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 num ber of reaction textures including garnet coronas between calcite and scapolite; garnet-quartz coronas between scapolite and wollastonite an d between plagioclase and wollastonite; calcite-quartz intergrowths in wollastonite; and calcite-plagioclase symplectites in scapolite. Thes e textures have been modelled using petrogenetic grids for reactions i n the CaO-Al2O3-SiO2-CO2 system, but with reduced mineral activities t o account for additional components in real mineral compositions. Such fixed-composition reduced-activity grids are strictly valid only at t he point in P-T-a(CO2) space where an assemblage last equilibrated, an d do not show the true positions of reactions away from this point bec ause mineral compositions change with reaction progress. In this case, however, mineral compositions close to end-member values and low exte nts of reaction progress mean that compositional change was limited an d the grids are good approximations to true pseudosections over the en tire P-T-a(CO2) range of interest. The grids show that the textures ar e consistent with near-isobaric cooling from about 850 to 700 degrees C at 7 kbar, a P-T path compatible with thermobarometric studies of ot her lithologies from the area. Phase relationships indicate that CO2 a ctivities were buffered by the local mineral assemblage during peak an d retrograde metamorphism, either under fluid-absent conditions or wit hin a non-pervasive fluid phase. Previous studies of garnet coronas in scapolite-wollastonite calc-silicates have used qualitative grids bas ed on limited experimental data to invoke garnet growth during water i nfiltration at high temperature, but the grids used here show that gar net coronas can form on cooling, without any need for water influx.