Precambrian alkaline potassic-ultrapotassic, mafic-ultramafic magmatism in Peninsular India

Abstract

Potassic–ultrapotassic, mafic–ultramafic (PUMU) alkaline magmas, typified by the abundance of potassium over sodium, constitute an extremely important alkaline rock group that includes rare and exotic members such as kimberlites, lamproites and lamprophyres. Their study is of great significance to our understanding of both large- and small-scale geodynamic processes taking place in the continental shield. Peninsular India is one of the largest repositories of some of the oldest Precambrian alkaline PUMU rocks in the world and this paper reviews their distribution, age, petrology, geochemistry, and petrogenesis with comments on their economic potential. Analysis of the distribution and available age data of the Precambrian PUMU rocks in various geological domains of peninsular India sheds highly interesting aspects viz. they range in age from Archaean to Neoproterozoic with maximum distribution in the Eastern Dharwar craton, where they are essentially of Mesoproterozoic age. On the other hand, those occurring in the Southern Granulite terrain, Eastern Ghats Mobile belt and Bastar -Bhandara craton are exclusively of Neoproterozoic age whilst only Archaean PUMU rocks are as yet known from the Western Dharwar craton. Regional and local geological lineaments played an important role in controlling the emplacement of PUMU rocks. Crustal contamination is shown to be of negligible influence on the geochemistry of Precambrian Indian kimberlites and lamproites. All of them, with the exception of the Majhgawan and Hinota pipes, are found to be similar to their archetypal varieties contrary to some recent claims. To account the observed geochemistry, the kimberlites and lamproites are inferred to have been derived from a refractory (depleted), but subsequently metasomatised (enriched), source regions in the sub-continental lithospheric mantle. Kimberlites of Siddanpalle and Krishna lamproites, however, are inferred to have been derived from a relatively much shallower source regions. Based on the multiple ages of eruption of lamproites in and around the Cuddapah Basin spread over a vast area, a ‘long lived’ ultrapotassic alkaline magma reservoir of Precambrian age has been inferred. Isotopic studies of the kimberlites and lamproites demonstrate the on-set of metasomatic (enrichment) processes in Indian lithosphere at least >2Ga as well as the existence of Proterozoic mantle heterogeneity. Precambrian lamprophyres of India exhibit primitive as well as evolved (differentiated) nature with only limited incidences of crustally contaminated magmas. They have indistinguishable chondrite normalized REE patterns as kimberlites and lamproites thereby implying similar processes were involved in their genesis with the lamprophyres possibly being derived from relatively much shallower depths. Non-eruption of perceptible Proterozoic PUMU rocks in the Western Dharwar craton is envisaged as a direct consequence of lack of any significant thermal perturbation(s) experienced by the underlying lithospheric (metasomatised?) mantle subsequent to its stabilization during the Archaean. Some future thrust areas of research are also identified.