Wollastonite--Scapolite Assemblages as Indicators of Granulite Pressure-Temperature-Fluid Histories: The Rauer Group, East Antarctica
Mid-Proterozoic (∼ 1000 Ma) granulite facies calc-silicates from the Rauer Group, East Antarctica, contain grossular-wollastonite-scapolite-dinopyroxene ( + quartz or calcite) assemblages which preserve symplectite and corona textures typically involving the growth of secondary wollastonite. The tex...
| Published in: | Journal of Petrology |
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| Main Authors: | , |
| Format: | Text |
| Language: | English |
| Published: |
Oxford University Press
1992
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| Subjects: | |
| Online Access: | http://petrology.oxfordjournals.org/cgi/content/short/33/3/693 https://doi.org/10.1093/petrology/33.3.693 |
| Summary: | Mid-Proterozoic (∼ 1000 Ma) granulite facies calc-silicates from the Rauer Group, East Antarctica, contain grossular-wollastonite-scapolite-dinopyroxene ( + quartz or calcite) assemblages which preserve symplectite and corona textures typically involving the growth of secondary wollastonite. The textures include (1) wollastonite rims between quartz and calcite; (2) wollastonite-plagioclase rims and intergrowths between quartz and scapolite; (3) wollastonite-scapolite-clinopyroxene inter-growths replacing grossular; and (4) wollastonite-plagioclase symplectites replacing grossular or earlier symplectites (3). Reactions between grossular, scapolite, wollastonite, calcite, quartz, anorthite, and vapour, have been modelled in the CaO-Al 2 O 3 SiO 2 -H 2 O-CO 2 and more complex systems using the internally consistent data-set of Holland & Powell (1990). Reactions producing scapolite and wollastonite consume vapour as temperature increases (i. e., carbonation), in agreement with the results of Moecher & Essene (1990α). These calc-silicates can therefore behave as fluid sinks under high-grade conditions. Conversely, they may be important fluid sources on cooling and contribute to the formation of post-metamorphic CO 2 rich fluid inclusions in isobarically cooled granulites. P-T-α CO2 diagrams calculated for typical phase compositions (e. g., garnet, scapolite) demonstrate that the observed textures are a record of near-isothermal decompression at 800–850 � C, consistent with P—rpath determinations based on other rock types from the Rauer Group. For example, texture (2) results from crossing the reaction Scapolite + Quartz = Wollastonite + Plagioclase + V on decompression, at ∼ 6. 5–7 kb, 820 � C, and a CO2 of 0–4–0–5. Furthermore, correlations between modes of product phases (e. g., wollastonitexlinopyroxene) and reactant garnet composition preclude open-system behaviour in the formation of these textures, consistent with post-peak vapour-absent reactions such as Grossular + Calcite + Quartz = Wollastonite + ... |
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