The Nature of Mesoproterozoic Central Indian Tectonic Zone with Exhumed and Reworked Older Granulites

Abstract

The ESE-WNW trending Central Indian Tectonic Zone (CITZ) divides the Indian subcontinent into southern and northern crustal provinces. Its narrow northern belt consists of closed early Proterozoic Mahakoshal rift zone, which is presently confined between two Moho reaching faults that characterize the Son-Narmada (SONA) lineament. The Mesoproterozoic Sausar Mobile Belt (SMB), the Chhotanagpur Granite Gneiss Complex (CGGC) and the Gneissic Complex of Northeast India represent the wider southern belt. The Mahakoshal rift belt has a thick pile of mafic-ultramafic rocks and possibly oceanic components. The basin closed due to a south-directed subduction creating a magmatic arc that was sutured to its southern boundary during 1.8-1.7 Ga. A south dipping ductile shear zone, known as the Son-Narmada South Fault affects both its supracrustals and intrusive granitoids. The Son-Narmada South Fault generally delimits the northern boundary of the Gondwana Basins, whereas the Son-Narmada North Fault, delineates the northern boundary of the Mahakoshal Mobile Belt and the southern boundary of the Meso-Neoproterozoic Vindhyan Basin. The SMB, representing the southern belt of the CITZ in the central sector, comprises strongly folded and metamorphosed non-volcanic and Mn-rich Sausar sediments, intermixed with reworked basement gneisses, migmatites and granulites representing the pre-Sausar assemblage. The Sausar Orogeny closed during terminal Mesoproterozoic (ca. 1.0 Ga). On the other hand, metamorphism in the basement gneiss, migmatites, and northern and southern granulite belts pre-dates the Sausar Orogeny. The earliest structure of the SMB truncates and rotates the N-S oriented structures of the southward-located Sakoli and the Dongargarh belts of Neoarchaean-Palaeoproterozoic age. The northern granulite belt of the SMB and the CGGC, and the high-grade supracrustals from the Sonapahar area in NE India, all indicate very high P-T conditions during 1.7-1.5 Ga peak metamorphism, which was followed by decompressional cooling. The setting resembles continental collision, which was followed by tectonic exhumation and denudation. On the other hand, the southern granulite belt from the SMB documents a late Archaean (2.6 Ga) event and a later 1.4 Ga event. The Dalma Volcanic Belt located close to the southern tectonized boundary of the CGGC belt, and possibly the assemblage of copious mafic enclaves within the ca. 550 Ma granite plutons from NE India, represent back arc-like setting during 1.6-1.5 Ga. The setting is caused due to the southward subduction of the north Indian block beneath the southern block leading to continental collision. The metasedimentary belt, located further to the south of the Dalma Volcanic Belt and flanking the Archaean Singbhum Craton to the north, on the other hand, is Palaeoproterozoic in age, but has been metamorphosed during ca. 1.6 Ga because of the northern collision-related orogeny. Reworking and incorporation of basement gneisses and granulites from the mid-crust level to the younger Sausar cover sediments denote renewed spell of thrust tectonics that remobilized the collision zone rocks of the earlier orogeny. The tectonic transport was from the north to the south with north-dipping thrusts. Thus there was a reversal in the direction of convergence from the preceding orogeny. The Sausar Orogeny closed during ca. 1.0 Ga. It caused anatectic event producing granitoid plutons, migmatites and charnockites in the CGGC belt, and migmatites in the Gneissic Complex in NE India.