Geochemistry and Tectonic Setting of the Precambrian Mahakoshal and Sonakhan Greenstone Belts of the Central Indian Shield

Abstract

Metabasalts of the Paleoproterozoic Mahakoshal and the Late-archean to Paleoproterozoic Sonakhan greenstone belts of the Central Indian Shield have been studied for major elements and trace elements, including rare earth elements (REE) to understand the petrogenesis and tectonic environment. Metabasalts of the lower Saleemanabad Formation of the Mahakoshal belt are grouped into two types on the basis of difference in their geochemical characteristics. Type 1 metabasalts consist of relatively lower SiO2, Al2O3, K2O, and TiO2 and higher Fe2O3, MgO, CaO, and Na2O concentrations as compared to Type 2. Both types of metabasalt depict a back arc basalt character by showing enrichments in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and in some high field strength elements (HFSEs) like Zr and Hf, but show depletions in other HFSEs and heavy rare earth elements (HREEs) in comparison to the N-MORB. This is further supported by REE patterns and values of some key elemental ratios like Ba/Nb, Nb/Ta, La/Nb, Ta/Yb, Th/Yb, Eu/Eu*, Nb/Nb* which fall in the range of back-arc basalts. The lower Baghmara Formation of the Sonakhan greenstone belt consists of two distinct units of metabasalts. The lower and the upper metabasalts of the Sonakhan greenstone belt show geochemical similarity (except for the values of Zr and Hf) with the Type 1 and Type 2 metabasalts of the Mahakoshal greenstone belt respectively. The overall geochemical characteristics of the Sonakhan lower and upper metabasalts depict arc character to them. We interpret the Type 1 metabasalts of the Mahakoshal greenstone belt and the lower metabasalts of the Sonakhan greenstone belt were generated by the higher partial melting of a mantle source fluxed with lower magnitude of subduction input and relatively distal to the arc, whereas the Type 2 metabasalts of the Mahakoshal greenstone belt and the upper metabasalts of the Sonakhan greenstone belt were generated by lower partial melting at a shallow depth involving high subduction input and proximal to the arc. The present study reveals that these greenstone belts of the Central Indian Shield developed via subduction processes during Late-archean to Paleoproterozoic which in turn gives a broad hint regarding the amalgation of cratonic nuclei and the formation of Indian peninsula by accretionary tectonics.