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The ultramafic-mafic volcanic rocks of Archean greenstone belts are important archives for lithospheric and asthenospheric processes of the early Earth. Despite decades of research on this context, many issues still remain unsolved. For example, the process of komatiite magma genesis and the genetic relationship among komatiites, komatiitic basalts and tholeiitic basalts in Archean greenstone belts are not clearly understood. The metavolcanic rocks of the Badampahar greenstone belt (BGB), Singhbhum Craton are studied by major-trace element geochemistry to address the said problems and better understand the evolution of melts in Archean lithosphere. Our research suggests that the protoliths of the metavolcanic rocks were komatiites (both Al -depleted and -undepleted), komatiitic basalts and tholeiitic basalts. The Al-heavy rare earth element (HREE) depleted komatiites were formed by moderate degree mantle melting at a higher depth and the Al-HREE undepleted komatiites are products of moderate to high degree mantle melting at a shallower depth. The melting-assimilationfractional crystallization modelling result shows that komatiitic basalts were generated from Al-undepleted komatiites, and tholeiitic basalts were generated from evolved komatiitic basalts by assimilation and fractional crystallization processes. The older age limit of the BGB is determined to be 3.25 Ga. and the basement of sedimentation and volcanism was composed of plutonic felsic rocks.

Abstract:In the East Indian Shield, occurrence of titaniferous magnetite deposits associated with the Archean Greenstone belt occur in Kumhardubi, Betjharan and Nuasahi areas of Odisha and Dublabera area of Jharkhand. The ore bodies comprise lenses, veins, bands and patches within gabbroic rocks. Petrogenetic studies have revealed the primary and secondary mineral constituents of the ores such as titanomagnetite, ilmenite, hematite, spinel, cobaltite, goethite, martite, rutile and silicate gangue minerals. Various crystallographic intergrowths are resulted from exsolution & oxidation at different temperatures during cooling of the sub-solidus magma. Chemical analyses show that the ore contains 10.35 -17.68 wt.% TiO2, 0.148 -0.227 wt.% V2O3 and 32.75 -67.39 wt.% Fe2O3. Different geochemical composition diagrams confirm their tholeiitic origin. The formation of the massive ore bodies is referred to late magmatic crystallization from tholeiitic magma followed by Fe-Ti enriched residual liquid injection within the host rocks. Syn to late formation of the magnetite ores along with gabbroanorthositic intrusive with respect to the Archean Greenstone Belt of East Indian Shield is suggested.

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 (~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.