Pan-African charnockite formation in Kerala, South India

Abstract Sm-Nd mineral ages of gneisses and associated granulites from the Ponmudi incipient charnockite locality (South India) indicate that granulite metamorphism occurred at, or shortly after, 558 Ma. Proterozoic ages recorded by garnet separates reflect a detrital age or an earlier metamorphic event preserved by inclusions within garnet. The age of post-metamorphic uplift (440–460 Ma) is constrained by Sr isotope equilibration between biotite and plagioclase. Since charnockite formation and subsequent uplift north of the Palghat-Cauvery shear zone had terminated by earliest Proterozoic time, these results confirm two distinct periods of granulite formation in South India and suggest that the Palghat-Cauvery shear zone represents the boundary between two blocks of strongly contrasting geological histories. Both incipient charnockite formation and subsequent uplift at Ponmudi may be correlated with the tectonothermal evolution of the Highlands Group in Sri Lanka. The similarity between Nd and Sr model ages for charnockites and gneisses from Ponmudi indicates that no significant Rb-Sr fractionation has occurred during the crustal history of these incipient charnockites. Pb isotopic ratios suggest that Th-U ratios were fractionated during charnockite formation at about 500 Ma. In contrast to charnockites found north of the Palghat-Cauvery shear zone, fractionation of U-Pb during the Archaean did not occur in the Ponmudi granulites.