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). Inthe 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 PatuxentFormation. 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). TheSWEAT 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

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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 1180 and 700 Ma). Because of ships and tectonic history of the Pensacola Mountains some the assumed tectonic importance of these rocks in models of which will be discussed below. involving Neoproterozoic rifting along the Transantarctic Mountains andbreak-up of a Neoproterozoicsupercon- Stratigraphic and tectonic implications. The implications of tinent involving Laurentia, we have undertaken U-Pb the early Ordovician age of the Gorecki Felsite Member for dating of zircons separated from a felsite from the Gorecki the rest of the PatuxentFormation andto stratigraphic Felsite Member of the Patuxent Formation. relationships elsewhere in the Transantarctic Mountains are notclearcut. As theseigneous rocks wereconsidered by Gambacorta Formation. The Gambacorta Formation, which Schmidt et al. (1978) to be interbeddedwith the sedimentary conformablyoverlies and is interbedded within the upper rocks ascribed to the PatuxentFormation in the Schmidt part of the Nelson Limestone in theeastern part of the and Williams hills, our new age for theGorecki Felsite Neptune Range, is a 1500 m thick sequence of felsic volcanic Member implies that atleast some of the Patuxent rocks. Although stratigraphic relationships indicate that the Formation is Cambrian and earlyOrdovician in age. volcanic rocks arelate mid-Cambrian or youngerin age, However, the Patuxent Formation cannot be entirely of this attempts to obtain an absolute age for the formation have age, because within the eastern part of the Neptune Range notbeen successful. The most widely quoted value is a in the Pensacola Mountains, deformed sedimentary rocks of Rb-Sr whole rock age of 510 f 35 Ma (Faure et al. 1979) the Patuxent Formation are older than the overlying middle based on a combined isochron for the volcanic rocks and Cambrian limestone, and,elsewhere in the Transantarctic inferred co-magmatic granites. The volcanic rocks on their Mountains,deformed rocks correlated with the Patuxent own give an age of 568 f 39 Ma. Formation are older than limestones of early Cambrian age

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Table 1. U-P6 data for the Gambacorta Felsite Member (Gambacorta Formation) and the Gorecki Felsite Member (Patuxent Formation)

Concentrations' Atomic Ratios

No. WeightFraction' U Common Pb 206pb3 Yb4 206pb4 Tb '"Pb ~'07Pb p' (W) (PPm)(PPm) Pb2MPb (PP) '"Pb 238U '"Pb235 U '"Pb Age (M4

Gambacorta Felsite, sample R.4723.11. 1A 120~m,2"NM, -TNDM 122.50.0216 10.8 0.2111560 2.2 0.0806+2 0.6367+24 0.05729+15503*6 0.72 2A 100~m,12.4143.70.0788 -2"DM2"NM, 5.9 4622 0.180 0.0808i3 0.6375+230.05723i8 5003~3 0.93 2B 180pm, TNM, -2"DM 137.00.1300 11.8 0.1754874 12.0 0.0811i20.6413i19 0.05736i.7 505i3 0.92

Gorecki Felsite, sample R.4712.98. 2"NM, -2"NM 0.0127 177.4 16.0 11.0 765 0.1771A 765 11.0 EOpm,16.0 177.40.0127 -2"NM 2"NM, 0.0805i2 0.6390*23 0.05756+13 513+5 0.76 2A 0.1551655150~m, 17.0 26.1-2'M,lO"NM 285.4 0.0227 0.0853+20.6949+220.059113+9 0.87571*3 3A 100Fm, 1O0M,15"NM0.1782778 0.2 15.1176.7 0.0129 0.0804+20.6353+200.057316iY 504+3 0.87

