The onset of the right-lateral strike-slip setting recorded in magnetic fabrics of A-type granite plutons of the Ribeira belt (SE Brazil)

Abstract

Strike-slip shear zones formed during continental collisions accommodate the convergence and indentation of rigid cratonic blocks. In high-temperature settings of deeply eroded orogens, such as the Neoproterozoic Ribeira belt in southeastern Brazil, the shear deformation is associated with syntectonic granitic magmatism. Anisotropy of magnetic susceptibility (AMS) and zircon U-Pb results from late A-type granites were used to investigate the timing of the strike-slip deformation. Some of these plutons show reddish colors, local rapakive textures and magnetic susceptibilities that are typically lower than older batholiths. Rock magnetic data and SEM analysis indicate that AMS of the A-type plutons is dominated by coarse magnetite and locally (titano)hematite, the latter formed by oxidizing hydrothermal fluids in the final stages of magmatic crystallization. The shape preferred orientation of mafic minerals and feldspars measured in the reddish granite facies , however, indicate that AMS reproduced the magmatic fabric defined by the silicates. Magnetic fabric orientations vary from concordant (Ouro Verde and Capão Bonito plutons) to partially concordant (Sguário pluton) with respect to the host rock shear zones, and define the transition from concentric magnetic lineations and foliations experienced by older I-type batholiths. These A-type plutons constrain the onset of the ductile transpressional deformation at ca. 595 Ma. The magmatism and shear deformation persisted up to 550 Ma in the western Ribeira belt when more brittle transtensional conditions prevailed. These results are consistent with the convergence and indentation of the Congo-São Francisco craton that partitioned the deformation across the orogen and promoted the belt lateral escape mediated by dextral shear zones.