High-temperature deformation processes in migmatites from the Atuba Complex, southern Ribeira Belt: Constrains from microstructures and EBSD analyses

Abstract

The Atuba Complex occurs in the southern region of the Ribeira Belt, in the eastern Paraná State. It comprises part of the Curitiba Terrane and is mainly composed of stromatic metatexites and migmatites subject to varying degrees of mylonitisation. The present work aims to perform microstructural analyses on ductile tectonites, as well as assess the prevailing deformation mechanisms by characterizing quartz crystallographic textures and microstructures, in order to understand in a more accurate way the structural and tectonic evolution of the Atuba Complex and of the Curitiba Terrane. The rocks have two phases of migmatization (granodiorite-tonalitic and granitic leucosomes) and constitute two deformation phases (Dn-1 and Dn), represented by the Sn-1 and Sn foliations, generated by ductile shear. Sn-1 foliation may be associated with Paleoproterozoic overthrusting tectonics and Sn foliation may be the result of Neoproterozoic sinistral transpressive tectonics. The electron backscatter diffraction (EBSD) results related to foliation Sn show that the deformation was accommodated by dislocation creep achieving dynamic recrystallization in quartz. Microscopically, the quartz may exhibit lobate grain boundaries, suggesting grain boundary migration (GBM), and may also show microstructures indicative of recrystallization by subgrain rotation (SGR) and, as very fine grains developed along grain boundaries, suggesting dynamic recrystallization by bulging (BLG), pointing to deformation superposition under greenschist facies conditions. The feldspar crystals show undulose extinction, subgrains and new grains, formed mainly by bulging and subgrain rotation. The domain comprised by the Sn foliation show activation of the basal slip with rhombohedral component or rhombohedral slip system for quartz, which indicate the deformation occurred at low temperature conditions (<500 °C) and may be associated with retrometamorphism that occurred in the Atuba Complex.