Objectives of the present study are to date the three rockunits using the Rb-Sr (rubidium-strontium) whole-rockisochron method described by Faure (1977) and to datemineral separates (biotite, hornblende) using Rb-Sr andargon-40/argon-39 techniques. This will facilitate an inter-pretation of the cooling history of the area based on theretention temperature of each mineral for the respectiveisotopes. The analytical data will be used along with fieldand petrographic observations to interpret the age, origin,and thermal history of these rock suites and to relate this tothe tectonic evolution of the Transantarctic Mountains.

Over the past year we have prepared the rock samplesfor the laboratory analyses, determined rubidium andstrontium concentrations of whole-rock samples by X-rayfluorescence, and determined the isotopic composition ofstrontium using a solid-source mass spectrometer.

A seven-point isochron has been generated for the Car-lyon Granodiorite (figure) using the computer programfrom Faure (1977), which is based on the program of York(1969). A date of 568.2 ± 9.1 million years has been calcu-lated as the estimated age of this suite, using a rubidium-87decay constant of 1.42 X 10_11 per year. The samples wereweighted according to the reciprocal of the squares of theirresiduals, which is a measure of their deviation from thebest fit line. Using this method, 90 percent of a normalpopulation would be included in the calculation at the 95percent confidence level.

Rubidium-strontium whole rock isochron of the CanyonGranodiorite from the Brown Hills.

The initial strontium-87/strontium-86 ratio was calcu-lated as 0.71222 ± .00015. It has been demonstrated byFaure (1977) that the initial 87Sr/ 86Sr ratio of mantle-derivedrocks would lie between 0.702 and 0.706. Since the calcu-lated initial ratio is well above this range, it suggests thatthe Carlyon Granodiorite is either a product of re-mobilization of crustal material, or has been contaminatedwith radiogenic strontium derived from the crust.

The Carlyon is a medium- to coarse-grained, biotite-hornblende-andesine granodiorite. It is commonly porphy-ritic, with a strongly developed foliation, sometimes almostgneissic. These textural and mineralogical criteria suggestsimilarities with the Olympus Granite Gneiss of WrightValley (77°33'S 161'30'E) (Faure, Jones, and Owen 1974)and the Lonely Ridge Granodiorite of the Nilsen Plateau(Faure, Murtaugh, and Montigny 1968). Their similar initial87Sr/ 86Sr ratios also support the possibility that these rocksare equivalents.

This research was supported by National Science Foun-dation grant DPI' 77-21505.

References

Faure, G., 1977. Principles of isotope geology. New York: Wiley andSons.

Faure, G., Jones, L. M., and Owen, L. B. 1974. Isotopic compositionof strontium and geologic history of the basement rocks inWright Valley, southern Victoria Land. New Zealand Journal ofGeology and Geophysics, 17, 611-627.

Faure, G., Murtaugh, J . M., and Montigny, R. 1968. The geologyand geochronology of the basement complex of the CentralTransantarctic Mountains. Canadian Journal of Earth Science, 5,555-560.

Grindley, G. W., and Laird, M. G. 1969. Geology of the Shackletoncoast. Antarctic map folio series (Folio 12, Plate 14). New York:American Geographical Society.

Haskell, T. R., Kennett, J . P. and Prebble, W. M. 1964. Basementand sedimentary geology of the Darwin Glacier area. In R. J.Adie (Ed.), Antarctic Geology, Amsterdam: North-HollandPublishing Co.

Haskell, T. R., Kennett, J . P., and Prebble, W. M. 1965. Geology ofthe Brown Hills and Darwin Mountains, southern VictoriaLand, Antarctica. Transactions of the Royal Society of New Zealand,2(15), 231-248.

York, D. J. 1969. Least squares fitting of a straight line with cor-related errors. Earth and Planetary Science Letters (5), 320-324.

The paleoposition of Marie ByrdLand, West Antarctica

SANKAR CHATrERJEE

Department of GeosciencesTexas Tech UniversityLubbock, Texas 79409

Marie Byrd Land and a large part of Ellsworth Landrepresent the greatest paleotectonic enigma in Antarctica.Despite its extensive ice cover, Antarctica can be dividedinto an East Antarctic shield, consisting of a Precambrian toLower Paleozoic metamorphic complex, and West Antarc-tica, made up largely of Paleozoic and Mesozoic orogenicbelts (Elliot 1975). If the antarctic ice sheet melted, EastAntarctica, after isostatic adjustment, would be largelyabove sea level (Bentley 1965). West Antarctica would con-sist of three major islands or archipelagoes, Marie Byrd

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Land, Eight Coast, and the Antarctic Peninsula, with theEllsworth Mountains and a block extending southwardpossibly forming a fourth.

Geophysical data indicate that each island in West Ant-arctica appears to be a segment of continental crustal mate-rial averaging 30 kilometers thick (Woollard 1962). Whatare the relationships of these islands to East Antarctica? Ofwhat are these islands composed? Questions concerning thevarious crustal fragments of West Antarctica are of greatinterest in plate tectonic studies.

