Geochronologic studies in East Antarctica
Terrestrial geology and geophysics Geochronologic studies in East Antarctica: Age of a pegmatite in Mawson charnockite E. S. GREW Department of Earth and Space Sciences University of California Los Angeles, California 90024
W. I. MANTON Geology Department The University of Texas at Dallas Richardson, Texas 75080
The Mawson charnockite is a group of hypersthene granites and granodiorites exposed over an area of at least 2,000 square kilometers along the coast of Mac. Robertson Land in East Antarctica (Crohn, 1959; Trail, 1970; Sheraton, in press). P. A. Arriens obtained a rubidium-strontium (Rb-Sr) isochron age of 1,084 ± 37 million years (initial 87Sr/86 Sr ratio of 0.729) on the charnockite at Mawson Station (67°36'S162°53'E) (Sheraton, in press; Tingey, in press). Ravich and Krylov (1964) report a whole-rock potassium-argon (K-Ar) age of 535 million years on porphyroblastic charnockite from Mawson. This paper reports results obtained as part of a geochronologic study of the crystalline rocks of East Antarctica and the Transantarctic Mountains (Grew and Manton, 1977; Grew, 1978; Grew and Halpern, 1979; Grew and Manton, 1979). These results were obtained on samples collected from a pegmatite at Mawson in February 1978, when F. S. Grew was exchange scientist with the Australian National Antarctic Research Expedition (ANARE). Pegmatites in the charnockite at Mawson form isolated veins and pods that range in width from a few centimeters to 45 centimeters and are roughly perpendicular to the north-northeast trending foliation of the charnockite (Crohn, 1959; Grew, unpublished field notes). The pegmatites are associated with displacements of a few centimeters and with shearing of the charnockite. The pegmatite sampled for this study contains quartz, feldspar, garnet, apatite, hornblende, biotite, zircon, and perrierite. Zircon forms euhedral crystals a few millimeters long. The perrierite is metamict (amorphous to X-rays), dark brown, and translucent only in thin slivers.
It was identified by the X-ray diffraction pattern obtained after heating fragments in air and a reducing atmosphere at 700° to 1,000° C (Lima-de-Faria, 1962). The Mawson perrierite, as well as perrierite from Enderby Land (see below), melted at 1,000° C. The perrierite and zircon were picked by hand and analyzed for uranium and lead isotopes using methods outlined previously (Grew, 1978). The data are given in the accompanying table. Lead isotope ratios corrected for an 850-million-years-old lead are plotted in the figure. The zircon data are nearly concordant at 850 million years. The perrierite data also indicate an age of 850 million years (see figure). However, as the perrierite contains a large proportion of common lead, the values for the daughter to parent (Pb/U) ratios are associated with considerable uncertainty. We conclude that both minerals and the pegmatite crystallized 850 million years ago and that the perrierite lost some lead after that time. Available data are not sufficient to determine whether the lead loss is related to the whole-rock K-Ar date of 535 million years reported by Ravich and Krylov (1964). An age of 850 million years for the pegmatite is consistent with Arriens' Rb-Sr isochron age of 1,084 ± 37 million years on the charnockite. The Pb/U data indicate that minor plutonic activity and deformation occurred about 200-250 million years after closure of the Rb-Sr isotopic system in the charnockite. The pegmatite at Mawson is nearly coeval with a pegmatite dated by zircons at 895 million years in granulite-facies rocks at
016 014 0.12 3 CD
010
0-0,08 0 N
006 004 02
0.02 0
I •01
0,2 04 0.6 0.8 10 1.2 14 1.6 18 2.0
2O7Pb/235U
Concordia diagram for minerals in a pegmatite in Mawson charnockite at Mawson Station, East Antarctica.
