Paleogeographic constraints on source area and depositional systems in the Neoproterozoic Irecê Basin, São Francisco Craton

Abstract

Neoproterozoic carbonate sequences overlying the Middle Cryogenian glacial deposits in the Sao Francisco Craton are included in the Salitre Formation within the Irecê Basin. Stratigraphic, elemental, and stable isotope geochemistry, and SHRIMP U–Pb detrital zircon analyzes of the Salitre Formation's Unit B are presented. The lower and intermediate sections of Unit B register three different depositional zones: basin, outer ramp, and middle ramp settings. The carbon isotope record is close to 0.0‰ to slightly negative for δ13C, suggesting an ocean isotopic signature. Detrital zircon grains occur in two different age distributions: (a) zircon grains from lower Unit B have ages of ca. 3.38, 2.56, and 2.07 Ga; and (b) zircon grains from the middle Unit B have the same ages plus 1.78 and 1.50 Ga. The Archean and Paleoproterozoic detrital zircon age distribution resembles the age distribution of the Gavião Block within the São Francisco Craton, which is the substratum for the Irecê Basin deposition. This implies a local source area, which includes granite–gneiss and greenstone belt associations. The Mesoproterozoic zircon grains in the São Francisco Craton are chrono-correlated to volcanic rocks preserved in the Espinhaço Group. The change in the detrital zircon age distribution is followed by a change in the carbonate facies, suggesting a possible unconformity from lower to middle Unit B. The youngest detrital zircon grain indicates the maximum depositional age of Salitre Formation of ca. 670 Ma. Results demonstrate that the Irecê Basin formed in a gulf-like basin as an extension of the Neoproterozoic passive margin at the end of the Sturtian or Marinoan glaciation.