Journal of the Geological Society, London, Vol. 1511, 1993, pp. 677-682, figs. 4 Printed in Northern Ireland

An active Neoproterozoic margin: evidence from the Skelton Glacier area, Transantarctic Mountains

A. J. ROWELL l, M. N. REES 2, E. M. DUEBENDORFER 2, E. T. WALLIN 2, W. R. VAN SCHMUS l, & E. I. SMITH 2 1Department of Geology, University of Kansas, Lawrence, KS 66045, USA 2Department of Geoscience, University of Nevada, Las Vegas, NV 89154, USA

Abstract: Metamorphosed supracrustal rocks in the central Ross Sea sector of the Transantarctic Mountains are of Neoproterozoic age and not Cambrian. They include pillow basalts with a mantle separation age of 700-800 Ma. In the Skelton Glacier area, these rocks experienced two strong phases of deformation that produced folds and associated foliations. Both rocks and structures are cut by a 551 4-4 Ma unfoliated quartz syenite (late Neoproterozic). The deformation and limited geochemical data suggest an active Antarctic plate margin whose late Neoproterozoic history is markedly different from that of the temporally equivalent rift to drift transition recorded along the autochthonous western margin of Laurentia. If these two cratons were ever contiguous, separation occurred by c. 700-800 Ma.

The Transantarctic Mountains are the product of late and renamed numerous times subsequently (Findlay 1990; Palaeogene-Neogene uplift (Webb 1991) along a line Smillie 1992). Metasedimentary strata in the Skelton Glacier subparallel to the boundary between the East Antarctic area are typically at greenschist facies and consist of the craton and the Ross orogen. Cambro-Ordovician granitoids Anthill Limestone overlain by the Cocks Formation (c. 500 Ma), late-stage products of the Ross orogeny, have (Skinner 1982), together forming the Skelton Group. been recorded from along much of the length of the range Metamorphic grade and extensive deformation preclude (Craddock 1972). We discuss here results from the central detailed sedimentological and stratigraphic study of these segment of the Ross Sea sector of the Transantarctic rocks. The Anthill Limestone includes a thick succession Mountains together with some of their regional, and indeed consisting largely of quartzites followed by several hundred global, implications. metres of strongly recrystallized limestone. The Cocks The Ross Sea sector of the range may be divided along Formation (Skinner 1982) contains numerous polymictic its length into three segments by major structures oblique to conglomerates, particularly in its lower part. Some of the its trend (Fig. 1). The northern boundary of the central conglomerates are interbedded with limestones that are only segment is not well understood, but seemingly is formed by a few metres thick. Dark green-grey argillites and the Priestley Fault and the megashear that continues it to metasandstones, some probably volcaniclastic, are the most the Scott Coast (Skinner 1983, 1987). North of this fault abundant sedimentary rock types; they are locally associated system, the mountains of northern Victoria Land expose with thin pillow basalts at the type locality. Strata of the Lower Paleozoic rocks of the accreted Bowers and Skelton Group have been correlated with the higher-grade Robertson Bay terranes (Bradshaw 1987; Borg et al. 1987) metamorphic rocks of the Koettlitz Group towards the north that are docked against the Wilson terrane, which is (Findlay et al. 1984). probably autochthonous (Roland 1991). The Byrd Fault The different conclusions of earlier workers regarding forms the southern boundary of the central segment the age of these metasediments depend largely on disparate (Skinner 1983). South of this fault, strongly folded, thick interpretations of a published U/Pb zircon age for the fossiliferous Lower Cambrian limestones crop out discon- foliated Olympus Granite-gneiss (Deutsch & Grogler 1966), tinuously between Beardmore and Byrd glaciers (Fig. 1). part of the Granite Harbour Complex. Using current decay Their fossil fauna provides modest biostratigraphic con- constants, Skinner (1983) concluded that zircon crystal- straint on timing of multiple phases of Lower Palaeozoic lization occurred in this rock at 589 + 13 Ma. He regarded Ross orogenic deformation that affects rocks between these the Skelton Group, and its correlatives, as upper two glaciers (Rowell & Rees 1989; Rowell et al. 1992). Precambrian and concluded that its two earliest phases of deformation were also Precambrian. Other plutons, which Skinner regarded as younger, were emplaced both during Prior controversy and after a third phase of deformation. He contended that Previous studies of basement rocks between Byrd and these latter plutons and deformation were related to the Priestley glaciers have yielded markedly different and Ross orogeny and that some intrusions were Ordovician in incompatible interpretations of both the age of the rocks age. His interpretation of the zircon data from the Olympus and their time of deformation, although the distribution of Granite-gneiss was challenged by Findlay (1985, 1990), who rock types is reasonably well understood. These rocks argued that the zircons probably had inherited cores with consist of metasedimentary and metavolcanic strata intruded ages markedly different from that of their rims. This view by a variety of granitoids that were originally assigned to the was supported by preliminary Rb-Sr mineral and whole-rock Granite Harbour Intrusive Complex (Gunn & Warren studies of foliated syntectonic granites. These rocks 1962); rocks of this intrusive complex have been subdivided seemingly became closed systems at about 485 Ma (Graham 677

