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The Iglesia Valley Basin (Fig.1) is part of the so-called Iglesia-Calingasta-Uspallata depression, which correspond to a mostly NS regional intermountain structure from 29º30?S to 32º30?S, along 350 kilometers (Ramos et al., 1984). It is bounded by the Cordillera del Colangüil (Cordillera Frontal, San Juan Province) to the West and by the Precordillera to the East. At presents a suture separates Precordillera from Cordillera Frontal (Fig.1) (Cita) . Deformation and structural style are consequence of the simultaneous uplift of the Cordillera Frontal and the Precordillera. At these latitudes and simultaneously with the Miocene volcanic activity that took place at the orogenic front towards the East of the Andean Cordillera, the analyzed eruptive sequence here was accumulated at the back-arc position and it shows distinctive features with geochemical differences to the arc rocks. It is composed of a pyroclastic sequence with related volcanic and subvolcanic rocks exposed in some localities along the Iglesia Valley Basin and surrounding areas, between Precordillera and Cordillera Frontal, San Juan Province. The eruptive rocks here studied crop out along the Iglesia Valley and favored by recent fluvial erosion. They constitute an extensive series of low relief deposits made up by continental sediments known as Las Flores Formation and volcanic rocks, mainly pyroclastics named Lomas del Campanario Formation. Lomas del Campanario Formation, jointly with the sedimentary Las Flores Formation constitute the Iglesia Group (Wetten, 1975). According to this author, the lower section is characterized by an agglomerate of andesites, dacites, tuffs and volcanic bombs. In this work, Lomas del Campanario Formation pyroclastic rocks are analyzed along with lava flows and shallow intrusives known as Intrusivos Terciarios (Cardó y Diaz, 2003).