SM-ND AGE AND PETROGENESIS OF RAMPUR METAVOLCANIC ROCKS, NW HIMALAYAS - LATE ARCHEAN RELICS IN THE HIMALAYAN BELT

The supposedly Late Proterozoic Rampur metavolcanic (RV) rocks, forming a succession of metabasaltic flows interbedded with phyllites and quartzites, in the northwestern Himalayas yield a whole rock Sm-Nd isochron age of 2.51 +/- 0.09 Ga and an I(Nd)=0.509655 +/- 88 (epsilon(Nd)=+5.3 +/- 1.7). This new date is substantially older than the presumed age and it has an important consequence on the stratigraphic correlation in the region. Geochemical data characterise RV as continental tholeiites with chemical features that are usually attributed to fractional crystallysation and assimilation of crustal material by magma (e.g. dominantly oversaturated, low Mg numbers, Ni and Sr contents and negative Eu, Nb, P and Ti anomalies). Fe-Mg relations and large variations in the ratios of most-incompatible elements suggest derivation of magma from a variably enriched mantle source with Fe/Mg ratio greater than the ideal model mantle, pyrolite. Progressive increase in the degree of source enrichment during the course of magma extraction is marked by a concomittant decrease in Nb/La and Ti/Y ratios. Contrarily, highly positive and uniform initial epsilon(Nd) values not only reflect derivation of magma from a mantle source with long-term LREE depletion but also strongly discount any significant crustal contamination. In order to account for the apparently conflicting chemical and isotopic constraints, we suggest that the source enrichment was through hydrous silicate melt phase and contemporaneous with melting. Melt generation from an enriched source under the implied hydrous conditions followed by olivine and plagioclase fractionation appears consistent with the observed geochemical and isotopic features.