hornblende, and accessory foliation, we conclude that most of these mafic rocks titanite, allanite and zircon. Secondary chlorite and were emplaced relatively early in the plutonic history epidote replace biotite. Allibone et al., (1993b) place of the area, but none have been dated. It is not it in DV2 suite on the basis of field relations, textures uncommon for igneous pyroxene and hornblende to and the characteristic porphyritic xenoliths. be replaced by actinolite and/or chlorite in these rocks. MT FALCONER PLUTON The Killer Ridge outcrops consist of medium (Ghent & Henderson, 1968; Ghent, 1970) grained, unfoliated, hornblende biotite diorite which

2 forms the southernmost tip of Killer Ridge, up to a This small (10 km ) pluton crops out only at Mt height of 200 m above the junction of the Miller and Falconer on the north side of the Lower Taylor Crisp glaciers (see also Gunn & Warren, 1962, p. 97). Valley. It is surrounded by and intrudes Skelton The diorite intrudes megacrystic Evans Pluton Group metasediments, orthogneiss and diorite. The granodiorite, and younger undeformed Harker Pluton pluton is strikingly discordant to the structural trends granite, both of which occur as rafts within the of the metasediments and gneiss, and truncates mafic diorite. Rare pegmatitic phases of diorite have and granophyric Vanda dikes. It shows little evidence hornblende laths up to 3 cm long. This diorite, along of post-emplacement deformation or metamorphism. with the host granitoids, is cut by Vanda mafic dikes. Most of the pluton consists of homogeneous The western end of Hanson Ridge, including the medium to coarse grained biotite-hornblende quartz summit trig point, is composed of up to 75% monzonite with the assemblage K-feldspar> undeformed diorite, clearly intruding interlayered plagioclase, biotite>hornblende, accessory allanite, Skelton Group metasediments and biotite orthogneiss titanite, apatite, zircon and ilmenite (Ghent & which form the bulk of the ridge. The diorite Henderson, 1968). Near the margins, grain size (P62313) consists of pyroxene (e.g. augite), biotite, decreases in places, and elongate, oriented xenoliths hornblende and plagioclase, with minor quartz and K- and schlieren occur. Breccia zones mark the contact feldspar. It is weakly foliated near contacts, generally with an adjacent dioritic body. Pink siliceous dikes, fine to medium grained, and cut by pale grey biotite thought to be late-stage features, intrude Mt Falconer granodiorite dikes. Further east on Hanson Ridge, Pluton and country rock only near the pluton margins. Skelton Group encloses foliation-parallel dioritic to A few Vanda mafic dikes intrude Mt Falconer Pluton. gabbroic orthogneisses, which are texturally distinct K-Ar biotite ages of 451-461 Ma are a minimum, from the undeformed diorite. but because of the shallow emplacement (at not On the ridge east of the main mass of Staeffler greater than 10 km), cooling was probably swift and Ridge is a spectacular very coarse grained gabbro the actual intrusion age may not be much older with interlocking pyroxene crystals up to 2 cm long (McDougall & Ghent, 1970). These ages, the (P62292). The gabbro is finer grained and more felsic “discordant” field relations and the lack of on the margins, and is cut by microgranite and Vanda deformation are consistent with other young DV2 mafic dikes. Dikes of diorite off the gabbroic body plutons in the Dry Valleys area. intrude Skelton Group metasediments, and megacrystic gabbro also forms a dike on the eastern MINOR GABBROIC-DIORITIC INTRUSIONS end of Staeffler Ridge. The Staeffler Ridge gabbro Gunn & Warren (1962) described “all intrusions of closely resembles an elongated concordant gabbro post-tectonic hornblende-andesine diorite found in body mapped in the Clare Range by Turnbull et al. Victoria Land” as Delta Diorite (from Delta Bluff on (1994). the Skelton Glacier). Diorite bodies in the lower Diorite dikes, a boss, and possible sills intrude Wright Valley have been described by Keiller (1991) Denton Pluton on the south side of the lower Wright and Ellery (1989), and at Mt Falconer “diorite Valley (Ellery, 1989; Keiller, 1991). The diorite dikes hybrid” rocks are described by Ghent & Henderson trend east-west, compared with the average NE strike (1968). Aslund (1990) and Simpson & Aslund (1996) of Vanda dikes, and pre-date the Vanda Dike swarm have summarised the petrology of much more (Keiller, 1991). These diorites are similar to diorites extensive mafic bodies at Mt Dromedary in the Royal at Gneiss Point, where hornblende-biotite quartz Society Range, 90 km south of the Wilson Piedmont. monzodiorite forms dikes up to 20 m thick, intruding We revisited diorite outcrops at Gneiss Point, Mt orthogneisses and Skelton Group metasediments. In Newall and Killer Ridge which were previously coastal cliffs 4 km west of Gneiss Point, massive described by Gunn & Warren (1962), and discovered hornblende diorite (P62295) forms a sill up to 50 m new diorites on King Pin and Hanson Ridge, and a thick within marble and amphibolite schist (Gunn & 66 J. Forsyth et al.

