Microstructural evolution of the Seridó Belt, NE-Brazil: the effect of two tectonic events on development of c-axis preferred orientation in quartz

Abstract

The polyphase evolution of the Seridó Belt (NE-Brazil) includes D 1 crust formation at 2.3–2.1 Ga, D 2 thrust tectonics at 1.9 Ga and crustal reworking by D 3 strike-slip shear zones at 600 Ma. Microstructural investigations within mylonites associated with D 2 and D 3 events were used to constrain the tectono-thermal evolution of the belt. D 2 shear zones commenced at deeper crustal levels and high amphibolite facies conditions (600–650 °C) through grain boundary migration, subgrain rotation and operation of quartz 〈 c〉-prism slip. Continued shearing and exhumation of the terrain forced the re-equilibration of high-T fabrics and the switching of slip systems from 〈 c〉-prism to positive and negative 〈 a〉-rhombs. During D 3, enhancement of ductility by dissipation of heat that came from syn-D 3 granites developed wide belts of amphibolite facies mylonites. Continued shearing, uplift and cooling of the region induced D 3 shear zones to act in ductile-brittle regimes, marked by fracturing and development of thinner belts of greenschist facies mylonites. During this event, switching from 〈 a〉-prism to 〈 a〉-basal slip indicates a thermal path from 600 to 350 °C. Therefore, microstructures and quartz c-axis fabrics in polydeformed rocks from the Seridó Belt preserve the record of two major events, which includes contrasting deformation mechanisms and thermal paths.