Abstract

ANGELETTI, Melisa; FRISICALE, María Cristina  and  DIMIERI, Luis V. Petrografía, caracterización geoquímica y análisis microestructural del granito del cerro Siempre Amigos, sierras de Azul, Tandilia. Rev. Asoc. Geol. Argent. [online]. 2016, vol.73, n.2, pp.211-224. ISSN 0004-4822.

The Siempre Amigos hill is part of the Azul ranges, Tandilia. It is located 28 km to the south of the city of Azul and 3 km to the north of the Azul megashear, a subvertical shear zone which basement is deformed to mylonite and ultramylonites. The Siempre Amigos hill predominant lithology is a Paleoproterozoic granite deformed at the center and interdigitated towards the edges with a gneissic rock and amphibolite bodies, with similar subvertical mylonitic foliation N250º. All rocks show evidence of brittle-ductile deformation although they are affected with variable intensity: they are some less deformed amphibolites and gneiss together with granite reaching ribbon mylonite to striped gneiss textures. Analyzing their microstructure and deformation mechanisms, the deformation can be estimated as medium to high metamorphic grade, with temperatures between 400° to 600°C. The study of preferred crystallographic orientations of quartz gives evidence that there is a crystallographic preferred orientation near to the contacts of the granite with the gneissic rocks, indicating a marked pure shear symmetric deformation, and within the center of the outcrops there is a clear simple shear component. In geochemical terms, the granite is slightly peraluminous and has a volcanic arc syncollisional signature. In summary, the Siempre Amigos area seems to be a syntectonic transfer zone of magma that could happened at middle to upper crust of magmatic arcs with active deformation.

Keywords : Strain; Volcanic arc; Striped gneiss; Río de la Plata cratón.

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