Tortonian trachyte volcanoe in the Alta Sierra de Somún Curá Volcanic Complex. Northern Extrandean. Patagonia. Argentina

REMESAL, M. B.; CERREDO, M. E.; ALBITE, J.M.; SALANI, F. M.; PARICA, C. A.

Hamburgo

Simposio; 25th Latin-American Colloqui·um of Geosciences; 2019

Universidad de Hamburgo

This contribution deals with one of the chiefly trachyte volcanoes of ASSCVC. The polygenetic stratovolcano of Cerro Mimbre (CMTV), located in the southern complex, is the northernmost of three volcanoes arranged a long a NW-SE trend. CMTV is set within a local low relief area dominated by volcanic landforms with general SE gentle slope. Two semicircular escarpments in the uppermostCMTV are compatible with an explosion crater.Associated loose deposits of white color and fine texture, located among coulées, are indicative of explosive eruptive facies. At least tour lobular trachytic coulées emerge from the domes distributed in the CMTV; coulées locallypreserve marginal ridges. Trachyte coulées include autobrecciated facies of porphyric texture with anorthoclase phenocrysts and minor ferromagnesian minerals. Groundmass is trachytic composed of sanidine microliths and clinopyroxene. Trachyte lava flows are porphyric with feldspar phenocrysts (of oligoclase to anorthoclase composition) set in a trachyte groundmass with sorne bostonitic patches. Microliths of anorthoclase to sanidine composition, olivine,clinopyroxene (Mg-rich augite), F-apatite and magnetite complete the groundmass assemblage. Two long basaltic flows extend for severa! kilometers enveloping theCMTV. Three scoria eones surround the CMTV; within the main crater, there are two oxidized deposits of scoria that would correspond to a very eroded cone, younger than the trachyte coulées. The lowermost basaltic flows surrounding the main volcanic building are vesicle rich, either aphyric or porphyritic withplagioclase phenocrysts and olivine microphenocrysts set in an intergranular groundmass with ophitic patches made of clinopyroxene and plagioclase. Trachyte flows of the CMTV yielded a new whole rock K/Ar age (Fig. ld) of 9.8 ± 0.3 Ma, which fits well (within errors) with already published chronological data for other trachyte lavas of ASSCVC (121 9-13 Ma).