Early Proterozoic Melt Generation Processes beneath the Intra-cratonic Cuddapah Basin, Southern India

Early Proterozoic tholeiitic lavas and sills were emplaced during the initial phase of extension of the intra-cratonic Cuddapah Basin, southern India. 40Ar–39Ar laser-fusion determinations on phlogopite mica, from the Tadpatri Fm mafic–ultramafic sill complex, constrain the age of the initial phase of extension and volcanism in the basin at 1·9 Ga. Despite their Early Proterozoic age, the igneous rocks are unmetamorphosed, undeformed and remarkably fresh. They exhibit a wide range in MgO contents (4–28 wt %) and have undergone varying degrees of accumulation or crystal fractionation. Variable La/Nb ratios (1·2–3·7) and εNd values (1 to −10) suggest that some, but not all, of the mafic rocks have been affected by crustal contamination. This appears to have taken place in magma chambers at ∼9 kbar, i.e. the base of the continental crust. Forward modelling of major and trace elements (Fe and Nd) and inverse modelling of rare earth elements suggest that the primary Cuddapah melts were generated by ∼10–15% partial melting of a lherzolite mantle source. This corresponds to a mantle potential temperature of ∼1500°C. The thickness of the mechanical boundary layer predicted by the geochemical modelling is 70 km with a minimum initial lithospheric thickness of 120 km. This corresponds to a stretching factor of 1·6–1·8. Richter's (1988) secular cooling model for the Earth predicts that, at 1·9 Ga, the ambient mantle had a potential temperature of ∼1500°C (i.e. ∼200°C hotter than Phanerozoic mantle). If the cooling model is correct then Proterozoic lithospheric stretching and mantle melting beneath the intra-cratonic Cuddapah Basin could have been caused by passive rather than active rifting.