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. delta(13)C(PDB) values of the carbonate from the border of the basin range between - 10.5 and -2.7parts per thousand, 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 delta(13)C(PDB) values (-5.3 and +0.6parts per thousand). 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 delta(13)C(PDB) values ranging from +1.9 to +2.4parts per thousand. Dolostone and arenaceous dolostone of the upper parts of the Araras Formation have high delta(13)C(PDB) values (+4.1 to +9.6parts per thousand) which are followed by an abrupt decrease in delta(13)C values, down to -1.0parts per thousand. 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 delta(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 +16.0parts per thousand). On the other hand, the delta(13)C(PDB) profile observed across the Araras Formation exhibits an approximately 200 m thick section of carbonates with low delta(13)C(PDB) values. Carbonates of the Araras Formation have Sr-87/Sr-86 ratios ranging from 0.70753 to 0.70803, comparable to cap carbonates that overly Marinoan glacial deposits from other neoproterozoic sequences. (C) 2004 Elsevier B.V. All rights reserved.
