GEOLOGY AND PETROGRAPHY OF THE MIO-PLIOCENE VULCANISM OF SAN LUIS PROVINCE.

Abstract

The Mio-Pliocene vulcanism of San Luis Province (Argentina), outcropping along an 80 km long strip with a NW-SE trend, is represented by domes, volcanic cones, lava flows, dykes, sills and pyroclastic deposits associated to meridian fractures on the westem side of the area, and with calderas on the eastem one. Rocks can be grouped into andesites, latites, lati-andesites and trachytes bearing plagioclase, sanidine, amphibole, clinopyroxene, and rarely biotite and quartz. Pyroclastic rocks correspond to lapilli-breccias and lapilli-ash tuff originated by explosions during domes' growth. Lapilli-tuff and tuffites are also present. The lack of collapse structures on the westem side is assigned here to a deeper magma as well as to the strong structural control of the basement. Volcanic products displaying different evolution grades along each fracture ar absent. Calderas might be a consequence of shallower magmas. There is no evidence of magmatic evolution throughout the Cerros del Rosario. In the Sierra del Morro, the most differentiated rocks be long to a pre-caldera evento while those less evolved were emmited after the collpase. This might suggest a stratified magma chamber, or that the magma evolved at a shallower crustallevel after the first emmisions.