Objectives of the present study are to date the three rock The initial strontium-87/strontium-86 ratio was calcu- units using the Rb-Sr (rubidium-strontium) whole-rock lated as 0.71222 ± .00015. It has been demonstrated by isochron method described by Faure (1977) and to date Faure (1977) that the initial 87Sr/86Sr ratio of mantle-derived mineral separates (biotite, hornblende) using Rb-Sr and rocks would lie between 0.702 and 0.706. Since the calcu- argon-40/argon-39 techniques. This will facilitate an inter- lated initial ratio is well above this range, it suggests that pretation of the cooling history of the area based on the the Carlyon Granodiorite is either a product of re- retention temperature of each mineral for the respective mobilization of crustal material, or has been contaminated isotopes. The analytical data will be used along with field with radiogenic strontium derived from the crust. and petrographic observations to interpret the age, origin, The Carlyon is a medium- to coarse-grained, biotite- and thermal history of these rock suites and to relate this to hornblende-andesine granodiorite. It is commonly porphy- the tectonic evolution of the Transantarctic Mountains. ritic, with a strongly developed foliation, sometimes almost Over the past year we have prepared the rock samples gneissic. These textural and mineralogical criteria suggest for the laboratory analyses, determined rubidium and similarities with the Olympus Granite Gneiss of Wright strontium concentrations of whole-rock samples by X-ray Valley (77°33S 16130E) (Faure, Jones, and Owen 1974) fluorescence, and determined the isotopic composition of and the Lonely Ridge Granodiorite of the Nilsen Plateau strontium using a solid-source mass spectrometer. (Faure, Murtaugh, and Montigny 1968). Their similar initial A seven-point isochron has been generated for the Car- 87Sr/86Sr ratios also support the possibility that these rocks lyon Granodiorite (figure) using the computer program are equivalents. from Faure (1977), which is based on the program of York This research was supported by National Science Foun- (1969). A date of 568.2 ± 9.1 million years has been calcu- dation grant DPI 77-21505. lated as the estimated age of this suite, using a rubidium-87 decay constant of 1.42 X 10_11 per year. The samples were weighted according to the reciprocal of the squares of their References residuals, which is a measure of their deviation from the best fit line. Using this method, 90 percent of a normal Faure, G., 1977. Principles of isotope geology. New York: Wiley and Sons. population would be included in the calculation at the 95 percent confidence level. Faure, G., Jones, L. M., and Owen, L. B. 1974. Isotopic composition of strontium and geologic history of the basement rocks in Wright Valley, southern Victoria Land. New Zealand Journal of Geology and Geophysics, 17, 611-627. Faure, G., Murtaugh, J . M., and Montigny, R. 1968. The geology and geochronology of the basement complex of the Central Transantarctic Mountains. Canadian Journal of Earth Science, 5, 555-560. Grindley, G. W., and Laird, M. G. 1969. Geology of the Shackleton coast. Antarctic map folio series (Folio 12, Plate 14). New York: American Geographical Society. Haskell, T. R., Kennett, J. P. and Prebble, W. M. 1964. Basement and sedimentary geology of the Darwin Glacier area. In R. J. Adie (Ed.), Antarctic Geology, Amsterdam: North-Holland Publishing Co. Haskell, T. R., Kennett, J. P., and Prebble, W. M. 1965. Geology of the Brown Hills and Darwin Mountains, southern Victoria Land, Antarctica. Transactions of the Royal Society of New Zealand, 2(15), 231-248. Rubidium-strontium whole rock isochron of the Canyon York, D. J. 1969. Least squares fitting of a straight line with cor- Granodiorite from the Brown Hills. related errors. Earth and Planetary Science Letters (5), 320-324. The paleoposition of Marie Byrd Marie Byrd Land and a large part of Ellsworth Land represent the greatest paleotectonic enigma in Antarctica. Land, West Antarctica Despite its extensive ice cover, Antarctica can be divided into an East Antarctic shield, consisting of a Precambrian to Lower Paleozoic metamorphic complex, and West Antarc- SANKAR CHATrERJEE tica, made up largely of Paleozoic and Mesozoic orogenic belts (Elliot 1975). If the antarctic ice sheet melted, East Department of Geosciences Antarctica, after isostatic adjustment, would be largely Texas Tech University above sea level (Bentley 1965). West Antarctica would con- Lubbock, Texas 79409 sist of three major islands or archipelagoes, Marie Byrd 1980 REvIEw 17 Land, Eight Coast, and the Antarctic Peninsula, with the On the other hand, a direct relationship between western Ellsworth Mountains and a block extending southward Marie Byrd Land and the northern Victoria Land of East possibly forming a fourth. Antarctica has been suggested by several investigators (Lopatin, Krylov, and Alavpyshov 1974; Wade and Wil- Geophysical data indicate that each island in West Ant- banks 1972). The