Reinbolt Hills (70°28'S/72°27'E), 500 kilometers southeast of Mawson Station (Grew and Manton, 1977). The Mawson charnockite and metamorphic rocks at Reinbolt Hills are part of a late Proterozoic high-grade terrain that extends south from the coast near Mawson Station and that was affected by metamorphic and plutonic activity 800-1,100 million years ago (Tingey, in press). The pegmatites at Mawson Station and Reinbolt Hills thus appear to represent a late phase of this Proterozoic activity. The results obtained on perrierite from Enderby Land (Grew and Manton, 1979) and from Mawson Station suggest that perrierite may be suitable for age determination with Pb/U isotopes. To our knowledge, perrierite has not been used previously for this purpose. One difficulty that may limit the use of perrierite for age determination is common lead; two of the three samples we analyzed had a large proportion of 204Pb. To our knowledge, Mount Charles and Rippon Glacier in Enderby Land (Grew and Manton, 1979) and Mawson Station are the first reported occurrences of perrierite in Antarctica. Chevkinite, a mineral with a crystal structure closely related to that of perrierite, as well as to that of allanite, has also been reported from Enderby Land (Atrashenok et al., 1967). Perrierite and chevkinite may be more common in high-grade metamorphic terrains and in charnockite than is generally realized. Perrierite and chevkinite may take the place of allanite, which is a widespread accessory mineral in amphibolite-facies terrains and in noncharnockitic granitic rocks. We thank the Antarctic Division of the Australian Department of Science and Environment for permitting E. S. Grew to participate in ANARE in 1977-78. We also thank Warren Thomas and William Carlson of the University of California at Los Angeles for performing the heating experiments on perrierite under a reducing atmosphere. This research has been supported by National Science Foundation grant DPP 76-80957.
References Atrashenok, L. Ya., G. V. Avdzeyko, A. V. Klimov, A. Ya. Kryb y , and Yu. I. Silin. 1967. Sravnitelnyye dannyye po absoIyutonomu vozrastu porod Antarktidy (svintsovyy i argonyy metody) [Comparative data on absolute ages of antarctic rocks (lead and argon methods)]. In "Voprosy datirovki drevneyshikh (katarkheyskikh) geobogicheskikh obrazovaniy i osnovnykh porod" [Questions of the dating of the most ancient (Katarchean) geological fbrmations and of basic rocks], pp. 227-29. Moscow: lzdatelstvo. Nauka. Crohn, P. W. 1959. A contribution to the geology and glaciology of the western part of Australian Antarctic Territory. Bureau of Mineral Resources, Geology and Geophysics Australia Bulletin, report 52. Grew, E. S. 1978. Precambrian basement at Molodezhnaya Station, East Antarctica. Geological Society of America Bulletin, 89: 801-13. Grew, E. S., and M. Halpern. 1979. Rubidium-strontium dates from the Shackleton Range Metamorphic Complex in the Mount Provender area, Shackleton Range, Antarctica. journal of Geology, 87: 325-32. Grew, E. S., and W. 1. Manton. 1977. Age of zircons from pegmatite at Reinbolt Hills, Ingrid Christensen Coast, Antarctica (70°30'S, 72°30'E). Transactions of the American Geophysical Union, 58: 1250 [abstract]. Grew, E. S., and W. I. Manton. 1979. Archean rocks in Antarctica: 2.5 billion year uranium-lead ages of pegmatites in Enderby Land. Science (in press). Lima-de-Faria, J . 1962. Heat treatment of chevkinite and perrierite. Mineralogical Magazine, 33: 42-47. Ravich, M. G., and A. J . Krylov. 1964. Absolute age of rocks from East Antarctica. In Antarctic Geology, ed. R. J . Adie, pp. 579-89. Amsterdam: North Holland Publishing Company. Sheraton, j. W. In press. Origin of the charnockitic rocks of Mac. Robertson Land. In Antarctic Geoscience, ed. C. Craddock. Madison: University of Wisconsin Press. Tingey, R. J . In press. The geology and geological evolution of the Prince Charles Mountains, Antarctica. Antarctic Geoscience, ed. C. Craddock. Madison: University of Wisconsin Press. Trail, D. S. 1970. ANARE 1961 geological traverses on the Mac. Robertson and Kemp Land Coast. Bureau of Mineral Resources, Geology and Geophysics, Australia, report 135.
