Metamorphic evolution of metasedimentary rocks of the Feira Nova region: Tectonic implications for the Brasiliano Orogeny in eastern Borborema Province, Northeast Brazil

Abstract

Thermobarometric estimations and X-ray maps were combined to evaluate the metamorphic evolution of metasedimentary rocks (Surubim Complex) in the Feira Nova region, eastern Borborema Province (NE Brazil). Sillimanite-bearing biotite schists, garnet-biotite schists and gneisses were studied. X-ray maps show different growing phases and retrograde zoning patterns for biotite and garnet. High Ca and Mg contents in garnet cores along with high Ti in biotite are interpreted as equilibrium compositions at peak metamorphism. Lower contents of Ca and Mg at garnet rims and of Ti in biotite rims suggest cooling and decompression. Garnet-biotite geothermometer and garnet-Al2SiO5-quartz-plagioclase (GASP) geobarometer constrain high temperature (∼650–760 °C) and medium pressure (∼0.6–0.9 GPa; burial to ∼23–34 km) conditions for the metamorphic peak, which was synchronous to development of a flat-lying foliation and local anatexis. Garnet rims indicate exhumation to ∼19-11 km (∼0.4–0.3 GPa) at ∼590-520 °C, probably during a transpressional regime. The data points to low exhumation and cooling rates. The lack of extensional fabrics in the region right after the Brasiliano Orogeny supports erosional unroofing rather than extensional collapse. Based on kyanite occurrences nearby, we infer a clockwise P-T path related to thickening in a predominant intracontinental setting, followed by exhumation with low erosional and cooling rates.