THE ARCHITECTURE AND EVOLUTION OF A SYN-OROGENIC GRENVILLIAN-AGE METAMORPHIC CORE COMPLEX, EASTERN INDIA

Detailed structural-metamorphic investigations and monazite chemical age determinations in the Giridih-Dumka-Deoghar-Chakai area reveal regional scale enclaves of early Mesoproterozoic granulite facies anatectic gneisses (1.6–1.4Ga) and granitoids (~1.4Ga) with steep N-trending fabrics within expansive Grenvillian-age (1.0–0.9Ga) granitoids lacking mesoscale fabrics constitute the basement in the Chottanagpur Gneiss Complex (CGC), eastern India. Non-anatectic amphibolite facies allochthonous supracrustal rocks (1.0–0.9Ga, with rare 1.2Ga older ages) tectonically overlie the basement and form a shallow-dipping carapace interleaved with granitoid mylonites at the cover-basement boundary. In the carapace, the steep-dipping gneisses are recumbently folded. CKFMASHT P-T path deduced for the supracrustal unit reveals mid-crustal prograde heating (620−680°C) induced by heat released from cooling Grenvillian-age granitoids and post-dating the flat-lying fabrics in the carapace. Steep-dipping transpressional shear zones with gently-plunging stretching lineations transect the basement-carapace composite. Kinematic vorticity values (Wm=0.59–0.74) suggest a marginally larger component of pure shear involved in the formation of the fabrics within the carapace and the shear zones.

The late Grenvillian age of the regional scale metamorphic core complex (MCC) formation coincides with the accretion of the CGC with the Meso/Neoarchean Singhbhum craton in the South. Large-scale emplacement of the Grenvillian granitoids marginally preceded thrusting of the supracrustal unit over the CGC basement. The collapse of the thickened crust acting against the granitoid emplacement-assisted buoyant rise of the basement resulted in strain localization and horizontal extension along detachment zone forming the shallow-dipping carapace. The accretion-induced compression strain was accommodated by the left-lateral steep-dipping shear zones.