MAGMATIC EVOLUTION OF THE COPAHUE-CAVIAHUE VOLCANIC COMPLEX

The Copahue-Caviahue volcanic complex is located in the Southern Volcanic Zone of the Andes, at 37°50'W and 71°05'S, 25km east of the volcanic front. The active Volcan Copahue is situated on the southwestern rim of the Caviahue Caldera, and last erupted in 2000. The eruptive history and magmatic evolution from the early Pliocene to present is preserved in the stratigraphy of the caldera walls, the intracaldera flows, and the active center. During the Pliocene, the Copahue-Caviahue volcanic complex was located on the main arc axis, which migrated west during the Pleistocene. Ar-Ar dating reveals that caldera formation occurred at around this time, possibly in response to a change from a compressional to an extensional ("{back-of-the-arc") stress regime. The petrology and major, trace and Sr-Nd-Pb isotope geochemistry of the lavas record the evolution of the complex from a number of polygenetic stratovolcanoes, through a period of non-catastrophic caldera collapse, followed by intracaldera volcanism, dome emplacement along the caldera ring fracture, and finally the growth of the present edifice of Volcan Copahue. Young back arc basaltic cones located to the east of the Copahue-Caviahue volcanic complex have incompatible element signatures intermediate to OIB and enriched MORB. Post caldera volcanism originates from a progressively more enriched mantle source and a decreasing slab derived component, indicated by increasing Nb/Zr and decreasing Ba/Nb through time.