Nowww

Griffin, N. L. 1%9. Paleomagnetic properties of the Dufek intrusion,Pensacola Mountains, Antarctica. Unpublished master's thesis,University of California-Riverside.

Himmelberg, G. R., and Ford, A. B. 1977. Iron-titanium oxides ofthe Dufek intrusion, Antarctica. American Mineralogist, 62,623-633.

Irving, E., Tanczyk, E., and Hastie, J. 1976. Catalogue of paleomag-netic directions and poles-3rd issue. Paleozoic results 1949-1975.(Geomagnetic Series 5). Ottawa: Geomagnetic Service of Canada.

McElhinny, M. W., and Cowley, J. A. 1978. Paleomagnetic direc-tions and pole positions-XV. Pole numbers 15/1 to 15/232.Geophysical Journal of the Royal Astronomical Society, 52, 259-276.

Uranium-lead ages of zirconsfrom Mount Provender,

Shackleton Range, TransantarcticMountains

E. S. GREW

$Department of Earth and Space SciencesUniversity of California

Los Angeles, California 90024

W. I. MANTON

Department of GeosciencesUniversity of Texas-DallasRichardson, Texas 75080

The bedrock exposures around Mount Provender (80023'S30°W) consist of amphibolite-facies metamorphic rocks ofthe Shackleton Range Metamorphic Complex and of clastic

sedimentary rocks of the Blaikiock Glacier Group (Clarkson1972; Stephenson 1966). Solovyev and Grikurov (1978) andClarkson, Hughes, and Thomson (1979) described a MiddleCambrian fauna from erratics of nearly unmetamorphosedrocks in moraine near Mount Provender. Grew and Hal-pern (1979) obtained rubidium-strontium (Rb-Sr) whole-rock isochron ages of 583 ± 48 million years and 656 ± 66million years on metamorphic rocks from this area.

We present uranium-lead (U-Pb) isotopic data and ageson zircons (tables 1 and 2) separated from the samplesanalyzed by Grew and Halpern (1979); descriptions andprecise locations of the samples are given by these authors.

The zircons are clear, euhedral, elongated crystals. Mostpassed through 100-mesh sieve cloth but were retained bythe 200-mesh cloth. No detrital cores were observed.

Data on zircon from sample 1006-2 are concordant at 500million years, while data on samples 10454 and 1029-7 lieclose to a chord 0-500 million years and data on 10174 lieclose to a chord 0-550 million years (see figure). The zirconU-Pb ages and total rock Rb-Sr isochron ages on samples1006 and 1017 (no. 1-5) are consistent within the analytical

Table 1. Uranium and lead concentrations and isotopic ratios on rocks from the Mount Provender area,Shackleton Range, Antarctica

Sample

suiteUPb

and no.(ppm)(ppm)20613b/20412b206Pb/207Pb206Pb/208Pb207PbI235U206Pb/238U

1006-224419.71.83 X 10315.36 0.87060.63270.0803

1017-413633.76.87 X 10316.4510.07 0.54030.0667

1029-757579.11.59 X 10315.1311.36 0.56790.0722

1045-474371.34.19 X 1016.35 8.2120.58990.0742

Common lead correction assuming a 500-million-year lead.

Table 2. U-Pb ages of zircon., Rb-Sr isochron ages, and a biotite Rb-Sr age of rocksfrom the Mount Provender area, Sheckleton Range, Antarctica

Age (million years)Sample

suiteNumber238U-206Pb 235U-207Pb 206Pb-20712b

Rb-Sr age Rb-Sr ageisochron biotite

Number(million years)Number(million years)a10062

515

512

4971-5'600--10174

431451

5561-5583 ± 483519 ± 1510178b -6-8656±66--1029'7466

470

490----1045C4

477484

518----

• Grew and Halpern (1979).b Too few zircons for analysis.C Rb/Sr ratios are 0.06-0.19 and are too low for Rb-Sr whole-rock age determination (Grew and Halpern 1979).

