Tectono-Magmatic and Metamorphic Evolution of the Shillong Back-Arc Basin in the Assam-Meghalaya Gneissic Complex, Northeast India

Abstract

The present investigation proposes the first geodynamic model to explicate the intricate relationships among the recently reported high Mg-andesites (HMAs) and rhyolites along with the associated metadolerites and granites related to the Shillong back-arc basin (1900–1400 Ma) of the Assam-Meghalaya Gneissic Complex (AMGC), Northeast India. This model aims to provide critical insights into the complex subduction geodynamics, magmatic processes, and crust–mantle interactions that occurred in the AMGC when the South Indian Block subducted beneath the North Indian Block along the Central Indian Tectonic Zone (CITZ) during the Columbia supercontinent amalgamation. The genesis of the HMAs is attributed to the lithospheric mantle, which underwent metasomatism via an influx of exceedingly higher degree of sediment-derived fluids originating from the subducting slab, while the doleritic melt is sourced from a mixed lithospheric-asthenospheric mantle, where slab-derived fluids with a lower sediment flux facilitated the decompression melting of the upwelling asthenosphere, leading to the generation of mafic magma at deeper mantle depths. The rhyolites are characterized as A-type in nature, giving an average zircon saturation temperature of 842°C. They were generated through underplating of the high-temperature mafic magma beneath the crust, which induced substantial thermal flux, driving crustal anatexis and the generation of A₂-type felsic magma in the back-arc setting. The basin's geodynamic significance is corroborated by its complex metamorphic and polyphasic deformational history. Geothermobarometric calculations involving the metadolerites yielded pressure–temperature estimates of 5.62–8.64 (average 7.02) kbar and 655–701 (average 683)°C, which assign an amphibolite facies metamorphism to these rocks, remnants of their peak metamorphic conditions during the Pan-African tectono-thermal episode (~500 Ma). Our postulated geodynamic model for the Paleo-Mesoproterozoic Shillong Basin of the AMGC, which we consider as the easternmost continuation of the CITZ, carries substantial significance in comprehending the broader geodynamic scenario operative during the creation of the Greater Indian Landmass during the Proterozoic.