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

The Neoproterozoic Borborema Province (BP) in northeastern Brazil is charac terized by a regional flat-lying foliation and by abundant magmatic rocks c ommonly spatially associated with large transcurrent shear zones. Combined field, micropetrographic, thermobarometric and magmatic fabric studies carr ied 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 regio nal deformation, with a top-to-the-NE displacement, was followed by the dev elopment of conjugate strike-slip shear zones; (2) similar low-pressure (<6 00 MPa)/high-temperature (>650 degrees C) metamorphic conditions occurred d uring the two events; (3) plutons intruded the flat-lying foliation; (4) ma gma emplacement slightly predated strain localization in transcurrent shear zones; but (5) plutons underwent strike-slip deformation before complete c rystallization. Therefore, regional deformation, pluton emplacement and she ar zone development were successive events occurring over a relatively shor t time span. Ar-40/Ar-39 laser dating of amphibole (584 Ma) and biotite (54 5-553 Ma) single grains supports slow cooling (approximate to 5 degrees C/M a) of country rocks through the argon closure temperatures of these mineral s. Complex, sometimes discordant, amphibole ages bracketed between 552 and 575 Ma for shear zones may be attributed to episodic activity and/or dike s warm intrusions into them, which disturbed the Ar system. Biotite ages for the shear zones (533-545 Ma) also imply a relatively low cooling rate. Thes e data show that the massive injection of magmas in the continental crust o f the BP during the Brasiliano/Pan-African orogeny produced a long-lived th ermal anomaly of regional extent. The 40Ar/39Ar ages in the Caruaru area ar e older than in other segments of the BP with similar geological characteri stics. This indicates a tectono-thermal history for the BP that is more com plex than previously recognized, with either diachronous deformation, magma tism and associated low-P, high-ir metamorphism, or contrasting cooling of the different domains, possibly due to differential vertical movements. (C) 2000 Published by Elsevier Science B.V. All rights reserved.