U-Pb LA-ICP-MS detrital zircon ages of the S?o Jo?o del Rei and Carandaí basins: New evidence of intermittent Proterozoic rifting in the S?o Francisco paleocontinent

摘要

The Tiradentes Formation (S?o Jo?o del Rei basin) is a shallow marine quartzite succession resting on the Paleoproterozoic crust of the southern part of the S?o Francisco Craton. A regional angular unconformity separates the Tiradentes strata from the younger Carandaí, Barroso and Prados Formations (Carandaí basin). The Carandaí Formation is constituted of fault-bounded diamictites containing lithoclasts derived from basement rocks and the Tiradentes Formation. The Barroso Formation is a sag basin limestone succession, whereas the Prados Formation is a pelitic succession that fills a NE-SW-trending aulacogen. Weak deformation and metamorphism (600-590. Ma) overprinted the S?o Jo?o del Rei and Carandaí sequences due to the Brasiliano Orogen and, likewise, the overlying Andrelandia Megasequence (<. 950. Ma). Detailed field geology and U-Pb LA-ICP-MS detrital zircon ages from the Tiradentes quartzites and the Carandaí diamictites indicate that these units are associated with two separate Mesoproterozoic rifting episodes. The youngest zircon grains extracted from the basal and top Tiradentes quartzites (1536. ±. 33. Ma and 1540. ±. 45. Ma) estimate the maximum age of the initial extension and sedimentation. The timing of the second extensional phase is marked by the youngest zircon age for the Carandaí diamictites (1379. ±. 3. Ma). These rifting episodes allow a tectonic relationship with two basin cycles of the Espinha?o intracratonic rift system, as suggested by geologic correlations and geochronology. The Prados pelites represent renewed rifting genetically related to the Andrelandia Megasequence; one sheared quartzite that is probably related to the lower unit of the Andrelandia sequence contains zircon grains as young as 1145. ±. 79. Ma. This maximal depositional age also allows a tectonic correlation with the ultimate extensional episode of the Espinha?o system, according to geologic and geochronologic evidence. From a tectonic view point, the studied rift sequences are related with the evolution of the Espinha?o intracratonic system, allowing a better understanding of intermittent extensional and basin tectonics in the S?o Francisco/Congo paleocontinent at Mesoproterozoic times.