Petrogenesis of high-Mg andesites from the Proterozoic Shillong Basin, Northeast India: evidence for continuation of the Central Indian Tectonic Zone to the Assam-Meghalaya Gneissic Complex and its implications for the Columbia supercontinent reconstruction

This study provides comprehensive geochemical and petrological data for the newly discovered high-magnesium andesites (HMAs) from the Shillong Group of rocks in the Proterozoic Shillong Basin of the Assam-Meghalaya Gneissic Complex (AMGC). It is the first report of HMAs from the easternmost part of the Indian shield region. The AMGC holds immense geodynamic significance because this region has recorded magmatism related to Columbia, Rodinia, and Pangea supercontinental events. The present work discusses geochemical signatures of the Shillong HMAs characterized by calc-alkaline traits with high-K contents and significant concentrations of TiO2 (0.54-0.76 wt %) higher than boninites (TiO2 < 0.5 wt %), and Mg# (53-56). They exhibit high LILE/HFSE and LREE/HREE ratios, which are distinctive features of magmas generated in subduction zones. Tectonic discrimination diagrams clearly suggest that the Shillong HMAs were generated in a back-arc basin regime. We argue that the Shillong HMAs were derived by 5% to 10% partial melting of a depleted lherzolite mantle source within the spinel - garnet transition zone. We also argue that the Shillong HMAs may have formed as a result of the interaction between subducted sediment-derived melts and mantle. The Shillong HMAs have undergone low- to medium-grade greenschist facies metamorphism characterized by significant mineral alteration, however, they still preserve bulk original alteration-resistant trace element compositions. From a regional perspective, the synchronous nature of origin and geotectonic setting, associated geochronological ages of formation, and regional structural trends suggest that the Shillong Basin is the easternmost continuation of the Mahakoshal Mobile Belt and specifically, the Bathani volcano-sedimentary sequence of the Central Indian Tectonic Zone (CITZ); and together they form the northern fragments of the same suture zone consisting of the CITZ, the Chotanagpur Granite Gneiss Complex, and the AMGC, which demarcates the northern and southern Indian blocks of the Greater Indian Landmass.