The paleoposition of Marie Byrd Land, West ... amazonaws com
Objectives of the present study are to date the three rock units using the Rb-Sr (rubidium-strontium) whole-rock isochron method described by Faure (1977) and to date mineral separates (biotite, hornblende) using Rb-Sr and argon-40/argon-39 techniques. This will facilitate an interpretation of the cooling history of the area based on the retention temperature of each mineral for the respective isotopes. The analytical data will be used along with field and petrographic observations to interpret the age, origin, and thermal history of these rock suites and to relate this to the tectonic evolution of the Transantarctic Mountains. Over the past year we have prepared the rock samples for the laboratory analyses, determined rubidium and strontium concentrations of whole-rock samples by X-ray fluorescence, and determined the isotopic composition of strontium using a solid-source mass spectrometer. A seven-point isochron has been generated for the Carlyon Granodiorite (figure) using the computer program from Faure (1977), which is based on the program of York (1969). A date of 568.2 ± 9.1 million years has been calculated 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 residuals, which is a measure of their deviation from the best fit line. Using this method, 90 percent of a normal population would be included in the calculation at the 95 percent confidence level.
Rubidium-strontium whole rock isochron of the Canyon Granodiorite from the Brown Hills.
The paleoposition of Marie Byrd Land, West Antarctica SANKAR CHATrERJEE
Department of Geosciences Texas Tech University Lubbock, Texas 79409 1980 REvIEw
The initial strontium-87/strontium-86 ratio was calculated as 0.71222 ± .00015. It has been demonstrated by Faure (1977) that the initial 87Sr/ 86 Sr ratio of mantle-derived rocks would lie between 0.702 and 0.706. Since the calculated initial ratio is well above this range, it suggests that the Carlyon Granodiorite is either a product of remobilization of crustal material, or has been contaminated with radiogenic strontium derived from the crust. The Carlyon is a medium- to coarse-grained, biotitehornblende-andesine granodiorite. It is commonly porphyritic, with a strongly developed foliation, sometimes almost gneissic. These textural and mineralogical criteria suggest similarities with the Olympus Granite Gneiss of Wright Valley (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 initial 87Sr/ 86Sr ratios also support the possibility that these rocks are equivalents. This research was supported by National Science Foundation grant DPI' 77-21505. References Faure, G., 1977. Principles of isotope geology. New York: Wiley and Sons. 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. York, D. J . 1969. Least squares fitting of a straight line with correlated errors. Earth and Planetary Science Letters (5), 320-324.
Marie Byrd Land and a large part of Ellsworth Land represent the greatest paleotectonic enigma in 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 Antarctica, made up largely of Paleozoic and Mesozoic orogenic belts (Elliot 1975). If the antarctic ice sheet melted, East Antarctica, after isostatic adjustment, would be largely above sea level (Bentley 1965). West Antarctica would consist of three major islands or archipelagoes, Marie Byrd 17
Land, Eight Coast, and the Antarctic Peninsula, with the Ellsworth Mountains and a block extending southward possibly forming a fourth. Geophysical data indicate that each island in West Antarctica appears to be a segment of continental crustal material averaging 30 kilometers thick (Woollard 1962). What are the relationships of these islands to East Antarctica? Of what are these islands composed? Questions concerning the various crustal fragments of West Antarctica are of great interest in plate tectonic studies. Marie Byrd Land lies at the southern end of the Pacific Ocean basin, in a rather crucial position with regard to largescale tectonic trends and reconstructions. Andean structural and magmatic characteristics, for example, can be easily traced along the Antarctic Peninsula, but these are not evident for the mountains of Marie Byrd Land or for the Ellsworth Mountains. On the other hand, typical shield material and continental Gondwana sediments, which are present in East Antarctica as well as in all southern continents, are lacking in Marie Byrd Land. The prevolcanic rocks in Marie Byrd Land are unfossiliferous metaclastics, metavolcanics, and a variety of granitic intrusives, providing few clues regarding intra- and intercontinental relationships. The presence of the Cenozoic alkaline volcanics of Marie Byrd Land and the active volcanism of Mount Erebus near McMurdo Sound support indirect evidence of compressive plate margin between West and East Antarctica (Molnar, Atwater, Mammerickx, and Smith 1975). However, because of the absence of an ophiolite suite and paired metamorphic belts, the plate tectonic models are still open to debate (Elliot 1975). Limited paleomagnetic data indicate that Marie Byrd Land and New Zealand were disconnected from East Antarctica and Australia in the late Cretaceous and have drifted into their present positions (Scharnberger and Scharon 1972). However, evidence of the closing of an ocean basin between Marie Byrd Land and East Antarctica during the Cenozoic is lacking. Herron and Tucholke (1974) suggested that a spreading center may have been active beneath Marie Byrd Land and could account for the Cenozoic volcanics in this area. Pillow lavas and glass-rich tuff-breccia deposits (hyaloclastites) can be produced in a nonmarine environment by subglacial eruption. LeMasurier (1972) recognized hyaloclastites in Marie Byrd Land and concluded that an ice sheet of substantial thickness has existed continuously in Marie Byrd Land since Eocene time. This may indicate the timing of drifting of West Antarctica into the present polar position.