Marie Byrd Land lies at the southern end of the PacificOcean basin, in a rather crucial position with regard to large-scale tectonic trends and reconstructions. Andean structuraland magmatic characteristics, for example, can be easilytraced along the Antarctic Peninsula, but these are not evi-dent for the mountains of Marie Byrd Land or for the Ells-worth Mountains. On the other hand, typical shield materialand continental Gondwana sediments, which are present inEast Antarctica as well as in all southern continents, arelacking in Marie Byrd Land. The prevolcanic rocks in MarieByrd Land are unfossiliferous metaclastics, metavolcanics,and a variety of granitic intrusives, providing few cluesregarding intra- and intercontinental relationships.

The presence of the Cenozoic alkaline volcanics of MarieByrd Land and the active volcanism of Mount Erebus nearMcMurdo Sound support indirect evidence of compressiveplate margin between West and East Antarctica (Molnar,Atwater, Mammerickx, and Smith 1975). However, becauseof the absence of an ophiolite suite and paired metamorphicbelts, the plate tectonic models are still open to debate(Elliot 1975).

Limited paleomagnetic data indicate that Marie ByrdLand and New Zealand were disconnected from East Ant-arctica and Australia in the late Cretaceous and have driftedinto their present positions (Scharnberger and Scharon1972). However, evidence of the closing of an ocean basinbetween Marie Byrd Land and East Antarctica during theCenozoic is lacking. Herron and Tucholke (1974) suggestedthat a spreading center may have been active beneath MarieByrd Land and could account for the Cenozoic volcanics inthis area.

Pillow lavas and glass-rich tuff-breccia deposits (hyalo-clastites) can be produced in a nonmarine environment bysubglacial eruption. LeMasurier (1972) recognized hyalo-clastites in Marie Byrd Land and concluded that an ice sheetof substantial thickness has existed continuously in MarieByrd Land since Eocene time. This may indicate the timingof drifting of West Antarctica into the present polar position.

On the other hand, a direct relationship between westernMarie Byrd Land and the northern Victoria Land of EastAntarctica has been suggested by several investigators(Lopatin, Krylov, and Alavpyshov 1974; Wade and Wil-banks 1972). The Swanson Formation of Marie Byrd Landand the Robertson Bay Group in northern Victoria Landshow strong similarities in lithologies, deformational pat-terns, and metamorphic histories. The 1,000-kilometer gapacross the Ross Sea between two sectors of Antarctica couldbe the result of depression of that central part by blockfaulting.

Thus there are two contrasting views of the paleopositionof Marie Byrd Land. One group believes that Marie ByrdLand was welded to Antarctica by plate convergence dur-ing Mesozoic or later time, though the suture zone is notwell defined. The other thinks Marie Byrd Land and north-ern Victoria Land represent segments of a single contin-uous geologic province. The question remains unresolved.

This study was initiated by F. Alton Wade and is fundedby National Science Foundation grant DPP 77-19566.

References

Bentley, C. R. 1965. The land beneath the ice. In R. J . Adie (Ed.),Antarctic Geology and Geophysics. Oslo: Universitetsforlaget.

Elliot, D. H. 1975. Tectonics of Antarctica: A review. AmericanJournal of Science, 275A, 45-106.

Herron, E. M., and Tucholke, B. E. 1974. Sea-floor magnetic patternsand basement structure in the south eastern Pacific. In P. Wors-tell (Ed.), Initial reports of the Deep Sea Drilling Project, Vol. 35.Washington, D.C.: U.S. Government Printing Office.

LeMasurier, W. E. 1972. Volcanic record of Cenozoic glacial historyof Marie Byrd Land. In R. J . Adie (Ed.), Antarctic Geology andGeophysics. Oslo: Universitetsforlaget.

Lopatin, B. G., Krylov, A. IA., and Alavpyshov, 0. A. 1974. Majortectonomagmatic stages in the development of Marie Byrd andEight Coast (West Antarctica) according to radiometric data. In-terdepartmental Committee on the Study of the Antarctica, 13, 52-60.

Molnar, P., Atwater, T., Mammerickx, J ., and Smith, S. M. 1975.Magnetic anomalies bathymetry and the tectonic evolution ofthe South Pacific since late Cretaceous. Geophysical Journal of theRoyal Astronomical Society, 40, 383-420.

Scharnberger, C. K., and Scharon, L. 1972. Paleomagnetism andplate tectonics of Antarctica. In R. J . Adie (Ed.), Antarctic Geologyand Geophysics. Oslo: Universitetsforlaget.

Wade, F. A., and Wilbanks, J . R. 1972. Geology of Marie Byrd andEllsworth Lands. In R. J . Adie (Ed.), Antarctic Geology and Geo-physics. Oslo: Universitetsforlaget.

Woollard, G. P. 1962. Crustal structure in Antarctica. In H. Wexler,M. J . Rubin, and J . E. Caskey (Eds.), Antarctic Research: TheMathew Fontaine Maury Memorial Symposium. Washington, D.C.:American Geophysical Union.

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