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/ Rb/Sr results in the region, support a late Precambrian age 8 °E 180 ° for the basement.

New field and isotopic results Our results demonstrate that the basement in the central segment of this sector of the range is of Neoproterozoic age. Furthermore, we show that in contrast with earlier interpretations, there are no regionally developed folds or fabrics that can be attributed unequivocally to Ross orogenesis in the Skelton Glacier area. The major deformation in this region is Proterozoic.

.... The Skelton Group strata that we mapped (Fig. 2) are [~~' "" Priestley Fault intruded by diabase dykes and by unfoliated granitoid rocks. The lack of structural fabric in these granitoids suggests they could be referred loosely to the Skelton Granodiorite (Gunn & Warren 1962) and that they are younger than the Olympus Granite-gneiss (Skinner 1983). Detailed study of the relationships of the various unfoliated granitoids in the region has not been attempted. Although the southernmost two areas of granitoid rocks in the study area (Fig. 2) may be exposures of a single pluton, it is possible that they represent two small stocks. Both were sampled and the rocks dated by the U-Pb method. One of the samples (GB88-1, a granite from the southwestern outcrops) exhibited complex isotopic systematics characterized by slightly discordant 2°Tpb/2~Pb ages that range from 531 to 543 Ma. It will not be discussed further. The other sample (CG88-1, from the southeastern outcrops), a quartz syenite, yielded an uncomplicated discordia (Fig. 3; Table 1) with a precise upper intercept age of 551 + 4Ma. Optically, the zircons used for this determination were pink, transparent, and inclusion-free, with no evidence of cores or overgrowths and no indication that they consisted of a mixed population. The four fractions exhibited typical isotopic systematics with respect to their magnetic susceptibility. The lack of any age Fig. 1. Location map of main features of the Ross Sea sector of the shift following air abrasion (Krogh 1982) also suggests that Transantarctic Mountains with boundaries of its central segment. the zircons lack overgrowths. The upper intercept is readily Position of Fig. 2 is indicated and small inset map shows location of interpreted as the age of crystallization for this sample. area illustrated in Fig. 1 relative to the continent. Coarse ornament Although a numerical scale for the base of the Cambrian indicates position of the terranes of northern Victoria Land. (BT, is not firmly established (Compston et al. 1992), we consider Bowers terrane; RBT, Robertson Bay terrane; WS, WeddeU Sea; that the best estimates place it between 540 and 530Ma WT, Wilson terrane). (Conway Morris 1988). In this light, both the Skelton Group and the quartz syenite intruding it near the confluence of the Cocks and Skelton glaciers are unquestionably Precambrian, but it is possible to be more specific. We analysed a & Palmer 1987): a result broadly consistent with prior Rb/Sr whole-rock powder of a pillow basalt from the Cocks studies that also yielded late Cambrian-early Ordovician Formation (Sample CG 88-16) together with three mineral closure ages (Deutsch & Grogler 1966; Faure & Jones concentrates separated by magnetic properties (Table 2; Fig. 1974). Influenced by these isotopic data, Findlay (1990) 4). Magnetic splits of HNO3-washed rock powder were suggested that rocks of the metasedimentary Koettlitz prepared because the basalt is too fine-grained for mineral Group were correlative with the Lower Cambrian strata purification. The acid wash removed the fine powder and south of Byrd Glacier and that, by implication, the soluble components, leaving the principal silicate minerals deformation in this segment of the Transantarctic Mountains with pyroxene concentrated in the more magnetic split and was entirely of Palaeozoic age. Recent conflicting evidence, plagioclase enriched in the less magnetic fractions. These however, has been derived from the Dry Valley region, samples do not have sufficiently different Sm/Nd ratios to north of Skelton Glacier. Adams & Whitla (1991) define an isochron, but they can be used to define the determined Rb-Sr whole-rock isochron ages of 840 + 30 Ma mantle separation age, TDM, as 700-800 Ma (Fig. 4). These for the greenschist-amphibolite facies metasediments of the data imply a maximum crystallization age for the basalt of c. Asgard Formation; they suggested this was the time of 800Ma. Although an inherited, older crustal component sedimentation of the beds. These basement rocks, which could require that the age be younger, an older age is consist largely of impure marbles, are also commonly possible only if the mantle source is anomalously depleted. correlated with the Koettlitz Group (Findley et al. 1984). Borg et al. (1990) reported a comparable Sm-Nd isochron The data, although not compelling because of incompatible age of 762-1-24 Ma for basalt and gabbro from the Cotton

