Sr, Nd, Pb and O isotope composition of Late Cenozoic volcanics, northernmost SVZ (33-34oS).

Abstract

The volcanic centres of the northernmost segment of the SVZ are more similar to those of the CVZ between 16-28oS than to those further south in the SVZ, with respect to both the common presence of hydrous minerals, and the volumetric dominance of intermediate and silicic rocks. In the northern section of the SVZ, although the basalts and andesites have 87Sr/86Sr and 143Nd/144Nd ratios which are similar to those in the southern segment, they are more radiogenic with respect to Sr isotope composition and the rhyolites are significantly more radiogenic. In addition, the rhyolites have significantly higher delta 18O values than the associated mafic and intermediate rocks. The basalts and andesites from this zone are similar with respect to O isotope composition both to each other and to the intermediate and mafic rocks from the southern part of the SVZ. The Pb isotope compositions of basalts, andesites, dacites and rhyolites are similar throughout both the SVZ and Austral volcanic zone as far south as 52oS. The available geochemical data cannot distinguish unambiguously between two possible models of petrogenesis - 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.-J.M.H.