Contrasting carbon and oxygen isotopic evolution in metacarbonates from the Kerala Khondalite Belt, southern India

The Kerala Khondalite belt is a Proterozoic metasupracrustal granulite faci es terrain in southern India comprising garnet-biotite gneiss, garnet-silli manite gneiss and orthopyroxene granulites as major rock types. Gale-silica te rocks and marbles, occurring as minor lithologies in the Kerala Khondali te Belt, show different mineral assemblages and reaction histories of which indicate a metamorphic P-T- fluid history dominated by internal fluid buff ering during the peak metamorphism, followed by external fluid influx durin g decompression. The carbon and oxygen isotopic compositions of calcite fro m three representative metacarbonate localities show contrasting evolutiona ry trends. The Ambasamudram marbles exhibit carbon and oxygen isotope ratio s (delta C-13 similar to 0 parts per thousand and delta O-18 similar to 20 parts per thousand) typical of middle to late Proterozoic marine carbonate sediments with minor variation ascribed to the isotopic exchange due to the devolatilization reactions. The delta C-13 and delta O-18 values of simila r to -9 parts per thousand and 11 parts per thousand, respectively, for cal cite from calc-silicate rocks at Nuliyam are considerably low and heterogen eous. The wollastonite formation here, possibly corresponds to an earlier e vent of fluid infiltration during prograde to peak metamorphism, which resu lted in decarbonation and isotope resetting. Further, petrologic evidence s upports a model of late carbonic fluid infiltration that has partially affe cted the calc-silicate rocks, with subsequent isotope resetting, more towar ds the contact between calc-silicate rock and charnockite. At Korani, only oxygen isotopes have been significantly lowered (delta O-18 similar to 13 p arts per thousand) and the process involved might be a combination of metam orphic devolatilization accompanied by an aqueous fluid influx, supported b y petrologic evidence. The stable isotope signatures obtained from the indi vidual localities, thus indicate heterogeneous patterns of fluid evolution history within the same crustal segment.