Early Palaeozoic Rather Than Neoproterozoic Volcanism And
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Journal of the Geological Society, London, Vol. 152, 1995, pp. 417-420, 2 figs. 1 table. Printed in Northern Ireland FelsiteMember, Patuxent Formation) was previously thought to be Neoproterozoic in age and related to rifting, Early Palaeozoic rather than Neoproterozoic whereas the other (Gambacorta Formation) is known from volcanism and rifting within the Transantarctic stratigraphic evidence to be Cambro-Ordovician. Mountains PensacolaMountains. The PensacolaMountains (Fig. l), situated towards the South Atlantic-Weddell Sea end of the I. L. MILLAR'.' & B. C. STOREY' Transantarctic Mountains, had a complex history involving 'British Antarctic Survey, Natural Environmental three separate orogenic episodes (Schmidt et al. 1978): (1) Research Council, High Cross, Madingley Road, the Beardmore Orogeny, which folded Late Precambrian or Cambridge CB3 OET, UK LowerPalaeozoic sedimentary and volcanic rocks of the 'British Antarctic Survey, c/o NERC Isotope Patuxent Formation; (2) the Ross Orogeny, which folded an Geosciences Laboratory, Keyworth, Nottingham unconformably overlying Lower Palaeozoic succession that NG12 5GG, UK includes volcanic rocks of the Gambacorta Formation; (3) the GondwanianOrogeny, which folded an Upper Palaeozoic succession during Permian-Triassic times. PatuxentFormation. The PatuxentFormation is typically New U-Pb dating of zircons separated from felsic volcanic rocks of thePatuxent and Gambacorta formations from the Pensacola formed of well-bedded fine- to medium-grained sandstones Mountainsin theTransantarctic Mountains, Antarctica, yields with interbedded mudstones (Schmidt et al. 1978). In the earliest Ordovician agesof 500 k 8 and 501 f 3 Ma respectively. The Schmidt and Williams hills, on the western side of the datedfelsic volcanic rock of the Gorecki Felsite Member of the NeptuneRange in the Pensacola Mountains (Fig. l), Patuxent Formation is important tectonically, as the felsite, together bimodal volcanic rocks and mafic sills have also been withmafic volcanic rocks, werepreviously considered to provide assigned to the Patuxent Formation. Schmidt et al. (1978) key evidence of a Neoproterozoic rifting event prior to separation referred to the volcanic rocks as three different members, of Laurentia from East Antarctica. The new data do not necessarily the Pillow Knob Basalt Member, the Williams Basalt refutethis event butindicate apreviously unrecognized early Member andthe GoreckiFelsite Member within the Ordovician period of bimodal magmatism and extension along the Patuxent Formation andthey described the mafic sills, which Transantaractic Mountains. are up to 15 m thick, separately. A new interpretation of their tectonic setting, based primarily on an elemental and Keywords: Antarctica, Ordovician, volcanism, rifting, U/Pb. isotopic study, has suggested thatthe sedimentary and magmatic rocks formedin an intracontinental rift setting (Dalziel 1992; Storey et al. 1992). Within the context of the The SWEAT hypothesis links the Southwest US and East SWEAT hypothesis, this was most likely a prelude to the Antarctica-- as conjugate rift margins of Neoproterozoic separation of Laurentia from Antarctica in Late Neo- supercontinent(Moores 1991). Ithas led to dramatic proterozoic time. The Patuxent Formation was subsequently changesin the configuration of Neoproterozoic and Early deformed into tight upright folds prior to deposition of the Palaeozoic reconstructions (e.g. Dalziel et al. 1994), and a Lower Palaeozoic succession. revised framework for the interpretation of orogenic belts The main constraint onthe age of the Patuxent andsutures (fora review seeStorey 1993). The SWEAT Formation is provided by the unconformably overlying hypothesis was originally based on comparative geology, and Nelson Limestone which containsa Mid-Cambrian fauna the matching of Precambrianorogenic belts between (Palmer & Gatehouse 1972). Elsewhere along the Laurentia and Antarctica (Moores 1991; Dalziel 1991), with TransantarcticMountains, deformed sedimentary rocks of early Cambrian separation of the continents. The timing of the Beardmore Group, which have been correlated with the rift to drifttransition was subsequentlyconsidered tobe Patuxent Formation (Stump et al. 1986), are overlain by the Neoproterozoic (c. 750Ma), to accommodate a more global Lower Cambrian Shackleton Limestone (Laird et al. 1971) model linking opening of the Pacific Ocean between and it is these relationships that have led most authors to Laurentiaand Antarctica to earlyPalaeozoic collision of consider the Beardmore Group and Patuxent Formation to Laurentiaand SouthAmerica, and to amalgamation of be Late Neoproterozoic in age (Stump 1992). However, Gondwana at around 500 