Table 1. Uranium and lead contents and isotopic ratios of two minerals from a pegmatite at Mawson Station, East Antarctica
Mineral
U Pb 206pb*/238U 207Pb*/235U ppm ppm 206Pb/204Pb 206Pb/207Pb 206Pb/208Pb
Perrierite 962.7 612 114.5 5.208 0.2563 0.1222 1.117 Zircon .........1302 175 2.48 x 10 13.69 16.11 0.1363 1.264 *Radiogenic component
W. I. MANTON Geology Department The University of Texas at Dallas Richardson, Texas 75080
The Mawson charnockite is a group of hypersthene granites and granodiorites exposed over an area of at least 2,000 square kilometers along the coast of Mac. Robertson Land in East Antarctica (Crohn, 1959; Trail, 1970; Sheraton, in press). P. A. Arriens obtained a rubidium-strontium (Rb-Sr) isochron age of 1,084 ± 37 million years (initial 87Sr/86 Sr ratio of 0.729) on the charnockite at Mawson Station (67°36'S162°53'E) (Sheraton, in press; Tingey, in press). Ravich and Krylov (1964) report a whole-rock potassium-argon (K-Ar) age of 535 million years on porphyroblastic charnockite from Mawson. This paper reports results obtained as part of a geochronologic study of the crystalline rocks of East Antarctica and the Transantarctic Mountains (Grew and Manton, 1977; Grew, 1978; Grew and Halpern, 1979; Grew and Manton, 1979). These results were obtained on samples collected from a pegmatite at Mawson in February 1978, when F. S. Grew was exchange scientist with the Australian National Antarctic Research Expedition (ANARE). Pegmatites in the charnockite at Mawson form isolated veins and pods that range in width from a few centimeters to 45 centimeters and are roughly perpendicular to the north-northeast trending foliation of the charnockite (Crohn, 1959; Grew, unpublished field notes). The pegmatites are associated with displacements of a few centimeters and with shearing of the charnockite. The pegmatite sampled for this study contains quartz, feldspar, garnet, apatite, hornblende, biotite, zircon, and perrierite. Zircon forms euhedral crystals a few millimeters long. The perrierite is metamict (amorphous to X-rays), dark brown, and translucent only in thin slivers.
It was identified by the X-ray diffraction pattern obtained after heating fragments in air and a reducing atmosphere at 700° to 1,000° C (Lima-de-Faria, 1962). The Mawson perrierite, as well as perrierite from Enderby Land (see below), melted at 1,000° C. The perrierite and zircon were picked by hand and analyzed for uranium and lead isotopes using methods outlined previously (Grew, 1978). The data are given in the accompanying table. Lead isotope ratios corrected for an 850-million-years-old lead are plotted in the figure. The zircon data are nearly concordant at 850 million years. The perrierite data also indicate an age of 850 million years (see figure). However, as the perrierite contains a large proportion of common lead, the values for the daughter to parent (Pb/U) ratios are associated with considerable uncertainty. We conclude that both minerals and the pegmatite crystallized 850 million years ago and that the perrierite lost some lead after that time. Available data are not sufficient to determine whether the lead loss is related to the whole-rock K-Ar date of 535 million years reported by Ravich and Krylov (1964). An age of 850 million years for the pegmatite is consistent with Arriens' Rb-Sr isochron age of 1,084 ± 37 million years on the charnockite. The Pb/U data indicate that minor plutonic activity and deformation occurred about 200-250 million years after closure of the Rb-Sr isotopic system in the charnockite. The pegmatite at Mawson is nearly coeval with a pegmatite dated by zircons at 895 million years in granulite-facies rocks at
016 014 0.12 3 CD
010
0-0,08 0 N
006 004 02
0.02 0
I •01
0,2 04 0.6 0.8 10 1.2 14 1.6 18 2.0
2O7Pb/235U
Concordia diagram for minerals in a pegmatite in Mawson charnockite at Mawson Station, East Antarctica.
