REACTIONS AND TEXTURES IN WOLLASTONITE SCAPOLITE GRANULITES AND THEIRSIGNIFICANCE FOR PRESSURE TEMPERATURE FLUID HISTORIES OF HIGH-GRADE TERRANES

Reaction textures in wollastonite-scapolite calc-silicates provide imp ortant evidence for pressure-temperature-fluid histories in granulites . Pressure-temperature, T-a(CO2) and P-a(CO2) diagrams, calculated for appropriate mineral compositions using the internally consistent data set of Holland and Powell (1990), are presented and used to provide a framework for interpretation of the reaction textures. Garnet rims rep lacing wollastonite and scapolite in calc-silicate gneisses is a commo n texture in many granulite terranes. This texture can plausibly be de veloped in post-peak P-T histories dominated by cooling (e.g., near-is obaric cooling) and does not generally imply the infiltration of hydro us fluids. Examples from the Arunta Complex, Australia, and the Northe rn Prince Charles Mountains, Antarctica, are consistent with cooling w ithout fluid influx, at 7-8 kbar and a(CO2) of 0.3-0.5. Contrasting te xtures involving the replacement of garnet by wollastonite + scapolite or wollastonite + plagioclase symplectites, and the growth of wollast onite + plagioclase rims or coronas on scapolite-quartz boundaries, ca n be interpreted as essentially closed-system features and modelled th rough modal analysis of reaction products. In the case of the Rauer Gr oup, Antarctica, near-isothermal decompression from ca. 8 to 6 kbar at 850-800-degrees-C and a(CO2) in the range 0.35-0.45 is indicated by t hese textures, based on calculated P-T-a(CO2) grids in the CaO-Al2O3-S iO2-CO2 (CASV) and more complex systems. Calculated reactions producin g wollastonite + scapolite from garnet-bearing assemblages do not invo lve decarbonation, but may consume CO2 with increasing temperature in the absence of calcite. Hence, these calc-silicates may act as CO2 ''s inks'' on the prograde segments of clockwise P-T paths. Conversely, sc apolite-wollastonite granulites will act as sources of post-peak CO2-r ich fluids liberated upon cooling and potentially trapped elsewhere as post-peak carbonic fluid inclusions.