Recent Volcanism in the Puyehue--Cordon Caulle Region, Southern Andes, Chile (40 5 S): Petrogenesis of Evolved Lavas

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Abstract
Puyehue Volcano (40·5°S) in the southern volcanic zone (33°–46°) of the Andes is a largely basaltic stratovolcano constructed on a highly eroded, dominantly andesitic volcanic center. During growth of Puyehue Volcano there was a trend from basaltic to more siliceous lavas, and the most recent eruptions (1921–22, 1960) are Cordon Caulle rhyodacites and rhyolites erupted from fissures northwest of the volcano. These basaltic through rhyolitic lavas define a medium-K2O suite of tholeiitic affinity with trace element and Pb-isotopic signatures typical of volcanic rocks associated with subduction zones. Most of the evolved lavas, ranging from andesite to rhyolite, formed by low to moderate pressure (≤ 5 kb) fractional crystallization of a plagioclase-dominated anhydrous assemblage. Magma mixing produced aphyric basaltic andesites with anomalously high incompatible element contents and latestage andesites with disequilibrium phenocryst assemblages. The age progression from abundant basalt to younger, less voluminous, more silicic lavas reflects increasingly greater degrees of fractional crystallization which caused the apparent compositional gap between mixing end members to widen. There is no evidence in the silicic lavas for assimilation of geochemically distinctive continental crust. Puyehue basalts are surprisingly more heterogeneous in 87Sr/86Sr (0·70378–0·70416) and incompatible element abundance ratios (e.g., La/Sm, Ba/Nb) than the more evolved lavas. This geochemical variability may reflect subcrustal source heterogeneities or contamination by lower crust. The older basaltic andesites and andesites underlying the Puyehue edifice have Sr and Nd isotopic ratios and incompatible element abundance ratios within the range of Puyehue basalts. Apparently, similar sources and processes were involved in their genesis.

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