Petrology and Geochemistry of Acid Volcano-Plutonic Rocks from Riwasa and Nigana Areas of Neoproterozoic Malani Igneous Suite, Northwestern Peninsular India: An Understanding Approach to Magmatic Evolution

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This paper discusses the geochemical and petrological characteristics of acid volcano-plutonic suite of rocks exposed in Riwasa and Nigana areas of Malani Igneous Suite (MIS), Northwestern India. Geochemically, these acidic rocks having peraluminous and alkalic to alkali-calcic nature and classified as volcanic phase (Riwasa rhyolites), plutonic phase (Nigana granites) and dyke phase (micro-granites). Petrographically, rhyolites show porphyritic, granophyric, glomeroporphyritic, aphyritic, spherulitic and perlitic textures whereas granites show hypidomorphic, granophyric and microgranophyric textures. They are high in silica, A/CNK, total alkalis, Fe/Mg, Ga/Al, Zr, Rb, U, Th, Cu, REEs (except Eu) and low in CaO, MgO, Sc, Cr, Ni, Sr and Eu abundances, which have affinity with A-type granitoids. Their chemistry also support that they are high heat production (HHP) granitoids and their crustal origin. The enrichment of trace elements indicates a genetic link between fractional crystallization of silicate minerals and post magmatic fluid alterations in magmatic chamber. Negative anomalies of Ti, P, Sr and Eu in the multi-element spider diagrams indicate that the emplacement of these granites and associated acid volcanics were controlled by fractionation of feldspar (alkali and plagioclase) and crustal contamination in the magmatic melt arise upward. Normative values of silicate oxides further suggests that these rocks are formed between 2–7 kb pressure ranges and may be emplaced from shallow to greater depth ranges (15–30 km and 450–900°C). Furthermore, geochemical features in acidic rocks such as strong linear positive correlation between LREE, Zr, Nb, Ga, Y and Rb emphasize that the behavior and enrichment of these elements are largely controlled by post-magmatic processes in plume related associations. Hence, the geochemical data presented here, are therefore consistent with an intraplate, co-magmatic and A₂-subtype granite field emplaced in extensional tectonic regime of MIS.