Relictic textures in the Neoproterozoic meta-ignimbrites of the basement of Cerro del Corral, Sierras Australes de Buenos Aires

Abstract

Relicts of eutaxitic texture, devitrified vitric fragments and andesitic lithoclasts occur in low-grade and weakly deformed acid meta-magmatic rocks of the eastern margin of the Cerro del Corral (Sierras Australes de Buenos Aires, Argentina). On the basis of the pyroclastic characteristics and the presence of quartz, plagioclase and K-feldspar phenocrysts, we re-interpreted the protolith of these rocks as ignimbrites of fenodacitic to fenorhyolitic composition. Embayed quartz phenocrysts (up to 1.5 mm diameter) show micro-channels and rounded inclusion filled with the same fine-grained, devitrified and non-foliated material (with felsitic to spherulitic texture) of the pyroclastic matrix. This matrix is also preserved as small remnants attached to the phenocrysts (or wrapping them) and passing laterally into a mylonitic matrix (quartz and sericite-rich and moderately foliated), with granoblastic to granolepidoblastic texture. We emphasize the stratigraphical importanceand the geological implications for the basement of the Sierras Australes of Buenos Aires that can be derived from the re-examination of these rocks and their re-definition as meta-ignimbrites. The deposition of the meta-ignimbrites from pyroclastic ash flows occurred within a geological setting dominated by stratovolcanoes. This tectonic setting was probably associated with a Neoproterozoic magmatic arc belonging to the Brasiliano -Panafrican Cycle.