THE VOLCANISM OF THE WESTERN PART OF THE LOS-FRAILES MESETA (BOLIVIA)- A REPRESENTATIVE EXAMPLE OF THE ANDEAN VOLCANISM SINCE THE UPPER OLIGOCENE

The Los Frailes Meseta (Bolivia) is one of the large Tertiary ignimbri tic fields of the inner volcanic are from Central Andes (Central Volca nic Zone, CVZ), in the contact zone between the Altiplano to the west and the Eastern Cordillera to the east. Field observations and mineral ogical and geochemical studies (major and trace elements) lead to dist inguish two types of volcanism in the western border of the Meseta. Du ring the Middle Miocene and the Pliocene, the volcanic activity can be subdivided into three pyroclastic emission cycles, the Larco, Coroma and Pliocene ignimbrites, the first two being separated by the Quechua 2 orogeny. All these ignimbrites are very similar and correspond to p eraluminous rhyolites to rhyodacites. In the studied area, the Coroma cycle is the only one where an ignimbrite-less evolved resurgent dome association can be observed. Beside these ignimbrites, isolated small lava flows and domes overlay and/or intrude all the other formations. They are meta-aluminous lavas with a shoshonitic affinity. A Quaternar y age can be attributed to this second volcanism. These two volcanic t ypes are well-known in the CVZ and are related to the different deform ation stages, either compressional or extensional, which occur alterna tely in the Cordillera since 26 Ma.