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MAP SYMB(

Uthologic contact h .----"~z6 Fault, showing dip v u Overturned anticline and syn

-~ = 35 Mesoscopic Fz fold, showing plunge and downp~unge prot ss Mesoscopic F3 fold, showin~ p~unge and d6wnplunge pro4 8~_ _$_ S 1, S z foliation inclined & ve ,,o, S], cleavage inclined & verti,

Fig. 2. Geological map of the Skelton Group and associated intrusive rocks in Limestone the Skelton Glacier-Cocks Glacier area. ANTHILL F-r'Lr'~LrLr~ DIABASE The Skelton Group here consists of the LIMESTONE Anthill Limestone and Cocks Forma- Quartzite tion. Location of granitoid isotopic samples marked by stars: the northern one is CG88-1, the more southerly one is COCKS FORMATION GB88-1.

Plateau, with end (t) +6.76, showing that the DM curve is of Pearce et al. (1977), the basalt has continental rift appropriate for the region. Thus the basalt (CG88-16) must affinities. The quartz syenite may have been emplaced in a be older than 551+4Ma (the age of intrusive quartz subsequent volcanic arc environment, as is suggested by the syenite) and younger than or equal to c. 700-800 Ma, i.e., position of sample CG88-1 on a Rb versus Y + Nb discrimina- definitely Neoproterozoic. tion diagram (Pearce et al. 1984). Chemical analyses of the basalt and the quartz syenite Our mapping documents three phases of folding in the are limited to three samples (Table 3), so caution must be Anthill Limestone of the Skelton Group (Rees et al. 1990); exercised when inferring tectonic setting from these results. the folds are only in part comparable to those recognized in According to the MgO-FeO-Al203 discrimination diagram earlier studies (Skinner 1982). Two of the phases clearly predate the 551 +4Ma quartz syenite because both this intrusion and the diabase dykes cut F1 and F2 structures. Timing of the localized third phase relative to the 0.092 ' I ' I ' I ' I ' I ,~fl emplacement of the pluton, however, is uncertain. D3 0G88 -1 y structures in this area include a nonpenetrative cleavage, $3, and a set of associated symmetrical, open to tight, steeply 0.088 south-plunging folds that die out to the west and north. $3 ~O ~ ~nm(-1)aa clearly cross-cuts F2 folds and associated cleavage, and its

