Discovery of Early Tonian Calc‐alkaline and Shoshonitic Metamafic Rocks from the North Purulia Shear Zone, Chhotanagpur Gneissic Complex, Eastern India: Implications of Proterozoic Sub‐continental Lithospheric Mantle

Abstract

Reports of shoshonitic rocks in Precambrian terrains are relatively rare. Pl-Grt amphibolites and Hbl-Bt mafic granulites occurring in the migmatitic gneisses of the Chhotanagpur Gneissic Complex (CGC) show calc-alkaline and shoshonitic characteristics. Relict porphyritic, sub-ophitic and poikilitic textures are noted in these rocks. Their parent magma was emplaced during the waning phase of the regional metamorphism. Geochemically, these metamafics are similar to the Group III potassic and ultrapotassic rocks of Foley et al. (1987). The magma was derived from the metasomatized subcontinental lithospheric mantle (SCLM). Subduction-related sediment melts metasomatized the SCLM. Compositionally, the SCLM is a metasomatized phlogopite-amphibole-spinel-bearing harzburgite. 1%-5% batch melting of the SCLM could produce the parental magma of the mafic granulites. Pressures and temperatures of metamorphic equilibration were carried out by pseudosection modeling. Peak metamorphic assemblage (M 1 : Grt-Cpx-Pl-Qz) in garnetiferous amphibolite equilibrated at 740º C and 8.7 kbar. The Cpx-Pl corona appeared around the garnet during decompression (M 2 : 655°C, 6 kbar). The Hbl-Pl symplectites around garnet formed during isobaric cooling (M 3 : 580°C and 5.9 kbar). The emplacement of shoshonitic magma and subsequent decompression happened at the slab break-off stage of continental collision (~990 Ma).