C–O–Sr isotopic stratigraphy of cap carbonates overlying Marinoan-age glacial diamictites in the Paraguay Belt, Brazil

The Neoproterozoic Paraguay Belt, located along the southeastern border of the Amazon Craton and eastern border of the Rio Apa Block, comprises a thick passive margin succession of glaciomarine turbidites, carbonates and siliciclastic sedimentary rocks that were deformed during the Brasiliano/Pan-African Orogeny. The carbonate sequence comprises a 1300 m thick succession of platformal carbonate rocks (Araras Formation), which directly overlie Marinoan glacial diamictites (Puga Formation). The late Neoproterozoic age of this carbonate sequence is indicated by the presence of Cloudina lucianoi and Corumbela werneri found in the upper part of the Corumbá Group (Southern Paraguay Belt). and Sr isotope data were obtained on marine carbonate samples across three different sections of the Araras Formation. δ13CPDB values of the carbonate from the border of the basin range between −10.5 and −2.7‰, including a short stratigraphic interval of cap dolomite (22 m) directly overlying diamictites. The isotopic profile across the basal 200 m of deep shelf laminated microcrystalline limestone and clay-limestone (cap-carbonate), that overlie glacial diamictites, also show negative δ13CPDB values (−5.3 and +0.6‰). In contrast with the lower portion of the carbonate sequence, a 800 m thick unit of shallow-water dolostones forming the upper part of the carbonate sequence present uniform positive δ13CPDB values ranging from +1.9 to +2.4‰. Dolostone and arenaceous dolostone of the upper parts of the Araras Formation have high δ13CPDB values (+4.1 to +9.6‰) which are followed by an abrupt decrease in δ13C values, down to −1.0‰. rbon isotope profile of the Paraguay Belt is quite different from the Bambuı́ Group an other important Neoproterozoic sequences in Central Brazil, which also overlie glacial sediments. Negative δ13CPDB values are found only within the lowermost meters of these sequence and are followed by an important and extensive positive carbon isotope excursion (values ranging from +6.9 to +16.0‰). On the other hand, the δ13CPDB profile observed across the Araras Formation exhibits an approximately 200 m thick section of carbonates with low δ13CPDB values. ates of the Araras Formation have 87Sr/86Sr ratios ranging from 0.70753 to 0.70803, comparable to cap carbonates that overly Marinoan glacial deposits from other neoproterozoic sequences.