Proterozoic Crustal Evolutionary History of Northeast India: Geochronological and Geochemical Constraints From Late Paleoproterozoic Shillong Group Metavolcanics

ABSTRACT The Shillong Basin in the east‐central part of the Assam Meghalaya Gneissic Complex (AMGC) records a part of the Proterozoic crustal evolutionary antiquity of northeastern India. A comprehensive study on petrography, mineral chemistry, geochronology and geochemistry of the Shillong Metavolcanics (SM) is presented here for the first time for a deeper understanding of the tectonic processes responsible for Proterozoic volcanism and its link with the supercontinent built‐up processes. The SM exhibit calc‐alkaline to shoshonitic and metaluminous to strongly peraluminous nature, variable Eu anomaly (0.34–1.25), enriched LILE and LREE, depleted HFSE (Nb, Sr, P and Ti) and MREE, and distinctive features of within‐plate rift to active continental margin setting. It is generated through 1% to 20% partial melting of the spinel and garnet lherzolite and assimilation of the partial melt of lower crust in back arc regime. The U–Pb zircon age of meta‐rhyolite from the volcanic dominated middle Umiam Formation of Shillong Group is dated as 1699 ± 27 and 1621 ± 9.2 Ma, both showing more than 95% concordance. The age of metavolcanics constraints the basin development through back to Late Paleoproterozoic. The comprehensive study of the magmatic and metamorphic events of AMGC and Late Paleoproterozoic Shillong Metavolcanics in respect of Central Indian Tectonic Zone (CITZ) suggests its continuation towards northeastern margin of Indian Subcontinent. Further, the proximity to the continental margin and back arc volcanism indicates existence of collisional setting related to amalgamation of supercontinent Columbia during Late Paleoproterozoic.