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

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