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
Chatterjee, Nilanjan; Bhattacharji, Somdev
published: Jul 29, 1998
ArtNo. ESP154017401002, Price: 29.00 €
Mid-late Proterozoic dike intrusions of tholeiites and their differentiates occur around the intracratonic Cuddapah Basin in the Archean basement of South India. Tholeiitic flows and sills of similar age (⁓1.55 Ga) occur within the lower strata of the Cuddapah Basin. Whereas no fractional crystallization relations exist among the tholeiites outside the basin, basalts inside the basin may be related by fractional crystallization of olivine, clinopyroxene and plagioclase. A group of sills inside the basin contain gabbroic xenoliths and mafic xenocrysts. The bulk compositions of these sills indicate that they formed by accumulation of fractionated crystals from associated xenolith and xenocryst-free tholeiites. Fractionation and crystal-accumulation occurred at a depth of about l8 km (⁓5kb) as indicated by olivine-melt and two-pyroxene (in xenoliths) equilibria. Calculated olivine compositions (Fo78-79) in equilibrium with the tholeiites at 5kb and 1142-1154 °C agree well with composition of the olivine in the xenoliths (Fo78-82). The temperature range is also in agreement with temperatures (1019-l148°C 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 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 87Si/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 identical 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-TiO2basalts 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.