' Average grain size, and magnetic properties of zircon fractions on a Frantz LB-l Magnetic Barrier Separator, operated at a given tilt angle. NM: non-magnetic. NDM: non-diamagnetic. DM: diamagnetic. All fractions were abraded. 'Errors on sample weights, and therefore on U and Pb concentrations, are approximately 20%. Common Pb corrected for assumed blanks. 'Measured ratio corrected for fractionation and spike. 4Correctedfor fractionation, S ike, laboratorPb and U blanks, and initial commonPb (Stacey & Kramers 1975). Assumed Pb blankcomposition is: 206 g, Pb/''Pb = 18.30 207Pb/2 Pb = 15.56; 2dPb/2MPb = 37.63. Blanks during the analysis of Gambacorta Felsite samples are estimated at 6.5 pg for Pb and 2 pg for U. Blanks during Gorecki Felsite analysis are estimated at 4 pg for Pb and 2 pg for U. Errors are quoted at the 2a level, and refer to the last digits of isotopic ratios and ages. Data were reduced using the method of Ludwig (1989). Errors on measured ages propogated through the data reduction calculations were *2 standard errors of the mean. Correlation coefficient of zmPb/23sU to 206Pb/z'3U is calculated using the procedures and algorithm of Ludwig (1989).

(Laird et al. 1971). These relationshipsand our new data these rocks, which include thick pillow basalt sequences, lead us to conclude thateither the top of the Patuxent combined with geological evidence, that Storey et al. (1992) Formation was diachronous, or, more likely, thatthe based their interpretation of a continental rift setting. The sedimentary and volcanic rocks in the Schmidt and Williams basaltic lavas and sills are hypersthene-normative olivine hills are not part of the Patuxent Formation but belong to a tholeiites which have major and trace element abundances stratigraphically youngerformation. The new data also characteristic of present-daytransitional (T)-type MORB. make the model of Rowel1 et a1 (1992), who speculated that 500 values of +l to +2.5 are lower thanexpected for the Patuxent Formation could have been the slope or basin normal ocean floor tholeiites, and may be indicative of an deposit outboard of the Cambrian carbonate marine enriched asthenospheric or lithospheric mantle source that is platforms more tenable,although we are uncertain of a often tapped during initial rifting events. comparable tectonic setting where deformed clastic turbidite Cambrian rift sequences are not unique to this part of deposits are progressively overridden by a carbonate the palaeo-Pacific margin of Gondwana. Inthe Ellsworth platform sequence. Mountains, one of the displaced crustal blocks within West Our new U-Pb data also suggest thatthe closely Antarctica, thick basaltic lavas interbedded with sedimen- associated mafic volcanic rocks of the Patuxent Formation, tary rocks of mid-Cambrian age have geochemical on which we have no precise ages, may also beearliest characteristics which suggest formationin an extensional Ordovician in age. It was primarily on the geochemistry of setting (Vennum & Storey 1987). We suggest therefore that the early Ordovician felsites and associated mafic rocks in the Pensacola Mountains, early Ordovician porphyries in the 0.088 neighbouring Thiel Mountains (Pankhurst et al. 1983) and basaltic rocks in the Ellsworth Mountains formed during a protractedperiod of crustalextension during Cambrian- 0.086 - Ordovician times. It followed a period of pre-mid-Cambrian compression asindicated by the Beardmore folding event 3 - and was rapidly followed by compressional deformation, at 3 o.oe4 least in the Pensacola Mountains, during the Ordovician P Ross Orogeny. 0.082 - Implications for the SWEAT hypothesis. The early Ordovician age for at least the felsic component of bimodal 0.080 magmatism in the Pensacola Mountainsmeans that these rocks cannot now be considered as evidence in support of 0.078 theSWEAT hypothesis and the separation of Laurentia 0.62 0.64 0.700.66 0.68 from this margin in Neoproterozoic times. Equally, it does 20'Pb/n5U not refute the hypothesis. It is tempting to suggest that this Fig. 2. U-Pb zircon ages for the Gambacorta and Gorecki felsites. Cambrian-early Ordovician rifting and extensionalevent Data were plotted using 'Isoplot' (Ludwig, 1990). could represent the time of North American separation from

