Early Neoproterozoic accretion at the northern margin of the Chottanagpur Gneiss Complex, Eastern India

Abstract

The study based on structural mapping, monazite chemical age determinations and analyses of P-T pseudosections explores the hitherto undocumented accretion dynamics of the northern margin of the Chottanagpur Gneiss Complex (CGC), Eastern India. In the Gaya-Chatra-Koderma sector straddling the northern margin of the CGC, the CGC block comprises a shallow-dipping carapace of 1.5-1.4 Ga and 1.0-0.9 Ga mylonitized granitoids, intrusive into 1.6-1.5 Ga anatectic basement gneisses, and supracrustal rocks inter-leaved with recumbently folded gneisses. The carapace, structurally overlying the steeply dipping high-T fabrics in the basement anatectic gneisses exposed in the tectonic windows, formed due to top-to-the-north thrusting of the allochthonous CGC supracrustal rocks over the tilde 1.7 Ga undeformed granitoids and steeply dipping greenschist-facies metamorphic rocks (tilde 1.8 Ga) in the northern block. Continued N-S shortening caused steeply dipping ENE-striking sinistral and ESE-striking dextral shear zones that locally modify former structures in the CGC foreland. P-T pseudosection analyses indicate that locally developed Early Neoproterozoic garnet-bearing leucosomes in a hornblende-biotite granitoid formed by lower crustal decompression melting, whereas the structurally overlying mica schist in the shallow-dipping carapace experienced mid-crustal loading. We suggest the accretion strain accommodated by the foreland thrusts were contemporaneous with extensional detachment zones that exposed lower parts of the thickened orogen. The tectonic events in the CGC appear to correlate poorly with those in the Capricorn Orogen and the Albany Fraser Orogen in Western Australia.