Abstract

FRISICALE, María C.; DIMIERI, Luis V.; ARAUJO, Vanesa S.  and  DRISTRAS, Jorge A.. Deformation mechanisms in the mylonite/striped gneiss and mylonite/ultramylonite transition in Sierras de Azul, Río de la Plata craton, Buenos Aires. Rev. Asoc. Geol. Argent. [online]. 2010, vol.67, n.1, pp.4-18. ISSN 0004-4822.

At the western sector of the Azul Megashear, mylonitic rocks with variable deformation that includes protomylonites, mylonites, ribbon mylonites, striped gneiss and ultramylonites were identified. A detailed examination of deformation mechanisms acting on main minerals like feldspar, quartz, amphibole and pyroxene was done. The analysis of deformation mechanisms allowed to establish the metamorphic degree reached in this sector of the shear zone. Thus, two areas with different metamorphic conditions were recognized. One of them, that involve the outcrops of Sector Norte, Cerro Negro and Manantiales de Pereda, with a deformation produced under metamorphic conditions of amphibolite to granulite facies (temperatures higher than 600º); and the other with a less intense deformation produced under metamorphic conditions of greenschists facies, involving the outcrops of La Manuela and La Chiquita. Furthermore, meso and microscopic kinematic indicators such as S-C structures, fragmented rigid grains, rotated porphyroclasts and mineral-fish structures were analyzed in order to establish the sense of movement of the rock masses in this region and to compare it with results elsewhere in the Azul Megashear.

Keywords : Mylonite; Striped gneiss; Ultramylonite; Deformation mechanisms; Río de la Plata Craton.

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