Neoarchaean mafic volcanic rocks from the Southern Bastar greenstone belt, Central India: petrological and tectonic significance

Abstract

The Archaean southern Bastar Craton of India is an integral part of the Singhbhum protocontinent and includes a suite of geochemically diverse Neoarchaean mafic volcanics that occur as well-exposed extrusive masses capping hills of granite gneiss. All mafic volcanics have undergone greenschist facies metamorphic conditions, but most have preserved original igneous textures. Geochemical data indicate the mafic volcanic rocks can be divided into three distinct varieties; sub-alkaline basalt (SAB), basaltic andesite (BA), and boninite (BON). We interpret the geochemical data to indicate that these volcanics are genetically related through fractionation of BON to BA and SAB. Regional geology, metamorphism, distinctive sedimentary records, and small negative Nb anomalies in the mafic volcanic geochemistry suggest they were deposited in a stable continental rift environment. We interpret the geochemical characteristics of the mafic volcanics to most likely reflect variations in source characteristics, together with minor crustal contamination, rather than the process of volcanic-arc magmatism. Geochemical modelling suggest that the primary BON composition is consistent with about 15-20% batch-melting of a mantle source. BA and SAB represent fractional crystallisation of olivine from a high-Mg basaltic or boninitic magma. Compatible and incompatible trace element modelling suggests that all three rock types probably originated from a lherzolite mantle source.