Stratigraphy and petrology of the Northeastern sector of Sierra de Apas. Chubut Province

Abstract

The volcanic building of Sierra de Apas (located on the limit between the Rio Negro and Chubut provinces) is part of the Paleogene-Neogene intraplate magmatism of the northern Extrandean Patagonia. This contribution is focused on the northeastern of the Sierra where five effusive facies were recognized within the volcanic pile. The lowermost unit is represented by phenochryst-poor dark trachytes which classsify as trachytes and benmoreites. The dark trachytes are overlain either by light trachytes or by pyroclastic flows. The former are porphyritic rocks clasiffied as comendites and yielded an age of 19 ± 2 Ma. The pyroclastic flows are mainly pumiceous, of trachyandesite composition; both petrography and chemical features suggest a genetic link with the light trachytes.An exogene dome is represented by the Cerro Colorado, composed of a peralkaline rhyolite (comendite) which yielded an age of 24 ± 1 Ma. The uppermost effusive unit is represented by basaltic lava flows which are found resting upon either the light trachytes or the pyroclastic flows. Basaltic rocks may be grouped into porphyric and aphyric. On the basis of their chemical composition, the latter are transitional olivine basalts, whereas the former may be further divided in alkaline olivine basalts and hawaiites. Both stratigraphy and chemical data suggest that the described volcanic pile is the result of at least two evolutionary stages. One of them includes the basalts, light trachytes, pyroclastic flows and comendites which display an evolutionary trend likely resulting from fractional crystallization. The oldest dark trachytes would belong to another evolutionary stage.