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Antarcticaas in the original idea of Moores (1991) and Gondwanainteraction andthe origin of the Appalachian-Andean mountain system. Geological Society of America Bulletin, 106,243-252. Dalziel (1991)' and not in Neoproterozoic time (Dalziel FAURE,G., EASTIN,R., RAY,P.T., MCCLELLAND,D. & SCHULTZ,C.G. 1979. 1992). The data also tie in more closely with the original Geochronology of igneous and matamorphicrocks, centralTransant-

uostulated age of continental break-up based onthermal ~ ~ ~~ & I arcticmountains. In: LASKER.B. RAJA RAO. C.S. feds)\, Fourth subsidencecurves forthe westernCordilleran margin of International Gondwana Symposium. Hindustan Publishing Corporation, North America (Bond et al. 1984). But a Cambrian rifting Delhi, 805-813. LAIRD,M.G., MANSERGH,G.D. & CHAPPELL,J.M.A. 1971. Geology of the event is not consistent with the differing Cambrian fauna1 centralNimrod Glacier area, Antarctica. New Zealand Journal of provinces forthe Laurentian and Antarcticconjugate Geology and Geophysics, 14,427-468. margins(McKerrow et al. 1992). Instead, Cambro-a LUDWIG,K.R. 1989. PBDAT: a computer program for processing Pb-U-Th Ordovician extensional episode along the Antarctic margin isotopedata, version 1.20. United State GeologicalSurvey, Open-file reports 88-542. may more readily be explained as an extensional back-arc - 1990. Isoplot: a plotting and regression program for radiogenic isotope basin setting. This is consistent with the widely held view of data, version 2.03. United States GeologicalSurvey, Open-file reports an active convergent margin during Early Palaeozoic times 88-557. (see Stump 1992). MCKERROW,W.S., SCOTESE, C.R. & BRASIER,M.D. 1992. EarlyCambrian continental reconstructions. Journal of the Geological Society, London, 149,599-606. Conclusions. New U-Pb zircon dating of felsic volcanic MOORES,E.M. 1991. The southwest U.S.-East Antarctic(SWEAT) rocks, part of abimodal magmatic suite in the Pensacola Connection: a hypothesis. Geology, 19,425-428. PALMER,A.R. & GATEHOUSE,C.G. 1972. Early and Middle Cambrian trilobite Mountains, gives an earlyOrdovician age for rocks from Antarctica. US Geologlcal Survey Professional Paper, 4%-D. previously considered tobe Neoproterozoic in age. The PANKHURST,R.J., STOREY, B.C., MILLAR,I.L., MACDONALD, D.I.M. & bimodal volcanic rocks cannot now be used in evidence for VENNUM,W.R. 1988. Cambrian-Ordovician magmatismin the Thiel theSWEAT hypothesis andtheseparation of North Mountains,Transantarctic Mountains, and implications for the Beardmore orogeny. Geology, 16,246-249. America from Antarctica in Neoproterozoic Instead, POWELL,C. McA.,LI, Z.X.,MCELHINNY, M.W., MEERT,J.G. & PARK,J.K. they may be part of a protracted period of Cambrian and 1993. Paleomagneticconstraints on timing of theNeoproterozoic earlvOrdovician magmatism associated with an earlv breakup of Rodinia and the Cambrian formation of Gondwana. Geology, Palaeozoicback-arc kxtensional event. TheSWEAT 21, 889-892. hypothesis may still be applicable, although currently the Ross, GM. 1991. Tectonicsetting of theWindermere Supergrouprevisited. Geology, 19, 1125-1128. main evidencein support of previousa episode of ROWELL.A.J.. REES.M.N. & EVANS. K.R. 1992. Evidence of maior Middle Neoproterozoic rifting along this margin is a Sm-Nd age of Cambriandeformation in the Ross orogen,Antarctica. Geology, 20, 762 24 Ma (errorsricalcuiated to be *90 Ma by Storey et 31-34. al. 1992) obtained from a basaltic lava together with mineral SCHMIDT, D.L., WILLIAMS, P.L.NELSON, W.H. 1978. Geological map Of the Schmidt hills quadrangle and part of the Gambacorta Peak quadrangle, separatesfrom an associated gabbro within the Goldie Pensacola Mountains, Antarctica. US Antarctic Research Program Map Formation in the central Transantarctic Mountains (Borg et A-& United States Geological Survey. .I al. 1990). STACEY,J.S. & KRAMERS,J.D. 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Science Letters, 26, 207-221.

