Tectono-thermal evolution, magma emplacement, and shear zone development in the Caruaru area (Borborema Province, NE Brazil)

Abstract

The Neoproterozoic Borborema Province (BP) in northeastern Brazil is characterized by a regional flat-lying foliation and by abundant magmatic rocks commonly spatially associated with large transcurrent shear zones. Combined field, micropetrographic, thermobarometric and magmatic fabric studies carried out on plutons, country rocks, and shear zones in the eastern domain of the province (the Caruaru area) reveal that: (1) an early episode of regional deformation, with a top-to-the-NE displacement, was followed by the development of conjugate strike–slip shear zones; (2) similar low-pressure (<600 MPa)/high-temperature (>650°C) metamorphic conditions occurred during the two events; (3) plutons intruded the flat-lying foliation; (4) magma emplacement slightly predated strain localization in transcurrent shear zones; but (5) plutons underwent strike–slip deformation before complete crystallization. Therefore, regional deformation, pluton emplacement and shear zone development were successive events occurring over a relatively short time span. 40Ar/ 39Ar laser dating of amphibole (584 Ma) and biotite (545–553 Ma) single grains supports slow cooling (≈5°C/Ma) of country rocks through the argon closure temperatures of these minerals. Complex, sometimes discordant, amphibole ages bracketed between 552 and 575 Ma for shear zones may be attributed to episodic activity and/or dike swarm intrusions into them, which disturbed the Ar system. Biotite ages for the shear zones (533–545 Ma) also imply a relatively low cooling rate. These data show that the massive injection of magmas in the continental crust of the BP during the Brasiliano/Pan-African orogeny produced a long-lived thermal anomaly of regional extent. The 40Ar/ 39Ar ages in the Caruaru area are older than in other segments of the BP with similar geological characteristics. This indicates a tectono-thermal history for the BP that is more complex than previously recognized, with either diachronous deformation, magmatism and associated low- P, high- T metamorphism, or contrasting cooling of the different domains, possibly due to differential vertical movements.