Granulite-Facies Metamorphism at Molodezhnaya Station, East Antarctica
Granulite-facies quartzofeldpathic gneisses metamorphosed 1000 m.y. ago are exposed around Molodezhnaya Station (67°40'S, 46°E) in East Antarctica. In addition to quartz, K-feldspar, and plagioclase, the fourteen samples studied in detail consist of the assemblages biotite-orthopyroxene-magneti...
| Published in: | Journal of Petrology |
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| Main Author: | |
| Format: | Text |
| Language: | English |
| Published: |
Oxford University Press
1981
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| Online Access: | http://petrology.oxfordjournals.org/cgi/content/short/22/3/297 https://doi.org/10.1093/petrology/22.3.297 |
| Summary: | Granulite-facies quartzofeldpathic gneisses metamorphosed 1000 m.y. ago are exposed around Molodezhnaya Station (67°40'S, 46°E) in East Antarctica. In addition to quartz, K-feldspar, and plagioclase, the fourteen samples studied in detail consist of the assemblages biotite-orthopyroxene-magnetite, biotite-garnet-orthopyroxene-ilmenite± magnetite, biotite-garnet ± ilmenite ± magnetite, biotite-garnet-sillimanite-ilmenite ± rutile, and biotite-garnet-cordierite-ilmenite-(sillimanite-rutile). Garnets are pyrope-almandine (13 to 34 mol per cent pyrope). Biotite (X Fe = 0.33 to 0.57) is rich in TiO 2 (4 to 6.3 wt per cent) and its Al 2 O 3 content depends on the mineral assemblage. Orthopyroxene (X Fe = 0.45 to 0.60) contains 1.5 to 3.0 weight per cent Al 2 O 3 . By and large, the minerals are chemically homogeneous and compositional variations are systematic, which indicate crystallization under equilibrium conditions. On the basis of the compositions of coexisting garnet-biotite, garnet-cordierite, garnet-plagioclase (with sillimanite), and garnet-plagioclase-orthopyroxene, temperatures and pressures during the granulite-facies metamorphism are estimated to be 700°C ± 30°C and 5.5 ± 1 kb. Water pressure apparently was significantly less than total pressure. Alteration during events following the granulite-facies metamorphism has resulted in chemical zoning in garnet, in which grain edges are more iron-rich than cores, heterogeneous biotite compositions, and anomalous trends involving MnO. Temperatures based on biotite and garnet-edge compositions range from 410 to 580°C. Differences in the chemical potential (μ) of water and oxygen in the fluid phase can explain compositional variations among the three sillimanite-bearing samples and the relatively iron-rich compositions of garnet and biotite associated with cordierite. Apparently, the water released by the formation of cordierite remained in the rock, forcing μH 2 O to increase as cordierite formed. Buffering of fluid phase composition by the mineral assemblage ... |
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