Precambrian mafic magmatism in Bundelkhand craton

Abstract

Precambrian mafic magmatism of the Bundelkhand craton is represented by the mafic dyke swarms that have extensively traversed the gneisses and granitoids and metamorphosed mafic-ultramafic rocks associated with greenstone-gneiss, occurring as small linear isolated basins. The mafic dykes were emplaced (during ~2.1 to ~2.0 Ga) in three phases, two in NW-SE direction and the third in NE-SW direction, depicting episodic events subsequent to the stabilization of the craton (at ~2.5 Ga). The former represent culmination of tectono-magmatic event in the Bundelkhand region. The mafic-ultramafic suite of rocks associated with greenstone-gneiss units, range in composition from basalt through basaltic-andesite to basaltic-komatiite. The basaltic-komatiites display remarkable geochemical similarity with boninites. The dykes are of varying compositions and are mostly low-Ti tholeiites and range from sub-alkaline basalt through basaltic andesite and dacite. They show enrichment of light rare earth elements (LREE) and large ion lithophile elements (LILE) and depletion of high field strength elements (HFSE). Incompatible trace elemental ratios indicate that crustal contamination alone cannot be the dominant process for the evolution of the dykes. Ti/Zr and Ti/Y ratios of the dykes reflect near primary chemical characteristics of the mantle sources. Large variations observed in Nb/La and La/Sm ratios possibly reflect varying influence of the crustal material and also some LREE-LILE enrichment processes. To explain the observed geochemical characteristics, it is proposed that the mantle source region was contaminated by sediments/ crustal components probably via subduction processes. Geochemical characteristics of the dykes swarm probably reflects enriched mantle source characteristics followed by fractional crystallisation of the magma. Our study indicates important role of subduction processes in the evolution of the Precambrian crust and subcontinental lithosphere in the Bundelkhand and neighbouring shield and Himalayan areas.