Reactions and textures in wollastonite-scapolite granulites and their significance for pressure-temperature-fluid histories of high-grade terranes

Abstract

Reaction textures in wollastonite-scapolite calc-silicates provide important evidence for pressure-temperature-fluid histories in granulites. Pressure-temperature, T-aCO2 and P-aCO2 diagrams, calculated for appropriate mineral compositions using the internally consistent dataset of Holland and Powell (1990), are presented and used to provide a framework for interpretation of the reaction textures. Garnet rims replacing wollastonite and scapolite in calc-silicate gneisses is a common texture in many granulite terranes. This texture can plausibly be developed in post-peak P-T histories dominated by cooling (e.g., near-isobaric cooling) and does not generally imply the infiltration of hydrous fluids. Examples from the Arunta Complex, Australia, and the Northern Prince Charles Mountains, Antarctica, are consistent with cooling without fluid influx, at 7–8 kbar and aCO2 of 0.3–0.5. Contrasting textures involving the replacement of garnet by wollastonite+scapolite or wollastonite+plagioclase symplectites, and the growth of wollastonite+plagioclase rims or coronas on scapolite-quartz boundaries, can be interpreted as essentially closed-system features and modelled through modal analysis of reaction products. In the case of the Rauer Group, Antarctica, near-isothermal decompression from ca. 8 to 6 kbar at 850–800°C and aCO2 in the range 0.35–0.45 is indicated by these textures, based on calculated P-T-aCO2 grids in the CaOAl2O3SiO2CO2 (CASV) and more complex systems. Calculated reactions producing wollastonite+scapolite from garnet-bearing assemblages do not involve decarbonation, but may consume CO2 with increasing temperature in the absence of calcite. Hence, these calc-silicates may act as CO2 “sinks” on the prograde segments of clockwise P-T paths. Conversely, scapolite-wollastonite granulites will act as sources of post-peak CO2-rich fluids liberated upon cooling and potentially trapped elsewhere as post-peak carbonic fluid inclusions.