Swanson Formation of Marie Byrd Land arctica appears to be a segment of continental crustal mate- and the Robertson Bay Group in northern Victoria Land rial averaging 30 kilometers thick (Woollard 1962). What show strong similarities in lithologies, deformational pat- are the relationships of these islands to East Antarctica? Of terns, and metamorphic histories. The 1,000-kilometer gap what are these islands composed? Questions concerning the across the Ross Sea between two sectors of Antarctica could various crustal fragments of West Antarctica are of great be the result of depression of that central part by block interest in plate tectonic studies. faulting. Marie Byrd Land lies at the southern end of the Pacific Thus there are two contrasting views of the paleoposition Ocean basin, in a rather crucial position with regard to large- of Marie Byrd Land. One group believes that Marie Byrd scale tectonic trends and reconstructions. Andean structural Land was welded to Antarctica by plate convergence dur- and magmatic characteristics, for example, can be easily ing Mesozoic or later time, though the suture zone is not traced along the Antarctic Peninsula, but these are not evi- well defined. The other thinks Marie Byrd Land and north- dent for the mountains of Marie Byrd Land or for the Ells- ern Victoria Land represent segments of a single contin- worth Mountains. On the other hand, typical shield material uous geologic province. The question remains unresolved. and continental Gondwana sediments, which are present in East Antarctica as well as in all southern continents, are This study was initiated by F. Alton Wade and is funded lacking in Marie Byrd Land. The prevolcanic rocks in Marie by National Science Foundation grant DPP 77-19566. Byrd Land are unfossiliferous metaclastics, metavolcanics, and a variety of granitic intrusives, providing few clues regarding intra- and intercontinental relationships. References Adie (Ed.), The presence of the Cenozoic alkaline volcanics of Marie Bentley, C. R. 1965. The land beneath the ice. In R. J . Universitetsforlaget. Byrd Land and the active volcanism of Mount Erebus near Antarctic Geology and Geophysics. Oslo: American McMurdo Sound support indirect evidence of compressive Elliot, D. H. 1975. Tectonics of Antarctica: A review. Journal of Science, 275A, 45-106. plate margin between West and East Antarctica (Molnar, Herron, E. M., and Tucholke, B. E. 1974. Sea-floor magnetic patterns Atwater, Mammerickx, and Smith 1975). However, because and basement structure in the south eastern Pacific. In P. Wors- of the absence of an ophiolite suite and paired metamorphic tell (Ed.), Initial reports of the Deep Sea Drilling Project, Vol. 35. belts, the plate tectonic models are still open to debate Washington, D.C.: U.S. Government Printing Office. (Elliot 1975). LeMasurier, W. E. 1972. Volcanic record of Cenozoic glacial history of Marie Byrd Land. In R. J. Adie (Ed.), Antarctic Geology and Limited paleomagnetic data indicate that Marie Byrd Geophysics. Oslo: Universitetsforlaget. Land and New Zealand were disconnected from East Ant- Lopatin, B. G., Krylov, A. IA., and Alavpyshov, 0. A. 1974. Major arctica and Australia in the late Cretaceous and have drifted tectonomagmatic stages in the development of Marie Byrd and into their present positions (Scharnberger and Scharon Eight Coast (West Antarctica) according to radiometric data. In- 1972). However, evidence of the closing of an ocean basin terdepartmental Committee on the Study of the Antarctica, 13, 52-60. between Marie Byrd Land and East Antarctica during the Molnar, P., Atwater, T., Mammerickx, J ., and Smith, S. M. 1975. Cenozoic is lacking. Herron and Tucholke (1974) suggested Magnetic anomalies bathymetry and the tectonic evolution of the South Pacific since late Cretaceous. Geophysical Journal of the that a spreading center may have been active beneath Marie Royal Astronomical Society, 40, 383-420. Byrd Land and could account for the Cenozoic volcanics in Scharnberger, C. K., and Scharon, L. 1972. Paleomagnetism and this area. plate tectonics of Antarctica. In R. J . Adie (Ed.), Antarctic Geology and Geophysics. Oslo: Universitetsforlaget. Pillow lavas and glass-rich tuff-breccia deposits (hyalo- Wade, F. A., and Wilbanks, J . R. 1972. Geology of Marie Byrd and clastites) can be produced in a nonmarine environment by Ellsworth Lands. In R. J . Adie (Ed.), Antarctic Geology and Geo- subglacial eruption. LeMasurier (1972) recognized hyalo- physics. Oslo: Universitetsforlaget. clastites in Marie Byrd Land and concluded that an ice sheet Woollard, G. P. 1962. Crustal structure in Antarctica. In H. Wexler, of substantial thickness has existed continuously in Marie M. J. Rubin, and J. E. Caskey (Eds.), Antarctic Research: The Byrd Land since Eocene time. This may indicate the timing Mathew Fontaine Maury Memorial Symposium. Washington, D.C.: of drifting of West Antarctica into the present polar position. American Geophysical Union. 18 ANTARCTIC JOURNAL.