Wollastonite--Scapolite Assemblages as Indicators of Granulite Pressure-Temperature-Fluid Histories: The Rauer Group, East Antarctica |
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Authors: | HARLEY, SIMON L. BUICK, IAN S. |
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Affiliation: | 1Department of Geology and Geophysics, Grant Institute of Geology, University of Edinburgh, Kings Buildings West Mains Road, Edinburgh EH9 3JW, Scotland 2Department of Geology, University of Tasmania GPO Box 252C, Hobart, Tasmania, Australia |
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Abstract: | Mid-Proterozoic ( 1000 Ma) granulite facies calc-silicates fromthe Rauer Group, East Antarctica, contain grossular-wollastonite-scapolite-dinopyroxene( + quartz or calcite) assemblages which preserve symplectiteand corona textures typically involving the growth of secondarywollastonite. The textures include (1) wollastonite rims betweenquartz and calcite; (2) wollastonite-plagioclase rims and intergrowthsbetween quartz and scapolite; (3) wollastonite-scapolite-clinopyroxeneinter-growths replacing grossular; and (4) wollastonite-plagioclasesymplectites replacing grossular or earlier symplectites (3). Reactions between grossular, scapolite, wollastonite, calcite,quartz, anorthite, and vapour, have been modelled in the CaO-Al2O3SiO2-H2O-CO2and more complex systems using the internally consistent data-setof Holland & Powell (1990). Reactions producing scapoliteand wollastonite consume vapour as temperature increases (i.e., carbonation), in agreement with the results of Moecher &Essene (1990). These calc-silicates can therefore behave asfluid sinks under high-grade conditions. Conversely, they maybe important fluid sources on cooling and contribute to theformation of post-metamorphic CO2rich fluid inclusions in isobaricallycooled granulites. P-T-CO2 diagrams calculated for typical phase compositions (e.g., garnet, scapolite) demonstrate that the observed texturesare a record of near-isothermal decompression at 800850 C, consistent with Prpath determinations based on otherrock types from the Rauer Group. For example, texture (2) resultsfrom crossing the reaction Scapolite + Quartz = Wollastonite + Plagioclase + V on decompression, at 6. 57 kb, 820 C, and aCO2 of0405. Furthermore, correlations betweenmodes of product phases (e. g., wollastonitexlinopyroxene) andreactant garnet composition preclude open-system behaviour inthe formation of these textures, consistent with post-peak vapour-absentreactions such as Grossular + Calcite + Quartz = Wollastonite + Scapolite occurring on decomposition at high temperatures (>800C). Reaction textures developed in calc-silicates from other granuliteterranes often involve the formation of grossular ( + quartz calcite) as rims on wollastonite-scapolite, or replacementof wollastonite by calcite-quartz. These textures have developedprincipally in response to cooling below 780810 C andmay be signatures of near-isobaric cooling. Infiltration ofhydrous fluid is not a necessary condition for the productionof garnet coronas in wollastonite-scapolite granulites. *Present address: Department of Earth Sciences, University ofMelbourne, Parkville, Victoria 3052, Australia |
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