Abstract
Active andesitic-type volcanism in the southern Andes (lat. 42–36° S) is characterized by low Sr- and O-isotopic compositions which fall in the range 87Sr/86Sr = 0.7036–0.7043, δ18O = 5.2–6.8‰. In contrast, volcanic centres in the Central Andes (lat. 24–21° S) are significantly enriched in both 87Sr and 18O. We present the first combined Sr- and O-isotope study of the southern volcanic zone (SVZ) in Chile (crustal thickness <40 km)1 and of the southern part of the central volcanic zone (CVZ) in the borderlands of Bolivia, Chile and Argentina (crustal thickness ∼60–70 km)1. Only one of 13 volcanic centres studied (Lascar) exhibits a correlated increase in both 87Sr and 18O parallel with the basalt–andesite–rhyolite compositional change, indicative of contamination by upper crust through a fractional crystallization–assimilation mechanism. The other centres of the CVZ, for which there is an overall Sr–O correlation, document the variable contamination of mantle-derived magmas at depth in the lower crust which had been thickened by Mesozoic and earlier plutonism.
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