Tectono-magmatic evolution of the intra-cratonic Cuddapah Basin, India

Mafic-ultramafic igneous rocks from the Early Proterozoic intra-cratonic Cuddapah Basin of southern India have been studied for their petrological and geochemical characteristics. A field reconnaissance survey was accompanied by sample collection from several stratigraphic horizons in the sedimentary basin. ⁴⁰Ar-³⁹Ar laser fusion age determinations on phlogopite mica in the Tadpatri mafic-ultramafic sills have provided firm constraints on the age of the earliest phase of the mafic magmatism in the basin at 1.9 Ga. Petrographical and mineralogical studies of the Vempalle have Tadpatri sills from the Cuddapah Basin have provided some insight into the complex magma chamber processes that may have modified their parental melt compositions. This has been further investigated by studying variations in the major, trace and rare-earth element concentrations of the lavas and sills. Geochemical modelling using the major and trace element concentrations of mafic rocks from the Cuddapah Basin has provided firm constraints for the melt generation processes during the Early Proterozoic. Both forward and inverse geochemical modelling of the lavas and sills suggest that the former were generated by ~ 8-12% partial melting of a lherzolite mantle source whereas the sills were generated by larger degrees (~ 12-18%) of partial melting at a mantle potential temperature (T_p) of ~ 1500^oC. The thickness of the mechanical boundary layer predicted by the geochemical modelling, after lithospheric stretching, is 70 km with a minimum initial lithospheric thickness of 120 km. This corresponds to a lithospheric stretching factor (β) of 1.6-1.8.