Warren, 1962). Although appearing massive in hand the plutonic rocks into two broad compositional specimen, in thin section the diorite is weakly groups: granitic (SiO2 >60wt%) and gabbroic (SiO2 foliated. <60wt%), but acknowledge that there is probably a Much of King Pin nunatak is composed of petrologic continuum between them (see below). variably foliated biotite-hornblende-pyroxene diorite Ferrar Dolerite samples, and a mafic Vanda Dike (P62347). Pyroxene was formerly the dominant mafic sample, are included for comparison with the phase but has been partly replaced by hornblende. gabbroic group. Vanda dike chemistry is not The diorite contains rafts and inclusions of Skelton considered in detail here, except to note that a variety Group, gneissic megacrystic granite, and deformed of compositions is recorded (Keiller, 1991; Wu & and strained amphibolite, in places forming a complex Berg, 1992); the suite is characterised in general intrusion/injection breccia within the diorite. At the terms by high-K calc-alkaline chemistry, probably western end of the King Pin, the diorite clearly related to the monzonitic DV2 suite of Allibone et al. intrudes mafic megacrystic orthogneiss, probably (1993b). Skelton Group paragneisses are included in Evans Pluton. The diorite is cut by undeformed table 1 for comparison with orthogneiss samples. granitoid dikes, some megacrystic, which may Plutonic rocks in the study area have a wide range originate from the adjacent Brownworth Pluton. of silica contents (Fig. 8A, B). Ferrar Dolerite In the Decker Glacier area, diorite occurs as rafts samples have expectedly low total alkali contents and within Brownworth Pluton. Nearby, around the lower a Vanda dike is significantly more alkaline (Fig. 8A). Newall Glacier (possibly the “east ridge of Mt Apart from these rocks, no extremes of Na2O and Newall” locality of Gunn & Warren, 1962), variably K2O content are seen in the dataset, and most foliated monzodiorite containing hornblende, pyroxene samples straddle the alkaline/subalkaline dividing line and biotite may be part of the Flint Pluton or a on the total alkalies-silica diagram (Fig. 8A). younger intrusion. A diorite body is intruded by the GRANITIC ROCKS Mt Falconer Pluton (Ghent & Henderson, 1968). Diorite and related “hybrid” rocks range in We have attempted to classify the granitic rocks in composition from hornblende-biotite (± pyroxene) terms of the three I-type Dry Valley granitoid suites diorite, through quartz monzonite to pyroxene-bearing proposed by Allibone et al. (1993b). Rocks from the quartz syenite. The amounts of quartz and alkali calc-alkaline DV1a, adakitic DV1b and monzonitic feldspar vary irregularly over short distances, a DV2 suites tend to converge in composition at high characteristic also noted in most of the dioritic rocks silica contents, but the binary diagrams used to we have examined. discriminate these suites (Figs. 8A-E) can be used with some success. In most cases our geochemical MINOR GRANITIC INTRUSIONS results are consistent with field and petrographic observations. It is clear that none of our sampled During this study we observed, sampled and granitoids have the distinctively high Nb+Y contents analysed several small granitoid bodies. Their extent of the Glee Intrusives and Skelton Glacier Alkaline was not mapped because of poor exposure and/or Province of the Royal Society Range (Read et al., small size, and they are not shown on figure 2. 