Paleoarchean partial convective overturn in the Singhbhum Craton, Eastern India

Analyses of fold superposition structures and kinematics of shear zones along a 120 km long E-W corridor across the Singhbhum Craton (Eastern India) are combined with U-Pb-Th chemical ages of texturally constrained monazites in supracrustals, gneisses and granitoids to reconstruct the Paleoarchean tectonic history of the western part of the Craton. Metamorphic sub-greenschist facies shallow marine to platformal sediments of the Iron Ore Group (IOG) and the accordantly deformed Jagnnathpur meta-lavas were accreted onto a basement composed of >3.4 Ga amphibolite facies para-amphibolites/quartzites/muscovite schists (Older Metamorphic Group, OMG) and anatectic Older Metamorphic Tonalite Gneisses (OMTG). Crustal shortening was followed by emplacement-ascent of the Singhbhum granitoid pluton, SGP (3.2-3.3 Ga) synchronous with sinking of supracrustal screens along N/NNE-trending steep-dipping sinistral-normal ductile shear zones. Deformation microstructures indicate the bulk of the pluton responded to exhumation-strain by low-T(<= 500 degrees C) ductile flow of quartz and K-feldspar, with plagioclase being largely unaffected. But granitoids along the western margin of pluton emplaced concurrent with westward-retreating exhumation of the thickened crust was deformed at T >= 650 degrees C. Since all granitoids in the pluton share the same N/NNE-trending tectonic fabric, microstructural features imply phased emplacement of the pluton during exhumation strain. The transition from crustal shortening to exhumation interrupted by phased granitoid emplacement between 3.2 and 3.4 Ga is explained by a two-stage partial convective overturn model involving structures typical of supracrustal-down, granitoid-up sense of movement. The tectonic restoration in the western part of the Singhbhum Craton is new and complements growing evidence that accretion and convective overturn may have operated during the Paleoarchean. (c) 2013 Elsevier B.V. All rights reserved.