Strain localization in quartzo-feldspathic mylonites from Patos shear zone revealed by EBSD data

Abstract

Understanding how strain is localized and concentrated in the mid-crust is critical to geological research. Investigating the microstructure and texture of quartzo-feldspathic mylonites is essential in this context. The Patos shear zone (PSZ), stretching ∼600 km long and responsible for the deformation of the paleoproterozoic basement in the Borborema Province under varying temperature conditions, is a significant shear zone for exploration. Combining techniques, including optical and scanning electron microscopy and Electron Backscatter Diffraction (EBSD) our research focused on quartzo-feldspathic mylonites to comprehend the solid-state processes responsible for strain localization within the Patos shear zone. As materials undergo uplift and deformation transition from a submagmatic to a solid-state flow, strain is concentrated within increasingly narrow zones. Our results demonstrated that after melt solidification, biotite-rich layers at temperatures exceeding 650 °C become weaker domains that accommodate strain through dislocation creep in feldspar, biotite and to a lesser extent in quartz, with the activation of prism-[c] slip system. During late reactivation, under upper greenschist/lower amphibolite facies, dislocation creep, strain-induced myrmekitization, solution-precipitation mechanisms are the dominant mechanisms for grain-size reduction in feldspar, with the resulting fine-grained matrix deforming via dislocation-accommodated and fluid-assisted grain boundary sliding (GBS). Quartz undergoes recrystallization as it transitions from grain boundary migration to subgrain rotation, with the activation of prism-<a> slip system. At temperatures below 450 °C, in the southern Patos shear zone, quartz ribbons experience bulging recrystallization with the activation of prism-<a> and rhomb<a> slip systems. Feldspar porphyroclasts undergo fracturing, myrmekitization and dissolution-precipitation creep. Rock rheology is controlled by fine-grained polyphase aggregates deforming via Dislocation-accommodated GBS. Finally, our study highlights the role of discontinuities during low-temperature deformation, such as the contact between rheologically contrasting layers and transgranular fractures, which serve as a precursor that led to shear zones nucleation in the northern block of the Patos shear zone.