The origin of ∼ 2.5 Ga arc-related magmatic charnockites of Karimnagar, SE India, and implications for tectonic assembly in the Dharwar craton

Abstract

In the Indian peninsula, the Singhbhum, Bastar, and Dharwar cratons in the South India Block (SIB) constitute a contiguous mass of >2.5 Ga crystalline rocks. Did the cratons develop as a coherently evolved mass since their origin, or do these cratons constitute an assembly of disparately evolved cratons? Variably-deformed charnockites in the Karimnagar granulite belt and associated blastoporphyritic granitoids at the NE fringe of the Eastern Dharwar Craton (EDC) contain enclaves of mafic granulites, high-Al metapelites and anatectic quartzofeldspathic gneisses. The charnockites are demonstrably intrusive into the enclave suite. The enclave suite exhibits steeply-plunging reclined folds; the axial planes of the folds coincide with the N-striking tectonic fabrics in the Karimnagar charnockite/granitoids. The foliated charnockites display magmatic flow texture defined by trains of euhedral alkali feldspar, contain euhedral-subhedral pyroxene phenocrysts, and the quartz grains exhibit abundant chessboard microstructure. The weakly-strained euhedral laths of feldspars and pyroxenes phenocrysts share high-energy boundaries between themselves, and with quartz. Emplacement temperatures of the magmatic charnockites at ∼900 °C are obtained from Al-in-Opx thermometry. Whole rock chemistry is consistent with an arc-related origin for most charnockites. In zircons within charnockites, variably zoned cores yield ages of 2680 ± 15 Ma and 2504 ± 12 Ma in the UPb Concordia plot. Recrystallized domains in zircon grains yielded upper intercept ages constrained between 2510 ± 4 Ma and 2509 ± 3 Ma, identical with the U-Th-Pb chemical ages (2502–2508 Ma) retrieved from monazites. The zircon εHf(t) values (− 4.85 to 1.31) suggest the ∼2.5 Ga magmatic charnockites were derived from <3.0 Ga crustal sources. The late Neoarchean magmatic charnockites in the EDC margin were emplaced in a 2.7–2.5 Ga convergent tectonic setting, and the high-T magmatic charnockites formed due to delamination of a subducting (E-W shortening) oceanic crust. The subduction possibly relates to the late Neoarchean growth of the Dharwar craton involving the assembly of disparately-evolved crustal blocks, now parts of the Dharwar craton. The findings suggest that the emplacement of Karimnagar magmatic charnockites in a contractional setting is unrelated to an accretion between the Eastern Dharwar and the Bastar cratons, as suggested by earlier workers.