GEOCHEMISTRY AND PETROGENESIS OF THE BASAL ARAVALLI VOLCANICS NEAR NATHDWARA, RAJASTHAN, INDIA

The early Proterozoic Aravalli supergroup of Rajasthan near Nathdwara includes a thick basal unit of mafic volcanics with intercalated quartzites. The volcanics have been metamorphosed from greenschist to amphibolite facies which, together with intense deformation, obliterated original mineralogy and textures. Based on geochemical criteria we infer that the volcanics consist predominantly of tholeiitic rocks with minor amounts of komatiitic rocks which are referred to here as picritic because of the absence of textural criteria. Modelling of both major and trace element data suggests that the picritic magmas were generated by different extents of adiabatic melting of mantle sources over a range of P-T conditions, at pressures as high as 50 kbar. Their sources were enriched in light REE elements. The tholeiite series could not be related to the picrites either by fractional crystallization processes or by considering that the former represent lower extents of melting of sources similar to those for the latter under any physical conditions. The tholeiites require shallow lithospheric sources with enrichment in light REE and [Fe]/[Mg] ratios which are significantly higher than those of a pyrolite. Addition of melts from deeper levels to the shallow source regions of the tholeiite is considered a possible mechanism for the generation of tholeiitic magmas. Source characteristics and physical conditions of magma generation indicate that the basal Aravalli volcanism could be related to deep mantle plumes.