The petrogenesis and setting of Archaean metavolcanics from Karnataka State,South India

Major and trace element data for ultrabasic to silicic metavolcanics from five Archaean volcanic-sedimentary belts in South India are summarized. Their total alteration to greenschist and amphibolite facies assemblages poses problems for petrogenetic interpretation of these data. However, in the absence of unmetamorphosed lavas, some interpretation is essential.The data suggest that volcanism in each belt was petrogenetically similar. The presence of both light rare-earth element-enriched and -depleted lavas in individual volcanic sequences, together with variations in $\text{Zr}\text{Y}$ ratios is interpreted as a consequence of fractionation within ultramatic mantle diapirs, induced by upward vapour migration. Other chemical variations probably resulted from fractional crystallization of mafic phases from parental high MgO, high Cr magmas, the influence of plagioclase fractionation only being noticeable in late andesites and dacites. The consistent depletion of all basaltic samples in the high field strength elements Mb and Ta relative to the low field strength element Th and to La further emphasises the importance of a vapour phase in their petrogenesis. In this regard they resemble modern orogenic basalts. The favoured tectonic setting is that of ensialic, multiple back-arc basins above a thinner lithosphere than in modern back-arc environments, which rode over a shallow-angled subduction zone.