Late Archaean mantle metasomatism below eastern Indian craton: Evidence from trace elements, REE geochemistry and Sr-Nd-O isotope systematics of ultramafic dykes

Trace, rare earth elements (REE), Rb-Sr, Sm-Nd and O isotope studies have been carried out on ultramafic (harzburgite and lherzolite) dykes belonging to the newer dolerite dyke swarms of eastern Indian craton. The dyke swarms were earlier considered to be the youngest mafic magmatic activity in this region having ages not older than middle to late Proterozoic. The study indicates that the ultramafic members of these swarms are in fact of late Archaean age (Rb-Sr isochron age 2613 +/- 177 Ma, Sr-i similar to 0.702 +/- 0.004) which attests that out of all the cratonic blocks of India, eastern Indian craton experienced earliest stabilization event. Primitive mantle normalized trace element plots of these dykes display enrichment in large ion lithophile elements (LILE), pronounced Ba, Nb and Sr depletions but very high concentrations of Cr and Ni. Chondrite normalised REE plots exhibit light REE (LREE) enrichment with nearly flat heavy REE (HREE; (SigmaHREE)(N) similar to 2-3 times chondrite, (Gd/Yb)(N) similar to 1). The epsilon(Nd)(t) values vary from +1.23 to -3.27 whereas delta(18)O values vary from +3.16parts per thousand to +5.29parts per thousand (average +3.97parts per thousand +/- 0.75parts per thousand) which is lighter than the average mantle value. Isotopic, trace and REE data together indicate that during 2.6Ga the nearly primitive mantle below the eastern Indian Craton was metasomatised by the fluid ( silicate melt) coming out from the subducting early crust resulting in LILE and LREE enriched, Nb depleted, variable epsilon(Nd), low Sr-i(0.702) and low delta(18)O bearing EMI type mantle. Magmatic blobs of this metasomatised mantle were subsequently emplaced in deeper levels of the granitic crust which possibly originated due to the same thermal pulse.