Geochemistry and petrogenesis of late Archaean Aravalli volcanics, basement enclaves and granitoids, Rajasthan

Abstract

Major, trace and rare earth element data are presented for the basal volcanics of the Aravalli supracrustal sequence, for the mafic enclaves within the Banded Gneiss Complex (BGC) adjacent to the supracrustals, and for granitoids within the BGC and their petrogenesis discussed with regard to the problematic relationship between basement and cover. In terms of major elements the Aravalli lavas range from magnesian (22% MgO) komatiites (or picrites) to Fe-rich tholeftes, but differ from a typical Archaean greenstone volcanic sequence in that their trace element patterns are relatively enriched in incompatible and light rare earth elements. Moreover the range of patterns observed requires a number of distinct mantle sources as well as different degrees of melting. There are two distinct types of mafic enclave within the BGC. The larger lenses correspond quite well with the range of variation seen in the Aravalli sequence, and could be feeders; the other type, close to the BGC/Aravalli junction, has more alkalic characteristics (high ^Nb/Zr) and could represent rift volcanics, or tectonically accreted fragments of ocean islands. The data suggest that the Aravalli sequence is not an accreted oceanic terrane, but are more consistent with interaction of a deep mantle plume with rather mature sub-continental lithosphere. The early Archaean age of parts of the BGC, the chemistry of the granitoids and the alkaline character of the enclaves are also broadly compatible with this model: the character of the Aravalli sequence is intermediate between that of an Archaean greenstone belt and a Phanerozoic flood basalt province.