GEOCHEMISTRY AND PETROGENESIS OF GARHWAL VOLCANICS - IMPLICATIONS FOR EVOLUTION OF THE NORTH INDIAN LITHOSPHERE

Middle Proterozoic magmatism in the Garhwal Himalayas consists of an extensive sequence of mafic volcanics plus associated mafic dyke sheets. Geochemical data reveal that the volcanic suite is tholeiitic, with the dyke sheets showing considerably more Fe-enrichment than the lava flows. All the rocks are distinctly enriched in incompatible elements and light REE relative to primordial mantle abundances, but have an equally distinct "continental" signature reflected in marked negative Nb, Sr, P and Ti anomalies in their mantle-normalised spidergrams. Although such geochemical characteristics are commonly thought to indicate contamination of the magmas with a continental crust component, the evidence is strongly in favour of the compositional characteristics being inherited from their mantle source. Other Proterozoic volcanics in northern India have closely similar geochemical features, as have other classic occurrences of younger continental flood basalt suites in adjacent areas of Gondwanaland. These geochemical characters are probably imposed upon the subcontinental lithosphere at the time of continent formation but, in the case of younger CFB suites, this may have been modified to a greater or lesser extent by subsequent additions of "plume" mantle material.