Geochemistry of biotites and host granitoid plutons from the Proterozoic Mahakoshal Belt, central India tectonic zone: implication for nature and tectonic setting of magmatism

The northern part of the central India tectonic zone (CITZ) is occupied by the Proterozoic Mahakoshal Belt, which is mainly comprised of granitoids and volcano-sedimentary lithounits. The granitoids (ca. 1880–1710 Ma) are exposed as small circular to elliptical-shaped, stock-like intrusive bodies, such as Nerueadamar granitoids (NG), Tumiya granitoids (TG), Jhirgadandi granitoids (JG), Dudhi granite gneiss (DG), Raspahari granitoids (RG), Katoli granitoids (KG), and Harnakachar granitoids (HG), collectively forming the granite gneissic complex (GGC). The geochemistry of biotites, host granitoids, and enclaves from these plutons has been investigated in order to understand the redox condition and likely tectonic affinity of host granitoids. The Al₂O₃–MgO–FeO^t contents and operated elemental substitution in biotites strongly suggest the diverse nature of host magmas such as calc-alkaline, metaluminous (I-type), peraluminous (S-type), and transitional between I- and S-types, which appear to have formed in subduction zone and syn-collisional tectonic settings. The transitional (I-S)-type granitoids inferred based on biotite compositions, however, represent both metaluminous (HG) and peraluminous (DG and KG) granitoids in terms of whole-rock molar A/CNK (Al₂O₃/CaO + Na₂O + K₂O) ratios. Ages of granitoid magmatism and its field association with contemporaneous volcano-sedimentary lithounits clearly mark the post-collisional tectonic setting, which contradicts the subduction-related tectonic setting inferred from biotites of JG and microgranular enclave (JE) hosted in JG. Whole-rock major and trace elements broadly suggest the existence of collision tectonics during the formation of granitoid plutons. The JG, KG, and DG contain a bt-Kf-mag-qtz assemblage, and their parental magmas evolved under moderate oxidizing environments (ƒO₂ = −12.03 to −13.27 bars). On the other hand, RG (bt-gt-Kf-pl-qtz), NG (bt-ms-Kf-pl-qtz), and TG (bt-ms-Kf-pl-qtz) represent pure crustal-derived magmas evolved in strongly reducing conditions formed under a syn-collisional tectonic setting as evident from their mineral assemblages and biotite and whole-rock compositions. Granitoid plutons of the Mahakoshal Belt were most likely formed during amalgamation of the Columbian supercontinent.