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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/4/677/4892513/gsjgs.150.4.0677.pdf by guest on 25 September 2021 678 A.J. ROWELL ET AL. / 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.