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