¢OOD orientation defines a broadly arcuate pattern that is parallel 0.084 to the pluton-wall rock contact. The quartz syenite exhibits neither a magmatic nor a deformational fabric. At the a. ~D microscopic scale, the limited deformation within it may be a magmatic rather than a tectonic phenomenon. These 0.080 J plutonic rocks exhibit only deformation bands, slightly ~"//"/ Upper Intercept = 550.5 +- 4 Ma undulose extinction in quartz, and minor kinking of ~,/ Lower Intercept = 139 _+ 73 Ma feldspars. Grains show no preferred orientation. Sparse

0.076 i I ~ I ~ I , I , I , I i intragrain microcracks are nonpenetrative at both the 1.62 0.64 0.66 0.68 0.70 0.72 0.74 0.76 microscopic and mesoscopic scales. Although marginal 207 Pb/235 U recrystallization of feldspars into polygonal aggregates i~ Fig. 3. Concordia diagram for zircon from quartz syenite near common, unambiguously magmatic interstitial quartz is Cocks Glacier, locality CG88-1. Ellipses indicate correlated pristine. Therefore, at least some feldspar recrystallization two-sigma uncertainties in isotopic ratios. Diagram created using may be late magmatic in origin rather than due to subsolidus Ludwig 1988b. deformation, and kinking of the feldspars also may be

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Table 1. U-Pb zircon isotopic data and apparent ages

CG88-1 Sample Concentrations# Measured~ Radiogenic isotopic ratios § Agesll (Ma) Fraction* weight 2°6pb /2°4pb (mg) 238U 2°6pb (ppm) (ppm) 2°Tpb /2°6 pb 2°spb /2°6 pb 2°6pb/238 U 207pb /235 U 2°6pb/238 U 207pb/235 U 207pb /206pb

nm(- 1) + 240aa 9.816 562.0 42.69 11 765 0.05848(08) 0.23205 0.08749(42) 0.7055 541 542 548 ± 3 m(- 1) + 240 8.929 1,008.1 74.90 14 925 0.05847(06) 0.21060 0.08561(40) 0.6901 530 533 547 ± 3 m(0) + 240 4.739 1,387.3 101.89 15 873 0.05841(06) 0.20920 0.08464(40) 0.6817 524 528 545 ± 3 m(1) +240 3.850 1,581.5 114.97 15 873 0.05832(06) 0.21185 0.08378(41) 0.6737 519 523 542 ± 3 Upper intercept age = 550.5-1- 4 Ma Model 1 solution, MSWD = 1.03

Isotopic ratios measured by static multicollection on a VG Sector mass spectrometer at the University of Kansas. Mass fractionation corrections of 0.1% bias per mass unit were applied to Pb data; analytical blanks ranged between 50 and 320 pg total Pb. * nm, nonmagnetic; m, magnetic; numbers in parentheses indicate side tilt used on Franz separator at 1.4 A; aa, air-abraded; 240 is mesh size. t U corrected for analytical blank; Pb corrected for blank and nonradiogenic Pb. ¢ Uncorrected for blank. § Pb corrected for blank and nonradiogenic Pb; U corrected for blank; two sigma uncertainties in last two digits given in parentheses. Uncertainties based on precision of measured ratios, uncertainty in mass fractionation correction, uncertainties in concentration and isotopic composition of spikes, and uncertainty in isotopic composition of nonradiogenic Pb. The isotopic composition of nonradiogenic Pb used to calculate radiogenic 2°Tpb and 2~pb was estimated using a two-stage model (Stacey & Kramers 1975) for evolution of terrestrial Pb (17.82+ 1:15.58+0.1:37.60+ 1). Blank Pb isotopic composition used was 18.67 + 1:15.66 + 0.1:38.39 + I. Fractions spiked with a mixed Z°Spb/235U tracer. II U decay constants (Steiger & Jager 1977). Ages and two-sigma uncertainties (Ludwig 1988a).

