A Review of Petrography, Geochemistry and Genesis of High Magnesian Cumulate Ultramafics of Northwestern Fringe of Chotanagpur Gneissic Terrain, Eastern India

In the northwestern fringe of Chotanagpur Gneissic Terrain (CGT) number of occurrences of high MgO ultramafic cumulates are recorded emplaced adjacent to Gondwana basin. The ultramafics occur as veins, lenses and tongue like bodies in metaultramafites and exhibit close spatial association with amphibolites. Mineralogically they are composed of olivine, clinopyroxene, orthopyroxene, magnetite±Cr spinel±phogopite±sphene and are lehrzolitic in composition. Chemically they are characterized by high MgO, Ni,Cr and low TiO₂,alkalies and silica and exhibit komatiitic signature. Major oxide, Trace element and Rare Earth Element ratios suggest their similarity with Al-undepleted Munro type komatiite. Al-undepleted nature together with flat HREE distribution pattern and different elemental ratios suggest absence of garnet in the melting residue. Depth of magma generation estimated on the basis of major oxide ratios suggests melting depth of 150 km at pressure 6-7 Gpa for komatiite magma. They are postulated to be derived from a plume tail or from the innermost part of a plume head.