Proterozoic sedimentation and volcanism in the Singhbhum crustal province, India and their implications

Abstract

The Proterozoic volcano-sedimentary succession comprising successively younger Dhanjori, Chaibasa, Dhalbhum, Dalma and Chandil Formations of the Singhbhum crustal province, India records sedimentation and volcanism in a rapidly changing tectonic scenario. Cooling of the vast volume of Archaean Singhbhum granite possibly induced an isostatic readjustment. The associated tensional regime and deep-seated fractures controlled the formation of the Proterozoic Singhbhum basin. The Dhanjori Formation unconformably overlies the Singhbhum granite and is entirely terrestrial, dominantly fluvial. At the base, the conglomerate deposits, coarse-grained sandstone, and shale represent the distal fringe of an alluvial fan complex. The rest of the formation including the volcaniclastic rocks consists almost entirely of fining-upward fluvial cycles. The base of the Dhanjori Formation is a sequence boundary and the formation itself represents a lowstand systems tract. The Dhanjori volcano-sedimentary succession displays evidence of having passed through passive, as well as, active phases of continental rifting with an increasingly important influence of volcanism on the sedimentation through time. The Chaibasa Formation sharply overlies the Dhanjori Formation and a transgressive lag demarcates their contact. The basal part of the Chaibasa Formation immediately overlying the transgressive lag deposit represents a transgressive systems tract while offshore shales that sharply overlie the shallow marine sandstones up-section represent marine flooding surfaces. The shallow marine subtidal sandstones bear an excellent record of sandwave migration and provide a rare opportunity to unlock the Late Palaeoproterozoic lunar orbital periodicities. Unlike the Chaibasa, the overlying Dhalbhum Formation is entirely terrestrial (fluvial-aeolian) indicating that the Chaibasa-Dhalbhum contact is a sequence boundary. The Dalmas, overlying the Dhalbhum Formation, represents concordant lava outpourings without any break in sedimentation; these lavas are genetically related to mantle plume upwelling in an intracontinental rift setting. The volcano-sedimentary package lying north of the Dalma volcanic belt (Chandil Formation) is of Mesoproterozoic age. The entire Late Palaeoproterozoic volcano-sedimentary package displays post-depositional compressional deformation and greenschist to amphibolite facies metamorphism dated at ca. 1600 Ma, forming the so-called North Singhbhum fold belt. The volcano-sedimentary package lying south of the Dalma volcanic belt was pushed further south towards the Singhbhum granite batholith complex as a result of uplift related to the Dalma plume. The Singhbhum granite batholith acted as a rigid body. This gave rise to a compressional stress regime that induced shearing/thrusting at ca. 1600 Ma along the Singhbhum Shear Zone. The Dalma plume magmatism was possibly part of a ∼1600 Ma global tectono-thermal event.