attributed to magmatic processes. These observations are a major structure, because other than localized D3 folds, no consistent with pluton emplacement during or after D3 and structures exist that may be construed as evidence for early there is no compelling microstructural evidence for Palaeozoic deformation associated with the Ross orogeny in significant deformation after the quartz syenite was the Skelton Glacier region. Conceivably, the Byrd to emplaced. The possibility, however, that D3 post-dated Priestley fault sector of the Transantarctic Mountains (Fig. pluton emplacement can not be ruled out. Rheological 1) may represent part of the foreland that lay on the contrasts between the rigid intrusive body and mechanically cratonic side of the Ross orogen, rather than the mobile belt weaker wall rocks during D3 deformation also could have itself. produced the observed relationships (cf. Paterson & Tobisch On a global scale, our data impose constraints on 1988; Paterson et al. 1989). The relative timing of D3 and reconstructions that juxtapose the margin of East Antarctica intrusion of the 551 + 4 Ma quartz syenite therefore remains against that of western Laurentia (e.g. Moores 1991; Dalziel indeterminate with available data. 1991). Remnants of ocean-floor gabbro, basalt, and overlying continental-rise sedimentary rocks crop out in the central Transantarctic Mountains south of Byrd Glacier and Discussion and conclusions indicate that an ocean basin was well established along East At a regional level, our isotopic results and mapping Antarctica by c. 760 Ma (Borg 1991). We (Rees et al. 1992) demonstrate unequivocally that the geology of the central and others have previously concluded from this information segment of the Ross Sea sector of the Transantarctic that cratonic separation must have occurred prior to this Mountains is very different from that of rocks south of Byrd time. Our new data are compatible with such a model and Glacier. The evidence is consistent with the Byrd fault being the c. 700-800 Ma pillow basalts of the upper Skelton

Table 2. Sm-Nd data from metabasalt CG88-16

Sample* Sm Nd ft Sm-1475 Nd-1435 + eNd(o~§ eNd(oil TDM¶ (ppm) (ppm) Nd-144 Nd-144 2SE~ (Ga)

Whole rock 4.63 18.27 -0.2212 0.1532 0.512777 13 2.7 6.9 0.69 [email protected], [email protected] 3.91 15.69 -0.2351 0.1507 0.512746 12 2.1 6.6 0.74 [email protected], [email protected] 4.24 16.51 -0.2088 0.1556 0.512791 13 3.0 7.0 0.68 [email protected], [email protected] 2.46 10.66 -0.2909 0.1395 0.512668 13 0.6 6.2 0.78

* NM, non-magnetic; M, magnetic; numbers refer to current setting on Franz Isodynamic Separator. t f, fractionation factor: [(147Sm/144Nd)/0.1967 - 1]. $ Atomic ratios; Nd data normalized to 146Nd/144Nd = 0.72190; uncertainty is +2 standard errors of the mean in the last two digits for measured 143Nd/taaNd ratio. § Epsilon value relative to CHUR(0) = 0.512638. II 't' estimated at 760 Ma based on results of Borg et al. (1990) for basalt at Cotton Plateau. ¶ Based on depleted mantle evolution curve of DePaolo (1981).

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+10 Group may reflect this extensional event. Separation of about this age has also been suggested from the Pensacola Mountains, near the Weddell Sea terminus of the range (Storey et al. in press), but isotopic control on the timing is only very approximate. Subsequent Neoproterozoic polyphase folding and magmatism, seemingly in an arc setting, however, imply a radical change in tectonic regime in the Skelton Glacier area. Borg et al. (1990) and Borg & DePaolo (1991) invoked oblique accretion of one or more continental fragments along the East Antarctic margin ] / / a - between c. 750 and c. 570 Ma to explain the distribution of 4 4-I.-.Y.// i/ t / / v 1.7-2.0 Ga Nd crustal provinces. Conceivably, such accretion may have caused the deformation and 551 + 4 Ma

