Andean Cenozoic volcanism: Magma genesis in the light of strontium isotopic composition and trace-element geochemistry

Abstract

Subduction of the oceanic Nazca plate beneath the western edge of the South American plate is thought by most proponents of plate theory to have generated the tectonic forces and magmatic activity that have structured the Andean orogen. By this view, the Cenozoic andesitic magmas of the Andean stratovolcanoes originated within the oceanic crust of the descending plate or within the mantle adjacent to that plate. Accordingly, these Andean volcanic rocks should exhibit chemical and isotopic compositions similar to those of island-arc andesite of like origin. However, while major- and minor-element analyses reveal that the late Cenozoic volcanic rocks are chemically akin to those of the calc-alkalic suite of circum-Pacific island arcs, they exhibit significantly higher Sr/sup 87//Sr/sup 86/ ratios. The purpose of this paper is to examine these anomalously high isotopic ratios in the light of trace-element geochemistry and to develop from this a model for the genesis of the Cenozoic magma.