PETROLOGY OF THE MIO-PLIOCENE VOLCANISM OF THE SAN LUIS PROVINCE.

Abstract

During Mio-Pliocene epoch, an important volcanism developed in Sierras Pampeanas of San Luis and Córdoba. The outcrops at San Luis are represented by modal andesites, lati-andesites, latites and trachytes with plagioclase, sanidine, amphibole, clinopyroxene, and rarely biotite and quartz. The suite shows mesosilicic character (most rocks with 61-66 % of SiO,), low TiO, and MgO, high alkali contents, anel lack of iron-enrichment. ChemicaJJy, rocks are c1assified as trachytes, latites, andesites, dacites, shoshonites and rhyolites belonging to calc.:-alkaline, high-K calc-alkaline and shoshonite series, characterized by increasing K,O/N<\,O. Textural relationships and geochemistry of rocks are in agreement with a Mg-poor, water and potassium-rich magma, which crystallization was affected by repeated pressllre drops. Calc-alkaline OlltCropS,so far from the l\Iiocene magmatic arc, are related to horizontal movement of the subducted slab. High-K calc-alkaline and shoshonitic rocks would he younger and might he originated by low-grade melting in deep crustal levels. Geochemistry and mineral paragenesis in the Sierra del Morro could indica te either a stratified magma chamber (in which most differentiated products were the first to be emitted) or an evolution of the magma at different crustal levels; the most basic rocks representin,g a shallower magma, which lower water contents cOlllcl he attributed to the development of a pyroclastic event.