Plume Generated Mesoproterozoic Mafic-Ultramafic Magmatism in the Chotanagpur Mobile Belt of Eastern Indian Shield Margin

The Chotanagpur gneiss-Granulite complex (CGGC)-a mobile belt north of the Archaean Singhbhum cratonic nucleus and contiguous orogenic belt with the Singhbhum Proterozoic basin, is a vast tract of high-Grade rocks and gneisses with enclaves of granulite and metasedimentary rocks, and intrusive granites of Proterozoic age. Pervasive intrusions of mantle-Derived rocks of varied composition ranging from mafic-Ultramafic, sodic-Ultrapotassic alkaline rocks, massif anorthosite to younger tholentic basalts (Rajmahal) and dolerite at different geological periods ranging from Late Paleoproterozoic to Early Tertiary, give evidence of an active mantle in the prolonged history of evolution of this mobile belt. The present study is limited to metamorphosed mafic-Ultramafic rocks at the eastern sector of CGGC. The mafic-Ultramafic suite is represented by amphibolite, basic granulite and hornblendite. Compositions of the primary amphibole in these rocks range from hornblende to pargasitic hornblende, and plagioclase from An₄₀ to An₅₉. Positive correlation of Mg# between clinopyroxene and hornblende, and clinopyroxene and orthopyroxene, indicating that the Fe-Mg exchange K_D is constant between the ferromagnesian minerals, and the rocks to have attained equilibrium conditions of metamorphism. Calculated post-Peak metamorphic equilibrium pressures and temperatures are 3 8-5 4 kb and 643- 781°C, similar to conditions in the surrounding country rocks.

Chemically, the metabasic rocks studied are associated with the Bengal anorthosite massif at Saltora and have been grouped into low-Ti and high-Ti tholeiites. The former shows similarity with transitional basalts derived from T-MORB, while the latter is rich in incompatible elements and shows affinity with basalts derived from E-MORB or from recycled mantle fed by subducted oceanic crust. The nepheline normative ultramafic rock is the most depleted in incompatible elements and shows similarity in trace element contents with MORB. All these rocks show variable crustal contamination. Nevertheless, the bulk chemical compositions of the low-Ti rocks preserve evidence of low-Pressure fractional crystallisation involving olivine, plagioclase and clinopyroxene. The intra-cratonic Mesoproterozoic bi-Modal (tholeiitic-alkaline) magmatism in CGGC is analogous to Phanerozoic magma generation(Rajmahal tholentes-Ultrapotassic mafic-Ultramafic intrusions in the Gondwana basins) in a rift setting (Damodar graben /Shield margin faults) accompanied with crustal thinning. The trace element geochemistry of mafic-Ultramafic rocks gives evidence of plume-Generated magmatism in the Eastern Indian Shield margin during Mesoproterozoic time, which is correlatable with the global thermal event in the Precambrian shields.