Ñancuyique-Rucu Luán Domes, Somún Curá Magmatic Province, Argentine Patagonia.

SALANI, F. M.; REMESAL, M. B.

Conferencia; Soufriere Hills Volcano ? Fifteen Years Scientific Conference. Petrological and geochemical studies; 2011

Somún Curá Magmatic Province in northern Argentine Patagonia comprises an extensive basaltic plateau, mainly Oligocene, and several bimodal complexes, Miocene in age, corresponding to a post plateau eruptive stage. Whereas most of the complexes are composed of basaltic-trachyte lavas, in the southern border of the plateau (68º 30´W and 42º) it crops out a volcanic field (100 km3) known as Pire Mahuida Complex, characterized by a huge volume of acidic pyroclastic and lava facies, with minor amounts of basaltic lavas. This complex was developed through several stages from 21 to 18 Ma. Particularly, we focused the study in the rhyolite and dacitic lava domes which constitute the first effusive stage of the volcanic field. They are aligned along the northwestern side of an elliptical caldera (major axis 60 km and the minor axis 20 km), following a subcircular pattern, from Ñancuyique-Rucu Luán area to Pire Mahuida hills near the central region of the field. Emplacement of domes occurred in two continuous phases, from rhyolitic to dacitic composition. Rhyolite domes show circular shape, with 1.5 -2 km diameter and 200m high. Flow banding, ramp structures, vesiculation textures and litophysaes are characteristic features. Compositionally, they consist of poorly porphyritic rhyolites, with sanidine, quartz, biotite and trydimite. Dacite domes occurred as minor bodies of irregular shapes, and are composed of porphyritic lavas with hornblende and plagioclase with disequilibrium textures, apatite and opaque minerals. The suite corresponds to a calcalkaline sequence. Trace element chemistry and isotopic studies suggest a crustal genesis and an intra-plate tectonic setting with an arc signature. The geodynamic emplacement of Ñancuyique-Rucu Luán volcanism, is the result of the plate reorganization process that took place along the Pacific margin of the South America (~25 Ma) which favored an extensional regime in the external foreland.