2001) (Fig. 8F). Among these are two small (1-2 km) bosses of In the Wilson Piedmont area the Denton and biotite-augite quartz monzodiorite, one unfoliated and Evans plutons are grouped within the DV1a suite. intruded by (possibly part of) the Coleman Pluton DV1b suite rocks include the undifferentiated “early” west of Mt Coleman (P62326), the other weakly biotite orthogneisses that intrude Skelton Group, and foliated and intruding Flint Pluton in the central part the heterogeneous Mt Coleman garnet granite. The of Flint Ridge. Another small, fine grained biotite- small monzodiorite boss near Mt Coleman (P62326) bearing granitoid intrusion in the upper Newall has high Sr, Al O , Na O and low K O, Rb, Y, which Glacier is not seen in contact with any other units 2 3 2 2 are characteristic of the DV1b suite (Cox & Allibone (contacts are concealed under Ferrar dolerite or ice), 1991; Allibone et al., 1993b). The biotite ortho- but must intrude Bonney Pluton. It may be related to gneisses and samples from near Mt Coleman are the the nearby Valhalla Pluton. only DV1b suite rocks recorded in the Wilson Piedmont area. In contrast, several large plutons GEOCHEMISTRY & SUITE INTERPRETATION (Valhalla, St Johns, Suess) of this composition occur further inland. New major and trace element analyses of Wilson DV2 suite plutons include Brownworth, Harker Piedmont rocks are presented in table 1 and and Swinford. Analyses by Palmer (1990), Waters illustrated in figures 8 and 9. These supplement the (1993) and this study reveal metaluminous and already large published database of Southern Victoria sometimes strongly fractionated rocks with highly Land intrusive rocks (Palmer 1987, 1990; Allibone et evolved syenogranite compositions. Waters (1993) al., 1993b; Cox et al., 2000). assigned both Harker and Swinford plutons to the For convenience of discussion we have divided DV2 suite, noting that minor differences between Plutonic Rocks from the Cape Roberts Hinterland 67 sample was in situ (Y), not (N), almost (A) or unknown – XRF major and trace element data. Analyst John Hunt, Spectrachem Analytical, Wellington. Third column from left shows whether Wellington. Analytical, Analyst John Hunt, Spectrachem – XRF major and trace element data. Tab. 1 Tab. (U). 68 J. Forsyth et al. these rocks and the suite definition of Allibone et al., affinity, a result that is also supported by the presence (1993b) are the result of the highly evolved of hornblende. We provisionally regard the Gonville chemistry. & Caius Pluton as a late, unfoliated, evolved member Two plutons give ambiguous and/or problematic of the DV1a suite, similar to the Catspaw Pluton of suite correlations in terms of field relations, Allibone et al. (1993b) (Fig. 2). petrography and geochemistry. As mentioned above, Taken at face value, the 498±4 Ma U-Pb zircon the field evidence from the Gonville and Caius Pluton age for the Avalanche Bay Pluton is closer to the suggests that it pre-dates Vanda dike swarms, seeming DV1a suite (c. 500 Ma) than the younger Vanda dikes to rule out a DV2 correlation. The geochemistry of (484±7 Ma U-Pb zircon age of a dike from the our two and Palmer’s (1987) five samples of Gonville eastern Taylor Valley; Encarnacion & Grunow 1996). & Caius Pluton suggest either a DV1a or DV2 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 previously proposed 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 Al2O3. E. SiO2 vs Sr. 