1980 REVIEW 45

008

0.06

CD3

1)oJ

0.040('I

0.02

Fe mes0.20.40.60.8

207Pb/235UConcordia plot of U-Pb data from zircon in amphibolite-fades rocks of the Mount Provender area, Shackleton Range,Antarctica.

uncertainties associated with each set of data. These uncer-tainties are particularly large for the Rb-Sr data on samplesuite 1006 (Grew and Halpern 1979).

The zircons are interpreted to have crystallized duringthe amphibolite-facies event or to have lost all of their leadduring this event. The 500-to-550-million year dates recordthe times at which the zircons started retaining lead. Thisclosure of the U-Pb system in zircon occurred soon afterfinal Rb-Sr isotopic closure in total rocks and at about thesame time as its closure in biotite. An age in the range 500to 600 million years is indicated for the metamorphism of

the Mount Provender rocks, confirming the conclusionsreached by Grew and Halpern (1979).

Stephenson (1966) and Clarkson, Hughes, and Thomson(1979) suggest that the fossiliferous erratics in morainearound Mount Provender are derived from intermediatestrata in the Blaiklock Glacier Group, the basal beds ofwhich rest unconformably on the metamorphic rocks. Aminimum age of Middle Cambrian (500-540 million years)is thus implied for the metamorphism; this minimum isconsistent with the radiometric data.

This research was supported by National Science Foun-dation grant DPP 75-17390 to the University of Californiaat Los Angeles. Grew thanks V. N. Masolov, Deputy Chiefof the 22nd Soviet Antarctic Expedition (SAE) and othermembers of the 22nd SAE for their logistic support andcooperation.

References

Clarkson, P. D. 1972. Geology of the Shackleton Range: A prelimi-nary report. British Antarctic Survey Bulletin, 31, 1-15.

Clarkson, P. D., Hughes, C. P., and Thomson, M. R. A. 1979. Geo-logical significance of Middle Cambrian fauna from Antarctica.Nature, 279, 791-792.

Grew, E. S., and Halpern, M. 1979. Rubidium-strontium dates fromthe Shackleton Range Metamorphic Complex in the MountProvender area, Shackleton Range, Antarctica. Journal of Geology,87,325-332.

Solovyev, I. A., and Grikurov, G. E. 1978. Pervyye nakhodki sred-nekembriyskikh trilobitov v khrebte Shekiton (Antarktida)[First finds of Middle Cambrian trilobites in the ShackletonRange (Antarctica)]. Antarktika Dokiady Komissii, Izdatelstvo"Nauka," Moscow, 17,187-198.

Stephenson, P.]. 1966. Geology. 1. Theron Mountains, ShackletonRange and Whichaway Nunataks (with a section on paleo-magnetism of the dolerite intrusions by D. J. Blundell). Trans-antarctic Expedition, 1955-1958: Scientific Reports, 8, 1-79.

Petrologic studies in EnderbyLand with the Australian NationalAntarctic Research Expedition,

1979-80

E. S. GREW

Department of Earth and Space SciencesUniversity of California-Los Angeles

Los Angeles, California 90024

In 1979 I joined the Enderby Land party of the Aus-tralian National Antarctic Research Expedition (ANARE) fora second field season in Enderby Land (see figure; also see

Grew 1978a). The main objectives of the 1979-80 field pro-gram were to investigate (1) the regional distribution ofosumilite and of the apparently incompatible mineralassemblages mpphirine-quartz ± garnet and sillimanite-orthopyroxene-quartz and (2) the age relations and miner-alogy of pegmatites and of mylonite zones. Between 30November 1979 and 16 February 1980,! examined in detailthe bedrock at 10 localities, collected samples for petrologicstudies and radiometric age determination, and, where pos-sible, mapped geologic structures.

Ravich and Kamenev (1975) report the sillimanite-orthopyroxene association in quartzite at three localities(sites 459, 185, and 72), all of which I studied in detailduring the 1979-80 season. Two of these localities probablyare not part of a distinct sillimanite-orthopyroxene zone.Ravich and Kamenev's (1975) sample from one of theselocalities, Reference Peak (sample 459A), may represent the

46 ANTARCrIC JOURNAL