Resumen

FRISICALE, María C et al. Retrogradación de granulitas miloníticas y formación de filonitas en la megacizalla de Azul, Tandilia. Rev. Asoc. Geol. Argent. [online]. 2015, vol.72, n.3, pp.314-325. ISSN 0004-4822.

At the western end of the Azul megashear zone, which is part of the Sierras Septentrionales of Buenos Aires, an east-west belt of phyllonites occur on the north side of the cerro Negro. These outcrops of phyllonites are associated with deformed and retrograded granulites, showing a complete gradation between protomylonites and ultramylonites. An intense biotitization of amphibole and pyroxene of the original rocks becomes dominant, as well as the formation of white mica from feldspar, though in a lesser extent. Microtectonic studies and the identification of several periods of pseudotachylyzation indicate that high grade mylonitic rocks of the Azul megashear zone have undergone subsequent shear deformation under ductile-brittle conditions, accompanied by a remobilization of fluids that caused the retrogression of the original rocks, and the phyllonite formation.

Palabras clave : Sierras Septentrionales; Paleoproterozoic; Basement; Fluids.