Wollastonite and scapolite in Precambrian calc‐silicate granulites from Australia and Antarctica

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 CO 2 activity at a low value, so that any coexisting fluid phase must be H 2 O rich and CO 2 poor ( X co2 = 0.2‐0.3). In contrast, the H 2 O 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 CO 2 activities. In this rock, wollastonite has broken down to calcite‐quartz, to indicate isobaric cooling without influx of hydrous fluid.