Geochemistry and petrology of lavas from San Jose volcano, Southern Andes (33°45’S)

Abstract

San Jose volcano, one of the northernmost volcanoes of the Southern Andes, is composed of calcalkaline rocks with Si02 in the 56–63 wt% range. These rocks have some major-element, trace-element and Sr isotope compositions (0.70513-0.70551) that are intermediate between those of the Central and 37°-46°S Southern Andes andesites. The REE patterns of these rocks exhibit relatively high La/Yb normalized ratios (~10) with La about 80 times as much as in chondrites and Yb ranging from 6 to 9 times as much as in chondrites. The trace element abundances and SB systematics are consistent with a derivation of the most basic San José andesite from primary magmas, generated by relatively low degrees of partial melting of a gamet-peridotite source, through a crystal fractionation process dominated by olivine, clinopyroxene and plagioclase. The trace element composition of this sample is also consistent with the derivation from a source modified by subduction-related fluids. The relatively high Sr isotopic ratios of the samples suggest that mantle derived magmas are contaminated during their passage through the continental crust. © 1985, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.