Abstract
Abstract Amphibolite facies supracrustal rocks interleaved with granite mylonites constitute a shallowly dipping carapace overlying granulite facies anatectic basement gneisses in the Giridih-Dumka-Deoghar-Chakai area that spans ~11,000 km2 in the Chottanagpur Gneiss Complex (CGC). Steep N-trending tectonic fabrics in the gneisses include recumbent folds adjacent to the overlying carapace. The basement and carapace are dissected by steep-dipping sinistral shear zones with shallow/moderately plunging stretching lineations. The shear zones trend NNE in the north (north-down kinematics) and ESE in the south (south-down kinematics). Chemical ages in metamorphic monazites in the lithodemic units are overwhelmingly Grenvillian in age (1.0–0.9 Ga), with rafts of older domains in the basement gneisses (1.7–1.45 Ga), granitoids (1.4–1.3 Ga), and the supracrustal rock (1.2–1.1 Ga). P-T pseudosection analysis indicates the supracrustal rocks within the carapace experienced postthrusting midcrustal heating (640–690°C); the Grenvillian-age P-T path is distinct from the existing Early Mesoproterozoic P-T path reconstructed for the basement gneisses. Quartz opening angle thermometry indicates that high temperature (~600°C) persisted during deformation in the southern shear zone. Kinematic vorticity values in carapace-hosted granitoid mylonites and in steep-dipping shear zones suggest transpressional deformation involved a considerable pure shear component. Crystallographic vorticity axis analysis also indicates heterogeneous deformation, with some samples recording a triclinic strain. The basement-carapace composite was extruded along an inclined channel bound by the steep left-lateral transpressional shear zones. Differential viscous extrusion during crustal shortening coupled with the collapse of the thickened crust caused midcrustal flow along flat-lying detachments in the carapace.
Highlights
The formation of Metamorphic Core Complexes (MCCs) is conventionally related to extensional tectonic processes ([1] and references therein), proposed models vary in the details [2,3,4,5,6,7,8,9]
The reconstructed P-T path for the supracrustal rocks is similar to the one recorded by Maji et al [60] but is strikingly different from the Early Mesoproterozoic granulite facies clockwise P-T path documented by Karmakar et al [71], which is characterized by postpeak decompression and decompression-cooling for the Dumka anatectic basement gneisses
Documentation of domains of shallowly dipping foliations and recumbent/gently inclined folds truncated by steep-dipping ENEto ESE-trending shear zones (Figures 1 and 2) has recently been made in large tracts of the Chottanagpur Gneiss Complex (CGC) [62, 74]; these features are strikingly similar to the detachment zone of the proposed MCC in the Deoghar-Dumka-Chakai-Giridih area
Summary
The formation of Metamorphic Core Complexes (MCCs) is conventionally related to extensional tectonic processes ([1] and references therein), proposed models vary in the details [2,3,4,5,6,7,8,9]. The presence of regional scale strike-slip dominated shear zones is noted in a number of MCCs [10, 38,39,40,41,42,43,44,45] These shear zones are known to aid exhumation of the deep crustal rocks [6, 46,47,48,49,50,51,52,53,54], limited data exist on the field relations of these shear zones and their role in the formation of MCCs [55]. Kinematic analyses coupled with crystallographic data, monazite dating, and P-T pseudosection analyses illustrate the evolution of the proposed MCC
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