Unveiling the crustal structure beneath the Chotanagpur Granite Gneiss Complex, eastern India: Interpretation from the 3-D magnetotelluric study

Abstract

The Proterozoic Chotanagpur Granite Gneiss Complex (CGGC) is economically rich with mineral deposits such as Uranium, Bauxite, Iron, Copper, Gold, Diamond etc. Thermo-tectonic events, including the timing and mechanism of attendant magmatism, metamorphism, and metallogeny, remain poorly constrained in the CGGC. To examine the effect of magmatic processes that have originated from the mantle or deeper levels of the lower crust, a comprehensive magnetotelluric (MT) survey is being conducted in the CGGC region. This study aims to provide a detailed understanding of subsurface electrical conductivity to assess the impact of mantle-derived magmatic activity and subduction on the region's crustal structure. The crustal electrical resistivity structure of the CGGC is imaged using 16 magnetotelluric stations. The phase tensor skew shows the three-dimensional (3-D) nature of subsurface structures beneath the study region. 3-D modelling results show the resistive upper crust along the profile up to the depth of approximately 5 km and map several less resistive features in the mid-crust. The process of subduction significantly contributes to the buildup of magmatic fluids within the mid-crust. As the subducting slab releases fluids, it lowers the melting point of the overlying mantle rocks, facilitating partial melting. This resulting molten material can rise towards the mid-crust, transporting dissolved gases and volatiles, such as carbon, along with it. The low resistive features are observed from approximately 5–15 km depth with ∼10 Ω-m resistivity value. The uppermost crust is resistive due to the presence of granite and gneissic composition. Subduction related magmatic fluids at depths that accelerate carbon enrichment could be one reason for the less resistive features mapped in the mid-crust. From the 3-D inversion model, we conclude that no deep-seated faults are present in the study region. Faults extend up to the mid-crustal level, featuring layered structures. The low resistive zone in the mid crust corresponds to the corridors of paleo-fluid flow along crustal-scale structures established in response to terrane amalgamations.