SM-ND ISOTOPIC AND GEOCHEMICAL STUDY OF THE ARCHEAN TONALITE-AMPHIBOLITE ASSOCIATION FROM THE EASTERN INDIAN CRATON

We report the results of a Sm-Nd isotopic, major element and rare earth element (REE) study of the Older Metamorphic Group (OMG) tonalite-amphibolite association of the eastern Indian Craton. The Older Metamorphic Tonalite Gneisses (OMTG) have been previously dated to be 3.8 Ga using Sm-Nd isotope systematics, and 3.2-3.4 Ga by Rb-Sr and Pb-Pb dating. The results of this study indicate that the protoliths of the OMG amphibolites are 3.3 Ga (isochron age = 3.30 +/- 0.06 Ga, epsilon(Nd) = +0.9 +/- 0.7), and therefore, the OMTG, which intrude into the associated amphibolites, cannot be any older than 3.3 Ga. The amphibolites display light REE enrichment ((Ce/Yb)N = 2.2-6.7; La = 30-100 x chondrite) and nearly flat heavy REE patterns ((Tb/Lu)N = 1.2-1.9); the basaltic parents of the amphibolites were probably generated by the partial melting of a spinel lherzolite mantle. Strong linear relationships between the amphibolites and tonalites in Sm-147/Nd-144-Nd-143/Nd-144 space (isochron age = 3.29 +/- 0.04 Ga, epsilon(Nd) = +0.8 +/- 0.8) imply that they are genetically related. The tonalites display fractionated REE patterns (La = 100-300 x chondrite) with moderate heavy REE depletions ((Tb/Lu)N = 1.9-3.4). The isotopic, major element and REE data are consistent with the derivation of the OMTG from partial melting of OMG amphibolites or equivalent rocks at amphibole-garnet stabilization depths. An initial epsilon(Nd)(t) value of +0.9 +/- 0.7 for the amphibolites indicates the presence of a slightly depleted mantle source at 3.3 Ga with Sm-147/Nd-144 between 0.20 and 0.22. It is suggested that the growth of continental crust in the eastern Indian craton occurred in response to magmatic underplating in a plume setting.