Structural analysis combined with new geothermobarometric and geochronological results of the Além Paraíba shear zone, between Três Rios and Bananal, Ribeira Orogen, SE Brazil

Abstract

The Além Paraíba Shear Zone (APSZ) is composed of subvertical mylonites trending ENE/WSW with subhorizontal stretching lineation indicative of dextral movement. In the western part of the area mapped in this paper the shear zone, defined as an elongated body of mylonites, dies out. This constrains the possible displacement along the zone. The mylonites are of relatively high metamorphic grade, product of deformed orthogneisses, granulites, paragneisses and associated syn-tectonic granitoids. These rocks reached granulite facies conditions at temperatures and pressures in excess of 850 °C and 8 kbar. Geothermobarometric results indicate that the progressive ductile deformation related to the shear zone occurred under amphibolite facies conditions at temperatures of 714 ± 23 °C to 562 ± 38 °C and pressures between 7.1 ± 0.8 kbar and 4.3 ± 1.1 kbar. Zircon U-Pb dates by LA-ICPMS were obtained from three samples, one from a mylonitic orthogneiss with age 583.6 ± 7.1 Ma, one from a mylonitic paragneiss with age 591.5 ± 6.1 Ma, and one from a pegmatitic vein that cuts the mylonites obliquely and yields an age of 597.7 ± 9.4 Ma. Monazite crystals from the mylonitic orthogneiss were also analyzed by LA-ICPMS U-Pb, resulting in a spread of concordant dates ranging from 611 to 553 Ma. Taking these data together, we estimate the age of peak metamorphism to be between 595 and 590 Ma. The age of the monazite crystals suggests continuous crystallization and/or re-equilibration after the metamorphic peak, probably catalyzed by the activity of the shear zone and/or fluid migration under amphibolite facies conditions. Since the APSZ affects a 535 Ma granite, we argue that the shear zone was probably active in the time span between 580 and 530 Ma, after metamorphic peak conditions but still at relatively high temperatures. The temporal coincidence with the main collision in the Ribeira Orogen supports the idea that the shear zone is the result of oblique collision.