Composition and origin of Deccan basalts

The Deccan basalts are essentially composed of saturated tholeiitic lavas. However, undersaturated basalts, nephelinites, carbonatites, intermediate and acid differentiates have also been encountered in parts of western India (Upper Traps). Such unusual rocks are broadly aligned in two major rift zones in western India, the Narbada-Son and Cambay grabens and the faulted west coast. These rocks are younger than the earliest tholeiitic eruptions of central India (Lower Traps), but there are evidences of renewed eruptions of tholeiitic basalts in parts of western India. The earliest eruptions of Deccan basalts of quartz tholeiite composition have been derived by high degrees of partial melting of peridotite at moderate depth (37–41 km). The undersaturated basalts and nephelinites were possibly generated by low degrees of partial melting of garnet peridotite in the low velocity zone along the two tectonically active belts (rifts) in western India. The undersaturated basalts were formed prior to the break-up of Gondwanaland, when a concentration of pressure developed at the tectonically weak zones. The renewed eruptions of saturated tholeiites are thought to be post-tectonic, resulting from the release of stress when the tectonic events had ceased.