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 trace compositions (such as reported for the Swinford, element analyses of this pluton (Palmer, 1987; Fig. 8) Brownworth, Orestes and Harker Plutons by Allibone reveal monzonitic trends and element ratios and et al., 1993b); or a modification of the suite definition concentrations much more suggestive of a DV2 and/or approach to SVL granitoid classification may affinity than DV1. We provisionally propose a DV2 be required. Based on available evidence, figure 10 correlation of this pluton, noting that the intrusive age summarises the inferred age and composition range of Vanda dikes is extremely poorly constrained, relationships between the granitic plutons of the and that there is no petrogenetic reason why DV1 and Wilson Piedmont area. DV2 suites cannot have overlapped in time. These problems with the Gonville & Caius and GABBROIC ROCKS Avalanche Bay Plutons may be due to several causes. Our sampling may have been inadequate to properly Seven analyses of variably foliated and characterise the plutons; the plutons may have hybrid metamorphosed gabbroic and dioritic rocks (exclusive of 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 the mafic plutonic rocks are neither coarser grained equivalents of the Ferrar Dolerite, nor of the Vanda dikes. On a multi-element plot, appropriate for rocks of basaltic composition (Fig. 9A), they arguably form a coherent suite although P62295 and P62300 have somewhat higher Th, La, Ce and Zr than the other mafic rocks. The Wilson Piedmont mafic rocks do not obviously resemble Glee tholeiitic or alkaline mafic suites reported from the Royal Society Range to the south (Fig. 9B); specifically our rocks have higher Rb, Th, Ba, K and Zr contents than the tholeiitic suites reported by Aslund (1990) and lower Ti and Nb and higher K and Sr than Dromedary alkaline rocks. A comparison of multi-element plots of the Wilson Piedmont gabbroic-dioritic rocks with Dry Valleys granitoid suites (Fig. 9C) reveals a possible petrogenetic 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 be expected if the DV1a granitoids were more fractionated and/or evolved compositions related to the gabbroids. A rigorous petrogenetic treatment is beyond the scope of this field-based paper, but both Allibone et al. (1993a) and Cox et al. (2000) report rare mafic compositions (e.g. from Bonney Pluton) of DV1a suite rocks. We raise this correlation between mafic and DV1a felsic suites as a tentative hypothesis that should be more rigorously tested by radiometric dating and trace element and isotopic data and modelling.

STRUCTURE

The Wilson Piedmont Glacier region can be regarded as the eastern side of the Dry Valleys Fig. 9 – Multi-element diagrams normalised to primitive mantle structural block, contiguous with previously-mapped values of Sun & McDonough (1989). A. seven gabbroic-dioritic areas to the west. Skelton Group metasediments and plutonic rocks (<60wt% SiO2) from the Wilson Piedmont area (new associated orthogneisses form several NNW-trending data, this study). B. fields of gabbroic suites of southern Victoria belts which, although interrupted by younger plutons, Land (data of present study, Aslund, 1990). C. fields of granitic suites of Dry Valleys area (data of Allibone et al., 1993b; Cox et can be traced from the Ferrar Glacier as far north as al., 2000). the Convoy Range (Fig. 2). The oldest DV1a mega- 70 J. Forsyth et al.

Fig. 10 – Schematic drawing of the inferred relative timing of igneous intrusions in the Wilson Piedmont area, from field relationships, U-Pb geochronology, and suite correlations. Age is constrained by only six U-Pb zircon dates: Vanda dike 484±7 Ma, Avalanche Bay 498±4 Ma, Bonney 505±2 Ma (Encarnacion & Grunow 1996), Valhalla 488±2 Ma (Cox et al., 2000) and two orthogneisses 516±10 Ma and 531±10 Ma (Allibone & Wysoczanski, 2002).

