Depósitos piroclásticos gondwánicos en el sur de la sierra de Varela, provincia de San Luis: su petrografía y geoquímica

Gondwanic pyroclastic deposits in southern Sierra de Varela, San Luis province: petrography and geochemistry. This work presents the results of the petrographic and geochemical characterization of the pyroclastic deposits of the Cerro Varela Formation (Anisian), located in the southern portion of the Sierra de Varela, western San Luis province. This unit is linked regionally and chronologically to the Choiyoi Group and also fills the Beazley basin. The base of the Cerro Varela Formation overlies the Precambrian-Lower Paleozoic metamorphic basement and it is covered discordantly by the Upper Cretaceous Lagarcito Formation. In the study area two types of pyroclastic deposits area recognized: crystal-rich welded tuffs and fall deposits, both of rhyolitic composition. The crystal-rich welded tuffs comprise the majority of the outcrops, whereas the fall deposits are morerestricted because of their low degree of preservation. The pyroclastic flow deposits form red-colored layers with eutaxitic texture, columnar jointing and quartz and sanidine phenocrysts in a micrograin, felsitic and granophyric matrix with abundant iron oxides. Felsic volcanic lithic cognates and devitrified vitroclasts are also observed. The fall deposits are solid, orange-colored layers of fine grain size, with underformed vitreous fragments and quartzcrystaloclasts. The chemistry of the pyroclastic units is comparable to the middle to upper section transition of the Permo-Triassic Choiyoi Group in the Cordillera Frontal of Mendoza and San Juan. Their environment of generation is linked to rifting phenomena in a normal to thin crust where extension can be associated with the firststage of break-up of Gondwana during the Mesozoic.