Formation of Proterozoic tholeiite intrusives in and around Cuddapah Basin, South India and their Gondwana counterparts in East Antarctica; and compositional variation in their mantle sources
Mid-late Proterozoic dikes of tholeiites and their differentiates intrude the Archean crust around the intracratonic Cuddapah basin of South India. Inside the basin, mid-Proterozoic tholeiitic flows and sills related by fractional crystallization of olivine, clinopyroxene and plagioclase, occur within the lower strata. Some of the sills inside the basin contain gabbroic xenoliths and mafic xenocrysts. The bulk composition of these sills indicate that they formed by accumulation of fractionated crystals from associated xenolith and xenocryst-free tholeiites. Olivine-melt and two-pyroxene (in xenoliths) equilibria indicate that fractionation and crystal-accumulation occurred at a depth of about 18 km (~5 kb). Calculated olivine compositions (Fo78-79) in equilibrium with the tholeiites at 5 kb and 1142-1154oC agree well with the olivine in the xenoliths (Fo78-82). The temperature range is also in agreement with temperatures (1019-1148oC at 5 kbar) obtained from two-pyroxene thermometry of the xenoliths. The calculated pressure and temperature support the existence of a mid-level magma chamber under Cuddapah basin, postulated by previous workers. New trace element data on tholeiites, both inside and outside the basin, indicate that their mantle source was metasomatically enriched in Ba, Rb and K compared to mid oceanic ridge-basalt. Comparison with East Antarctic and other Proterozoic Gondwana tholeiites indicates that the metasomatic enrichment event may have occurred at ~2.9-2.7 Ga. Differences in trace element contents and ratios (e.g., Ti/Y, Ti/Zr, Y/Nb, Zr/Nb and Ba/Rb) of the temporally related Cuddapah tholeiites indicate that the sills inside the basin may have originated from a different mantle source than the dikes outside the basin. A comparison of major element fractionation relations, trace element ratios and published initial 87Sr/86Sr indicates that the tholeiitic sills inside the Cuddapah basin and the contemporary tholeiitic dikes of Napier Complex, Enderby Land, East Antarctica (studied by other workers) had a similar magmatic history and may have originated from a similar but not same mantle source. These observations support Proterozoic reconstruction studies of Gondwanaland. The Cuddapah-Enderby Land region mostly contains low P2O5-TiO2 basalts. The Jurassic division of high P2O5-TiO2 basalts and low P2O5-TiO2 basalts into two geographic domains of Gondwanaland does not apply to these Proterozoic basalts and the Jurassic high P2O5-TiO2 domain may have resulted from a change in the mantle source between the Proterozoic and the Jurassic.