Geochemical and Sm–Nd isotopic constraints on the petrogenesis and tectonic setting of the Proterozoic mafic magmatism of the Gwalior Basin, central India: the influence of Large Igneous Provinces on Proterozoic crustal evolution

Abstract

Palaeoproterozoic mafic magma intruded the Gwalior Group of sediments in the form of gabbroic sills. Huge sills extending more than 120 m in depths are exposed in quarries. They are coarse to medium grained, massive and have chilled margins. They predominantly consist of plagioclase feldspar, pyroxene with accessory quartz, iron oxides and apatite. Ophitic to sub-ophitic, porphyritic and intergrowth textures are common. These are tholeiitic, sub-alkaline magma types which have been generated by varying degrees of partial melting and have experienced fractionation of dominantly pyroxene + olivine ± plagioclase. They are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), and exhibit negative Nb and P anomalies and positive Pb anomalies. A mineral-whole-rock Sm-Nd isochron corresponds to an age of 2104 ± 23 Ma with an initial ¹⁴³Nd/¹⁴⁴Nd = 0.509938 ± 0.000023 and εNdt = -0.9 (t = 2000). Model ages (2.6-1.7 Ga) indicate that their mantle sources had a protracted evolution. Mantle source melting may have been triggered by mantle plume in an extensional intra-continental rift tectonic setting, facilitating emplacement of a Large Igneous Province (LIP) in the northern Indian Shield. Thus, Gwalior mafic magmatism represented by the studied sills is the manifestation of one such LIP in the northern Indian Shield. These magmatic activities are contemporaneous with the widespread mafic magmatic activities concomitant with the development of supra-crustal basins on Archaean cratons worldwide and the Indian Shield in particular, where upwelling plumes triggered largescale crustal extension, breaking-up of supercontinents and emplacement of LIPs.