18
On the other hand, a direct relationship between western Marie Byrd Land and the northern Victoria Land of East Antarctica has been suggested by several investigators (Lopatin, Krylov, and Alavpyshov 1974; Wade and Wilbanks 1972). The Swanson Formation of Marie Byrd Land and the Robertson Bay Group in northern Victoria Land show strong similarities in lithologies, deformational patterns, and metamorphic histories. The 1,000-kilometer gap across the Ross Sea between two sectors of Antarctica could be the result of depression of that central part by block faulting. Thus there are two contrasting views of the paleoposition of Marie Byrd Land. One group believes that Marie Byrd Land was welded to Antarctica by plate convergence during Mesozoic or later time, though the suture zone is not well defined. The other thinks Marie Byrd Land and northern Victoria Land represent segments of a single continuous geologic province. The question remains unresolved. This study was initiated by F. Alton Wade and is funded by 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. American Journal of Science, 275A, 45-106. Herron, E. M., and Tucholke, B. E. 1974. Sea-floor magnetic patterns and basement structure in the south eastern Pacific. In P. Worstell (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 history of Marie Byrd Land. In R. J . Adie (Ed.), Antarctic Geology and Geophysics. Oslo: Universitetsforlaget. Lopatin, B. G., Krylov, A. IA., and Alavpyshov, 0. A. 1974. Major tectonomagmatic stages in the development of Marie Byrd and Eight Coast (West Antarctica) according to radiometric data. Interdepartmental 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 of the South Pacific since late Cretaceous. Geophysical Journal of the Royal Astronomical Society, 40, 383-420. Scharnberger, C. K., and Scharon, L. 1972. Paleomagnetism and plate tectonics of Antarctica. In R. J . Adie (Ed.), Antarctic Geology and Geophysics. Oslo: Universitetsforlaget. Wade, F. A., and Wilbanks, J . R. 1972. Geology of Marie Byrd and Ellsworth Lands. In R. J . Adie (Ed.), Antarctic Geology and Geophysics. Oslo: Universitetsforlaget. Woollard, G. P. 1962. Crustal structure in Antarctica. In H. Wexler, M. J . Rubin, and J . E. Caskey (Eds.), Antarctic Research: The Mathew Fontaine Maury Memorial Symposium. Washington, D.C.: American Geophysical Union.
ANTARCTIC JOURNAL
Rubidium-strontium whole rock isochron of the Canyon Granodiorite from the Brown Hills.
The paleoposition of Marie Byrd Land, West Antarctica SANKAR CHATrERJEE
Department of Geosciences Texas Tech University Lubbock, Texas 79409 1980 REvIEw
The initial strontium-87/strontium-86 ratio was calculated as 0.71222 ± .00015. It has been demonstrated by Faure (1977) that the initial 87Sr/ 86 Sr ratio of mantle-derived rocks would lie between 0.702 and 0.706. Since the calculated initial ratio is well above this range, it suggests that the Carlyon Granodiorite is either a product of remobilization of crustal material, or has been contaminated with radiogenic strontium derived from the crust. The Carlyon is a medium- to coarse-grained, biotitehornblende-andesine granodiorite. It is commonly porphyritic, with a strongly developed foliation, sometimes almost gneissic. These textural and mineralogical criteria suggest similarities with the Olympus Granite Gneiss of Wright Valley (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 initial 87Sr/ 86Sr ratios also support the possibility that these rocks are equivalents. This research was supported by National Science Foundation grant DPI' 77-21505. References Faure, G., 1977. Principles of isotope geology. New York: Wiley and Sons. 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. York, D. J . 1969. Least squares fitting of a straight line with correlated errors. Earth and Planetary Science Letters (5), 320-324.