f / magmatism that we have documented. This late Neoproter- ozoic scenario for the margin of East Antarctica is 2 / significantly different from that proposed for the auto- chthonous margin of western Laurentia. It is now evident " CHUR that while the western margin of Laurentia was developing a 0 I I I J passive margin succession (Ross 1991) and undergoing the 0 200 400 600 800 1000 transition from rift to drift in the late Precambrian to early Cambrian (Levy & Christie-Blick 1991), the Skelton Glacier area of East Antarctica was an active plate margin. T (Ma) Consequently, if these continents were ever conjugate rift pairs, then their late Neoproterozoic histories require that Fig. 4. Nd evolution diagram for whole-rock (solid line) and there was considerable prior separation between western mineral concentrates (dashed lines) from basalt CG88-16 (Table 2). The depleted mantle (DM) evolution curve is from DePaolo (1981); Laurentia and at least the central Ross Sea segment of the CHUR evolution line is at e = 0. The solid circle (CPB) denotes Antarctica. the initial composition of Nd for basalt from the Cotton Plateau at 762 Ma (Borg et al. 1990); Nd evolution lines for CG88-16 are We thank P. Braddock for his assistance in the field and B. Storey projected back to 760 Ma for comparison. The data indicate for helpful review of an earlier version of the manuscript and CG88-16 was derived from a typical depleted mantle at c. making available a preprint of a paper in press. The work was 700-800 Ma. supported by National Science Foundation grants DPP 8715768 and DPP 9117444 to the University of Kansas and DPP 8716068 to the University of Nevada, Las Vegas. A. J. R. is grateful to the Council of Robinson College, Cambridge for the award of a Bye Fellowship Table 3. Chemical data and the Director of British Antarctic Survey for facilities provided CG88-16a CG88-16b CG88-1 during a sabbatical leave in the spring of 1990.

SiO2 51.55 51.10 64.33 TiO 2 1.99 1.89 0.37 References AI20 3 15.47 15.78 16.99 ADAMS, C. J: & WHITLA, P. F. 1991. Precambrian ancestry of the Asgard FeO 12.22 12.22 5.38 Formation (Skelton Group): Rb-Sr age of basement metamorphic rocks MnO 0.22 0.23 7.07 in the Dry Valley region, Antarctica. In: THOMSON,M. R. A., CRAME, J. A. & THOMSON, 1. W. (eds) Geological Evolution of Antarctica. MgO 5.36 5.03 0.22 Cambridge University Press, Cambridge, 129-135. CaO 9.37 9.64 0.98 BORG, S. G. 1991. North America, Australia, and Antarctica as a late NazO 3.39 3.73 0.15 Precambrian supercontinent: a discussion. Geological Society of K20 0.46 0.39 4.84 America, Abstracts with Programs, 23, A305. P205 0.08 0.08 0.31 -- • DEPAOLO, D. J. 1991. A tectonic model of the Antarctic Gondwana LOI 0.34 0.57 0.07 margin with implications for southeastern Australia: isotopic and geochemical evidence. Tectonophysics, 196, 339-358. TOTAL 100.45 100.06 100.64 -- -- & SMITH, B. M. 1990. Isotopic structure and tectonics of the central Transantarctic Mountains. Journal of Geophysical Research, 95, Trace elements in ppm 6647-6667. Nb bd bd 11 , STUMP, E., CHAPPELL, B. W., MCCULLOCH, M. T., WYBORN, D., Y bd bd 30 ARMSTRONG, R. L. & HOLLOWAY, J. R. 1987. Granitoids of northern Victoria Land, Antarctica: Implications of chemical and isotopic Rb 15.0 10.8 101 variations to regional crustal structure and tectonics. American Journal of Science, 287, 127-169. Major elements and Nb and Y by XRF in the BRADSHAW, J. D. 1987. Terrane boundaries and terrane displacement in Rock Chemistry Laboratory at the Univers- northern Victoria Land, Antarctica: some problems and constraints. In: ity of Nevada, Las Vegas; E. I. Smith and LEITCH, E. C. & SCHE|BNER, E. (eds) Terrane accretion and orogenic Yagang Wang analysts. Rb by instrumental belts. American Geophysical Union, Washington and Geological Society neutron activation analysis at the Phoenix of America, Boulder, Geodynamics volume 19, 199-205. COMPSTON, W., WILLIAMS, I. S., KIRSCHVINK, J. L., ZHANG ZICHAO & MA Memorial Laboratory, University of Michi- GUOGAN. 1992. Zircon U-Pb ages for the Early Cambrian time-scale. gan. Samples CG88-16a and 16b are two Journal of the Geological Society, London, 149, 171-184. hand samples of basalt from the same CONWAV MORRIS, S. 1988. Radiometric dating of the Precambrian-Cambrian outcrop. CG88-1 is a quartz syenite, bd, Boundary in the Avalon Zone. In: LANDING, E., NARBONNE, G. M. & below detection. MYROW, P. (eds) Trace Fossils, Small Shelly Fossils and the

