Applied Sciences Session:

Title:            Petrogenesis of Amphibolites from Northern Chotanagpur Plateau, Eastern Indian Peninsular Shield
Authors:       Vikash Kumar
Abstract:      The protoliths of amphibolites belonging to northern Chotanagpur plateau including the world renowned Koderma mica belt (Jharkhand State) of Precambrian eastern peninsular India geochemically bear strong tholeiitic affinities of the oceanic island arc setting and has been assigned to immature or initial stages of island arc system with no sign of volcanic ejecta and occurring as sill-flow complexes all suggesting their emplacement in the embryonic intraarc marginal basin. The Mg-rich nature of these rocks indicates that they have evolved from fractional crystallization of a primary magma of picritic basalt composition. Lack of spinifex texture and CaO/Al2O3 ratio < 1 precludes the possibility of komatiite as the precursor. The various critical constituents based on major, trace and REE data precisely support this contention. General agreement show evolution of magmas in island arc set up is more variable and diverse in nature and is a multistage and multisource phenomena ranging in composition from tholeiitic in the initial stage through calc-alkaline in mature to alkaline in the late stages of their development. Amongst the various petrogenetic models of arc magmatism the most favoured or lone viable source considered for the evolution of the Mg-rich basaltic rather tholeiitic suites is the melting of metasomatised mantle wedge triggered by volatile influx and the convection within upper mantle which seems the most plausible mode of genesis of the protoliths of the investigated amphibolites under moderate degree of partial melting (15-25%) of the lherzolite (enriched hydrous peridotite) in the subarc mantle wedge. Genetically significant REE ratios (La/Sm)N, (La/Yb)N and (La/Ce)N that reflect mantle source rock under fairly large partial melting also support the above observations. There is conspicuous absence of rocks of the composition of either adakite or boninite in the area which is highly important from petrogenetic point of view in the typical subduction related island arc set-up. Olivine- normative MgO-rich varieties (MgO~27%) of the studied rocks i.e. talc-tremolite schists evolved from tholeiitic magma by early
fractionation of ol+opx assemblages followed by cpx+plag. at lower temperature giving rise to the dominant quartz- & hypersthene- normative basalt and basaltic andesite. Calc-alkaline series are almost absent from amongst the studied amphibolites despite the fact that tholeiitic and calc-alkaline magma may co-exist under this set-up.

Keywords:   Petrogenesis, Amphibolites, Chotanagpur plateau, Precambrian, Eastern India
Pages:         594-606

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