Constraining the diagenesis of the Puga cap carbonate from U-Pb in-situ dating of seafloor crystal fans, southern Amazonian craton, Brazil

Cap carbonates overlying Marinoan (635 Ma) glacial deposits worldwide record extreme paleoceanographic changes that may have triggered the rise of macroscopic life. However, the lack of geochronological constraints on these successions hinders reconciling climate and environmental change. Their upper part is marked by anomalous aragonite precipitation associated with large transgression and oxygenation of the post--glacial ocean. Neomorphism of these deposits to low- -Mg calcite often includes centimetre--scale seafloor crystal fans. We used in--situ U--Pb dating and C and Sr isotopic analyses in the Puga cap carbonate to estimate the nature and timing of the neomorphism process. An age of 623 +/- 3 [9] Ma (delta C-13 similar to -6% and Sr-87/Sr-87 similar to 0.7071-0.7075) for a blocky calcite cement is consistent with solution--reprecipitation within the fans during final filling and diagenetic stabilization of the cap carbonate fabric. This result improves previous cap carbonate direct dating, providing a rare Lower Ediacaran chronostratigraphic reference.