ISOTOPIC EVIDENCE FOR INVOLVEMENT OF CO2-BEARING MAGMAS IN GRANULITE FORMATION

IT has been suggested 1,2 that CO2-bearing magmas play an important part in the formation of granulites. These magmas crystallize directly into granulites, and also expel fluids which promote the development of granulite-facies mineral assemblages in adjacent country rocks. Direct evidence for carbonic fluids remains elusive, however, as granulite-facies aureoles adjacent to charnockitic intrusives 1 may result either from the influx of carbonic fluids derived from the intrusives or simply from extraction of water into a vapour-absent melt. Here we report results of a detailed study of carbon isotope compositions of graphite associated with a charnockite dyke from the Ponmudi quarry of south India. Higher graphite abundances and an anomalously heavy carbon isotopic Composition (delta-C-13gr = -8.1 parts per thousand) at the dyke margins indicate that externally derived CO2-rich fluids were transported into the Ponmudi paragneisses by the intrusive, and precipitated nearly quantitatively as graphite on encountering reducing country rocks. The occurrence of large orthopyroxene crystals along the dyke margins suggests that these fluids also served to dehydrate the country rocks. This dyke may thus be representative of the widespread transport of carbonic fluids in this terrain by felsic magmas.