Lithostratigraphic, petrographic and geochemical analysis of the Uritusinga volcanics, Loja-Ecuador

Abstract

Cartography of ancient volcanic deposits is a topic of interest for the geoscientific community. The discussion regarding the origin of volcanic lithotypes and volcaniclastic lithofacies in recent decades has allowed us to understand the behavior and geodynamics of extinct, eroded volcanoes, which occupy a place in the regional stratigraphic record. In Ecuador, south of Loja on the Puntzara Grande - Malacatos road, Cerro Uritusinga hill includes unreported volcanic and volcaniclastic rocks. This research presents lithostratigraphic characterization, the description of volcanic lithotypes and volcaniclastic lithofacies, and geochemical analysis. The lithostratigraphy of the Uritusinga volcanic deposits includes: volcanic breccias (BVMP), volcanic conglomerates (CCSO), clast tuff (TGC), crystal tuff (TFC), massive fine tuff (TFM), and andesite (And). Thin section analysis reveals pilotaxic texture in volcanic breccias and fluid microtextures in fine massive tuffs. The geochemical composition was analyzed in four samples of lavas where the proportions of alkali (Na₂O + K₂O) evaluated as a function of silica (SiO₂) indicate that the rocks correspond to andesites. The K₂O and SiO₂ ratios indicate a calc-alkaline affinity. Th(_N) and Nb(_N) concentrations indicate that these lavas formed in a volcanic arc on a continental margin. Fission track ages reveal an age of 2,36 ± 0,36 Ma. Stratigraphic, petrographic, geochemical, and geochronological data allow linking the Uritusinga Unit to distal volcanic facies of Pleistocene volcanism regionally reported in the Salapa Formation.