THE OROCLINE-CONTROLLED CENOZOIC VOLCANIC ROCKS FROM THE WESTERN ANDES OF BOLIVIA

Four main periods of uplift and folding affected the thick Cenozoic volcanic sequence of the western Andean Cordillera from Bolivia since the Late Oligocene to the Upper Holocene epoch. These distrophic events are related to the installation of a major orocline flexure of the Altiplano-Puna-region, the so-called "Bolivian Orocline", as consequence of an A-type subduction of the Nazca plate whose slap dips at steep angles (near 30o) beneath this important segment of the central Andes of South America. The wester Andes of Bolivia conform an Inner volcanis arc displayed along the international boundary with Chile and Peru (between 17o-230S latiditude). Thi magmatic arc is characterized by prevalent emissions of high-k-calculakaline volcanics (high-K dacitesm, andersites and rhyolites), conforming this, a rift-controlled "shoshonotic" suite extended parallel to the proper cabalkaline volcanic rocks of the Chilean forearc environment. Nevertheless, scattered alcaline associations of volcanics have been also recognized on the northern part of the western Cordillera.