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

Abstract

Abstract 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 Corumba Group (Southern Paraguay Belt). Carbon and Sr isotope data were obtained on marine carbonate samples across three different sections of the Araras Formation. δ 13 C PDB 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 δ 13 C PDB 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 δ 13 C PDB values ranging from +1.9 to +2.4‰. Dolostone and arenaceous dolostone of the upper parts of the Araras Formation have high δ 13 C PDB values (+4.1 to +9.6‰) which are followed by an abrupt decrease in δ 13 C values, down to −1.0‰. The carbon isotope profile of the Paraguay Belt is quite different from the Bambui Group an other important Neoproterozoic sequences in Central Brazil, which also overlie glacial sediments. Negative δ 13 C PDB 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 δ 13 C PDB profile observed across the Araras Formation exhibits an approximately 200 m thick section of carbonates with low δ 13 C PDB values. Carbonates of the Araras Formation have 87 Sr / 86 Sr ratios ranging from 0.70753 to 0.70803, comparable to cap carbonates that overly Marinoan glacial deposits from other neoproterozoic sequences.