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/4/677/4892513/gsjgs.150.4.0677.pdf by guest on 27 September 2021 682 A.J. ROWELL ET AL.

Precambrian-Cambrian Boundary. New York State Museum, Bulletin, identification of magmatic and tectonic foliations in granitoids. Journal of 463, 53-58. Structural Geology, 11, 349-363. CRADDOCK, C. 1972. Geologic Map of Antarctica, l:5,(g)O,O00. American PEARCE, Z. H., GORMAN, B. E. & BIRKE1q', T. C. 1977. The relationship Geographical Society, New York. between major element chemistry and tectonic development of basic and DALZlEL, I. W. D. 1991. Pacific margins of Laurentia and East intermediate volcanic rocks. Earth and Planetary Science Letters, 36, Antarctica-Australia as a conjugate rift pair: Evidence and implications 121-132. PEARCE, T. H., HARRIS, N. B. W. & TRIDLE, A. G. 1984. Trace for an Eocambrian supercontinent. Geology, 19, 598-601. element discrimination diagrams for the tectonic interpretation of DE PAOLO, D. J. 1981. A neodymium and strontium isotope study of the granitic rocks. Journal of Petrology, 25, 956-983. Mesozoic calc-alkaline granitic batholiths of the Sierra Nevada and REES, M. N., DUEBENDORFER, E. M. & ROWELL, A. J. 1990. The Skelton Peninsular Ranges, California. Journal of Geophysical Research, 86, Group, southern Victoria Land. Antarctic Journal of the United States, 10470-10488. 24, No. 5, 21-24. DEUTSOI, S. & GROGLER, N. 1966. Isotopic age of the Olympus , WALLIN, E., ROWELL, A. J., & STUMP, E. 1992. Were East Antarctica Granite-Gneiss (Victoria Land-Antarctica). Earth and Planetary Science and western North America conjugate rift pairs in the Cambrian? Letters, 1, 82-84. Geological Society of America, Abstracts with programs, 24, 77. FAURE, G. & JONES, L. M. 1974. Isotopic composition of Strontium and ROLAND, N. W. 1991. The boundary of the East Antarctic craton on the geologic history of the basement rocks of Wright Valley, southern Pacific margin. In: TtlOMSON, M. R. A., CRAME, J. A. & TttOMSON, J. W. Victoria Land, Antarctica. New Zealand Journal of Geology and (eds.) Geological Evolution of Antarctica. Cambridge University Press, Geophysics, 17, 611-627. Cambridge, 161-165. FINDLAY, R. H. 1985. The Granite Harbour Intrusive Complex in McMurdo Ross, G. M. 1991. Tectonic setting of the WindermereSupergroup revisited. Sound: Progress and Problems. New Zealand Antarctic Record, 6, No. 3, Geology, 19, 1125-1128. 10-22, ROWELL, A. J. & REES, M. N. 1989. Early Palaeozoic history of the upper , 1990. Is the Larson Granodiorite Vcndian? - A Discussion. New Beardmore Glacier area: implications for a major Antarctic structural Zealand Antarctic Record, 10, No. l, 49-55. boundary within the Transantarctic Mountains. Antarctic Science, 1, --, SKINNER, D. N. B. ~£ CRAW, D. 1984. Lithostratigraphy and structure of 249-260. the Kocttlitz Group, McMurdo Sound, Antarctica. New Zealand Journal --, -- & EVANS, K. R. 1992. Evidence of major Middle Cambrian of Geology and Geophysics, 27, 513-536. deformation in the Ross orogen, Antarctica. Geology, 