The Neoproterozoic Dorsal de Cangucu strike-slip shear zone: its nature and role in the tectonic evolution of southern Brazil

The Dorsal de Cangucu Shear Zone (DCSZ) is part of a strike-slip fault syst em showing trends parallel to the Neoproterozoic Dom Feliciano Belt in sout hern Brazil. As an attempt to assess the role played by this fault system i n the tectonic evolution of the continental crust in southern Brazil, a re- evaluation of the main structural, magmatic and geochronological characteri stics of the best known shear zone of this system was conducted. Magmatism syntectonic to the strike-slip shear zone is represented by mantle-derived granodioritic magmas emplaced into transtensional segments. These were foll owed by crustal melts, represented by successively younger peraluminous gra nites. The porphyritic granodiorites have a mixed origin involving a parent al dioritic magma that suffered fractional crystallisation and assimilation of crustal rocks. They present a well-developed subvertical magmatic fabri c with northeast to north-south trending foliation and low plunging lineati ons defined by dimensional orientation of K-feldspar megacrysts. Partial me lting of the country rocks is the most likely petrogenetic process for the origin of the peraluminous granites. Microstructures produced by solid-stat e deformation under lower amphibolite- to greenschist-facies metamorphic co nditions exhibit ubiquitous kinematic indicators of sinistral displacement. The contribution of the transcurrent fault zones to crustal growth was lim ited to the emplacement of relatively small volumes of mantle-derived diori tic magma during their early stages of development. Large-scale tectonic co ntrol of the overall strain field responsible for the nucleation and sinist ral displacement of these faults is likely to be a far-field effect of conv ergence between the Kalahari and Zaire Cratons during the final stages of N eoproterozoic amalgamation of West Gondwana. (C) 1999 Elsevier Science Limi ted. All rights reserved.