Ma (Dalziel 1992). Rowel1 et al. (1992) have suggested that at least some of the Although the reliability of Neoproterozoicreconstruc- marine turbidites within the Patuxent Formation could be of tions isdifficult to test rigorously, the SWEAT hypothesis EarlyCambrian age, andrepresent deep water basin or hasexplained some unsolved geological problems in slope deposits to the Cambrian carbonate platforms. Until Cordilleran geology. It has also been broadly supported by now, this modelhas been difficult to testas the lack of palaeomagnetic data (Powell et al. 1993), and has been fauna1 evidence has prevented more precise dating of the reinforced by more detailedcomparisons and new sedimentary rocks, and direct dating of the magmatic rocks interpretations of the conjugate margins (Stump 1992; Ross within the Beardmore Group has proved difficult. 1991; Young 1992; Borg & DePaolo 1994). One such study Rb-Sr dating of the magmatic rocks hasproved has led to the recognition of a Neoproterozoic rifting event unsatisfactory due to open system behaviour. Faure et al. within the Pensacola Mountains and along the remainder of (1979) tabulated Rb-Sr whole rock dating results that the Transantarctic Mountains (Fig. 1; Storey et al. 1992). In included 809 f 38 Ma for the Gorecki Felsite Member and this paper we report U-Pb zircon ages for felsic volcanic 784 f 58 Ma for a dolerite sill in the Schmidt Hills. Borg et rocks from the Pensacola Mountains, one of which (Gorecki al. (1990) presented a Sm-Nd isochron age of 762 f 24 Ma 417 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/3/417/4890250/gsjgs.152.3.0417.pdf by guest on 02 October 2021 418 1. L. MILLAR & B. C. STOREY Results. Zircon separation and analysis was carried out at theNERC Isotope GeosciencesLaboratory, Keyworth, following analytical proceduresdescribed in Vaughan & Millar (in press). Analytical results are summarized in Table 1, and plotted in Fig. 2. A sample from the Gorecki Felsite Member of the Patuxent Formation yielded sparse zircons. The zircons were colourless, euhedral, with aspect ratios from 2:l to8:l. Unfractured grains with no visible inclusions were selected for analysis. Fraction 1A is slightly discordant, With 2mPb/235Uand 2MPb/23sUages of 502 and499Ma, respectively. Fraction 2A is highly discordant, with a 207Pb/2"Pb age of 571 Ma. This fraction must have contained undetectedinherited zircon cores. Fraction 3A touches concordia at around 499 Ma. The three analyses do not fall on a simple discordia, as indicated by a high MSWD (mean square of weighted deviates) of 5.5. Excluding the highly discordant point, and regressing fractions 1A and 3A only, gives an earlyOrdovician age of 498 f 6 Ma forthe emplacement of the Gorecki Felsite Member. However, it is possible that fractions 1A and 3A have lost some Pb, with 1A containing a little inheritance. The 2mPb/2"Pb age (504 f 3 Ma) of the near-concordant point (3A) might then representthe true age of the sample. Inorder to take account of both possible models, an age of 500 f 8 Ma is proposed forthe emplacement of the GoreckiFelsite Member. A sample from the Gambacorta Formation yielded more Lavas forming about m one thlrd of thesuccession abundant zircons thanthe Gorecki Felsite sample. The Doleriteabout sws forming 25 50 zircons were colourless to pale orange, euhedral, with aspect one third of the succession I m ' km ratios from 2:l to 6:l. Many grains were fractured, and melt Dominantly sedlmentary rock inclusions were common. However, unfractured grains with no visible inclusions were selected for analysis. The three Fig. 1. Geological sketch map of Neptune Range within the analysed fractionsoverlap concordia, giving an age of Pensacola Mountains (from Storey et al. 1992) together with 501 f 3 Ma. Fraction 2B, which contained rather coarser location map within Antarctica. EM, Ellsworth Mountains; PM, grains than the other two fractions, only just overlaps with Pensacola Mountains; TAM, Transantarctic Mountains; TM, Thiel concordia, and has a higher 207Pb/2MPbage than the other Mountains. fractions. It is possible that this fractioncontains minor inheritance of old radiogenic Pb. However, discarding this point would make little difference to the age. for a tholeiitic basalt in the Goldie Formation (Beardmore The new ages for the Gambacorta Formation and the Group) in the central Transantarctic Mountains, and based Gorecki Felsite Member of the Patwent Formation overlap primarily on the correlation of stratigraphy along this range, within analytical error and suggest that the felsites formed Storey et al. (1992) considered that the Patuxent Formation part of a regional earliest Ordovician magmatic event. The and associated igneousrocks were most likely of data have important implications for stratigraphic relation- Neoproterozoic age (between