GEOLOGY OF EL TATIO VOLCANIC AREA (CENTRAL ANDES - NORTHERN CHILE): PRELIMINARY STRATIGRAPHIC, TECTONIC AND PETROCHEMICAL DATA

The area of El Tatio (Chile), in the central sector of Andean Cordillera, is known since long time owing to the presence of a large geothermal field and to its location at the center of one of the widest Neogene ignimbrite provinces on Earth. Since the late Cretaceous and in particular starting from the late Miocene (about 10.4 Ma), intense and repeated episodes of silicic volcanism gave rise to a volcanic province, the Altiplano-Puna Volcanic Complex, between latitudes 21°-24° S in the Central Volcanic Zone. New stratigraphic studies of the El Tatio area have highlighted the occurrence of significant unconformities in the frame of the volcanic activity developing in the time span between Cretaceous and Holocene. Those unconformities allow volcanic history to be subdivided in four main eruptive epochs: • the late Cretaceous eruptive epoch is characterized by volcanic activity, probably occurring in a backarc geodynamic setting, which is documented by scarce outcrops in the northwesternmost sector of the studied area, of pyroclastics and lavas intercalated within the top portion of the underlying sedimentary succession; • in the upper Miocene-Pliocene, volcanic activity is mostly explosive and determines the emplacement of great volumes of ignimbrites (Rio Salado, Sifon and Puripicar and to the intrusion of the Copacoya subvulcanic body; • the Pleistocene eruptive epoch is characterized by the intensemost volcanic events which determine the emplacement of several volcanic edifices (the main ones are Quebrada Chucula, Tatio and Deslinde-Tatio) through mainly effusive volcanic activity. Those volcanic edifices are N-S and NNE-SSW-aligned (in the easternmost sector) following the main tectonic lineament developing on a regional scale which borders the margin of the Altiplano-Puna representing a segment of the Western or Main Range of the chilean Ande. During the Pleistocene eruptive epoch, an important explosive event also occurs determining the emplacement of widespread ignimbrites (Tatio ignimbrite) in the whole central and eastern sectors of the studied area; • finally, the recentmost Holocene eruptive epoch is characterized by the building of the Tocorpuri volcanic edifice which is located in the southeasternmost sector of the studied area. As regards tectonics, four thrust surfaces offset the above described volcanic successions and the volcanic edifices, probably extending in depth and cutting the volcanic conduits. Thrust surfaces strike in a grossly N-S direction and dip to the east. The direction of tectonic transport (vergence) is toward the west, as suggested by thrust geometry, displacement of the volcanic bodies and mesostructural analyses. Another structural lineament of regional importance is a system of en-echelon sinistal strike-slip faults, extending from southest to northwest and offsetting the thrust planes (Cizalla del Tatio fault). Notably, the displacement of thrust surfaces due to the strike-slip fault system decreases from west to east. This may suggests that the Cizalla del Tatio fault has been active together with the thrust faults, and the onset of the westernmost thrust planes, such as the Cablor and Cablor superior thrusts, should be older than the esternmost (Tocorpuri thrust). The different age of nucleation of thrusts is also suggested by the different ages of the displaced volcanic stratigraphic units. Moreover, the Cizalla del Tatio fault plays an important role in the hydrothermal activity of the well-known El Tatio geothermic field. The petrochemical study of volcanics from the El Tatio area puts in evidence the occurrence of prevailing dacites and subordinate andesites and rhyolites related to calcalkaline and HK-calcalkaline associations. Despite the limited range of shown compositions, the occurrence of strong differentiation allowed evolutive patterns to be evaluated.