GARNET CORONAS IN SCAPOLITE-WOLLASTONITE CALC-SILICATES FROM EAST ANTARCTICA - THE APPLICATION AND LIMITATIONS OF ACTIVITY-CORRECTED GRIDS

Calc-silicate boudins within Proterozoic granulite facies gneisses of the northern Prince Charles Mountains, East Antarctica, preserve a number of reaction textures including garnet coronas between calcite and scapolite; garnet-quartz coronas between scapolite and wollastonite and between plagioclase and wollastonite; calcite-quartz intergrowths in wollastonite; and calcite-plagioclase symplectites in scapolite. These textures have been modelled using petrogenetic grids for reactions in the CaO-Al2O3-SiO2-CO2 system, but with reduced mineral activities to account for additional components in real mineral compositions. Such fixed-composition reduced-activity grids are strictly valid only at the point in P-T-a(CO2) space where an assemblage last equilibrated, and do not show the true positions of reactions away from this point because mineral compositions change with reaction progress. In this case, however, mineral compositions close to end-member values and low extents of reaction progress mean that compositional change was limited and the grids are good approximations to true pseudosections over the entire P-T-a(CO2) range of interest. The grids show that the textures are consistent with near-isobaric cooling from about 850 to 700 degrees C at 7 kbar, a P-T path compatible with thermobarometric studies of other lithologies from the area. Phase relationships indicate that CO2 activities were buffered by the local mineral assemblage during peak and retrograde metamorphism, either under fluid-absent conditions or within a non-pervasive fluid phase. Previous studies of garnet coronas in scapolite-wollastonite calc-silicates have used qualitative grids based on limited experimental data to invoke garnet growth during water infiltration at high temperature, but the grids used here show that garnet coronas can form on cooling, without any need for water influx.