Microstructural and geochmical analysis of Paleoproterozoic pseudothachylytes in the Rio de la Plata craton, Tandilia belt, Argentina. s

FRISICALE, M.C., DIMIERI, L., DRISTAS, J.A., ARAUJO, V. Y FORTUNATTI, N.

GEOLOGICA ACTA

UNIV BARCELONA

Lugar: Barcelona; Año: 2012 vol. 10 p. 85 - 101

1695-6133

The current study focuses on the analysis of pseudotachylytes from the Azul Megashear Zone, located in the Ríode la Plata craton, central Argentina. This shear zone is a strip of mylonitic rocks of more than 40km in length andwith a maximum width of 2.5km. Glassy and microcrystalline pseudotachylytes are hosted by Paleoproterozoicgneiss mylonite, granitoid mylonite, granulite mylonite and striped gneisses. The pseudotachylytes occur asfault and injection veins characterized by sharp contacts with no gradation into the mylonites and show meltoriginfeatures such as partially melted quartz and feldspar clasts with embayed rims and flow structures. Theircomposition is similar to that of the rocks in which they are injected. In those cases involving banded hostrocks, slight chemical differences can be observed in the pseudotachylytes mainly in terms of their fabric andmineralogy. Most pseudotachylytes have textures deriving from post-solidification ductile deformation, but othersbearing mylonite-like features are related to viscous flow prior to complete solidification. There are indicationsthat some of these pseudotachylytes were formed during the ductile regime, others being generated in the brittleregime during shear zone reactivation following exhumation or uplift. The occurrence of several pseudotachylytegeneratingevents indicates intermittent aseismic/seismic slip at different crustal levels throughout this shear zone.