Damodar graben: A centre of contrasting magmatism in the eastern Indian shield margin

Fragmentation of the Protocontinental masses along the orthogonal type of mega-fractures during cratonisation processes is responsible in breaking-up of the Indian shield into major crustal blocks. Narmada-Son-Tapti (SONATA) geofracture is one such prominent megafracture dissecting the Indian shield across Central India that has given rise to a number of grabens, viz., Mahanadi, Godavari and Damodar. The E-W trending Damodar graben at the eastern Indian shield margin, has developed along the central axis of the Chhotanagpur Gneiss Granulitic Complex (CGGC, >2.0 Ga), which served as a basement for Gondwana sedimentation since Upper Carboniferous to Mesozoic period. This zone is marked by prolonged magmatic activity since Paleo-Proterozoic time, culminating into eruption of Rajmahal basalt during Early Cretaceous (116 Ma). The development of contrasting igneous rock association on either side of Damodar graben in a rift setting, suggest generation of magma from diverse source rock composition and depth. The emplacement of syenite, nepheline syenite, alkali granite, basic, ultrabasic rocks and massif anorthosite at the southern part of rift, and mafic (gabbro-anorthositetroctolite) and ultramafic magmatism together with rapakivi granite in the northern part, are manifestations of mid-Proterozoic events. The basic magmatic activity was followed by pervasive emplacement of K-rich peraluminous granites. However, the dyke swarms of dolerite, lamprophyre and lamproite of Early Cretaceous age in the Gondwana sediments of Damodar Valley Coal Field and the adjoining Bengal Basin give testimony to reactivation of deep seated basement dislocations. Such igneous rock kindreds of mantle origin are known for rift generated magmatism during Mesozoic period in the adjoining continental margins of Western Australia and East Antarctica which were once joined together with Eastern India, prior to the fragmentation of Gondwanaland. The phases of tholeiitic, anorthositic, granitic, alkaline and ultrapotassic magmatism in space and time through Paleo-Proterozoic to Early Tertiary age in Damodar graben are comparable with the Oslo graben of Norway and the Rhine graben of Germany. The geological history of CGGC is, therefore, controlled by magmatism in an intracontinental rift setting which shows reactivation over a period of about 2 billion years.