Charnockite formation in southern India

A stepped heating gas extraction technique has been developed which is capable of isolating CO2 released by fluid inclusions from that released by contamination and other sources. In some cases specific generations of inclusions may be extracted. This technique represents a significant advance in the measurement of carbon isotopes from fluid inclusions. Isotopic results are reproducible to ± 0.2‰ for gas-rich samples, but sample heterogeneity results in variable yield measurements (occasionally up to 200%). The technique has been applied to charnockites and related rocks from South India to constrain the role of CO2 in their petrogenesis. Results from a data base of 65 samples show that charnocidtes released more inclusion-CO2 than did associated amphibolite facies gneisses, implying that CO2 plays a significant role in charnockite formation. Field observations and theoretical phase equilibria, suggest incipient charnockites (partially transformed gneiss) form by sub-solidus transformation (induced by influx of CO2) and by melting (triggered by influx of mixed CO2-H2O). This melting reaction occurs at least 50°C below vapour-absent melting, so it may be an important mechanism for granulite and granite formation in the middle and lower crust. Massive charnockites (monotonous granulite) are believed to form mainly by direct crystallisation from a H2O-poor, CO2-rich melt. δ13C values support radiogenic and field evidence for at least two charnockite formation events in South India. The 2500 Ga event at the southern margin of the Archӕan Craton yields a range of δ13C values (-4%o to -13%o), tentatively interpreted as CO2 derived from subducting sediments. The younger event (500 Ma) affects the southern blocks (of probable early Proterozoic age), and is characterised by a bimodal distribution of δ13C values (-6‰ to -7‰ and -9‰ to -13‰). A sub-continental lithospheric source of CO2 (transported by magmas associated with crustal extension) is suggested by the heavier values. The lighter isotopes result mainly from mixing between this mantle source and organic graphite, but inclusion capture during an earlier event cannot be ruled out in a few cases. CO2-rich fluids are found to propagate by advective fluid flow through a microhydraulic fracture mechanism. A detailed case study of local charnockite formation indicates that isotope and reaction fronts are diffuse, almost over the entire distance of fluid flow (60 m), and fluid/rock disequilibrium suggests that fluid-rock ratios must be treated with care