Microstructures and K-Ar illite fine-fraction ages of the cataclastic rocks associated to the Camburu Shear Zone, Ribeira Belt, Southeastern Brazil

Abstract

The Camburu Shear Zone (CSZ) is recognized as a NE-SW trending dextral transcurrent fault and is related to the evolution of the Ribeira Belt, southeastern Brazil. The CSZ ductile regime dates from the Precambrian to the Cambro-Ordovician and it is characterized by a meter to hundreds of meters wide belt of quarzto-feldspathic mylonitic rocks organized in a positive-flower structure. Previous studies postulated that the fault underwent normal reactivation, probably during the Cenozoic, developing metric to centimetric thick intercalations of cemented cataclastic rocks. We concluded that these cataclastic rocks maintained their primary cohesion and were syntectonically silicified by feldspar-to-mica reactions. Cataclasis was shown by the fractal distribution of grains and fragments, dilatancy, rigid body rotation, cataclastic flow, microcracking and microfaulting. Two cataclastic events are described: (i) during the Lower Devonian (402 ± 6 Ma) in a ductile-to-brittle regime with temperatures up to 400°C and development of foliated protocataclasites, and (ii) during the Upper Carboniferous (310.9 ± 8 Ma) in a brittle regime with temperatures up to ~300°C and development of protocataclasites and crushed breccias. It is then suggested that the CSZ cataclasites were formed during two Paleozoic reactivation events associated to a far-field orogenic effect induced by the Precordillera and Chanica orogenic cycles in the western margin of the Gondwana Paleocontinent.