Incipient charnockite: Characterization at the type localities

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Fifty years after the discovery of incipient charnockite (orthopyroxene-bearing granitic rocks) partially replacing amphibole–biotite gneiss in southern India, this phenomenon has been found in transitional amphibolite facies to granulite facies terrains on every continent, and may be a characteristic feature of progressive deep-crustal metamorphism. The alteration is essentially a metasomatic transformation. Mass balance calculations for localities in southern India and Sri Lanka for which analytic data are most comprehensive show that the main reaction producing orthopyroxene in orthogneiss is breakdown of hornblende by reaction with biotite, with loss of FeO, MgO, TiO2, and CaO, as well as H2O from the host rock. There is concomitant gain of SiO2. No significant change of alkalis can be demonstrated as essential to the transformation, though advanced charnockitic alteration at the type locality of Kabbal, Karnataka may also show large increase of K2O, with removal of mafic constituents.Very similar orthopyroxene-bearing veins in garnet–biotite gneiss at the Ponmudi type locality in Kerala show definite, but more subtle metasomatism, with loss of FeO and MgO and increase of SiO2 and alkalis. Both the Kabbal and Ponmudi alterations occurred at mid-crustal levels: 5–6kbar and 700–750°C.It is postulated that both types of charnockitic alteration may have been produced as open-system processes by much the same kind of metasomatizing fluids – nearly coeval CO2-rich and ultrasaline fluids, based on fluid inclusion evidence. The paragneiss alteration is mainly biotite breakdown with some involvement of garnet, and leads to a distinct metamorphic facies type. The presence of graphite in some of the garnet–biotite gneisses at Ponmudi would have resulted in lower oxygen fugacity and, probably, lower H2O activity than in the similar Kabbal-type metamorphism. The fluid/rock ratios for both types were small (about 10 mass percent of the rocks), and the fluids, whatever their origin, were largely rock-governed.The source of the metasomatizing fluids has not been definitely identified. Carbon isotopes of fluid inclusions seem to show an igneous or primitive mantle source at both type localities, but ultrasaline fluids may be more plausibly explained by metasedimentary sources, and calcareous metasediments have been identified as a source of CO2 in some occurrences. Much remains to be learned about incipient charnockite and its significance in processes of crustal evolution.