20, 31-34. GRAIIAM, I. J. & PALMER, K. 1987. New Precise Rb-Sr mineral and SKINNER, D. N. B. 1982. Stratigraphy and Structure of Low-Grade whole-rock dates for I-type granitoids from Granite Harbour, south Metasedimentary Rocks of the Skelton Group, southern Victoria Land- Victoria Land, Antarctica. New Zealand Antarctic Record, 8, No. I, Does Teall Graywacke Really Exist? In: CRADDOCK, C. (ed.) Antarctic 72-80. Geoscience. University of Wisconsin Press, Madison, 555-563. GUNN, B. M. • WARREN, G. 1962. Geology of Victoria Land between the --, 1983. The granites and two orogenies of southern Victoria Land. In: Mawson and Mulock glaciers, Antarctica. New Zealand Geological OLIVER, R. L., JAMES, P. R. & JAGO, J. B. (eds) Antarctic Earth Science. Survey, Bulletin, 71, 157 pp. Australian Academy of Science, Canberra, 161~!63. KROGII, T. E. 1982. Improved accuracy of U-Pb zircon ages by the creation of --, 1987. Terra Nova Bay and the Deep Freeze Range- the southern more concordant systems using an air abrasion technique. Geochimica et aliochthonous border of north Victoria Land, Antarctica. Memoria della Cosmochimica Acta, 46, 637-649. Societa Geologica ltaliana, 33, 41-58. LEVY, M. & CIIRISTIE-BLICK, N. 1991. Tectonic subsidence of the early SMII.LIE, R. W. 1992. Suite subdivision and petrological evolution of Paleozoic passive continental margin in eastern California and southern granitoids from the Taylor Valley and Ferrar Glacier region, south Nevada. Geological Society of America Bulletin, 103, 1590-1606. Victoria Land. Antarctic Science, 4, 71-87. LUDWIG, K. R. 1988a. PBDAT for MS-DOS: A computor program for SrACEY, J. S. & KRAMERS, J. D. 1975. Approximation of terrestrial lead IBM-PC compatibles for processing raw Pb-U-Th isotope data. US isotope evolution by a two-stage model. Earth and Planetary Science Geological Survey Open File Reports, 81t-542. Letters, 26, 207-221. --, 1988b. ISOPLOT: A plotting and regression program for radiogenic SrEIGER, R. H. & JAGER, E. 1977. Subcommission on geochronology: isotope data for IBM-PC compatible computors. US Geological Survey Convention on the use of decay constants in geo- and cosmochronology. Open File Reports, 88-557. Earth and Planetary Science Letters, 28, 359-362. MOORES, E. M. 1991. Southwest U.S.-East Antarctica (SWEAT) STOREY, B. C., ALABASTER, T., MACDONALD, D. I. M., PANKtlURST, R. J. & connection: A hypothesis. Geology, 19, 425-428. DAt.ZIEL, I. W. D. Upper Protero~,;," -;~ related rocks in the Pensacola PATERSON, S. R. & TOalSOl, O. T. 1988. Using pluton ages to date regional Mountains, Antarctica: Precursors to supercontinent breakup? Tectonics, deformations: Problems with commonly used criteria. Geology, 16, in press. ll08-1111. WEaB, P. N. 1991. The Cenozoic history of Antarctica and its global impact. , VERNON, R. H. & ToBIscll, O. T. 1989. A review of criteria for the Antarctic Science, 2, 3-21.

Received 24 August 1992; revised typescript accepted 19 November 1992.

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