Weare erateful Yto our colleaeues v of theioint BAS-US West STOREY,B.C. 1993. The changing face of latePrecambrian and early AntarcticTectonics Project who assisted with the field work during Palaeozoicreconstructions. Journal of the Geological Society, London, 150,665-668. which the analysed samples were collected, and to G. Rogers for a -, ALABASTER,T., MACDONALD, D.I.M.,MILLAR, I.L., PANKHURST,R.J & thoughtful review. DALZEL.I.W.D. i992. Umer Proterozoicrift-related rocksin the L. Pensacola Mountains, Antarctica: Precursors to supercontinent breakup? References Tectonics, 11, 1392-1405. STUMP,E. 1992. The Ross Orogenof the Transantarctic Mountains in light of BOND,G., NICKESON,A. & KOMINZ,M.A. 1984. Break-up of a supercontinent the Laurentia-Gondwana split. GSA Today, 2, 12, 25-31. between625Ma and 555Ma: new evidence and implications for -, SMITH,J.H. & SELF, S. 1986. Timing of eventsduring the late continental histories. Earth and Planetary Science Letters, 70, 325-345. Proterozoic Beardmore orogeny,Antarctica: Geological evidencefrom BORG,S.G. & DEPAOLO,D.J. 1994. Laurentia, Australia, and Antarctica as a the La Gorce Mountains. Geological Society of American Bulletin, W, LateProterozoic supercontinent: Constraints from isotopicmapping. 953-%5. Geology, 22, 307-310. VAUGHAN,A.P.M. & MILLAR,I.L. In press. EarlyCretaceous magmatism --, & SMITH, B.M. 1990.Isotopic structure and tectonics of the central during extensional deformation within the Antarctic Peninsula magmatic Transantarctic Mountains. Journal of GeophysicalResearch, 95, arc. Journal of South American Earth Sciences. 6647-6667. VENNUM,W.R. & STOREY, B.C. 1987. Correlation of Gabbroic and Diabasic DALZIEL,I.W.D. 1991. Pacific margins of Laurentia and EastAntarctica- rocks from the Ellsworth Mountains, Hart Hills, and Thiel Mountains, Australia as aconjugate rift pair: Evidence and implications for an West Antarctica. In: MCKENZIE,G. D. (ed.) Gondwana Six: Structure, Eocambrian supercontinent. Geology, 19,598-601. tectonics, and geophysics. AmericanGeophysical Union, Geophysical -1992. Antarctica; a tale of two supercontinents?Annual Reuiew of Earth Monographs, 40,129-138. and Planetary Sciences, U), 501-526. YOUNG,G.M. 1992. Late Proterozoic stratigraphy and the Canada-Australia -, DALLASALDA. L.H. & GAHAGAN,L.M. 1994. PalaeozoicLaurentia- connection. Geology, U), 215-218.

Received 23 October 1994; revised typescript accepted 30 November 1994.

Note added in proof results will be published in:

Afterthis paper was in proof, we learnt that our American ROWELL,A.J., VAN SCHMUS,W.R., MCKENNA,L.W. & EVANS,K.R. In press. colleagues have independently arrived at somewhat similar conclu- EarlyPaleozoic continental-rise deposition off EastAntarctica: the sions and have obtained almost identical U-Pb zircon ages for the Patuxent Formation of the Pensacola Mountains. Antarctic Journal of the GambacortaFormation and the Gorecki Felsite Member. Their United States.

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