MULTI-STAGE DEFORMATION AND U-Pb GEOCHRONOLOGY OF CARBONATES IN THE ACAUÃ FORMATION, SERGIPANO BELT, NE BRAZIL

The Acauã Formation, located within the Estância Domain in the Sergipano Belt of the Borborema Province, comprises carbonates and slates that preserve evidence of multiple deformation regimes, including ductile, ductile-brittle, and brittle. Located along the western border of the Central Tucano Basin, this lithostratigraphic unit displays diverse structural and mineralogical characteristics that are crucial for understanding its tectonic evolution. The foliation dips gently to the NW and SE, forming regional open folds, and an anticline drag-fold related to the thrust fault propagation was formed at the contact of the massive and laminated carbonate facies. The massive facies are characterized by dark gray, very fine-grained dolostones lacking visible internal structures, while the laminated facies comprise light to medium gray dolostones with fine to silty grain size, well-defined laminations, and occasional "beef" structures (fibrous calcite veins). Petrographic analysis revealed a micritic dolomite matrix in both facies, with disseminated quartz, biotite and pyrite observed in the laminated facies. Cathodoluminescence analysis confirmed dolomite as the primary mineral phase in the matrix and identified two distinct vein generations: dolomitic and calcitic. These veins exhibit elongated crystal growth along their margins and blocky central fills, indicating a process of progressive dilation followed by abrupt opening. The veins acted as nucleation sites for faults, with their reactivation during deformation stages evidenced by the formation of normal and thrust faults, which are predominantly oriented NW-SE and NE-SW, respectively. U-Pb geochronology of carbonates provided constraints on the timing of deformation. The micritic dolostone matrix yielded an age of 601.5 ± 13.3 Ma, likely reflecting post-glacial carbonate deposition. A dolomitic vein near a thrust fault was dated at 508 ± 138 Ma, while slickenfibres on the fault surface yielded an age of 316 ± 83 Ma. Bed-parallel faults yielded a Lower Permian age of 291 ± 48 Ma. These results, although imprecise, suggest that the Acauã Formation carbonates formed during the Ediacaran, with vein formation initiated in the late Neoproterozoic and being reactivated during the Paleozoic era. The structural evolution highlights the significant role of mineralized veins played in fault nucleation and reactivation during regional tectonic events.