Hydrothermal alteration related to a deep mantle source controlled by a Cambrian intracontinental strike-slip fault: Evidence for the Meruoca felsic intrusion associated with the Transbraziliano Lineament, Northeastern Brazil

Abstract

One of the most prominent geological structures in Borborema Province, northeast Brazil, is the Transbraziliano Lineament that crosscuts most of the South American Platform and was active at least until the Devonian. This continental structure is responsible for the formation of rift and pull-apart basins in Northeastern Brazil, most of which filled with volcanic and continental sedimentary rocks (Parente et al., 2004). In the region of Sobral, Ceará State, this same continental structure controlled the intrusion of the Meruoca pluton and the formation of the Jaibaras Basin, which is bounded by strike-slip shear zones. Hydrothermal alterations seem to have been pervasive in Meruoca, as indicated by disturbances in both the Rb–Sr and U–Pb systems (Sial et al., 1981; Fetter, 1999) and by the large dispersion of anisotropic magnetic susceptibility (AMS) (Archanjo et al., 2009).In this paper, we address the origin of the hydrothermal fluids that affected the borders of the Meruoca batholith and their relationship with the activity of the Transbraziliano Lineament. These fluids were responsible for carbonate veins and Fe–Cu mineral concentrations that are commonly found associated with hydrothermally altered breccias. The carbon and oxygen isotope composition of these carbonate veins suggest that they may be related to CO2-bearing mantle-derived fluids that were channelized by the Transbraziliano Lineament. Based on oxygen isotopes, we argue that Fe–Cu concentrations may have formed in isotope equilibrium with the rhyolitic rocks at temperatures between 500 and 560 °C. This scenario points to magmatism as the main process in the formation of these rocks.We also report a K–Ar age of 530 ± 12 Ma for muscovite associated with the last ductile event that affected the Sobral-Pedro II Shear Zone and a U–Pb age of 540.8 ± 5.1 Ma for the Meruoca pluton. We further suggest that this granite is a late-kinematic intrusion that is most likely associated with the Parapuí volcanic rocks of the Jaibaras basin (535.6 ± 8.5 Ma, Garcia et al., 2010).