A PETROLOGICAL AND FLUID INCLUSION STUDY OF CALC-SILICATE-CHARNOCKITEASSOCIATIONS FROM SOUTHERN KERALA, INDIA - IMPLICATIONS FOR CO2 INFLUX

Recent discovery of wollastonite-bearing calc-silicate assemblages adj acent to gneiss-charnockite horizons in the supracrustal terrain of th e Kerala Khondalite Belt, southern India, provides an opportunity to e valuate the carbonic fluid infiltration model proposed for charnockite formation. Petrological and fluid inclusion studies across these hori zons in three representative localities are presented in this study. T he calc-silicate assemblages define peak metamorphic conditions of sim ilar to 800 degrees C at 5 kbar and define a low a(CO2). Adjacent char nockite assemblages developed through dehydration involving the breakd own of garnet, biotite and quartz to produce orthopyroxene under low a (H2O) conditions. Retrograde reactions preserved in the calc-silicate rocks, such as scapolite-quartz symplectites, and the partial breakdow n of wollastonite previously has been attributed to a near isothermal decompression during which infiltration of CO2-rich fluids occurred. F luid inclusion studies indicate that the earliest generation of fluids preserved in the calc-silicate assemblages are aqueous (with salinity similar to 8 wt % NaCl equivalent), consistent with mineral phase equ ilibria defining low a(CO2). The estimation of NaCl content in brines coexisting with scapolite, based on the Cl content of the scapolite, i ndicates the presence of up to 20 wt % NaCl during the formation of sc apolite consistent with the saline primary fluid inclusions. Primary c arbonic inclusions occur within the retrogressed calcite + quartz asse mblage after wollastonite, and are considered to represent the post-pe ak metamorphic carbonic fluid infiltration event, synchronous with the development of charnockites in the adjacent gneisses. These inclusion s have identical characteristics to those in the charnockites. We envi sage that the Kerala Khondalite Belt fluid regime was largely internal ly buffered during the prograde path, and that CO2 infiltration post-d ated peak metamorphism. Influx of CO2 was mostly structurally controll ed, and occurred along a near-isothermal uplift path. Graphite-bearing pegmatitic dykes with abundant CO2-rich inclusions in these localitie s attest to the transfer of carbonic fluids through magmatic conduits.