Marie Byrd Land and a large part of Ellsworth Land represent the greatest paleotectonic enigma in 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 Antarctica, made up largely of Paleozoic and Mesozoic orogenic belts (Elliot 1975). If the antarctic ice sheet melted, East Antarctica, after isostatic adjustment, would be largely above sea level (Bentley 1965). West Antarctica would consist of three major islands or archipelagoes, Marie Byrd 17
Land, Eight Coast, and the Antarctic Peninsula, with the Ellsworth Mountains and a block extending southward possibly forming a fourth. Geophysical data indicate that each island in West Antarctica appears to be a segment of continental crustal material averaging 30 kilometers thick (Woollard 1962). What are the relationships of these islands to East Antarctica? Of what are these islands composed? Questions concerning the various crustal fragments of West Antarctica are of great interest in plate tectonic studies. Marie Byrd Land lies at the southern end of the Pacific Ocean basin, in a rather crucial position with regard to largescale tectonic trends and reconstructions. Andean structural and magmatic characteristics, for example, can be easily traced along the Antarctic Peninsula, but these are not evident for the mountains of Marie Byrd Land or for the Ellsworth Mountains. On the other hand, typical shield material and continental Gondwana sediments, which are present in East Antarctica as well as in all southern continents, are lacking in Marie Byrd Land. The prevolcanic rocks in Marie Byrd Land are unfossiliferous metaclastics, metavolcanics, and a variety of granitic intrusives, providing few clues regarding intra- and intercontinental relationships. The presence of the Cenozoic alkaline volcanics of Marie Byrd Land and the active volcanism of Mount Erebus near McMurdo Sound support indirect evidence of compressive plate margin between West and East Antarctica (Molnar, Atwater, Mammerickx, and Smith 1975). However, because of the absence of an ophiolite suite and paired metamorphic belts, the plate tectonic models are still open to debate (Elliot 1975). Limited paleomagnetic data indicate that Marie Byrd Land and New Zealand were disconnected from East Antarctica and Australia in the late Cretaceous and have drifted into their present positions (Scharnberger and Scharon 1972). However, evidence of the closing of an ocean basin between Marie Byrd Land and East Antarctica during the Cenozoic is lacking. Herron and Tucholke (1974) suggested that a spreading center may have been active beneath Marie Byrd Land and could account for the Cenozoic volcanics in this area. Pillow lavas and glass-rich tuff-breccia deposits (hyaloclastites) can be produced in a nonmarine environment by subglacial eruption. LeMasurier (1972) recognized hyaloclastites in Marie Byrd Land and concluded that an ice sheet of substantial thickness has existed continuously in Marie Byrd Land since Eocene time. This may indicate the timing of drifting of West Antarctica into the present polar position.
18
On the other hand, a direct relationship between western Marie Byrd Land and the northern Victoria Land of East Antarctica has been suggested by several investigators (Lopatin, Krylov, and Alavpyshov 1974; Wade and Wilbanks 1972). The Swanson Formation of Marie Byrd Land and the Robertson Bay Group in northern Victoria Land show strong similarities in lithologies, deformational patterns, and metamorphic histories. The 1,000-kilometer gap across the Ross Sea between two sectors of Antarctica could be the result of depression of that central part by block faulting. Thus there are two contrasting views of the paleoposition of Marie Byrd Land. One group believes that Marie Byrd Land was welded to Antarctica by plate convergence during Mesozoic or later time, though the suture zone is not well defined. The other thinks Marie Byrd Land and northern Victoria Land represent segments of a single continuous geologic province. The question remains unresolved. This study was initiated by F. Alton Wade and is funded by 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. American Journal of Science, 275A, 45-106. Herron, E. M., and Tucholke, B. E. 1974. Sea-floor magnetic patterns and basement structure in the south eastern Pacific. In P. Worstell (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 history of Marie Byrd Land. In R. J . Adie (Ed.), Antarctic Geology and Geophysics. Oslo: Universitetsforlaget. Lopatin, B. G., Krylov, A. IA., and Alavpyshov, 0. A. 1974. Major tectonomagmatic stages in the development of Marie Byrd and Eight Coast (West Antarctica) according to radiometric data. Interdepartmental 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 of the South Pacific since late Cretaceous. Geophysical Journal of the Royal Astronomical Society, 40, 383-420. Scharnberger, C. K., and Scharon, L. 1972. Paleomagnetism and plate tectonics of Antarctica. In R. J . Adie (Ed.), Antarctic Geology and Geophysics. Oslo: Universitetsforlaget. Wade, F. A., and Wilbanks, J . R. 1972. Geology of Marie Byrd and Ellsworth Lands. In R. J . Adie (Ed.), Antarctic Geology and Geophysics. Oslo: Universitetsforlaget. Woollard, G. P. 1962. Crustal structure in Antarctica. In H. Wexler, M. J . Rubin, and J . E. Caskey (Eds.), Antarctic Research: The Mathew Fontaine Maury Memorial Symposium. Washington, D.C.: American Geophysical Union.
ANTARCTIC JOURNAL
