José Maurício Rangel da Silva

70 m.y. of synorogenic plutonism in eastern Borborema Province (NE Brazil): temporal and kinematic constraints on the Brasiliano Orogeny

Recent structural, petrological and geochronological work on plutons in eastern Borborema Province (NE Brazil) has helped to constrain its tectonic evolution during the Brasiliano/Pan-African Orogeny. 645-630 Ma-old plutons have been intensely affected by solid-state deformation. They show an early, shallowly to moderately-dipping foliation conformable with the regional fabric of country rocks, which is overprinted by a steeply-dipping foliation related to strike-slip shearing. Two compositional groups are recognized: (i) medium-K, epidote-bearing granodiorites, and (ii) coarse-grained granites and associated diorites. 590-580 Ma-plutons were emplaced before or at the very early stage of movement along large transcurrent shear zones, and include high-K calc-alkaline plutons and metaluminous syenites. The 590-580 Ma-old plutons underpin the change from a low-angle tectonic event to transpressive deformation. Once emplaced, these plutons in turn favored the localization of strain needed to promote nucleation of the spatially associated shear zones. Finally, two-mica leucogranites and dike swarms of widely varying compositions were emplaced around 570 Ma at the advanced stages of motion along transcurrent shear zones.

Geometry and Kinematics of the Afogados da Ingazeira Shear Zone, Northeast Brazil

Numerous transcurrent NE-SW, mainly sinistral, and E-W dextral-trending shear zones transect Borborema Province, northeastern Brazil. The most important kinematic event involving those shear zones reflects deformation related to the Brasiliano (Pan-African) tectonic cycle. The Afogados da Ingazeira shear zone (AISZ) is probably the most important of the NE-SW-trending lineaments, having continuity for over 250 km. Field work associated with petrographic studies (including quartz c-axis patterns), conducted in an area encompassing the north-central part of this shear zone, indicate that sinistral transcurrent deformation was responsible for the main banding/foliation (C-S) observed in the rocks. Mineral assemblages and microstructural features are suggestive of a deformational history starting under low- to medium-amphibolite facies (thermal peak conditions) and decreasing until greenschist facies.