Wollastonite and scapolite in Precambrian calc-silicate granulites from Australia and Antarctica |
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| Authors: | RG WARREN BJ HENSEN RJ RYBURN |
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| Institution: | Bureau of Mineral Resources, Box 378, Canberra, ACT 2601, Australia; Department of Applied Geology, University of New South Wales, Box 1, Kensington NSW 2033, Australia; Bureau of Mineral Resources, Box 378, Canberra, ACT 2601, Australia |
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| Abstract: | Abstract Scapolite, wollastonite, calcite, diopside, grossular-andradite garnet and sphene occur in calc-silicate rocks in the granulite terrain of the Arunta Block, central Australia. This assemblage buffers the CO2 activity at a low value, so that any coexisting fluid phase must be H2O rich and CO2 poor ( X co2 = 0.2-0.3). In contrast, the H2O activity in the surrounding felsic and mafic granulites was low. Thus fluid activities during granulite facies metamorphism were locally buffered in various rock units and fluid flow appears to have been restricted or fluid may have been absent. Late retrograde rims of garnet and garnet-quartz separate phases formed in the high-grade stage. Formation of these rims would have required either an influx of water-rich fluid or a decrease in pressure. Evidence from the surrounding granulites shows that in one locality, the calc-silicate rocks had undergone late isobaric hydration; in another locality, minor uplift had occurred soon after peak P-T conditions. In both, scapolite had partly broken down to plagioclase-calite. A calc silicate rock from the granulite terrain of Enderby Land, Antarctica, contains scapolite, wollastonite, calcite, diopside, quartz and sphene; this assemblage also indicates low CO2 activities. In this rock, wollastonite has broken down to calcite-quartz, to indicate isobaric cooling without influx of hydrous fluid. |
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| Keywords: | Key words: Calc-silicate rocks fluid-phase composition granulites scapolite wollastonite |
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