Sr, Nd, Pb and O Isotope Composition of Late Cenozoic Volcanics, northernmost SVZ (33–34°S)

Abstract

The three northernmost (33–34°S) volcanic centres in the SVZ share more petrochemical similarities with the volcanoes of the CVZ than the volcanoes further south in the southern Andes, including the volumetric predominance of intermediate rather than basic compositions, the presence of rhyolite and the common occurrence of hydrous minerals. Rhyolites from these volcanoes have radiogenic Sr and Nd isotope compositions and high δ18O values compared to spatially associated basalts and andesites, indicating incorporation of significant amounts of crustal components during their petrogenesis. Basalts and andesites from these volcanoes have similar Sr and Nd isotope compositions but contain more radiogenic Sr and less radiogenic Nd than rocks of equivalent bulk composition from the southern SVZ. However, δ18O values of rocks from both regions are similar. These data are consistent with the incorporation of crustal components either during passage of mantle-derived mafic magmas through the lower continental crust, or by their introduction by subduction processes into the mantle source region where these magmas were generated. Similar suggestions have been made concerning the petrogenesis of the volcanoes of the CVZ. The available geochemical data can not distinguish unambiguously between these two models; both processes may be important.