Rubidium-strontium age determination of part of the ...
rocks reaches deeper than into soils. Temperatures rise highest where rocks rest on fine-grained material (to a maximum of 30°C) (Miotke 1979b). Salt-enriched water in the soil is drawn to zones of maximum evaporation, where salt accumulation is especially favored. Salt crystallization within rocks causes salt fretting. I studied these processes with regard to tafoni forming at Bull Pass. X-ray analysis applied to salt samples from the Darwin Mountains mainly showed calcite, gypsum, thenardite, and mirabilite. Salt samples from Bull Pass showed halite, calcite, gypsum, and thenardite. Chemical analysis of salt compositions and thin-sections of salts in rock are in progress. Although running water on slopes is very rare and soils are dry, there seems to be a general movement indicated by a predominant downslope orientation of the long axle of debris (figure 2). The upper ends of large boulders sticking out of loose, fine-grained soils often are covered by slope debris; the lower ends in the shadow of restrictions, show a material deficit.
I- flttfll
Jc '
0
&
•••
axle
:
0 Slope 0
Figure 2. Slope creep at Bull Pass, Dry Valleys, Antarctica.
Wind activity also affects slope formation (Miotke 1979a). When fine grains are blown out, bigger rock particles start to slide, orienting their long axis downslope. Talus cones in the Darwin Mountains proved to be quite active. Their slopes are unstable; rocks start sliding and rolling when
Rubidium-strontium age determination of part of the basement complex of the Brown Hills, central Transantarctic Mountains ROBERT P. FELDER
and GUNTER FAURE
Institute of Polar Studies and Department of Geology and Mineralogy The Ohio State University Columbus, Ohio 43210 16
-
LI
p.
Figure 3. Bull Pass, Dry Valleys: The photograph illustrates the heating up of rocks by sun radiation.
walked on. Most of the slopes in the Darwin Mountains also show polygons The field party, working from 12 November 1978 to 14 January 1979, included Ran Gerson, of the University of Jerusalem Geographical Institute, and Bernd Janke of the University of Hannover Geographical Institute. This work was supported by National Science Foundation grant DPP 77-22182 and Deutsche Forschungsgemeinschaft. References Gerber, E. and Scheidegger, A. E. 1969. Stress-induced weathering of rock masses. Eclogae geologicae Helvetiae, 62(2), 401415. Miotke, F. -D. 1979. Formung und Formungsgeschwindigkeit von Windkantern in Victoria-Land, Antarktis. Polarforschung, 49(1), 30-43. (a) Miotke, F. -D. 1979. Zur physikalischen Verwitterung im Taylor Valley, Victoria-Land, Antarktis. Polarforschung, 49(2), 117-142. (b)
During the 1978-79 field season, we collected approximately 35 samples of the crystalline basement complex of the Brown Hills (158°33'E 79°46'S) to determine the age and cooling history of this portion of the Transantarctic Mountains. We did the field work between 5 November and 10 December 1978. Logistics included helicopter support from the Darwin Glacier camp and extended stays at various remote camps near the Darwin and Byrd Glaciers. The rocks in the study area have been described by Haskell, Kennett, and Prebble (1963, 1965) and by Grindley and Laird (1969), and have been subdivided on the basis of textural and structural criteria into the Carlyon Granodiorite, the Mt. Rich Granite, and the Hope Granite. Field observations indicate that the Carlyon grades into the Mt. Rich and that both are intruded by the Hope Granite. ANTARCTIC JOURNAL
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
I- flttfll
Jc '
0
&
•••
axle
:
0 Slope 0
Figure 2. Slope creep at Bull Pass, Dry Valleys, Antarctica.
Wind activity also affects slope formation (Miotke 1979a). When fine grains are blown out, bigger rock particles start to slide, orienting their long axis downslope. Talus cones in the Darwin Mountains proved to be quite active. Their slopes are unstable; rocks start sliding and rolling when
Rubidium-strontium age determination of part of the basement complex of the Brown Hills, central Transantarctic Mountains ROBERT P. FELDER
and GUNTER FAURE
Institute of Polar Studies and Department of Geology and Mineralogy The Ohio State University Columbus, Ohio 43210 16
-
LI
p.
Figure 3. Bull Pass, Dry Valleys: The photograph illustrates the heating up of rocks by sun radiation.
walked on. Most of the slopes in the Darwin Mountains also show polygons The field party, working from 12 November 1978 to 14 January 1979, included Ran Gerson, of the University of Jerusalem Geographical Institute, and Bernd Janke of the University of Hannover Geographical Institute. This work was supported by National Science Foundation grant DPP 77-22182 and Deutsche Forschungsgemeinschaft. References Gerber, E. and Scheidegger, A. E. 1969. Stress-induced weathering of rock masses. Eclogae geologicae Helvetiae, 62(2), 401415. Miotke, F. -D. 1979. Formung und Formungsgeschwindigkeit von Windkantern in Victoria-Land, Antarktis. Polarforschung, 49(1), 30-43. (a) Miotke, F. -D. 1979. Zur physikalischen Verwitterung im Taylor Valley, Victoria-Land, Antarktis. Polarforschung, 49(2), 117-142. (b)
During the 1978-79 field season, we collected approximately 35 samples of the crystalline basement complex of the Brown Hills (158°33'E 79°46'S) to determine the age and cooling history of this portion of the Transantarctic Mountains. We did the field work between 5 November and 10 December 1978. Logistics included helicopter support from the Darwin Glacier camp and extended stays at various remote camps near the Darwin and Byrd Glaciers. The rocks in the study area have been described by Haskell, Kennett, and Prebble (1963, 1965) and by Grindley and Laird (1969), and have been subdivided on the basis of textural and structural criteria into the Carlyon Granodiorite, the Mt. Rich Granite, and the Hope Granite. Field observations indicate that the Carlyon grades into the Mt. Rich and that both are intruded by the Hope Granite. ANTARCTIC JOURNAL
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