Reinbolt Hills (70°28'S/72°27'E), 500 kilometers southeast of Mawson Station (Grew and Manton, 1977). The Mawson charnockite and metamorphic rocks at Reinbolt Hills are part of a late Proterozoic high-grade terrain that extends south from the coast near Mawson Station and that was affected by metamorphic and plutonic activity 800-1,100 million years ago (Tingey, in press). The pegmatites at Mawson Station and Reinbolt Hills thus appear to represent a late phase of this Proterozoic activity. The results obtained on perrierite from Enderby Land (Grew and Manton, 1979) and from Mawson Station suggest that perrierite may be suitable for age determination with Pb/U isotopes. To our knowledge, perrierite has not been used previously for this purpose. One difficulty that may limit the use of perrierite for age determination is common lead; two of the three samples we analyzed had a large proportion of 204Pb. To our knowledge, Mount Charles and Rippon Glacier in Enderby Land (Grew and Manton, 1979) and Mawson Station are the first reported occurrences of perrierite in Antarctica. Chevkinite, a mineral with a crystal structure closely related to that of perrierite, as well as to that of allanite, has also been reported from Enderby Land (Atrashenok et al., 1967). Perrierite and chevkinite may be more common in high-grade metamorphic terrains and in charnockite than is generally realized. Perrierite and chevkinite may take the place of allanite, which is a widespread accessory mineral in amphibolite-facies terrains and in noncharnockitic granitic rocks. We thank the Antarctic Division of the Australian Department of Science and Environment for permitting E. S. Grew to participate in ANARE in 1977-78. We also thank Warren Thomas and William Carlson of the University of California at Los Angeles for performing the heating experiments on perrierite under a reducing atmosphere. This research has been supported by National Science Foundation grant DPP 76-80957.
References Atrashenok, L. Ya., G. V. Avdzeyko, A. V. Klimov, A. Ya. Kryb y , and Yu. I. Silin. 1967. Sravnitelnyye dannyye po absoIyutonomu vozrastu porod Antarktidy (svintsovyy i argonyy metody) [Comparative data on absolute ages of antarctic rocks (lead and argon methods)]. In "Voprosy datirovki drevneyshikh (katarkheyskikh) geobogicheskikh obrazovaniy i osnovnykh porod" [Questions of the dating of the most ancient (Katarchean) geological fbrmations and of basic rocks], pp. 227-29. Moscow: lzdatelstvo. Nauka. Crohn, P. W. 1959. A contribution to the geology and glaciology of the western part of Australian Antarctic Territory. Bureau of Mineral Resources, Geology and Geophysics Australia Bulletin, report 52. Grew, E. S. 1978. Precambrian basement at Molodezhnaya Station, East Antarctica. Geological Society of America Bulletin, 89: 801-13. Grew, E. S., and M. Halpern. 1979. Rubidium-strontium dates from the Shackleton Range Metamorphic Complex in the Mount Provender area, Shackleton Range, Antarctica. journal of Geology, 87: 325-32. Grew, E. S., and W. 1. Manton. 1977. Age of zircons from pegmatite at Reinbolt Hills, Ingrid Christensen Coast, Antarctica (70°30'S, 72°30'E). Transactions of the American Geophysical Union, 58: 1250 [abstract]. Grew, E. S., and W. I. Manton. 1979. Archean rocks in Antarctica: 2.5 billion year uranium-lead ages of pegmatites in Enderby Land. Science (in press). Lima-de-Faria, J . 1962. Heat treatment of chevkinite and perrierite. Mineralogical Magazine, 33: 42-47. Ravich, M. G., and A. J . Krylov. 1964. Absolute age of rocks from East Antarctica. In Antarctic Geology, ed. R. J . Adie, pp. 579-89. Amsterdam: North Holland Publishing Company. Sheraton, j. W. In press. Origin of the charnockitic rocks of Mac. Robertson Land. In Antarctic Geoscience, ed. C. Craddock. Madison: University of Wisconsin Press. Tingey, R. J . In press. The geology and geological evolution of the Prince Charles Mountains, Antarctica. Antarctic Geoscience, ed. C. Craddock. Madison: University of Wisconsin Press. Trail, D. S. 1970. ANARE 1961 geological traverses on the Mac. Robertson and Kemp Land Coast. Bureau of Mineral Resources, Geology and Geophysics, Australia, report 135.
Table 1. Uranium and lead contents and isotopic ratios of two minerals from a pegmatite at Mawson Station, East Antarctica
Mineral
U Pb 206pb*/238U 207Pb*/235U ppm ppm 206Pb/204Pb 206Pb/207Pb 206Pb/208Pb
Perrierite 962.7 612 114.5 5.208 0.2563 0.1222 1.117 Zircon .........1302 175 2.48 x 10 13.69 16.11 0.1363 1.264 *Radiogenic component
