Microstructural and geochemical analysis of Paleoproterozoic Pseudotachylytes in Río de la Plata craton, Tandil belt, Argentina (EN REVISIÓN)

MARÍA CRISTINA FRISICALE; LUIS V. DIMIERI; JORGE A. DRISTAS; VANESA ARAUJO; NATALIA FORTUNATTI

GEOLOGICA ACTA

UNIV BARCELONA

Año: 2010

1695-6133

Abstract This study focuses on the analysis of pseudotachylytes from the Azul Megashear Zone (AMSZ), located in the Río de la Plata craton, central Argentina. This shear zone is a strip of mylonitic rocks more than 40 kilometers long and a maximum width of 2.5 kilometers. Glassy and microcrystalline pseudotachylytes are hosted by Paleoproterozoic mylonitic gneisses, mylonitic granitoids, mylonitic granulites and striped gneisses. The pseudotachylytes occur as fault veins and injections veins both being characterized by sharp to irregular contacts and without any gradation into the mylonites. Partially melted quartz and feldspar clasts with embayed rims and flow structures provide evidence for a melt origin of these pseudotachylytes. The composition of the pseudotachylytes is similar to that of the rocks in which they are injected, yet in some cases that involve banded host rocks slight chemical differences can be observed mainly due to their fabric and mineralogy. Most pseudotachylytes have textures that resulted from post-solidification ductile deformation, although there are some pseudotachylytes bearing mylonite-like features that are related to viscous flow before complete solidification. Several evidences support that some of these pseudotachylytes were formed during a ductile regime, while others were generated in a brittle regime related with shear zone reactivation during exhumation or uplift. The occurrence of several pseudotachylyte-generating events indicates that intermittent aseismic/seismic slip has occurred repeatedly in this shear zone in different crustal levels.