Geochemical and petrological evidence for subduction - accretion processes in the Archean Eastern Indian Craton

The composition of sandstones often provides key evidence about the tectonic, weathering and transport processes operating on the surface at the time of deposition. Petrologic and geochemical analyses of little-metamorphosed middle-late Archean sandstones froth the Eastern Indian Craton show that the sedimentary rocks were derived from dominantly 3.3-Ga-old amphibolites of the Older Metamorphic Group (OMG) and tonalites of the Older Metamorphic Tonalite Gneisses (OMTG), the two oldest lithologic units of this craton. Chondrite-normalized rare earth element (REE) patterns of the sandstones show a light REE-enriched signature with (La/Sm)(N) varying from 5.2 to 6.7 with no Eu anomaly, while the heavy REEs display flat patterns with (Gd/Lu)(N) values of 0.9-1.6. Primitive mantle-normalized incompatible and compatible trace element plots of these sandstones demonstrate an overall similarity with global Proterozoic-Archean sandstones, including strong Nb-Ta negative anomalies. In an f(Sm/Nd) vs. epsilon(Nd)(0) diagram, the sandstones plot precisely between the regional amphibolites and tonalites. We infer from the REE abundances and the f(Sm/Nd) vs. epsilon(Nd)(0) plot that the sandstones represent a bimodal mechanical mixture of OMG and OMTG. The low Ce/Pb ratios of these rocks of 1-4 indicate a variably Pb-enriched Archean crust and that the Ce/Pb ratio acquired the continental crustal signature, distinctly different froth those of the bulk silicate earth and mantle values at least as early as mid-Archean. Strong Nb-Ta depletion relative to the primitive mantle suggests the sandstones were derived from subduction-related magmatic arc sources. This latter suggestion is strongly supported by the low Nb/Ta and high Zr/Sm ratios of these sandstones, identical to Archean tonalite-trondhjemites that require, based on recent trace-element partitioning results, their protoliths to have formed by subduction melting of low-magnesium amphibolites or metamorphosed hydrous basalts [1,2]. The average Nd model ages of the sandstones are greater than the Sm-Nd crystallization ages of the OMG and OMTG at similar to3.3 Ga. The geochemical data presented here can be collectively interpreted to suggest the presence of subduction-accretion processes operational in the mid-Archean Eastern Indian Craton. Because the OMG and OMTG, the source rocks of the sandstones, formed in a subduction-related arc setting, the basement rocks upon which this arc was constructed must have been older. This observation and the depleted mantle Nd model ages (T-DM) of the sandstones, ranging from 3.6 to 4.0 Ga, strongly indicate the presence of continental crust in this Eastern Indian craton older than 3.3 Ga and possibly as old as 4.0 Ga. (C) 2004 Elsevier B.V. All rights reserved.