crystic plutons such as Bonney, Wheeler, Denton and series of drillholes off Cape Roberts (CRP-1, Evans also tend to be elongated NNW or NW. CRP-2/2A and CRP-3), and is considered to be a Included in this group is an un-named deformed likely source for sediments cored in the Cape Roberts granodiorite in the Balham Valley (Isaac et al., 1996). holes. Petrographic comparisons between clasts from Of the DV1b plutons, some (e.g. Hedley, St Johns) the drillholes and rocks exposed inland support a share the NW trend but others (e.g. Valhalla, local provenance for basement clasts, and inferences Coleman) appear to be more structurally discordant. from sand detrital modes generally support this The NNW-trending fabric is generally cross-cut by conclusion (Talarico et al., 2000; Smellie, 2000). the discordant DV2 plutons, which include Few rock types reported from Cape Roberts Pearse/South Fork (Allibone et al., 1991; Isaac et al., drillholes can be matched to specific intrusions within 1996), Orestes, Brownworth, Harker and Mt Falconer. the Transantarctic Mountains, although such a match The basement is cut by numerous brittle faults on would be desirable in order to constrain the source a predominant NE trend (Mortimer et al., 2002 and area for the sediment. The commonest clast references therein) which, until recently, have been lithologies reported from all the Cape Roberts inferred to result from Cenozoic dextral transtensional drillholes (Talarico & Sandroni, 1998; Talarico et al., stress (e.g. Wilson, 1995). However, Mortimer et al. 2000; Cape Roberts Project Team 2000; Sandroni & (2002) suggested that many of these faults are likely Talarico, 2001) are grey and pink, undeformed, to be Palaeozoic in age – including some of those biotite ± hornblende monzogranites, which could be described at Doorly Spur (Fitzgerald, 1992; Wilson, derived from many different granitoid bodies within 1999). The prominent Vanda dike swarms share this the Dry Valleys region. Clasts with large K-feldspar NE strike, and dip subvertically. phenocrysts resemble rock types such as Swinford Based on our mapping of the basement granitoids and Brownworth plutons, while more equigranular of the Wilson Piedmont, and on previous mapping of material could be derived, for example, from Gonville the remainder of the Mackay Glacier to Ferrar Glacier and Caius Pluton. Some of the clast lithologies structural block, we are confident that there are no reported - calc-silicate rocks and schist - are clearly shear or high-strain zones of any significant extent derived from the widespread Skelton (Koettlitz) affecting the basement – beyond those which predate Group metasediments which host the granitoids. younger pluton injection. The later Palaeozoic Biotite orthogneiss (commonly intercalated with meta- marker, the Kukri Erosion Surface, and Jurassic sediments) is the most widespread type of gneiss Ferrar Dolerite sills also preclude significant vertical exposed onshore, and is the only type reported from fault movement, at least to within 10 km of the the drillholes, while the rarer hornblende-biotite Wilson Piedmont coastline (Mortimer et al., 2002). orthogneiss was not reported. Foliated biotite- hornblende granodiorites are most likely derived from deformed plutons such as Bonney and Evans. RELATION TO CAPE ROBERTS DRILLING Monzogranitic porphyry probably originated in the PROGRAMME RESULTS Vanda dike swarms of which several are known in the Dry Valleys/Wilson Piedmont area. Pink-grey The Wilson Piedmont Glacier region lies biotite haplogranites are most likely derived from the immediately onshore from the recently completed ubiquitous aplite dikes that cut most basement rocks. Plutonic Rocks from the Cape Roberts Hinterland 71

All the rock types above are so widespread along this Smith Lyttle for figure preparation. For discussions, reviews sector of the Transantarctic Mountains that no and improvements we thank A. Allibone, S. Cox, A. particular point sources and/or palaeoglacial Tulloch, N. Roland and F. Talarico. The study was funded catchments can be nominated. by the N.Z. Foundation for Research, Science & However, the uncommon grey biotite syenogranite Technology contract CO5523. GNS contribution number (two clasts; Talarico et al., 2000) is so unusual in 2260. composition that it matches only Harker and Orestes Editorial handling: C.A. Ricci plutons. Garnet-bearing biotite syenogranite (Talarico & Sandroni 1998) and garnetiferous biotite monzo- REFERENCES granite (Talarico et al., 2000) are clast types with few potential sources within the Dry Valleys, matching Allen A.D. & Gibson G.W., 1962. Geological investigations in only Coleman Pluton