© Terra Antartica Publication Terra Antartica 2002, 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 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 Ross Sea 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).