Komatiite Within Chhotanagpur Gneissic Complex at Semra, Palamau District, Jharkhand: Petrological and Geochemical Fingerprints

Komatiite near Semra village, southwest of Daltonganj in Palamau District of Jharkhand, occurs within tremolite actinolite schist of ultramafic parentage. The fragmented olivine phenocrysts show mutually parallel as well as angular alignment, representing relict spinifex texture. Mineralogically the Semra ultramafic is lehrzolite in composition. The cumulates lack visible deformation suggesting original magmatic crystallization of these ultramafic rocks. The present occurrence of Spinifex Textured Peridotitic Komatiite (STPK) in Chhotanagpur Gneissic Complex (CGC) at northwestern part of Eastern Indian Shield is of great significance as it opens up a possibility of the presence of Archaean rock in CGC, which is yet to be established.

These ultramafics are geochemically characterised by distinctive high MgO, Ni, Cr and poor in alkali, TiO₂, Ba, Cs, Rb, Nb, Hf and Y contents. It has low abundance of incompatible elements and is LREE depleted [(La/Yb)_n=0.74 - 1.07] with enriched flat HREE profile, representing chondrite like composition. This shows diagnostic convex upward REE profile. All these together with relict but distinct spinifex texture confirm the komatiitic character of Semra ultramafic. The chemical data plots for ultramafics confine to komatiite field in discriminate diagrams. The Mg# (>74) of the Semra ultramafics are comparable to primitive upper mantle Mg# (89.8) and high enough for rocks derived from ultramagnesian liquid of the mantle derived source. Considerably high Zr concentration in Semra ultramafic is attributed to Zr-enriched mantle source. Geochemically Semra ultramafics is comparable with "Al-undepleted" Munro type komatiites. Depleted incompatible trace elements also point towards Al-undepleted nature for the investigated ultramafics, which are characterised by distinctive low SiO₂ content in comparison to many well known komatiites of the world. The low K₂O content indicate plume related magmatism for its generation. Tectonic setting of this STPK with distinctive cumulus nature is suggestive of its emplacement in an extensional tectonic regime.