and some ortho-gneisses. southern Victoria Land, Antarctica. Part 6 - outline of the geology of the Victoria Valley region. New Zealand Journal of Smellie (2000) and others have pointed to Kirkpatrick Geology and Geophysics, 5, 234-242. Basalt as a significant sand source in CRP 2/2A; Allibone A.H., Cox S.C., Graham I.J., Smillie R.W., Johnstone major occurrences of this unit are currently restricted R.D., Ellery S.G. & Palmer K., 1993a. Granitoids of the Dry to the catchment of the Mackay Glacier. Significantly, Valleys area of southern Victoria Land, Antarctica: plutons, mafic plutonic rocks (gabbro, diorite) which form field relations and isotopic dating. New Zealand Journal of Geology and Geophysics, 36, 281-297. numerous bodies in the Skelton Glacier region have Allibone A.H., Cox S.C. & Smillie R.W., 1993b. Granitoids of the not been reported from the drillholes, perhaps Dry Valleys area, southern Victoria Land, Antarctica: indicating that the southerly-derived ice component Geochemistry and evolution along the early Paleozoic Antarctic suggested by Powell et al. (1998) may have bypassed craton margin. New Zealand Journal of Geology and Cape Roberts – or that such rocks do not travel well. Geophysics, 36, 299-317. Allibone A.H., Forsyth P.J., Sewell R.J., Turnbull I.M.& Bradshaw M.A., 1991. Geology of the Thundergut area, Southern Victoria Land, Antarctica, scale 1:50 000. New Zealand Geological CONCLUSIONS Survey miscellaneous map series 21, Wellington, Department of Scientific and Industrial Research. Granitoid intrusions, and their host rocks, have Allibone A.H., Smillie R.W., Cox S.C. & Johnstone R.D., 1992. The intrusive history of granitoid plutons and suites, Dry now been mapped from the furthest inland Dry Valleys area, south Victoria Land. In: Sixth International Valleys to the Ross Sea coast. The oldest rocks of the Symposium on Antarctic Earth Sciences, Tokyo. Ross Orogen in the Wilson Piedmont area, as further Allibone A.H & Wysoczanski R., 2002. Initiation of magmatism west in the Dry Valleys, are Skelton Group during the Cambro-Ordovician Ross Orogeny in southern Victoria Land, Antarctica. Geological Society of America metasediments. Associated orthogneisses are Bulletin, 114, 1007-1018. extensive, particularly along the coast, and biotite Angino E.E., Turner M.D. & Zeller E.J., 1962. Reconnaissance orthogneiss predominates over the hornblende-biotite geology of Lower Taylor Valley, Victoria Land, Antarctica. variety. Granitoid plutons previously described in the Geological Society of America Bulletin, 73, 1553-1562. Dry Valleys can be extended eastwards to the Wilson Aslund T., 1990. Metamorphism and magmatism of mafic intrusives at Dromedary massif, Antarctica. Unpublished MSc Piedmont, and the system of mapping plutons and thesis lodged in the library, University of Otago, Dunedin. grouping analyses into I-type suites generally works Cape Roberts Project Team, 2000. Studies from the Cape Roberts well. Rocks of the calc-alkaline DV1a suite, adakitic Project, Ross Sea, Antarctica – Initial Report on CRP-3. DV1b suite and monzonitic DV2 suite occur in the Chapter 4 – Petrology. Terra Antartica, 7(1/2), 107-131. Wilson Piedmont area. Mafic bodies, particularly Cook Y.A. & Craw D., 2001. Amalgamation of disparate crustal fragments in the Walcott Bay - Foster Glacier area, South diorite, are more common in the Wilson Piedmont Victoria Land, Antarctica. New Zealand Journal of Geology than in the Dry Valleys, and from field relations and and Geophysics, 44, 403-416. deformation they appear to span a wide range of Cox S.C., 1993. Inter-related plutonism and deformation in South ages. The deformation and metamorphism of the Victoria Land, Antarctica. Geological Magazine, 130, 1-14. Skelton Group and associated orthogneisses, and the Cox S.C. & Allibone A.H., 1991. Petrogenesis of orthogneisses in the Dry Valleys region, South Victoria Land. Antarctic Science, intrusion of DV1a, DV1b, DV2 and mafic suites can 3, 405-417. all be considered as episodes within the broader Ross Cox S.C. & Allibone A.H., 1995. Naming of igneous and Orogeny. Recent interpretations of the precise timing metamorphic rock units in Antarctica: recommendation by the and overlap of Ross events in the Dry Valleys are SCAR Working Group on Geology: discussion. Antarctic Science, 7, 303-306. given by Cox et al. (2000) and Allibone & Cox S.C., Parkinson D. ., Allibone A.H. & Cooper A.F., 2000. Wysoczanski (2002). Alkaline plutonic rocks, such as Isotopic character of Cambro-Ordovician plutonism, southern those reported from the Royal Society Range to the Victoria Land, Antarctica. 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