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The large Pliocene Caviahue caldera and associated active Copahue volcano are major volcanic features on the northwestern side of the Neuquén Basin. Chemical and petrographic data from the volcanic complex show a compositional range from basaltic andesite to rhyolite, predominance of two-pyroxene andesites and dacites, and a major quartz-biotite–bearing rhyolitic ignimbrite, which is part of the Riscos Bayos ignim-brite complex. Caldera wall sequences are dominated by lava and debris flows that show no consistent temporal trend toward more evolved magmas. The lavas at the top of the caldera wall series are among the most mafic in the region. The Copahue rocks are largely mafic two-pyroxene andesites, enriched in large ion lithophile elements and high field strength elements compared to the older Caviahue rocks. The intracaldera silicic rocks differ in composition from the Riscos Bayos ignimbrite sequence, which fills a paleovalley southeast of the caldera. The volume of these ignimbrites is insufficient to explain the formation of the Caviahue caldera. Cinder cones and lava flows east of the Caviahue complex consist of olivine-rich basalts. Isotopic data (Pb, Sr, Nd) show that all of the volcanic rocks in the Caviahue-Copahue volcanic complex have crustal components in their magma sources (i.e., subducted sediment and assimilated continental crust). The Caviahue series shows an increase in Sr and Pb isotopic ratios from mafic to silicic members, suggesting open-system evolution in the crust, although the isotopic variations are very small. The Copahue rocks are chemically and isotopically distinct from the Caviahue series; the He isotopic composition of geothermal gases from Copahue is close to mantle values, despite the evidence for sediment involvement in the other radiogenic isotopic systems. The compositional differences between the Caviahue and Copahue series may be related to crustal processes, involving continental crust with a different chemical and isotopic composition for the two series. Alternatively, the change from Caviahue to Copahue volcanism might have been the result of a change in the element extraction process from the subducted complex: a contribution of sediment melting for Copahue versus volatile flux-induced mantle melting for the Caviahue series. The switch from a dominantly flux-melting regime (Caviahue series) to a drier decompressional melting regime led to smaller degrees of melting, as indicated by higher La/Yb values in the Copahue rocks. The low Ba/La values and high He isotope ratios in the Copahue series suggest an earlier phase of volatile fluxing and element loss from the subducted sediment complex, which probably took place below the main volcanic arc west of Copahue. As a result, the Copahue volcano east of the main volcanic front erupted magmas that formed in a drier mantle environment dominated by decompressional melting with a more significant component of subducted sediment melt.

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