Deformation and metamorphic evolution of Chotanagpur Gneissic Complex (CGC), East Indian Shield

Formation and Evolution of different rock types during growth of Indian shield and mobile belts gives us opportunity to understand tectono-metamorphic implications of Indian subcontinent in Precambrian time. CGC is one such well-preserved fold belt formed during Proterozoic time period which serves valuable knowledge about the evolutionary history of Peninsular India through its rock record. It is located in the eastern part of Indian subcontinent and vastly occupied by Precambrian granite gneiss. From our field observation along Purulia shear zone (PSZ) and published data from different parts of CGC, we observed six stages of deformational and metamorphic evolution based on overprinting relationship of deformation, metamorphic and igneous intrusions.

During stage-I, oldest 1870 Ma Ultra High Temperature (UHT) Metamorphic event (M₁) happened and it is observed in form of granulite enclaves in E and SE regions of CGC. In stage-II, high-grade metamorphism (M₂) defines by regional UHT metamorphism and partial melting of supracrustals during collisional orogeny that causes formation of migmatitic charnockite gneiss by intrusion of granitoid into older M₁ granulites. In Northern part of CGC gray granites (porphyritic) intruded into unknown felsic basement with pelitic/calc-silicate supracrustals at 1750-1660 Ma. In this stage S₁ gneissic band developed in the regionally extensive gneisses during D₁ deformation. Stage-III is defined by post-D₁ magmatism where gabbro-anorthosite, porphyritic granitoid, syenite within ∼1650 Ma high grade basement gneiss intruded at ~1550-1500 Ma. In Stage-IV, Paleoproterozoic basement along with the post D₁ intrusive deformed under granulite facies metamorphism (M₃) in continent-continent collisional setting causes development of regional thin gneissic banding (S₂) along E-W related to D₂ and D₃ deformations during 1000–950 Ma. Stage-V is defined by Post- D₃ Intrusion of nepheline syenite, alkali syenite, porphyritic granite and mafic dyke during rifting stage of Grenvillian basement crosscutting all the preexisting fabrics during 950-900 Ma. Stage-VI is defined by upper amphibolite-facies metamorphism (M₄) to produce amphibolite, foliated granite and augen gneiss. Pegmatite & leucogranite emplaced parallel to the axial planes of F₁-F₃ folds interpreted from the mafic dykes in the eastern part of CGC. This causes development of the S₃ fabric in N-S orientation overprinted early granulite fabrics because of dominant F₂ folding indicates strong E-W compression during 850-780 Ma & 870-780 Ma.