THE CAVIAHUE-COPAHUE VOLCANIC COMPLEX, PROVINCE OF NEUQUEN, ARGENTINA

The Caviahue volcanic complex is found at about 38 oS in the transition zone from the high-Andes to the back-arc alkali-basalt cinder cone fields. The system consists of a large caldera-like depression of 15 by 18 km, with walls of ~0.6 km high at the northern and eastern sides. The active volcano Copahue occurs at the west side of the depression. The oldest rocks are about 1.8 Ma and the caldera age is unknown. The north wall is made of lava flows, dikes, sills and dome rocks whereas the eastern wall consists of debris flows, minor ash flows, lava flows and one large pyroclastic flow unit at the top. The composition of the volcanic rocks ranges from arc basalts through trachy-andesites to trachy-dacites, with MgO contents from 4.8-0.2 % and SiO2 contents from 51.1 to 68.7 %. The mineralogy consists of ol+cpx+opx+plag+mag in the mafic rocks to plag+opx+oxides in the more evolved rocks, whereas the main pyroclastic flow is the only unit in the whole complex that has a hydrous phase (biotite). Olivines range from 63-80 % Fo, cpx are Mg-rich, and plagioclases are andesines and labradorites. Glass inclusion data from Copahue rocks indicate low water concentrations (~1 % H2O), in agreement with the paucity of hydrous phases in the rocks. The Caviahue rocks are relatively rich in incompatible elements, with Rb from 36-116 ppm, and K2O from 0.9-4.8 %. The rocks all show an arc signature with negative Ti - Nb anomalies, indicating that these rocks belong to the Andes and not to the back-arc alkali-basalt province. Lead isotope and 129I data from the Copahue hydrothermal system (Fehn et al., 2002) indicate that the Copahue magmas carry a subducted sediment component. The volcanic complex is positioned at the extreme eastern margin of the arc, far away from the trench, which may be the cause for the water-poor character (slab already devolatilized at this remote end). The caldera volume (~ 160 km3) is an order of magnitude larger than the volume of the one major pyroclastic flow. The depression may not be collapse-related, but instead be a geomorphic-constructive feature related to the build-up of interfingering volcanoes and ice accumulation in an old depression.