Petrology, geochemistry and U-Pb zircon geochronology of Fe-Ti oxide ore-bearing mafic sill from Saltora-Mejia area of Chotanagpur Granite Gneissic Complex, eastern India: Implication for late tectonic emplacement of mafic rock of E-MORB nature in an orogenic belt

An east-west-trending medium-grained mafic sill containing co-genetic Fe-Ti oxide ore lenses is found disposed within granite gneisses around Saltora-Mejia area in the eastern part of the Chotanagpur Granite Gneissic Complex (CGGC) of eastern India. CGGC is considered as a Proterozoic mobile belt as it witnessed multiple phases of deformation and high- grade metamorphism during 1.8-0.8 Ga. Occurrence of such Fe-Ti oxide ore-bearing mafic sill is unique in the entire CGGC which is a vast Proterozoic orogenic belt and has witnessed many phases of voluminous mafic and felsic magmatisms. The mafic rock is of gabbronorite composition which contains plagioclase, clinopyroxene, orthopyroxene as major constituent primary minerals and amphibole as late magmatic mineral. The rock shows sub-ophitic, intergranular, mosaic and poikilitic texture (defined by larger pargasitic grain). The gabbronorite shows iron enriched tholeiitic character, low Mg#, low abundances of Ni and Cr, slight enrichment in LILE, LREE and slight depletion in HFSE like Nb and Ti. The computed melt in equilibrium with the studied gabbronorite shows transitional orogenic to anorogenic, within-plate and E-MORB-like geochemical character. In this study, the U-Pb zircon crystallization age (similar to 960 Ma) of the Saltora-Mejia gabbronorite is reported for the first time which coincides with the late tectonic stage of the most pervasive orogenic activity in the CGGC around 1.2-0.9 Ga. Transitional orogenic to anorogenic geochemical character, late tectonic evolution and other field and laboratory evidences together suggest evolution of the Saltora-Mejia gabbronorite sill in a late tectonic extensional environment which might have been facilitated by delamination of a subducted plate and upwelling of asthenospheric mantle during the waning stage of a major orogeny in the CGGC.