Dolomitization and isotope stratigraphy of the Vazante Formation, Sao Francisco Basin, Brazil

The Vazante Formation consists of approximately 1700 m of mainly microbial mats and stromatolitic reefal lenses that were deposited on shallow marine platform and have been entirely dolomitized. Samples representing different dolomite generations were taken from three boreholes covering the entire spectrum of the Vazante carbonates. Dolomites can be classified, based on petrography and geochemistry, into four generations ranging in crystal size between similar to 4 mum (almost micritic) and 3 mm and occurring as both replacements and cements. The Sr/Ca molar ratios, calculated for the dolomitizing fluid (0.0006-0.0138), suggest a contribution from a non-marine, possibly meteoric, water component. The delta O-18 and delta C-13 values vary from - 0.1 to - 14.3 parts per thousand (PDB) and 0.2 to - 2.3 parts per thousand (PDB), respectively. Fluid-inclusion study suggests that dolomitization must have commenced at a temperature lower than about 50 degreesC. A mixing-zone model of dolomitization for Dolomites I-III satisfies the constraints from elemental chemistry, delta O-18, Sr-87/Sr-86 and fluid inclusions. The petrographic and chemical criteria of the latest generation, Dolomite IV, reflect conditions of deep burial environment at temperatures above 120-130 degreesC. The VC values show only small variations at the base of the formation, followed by a major negative plunge (similar to 4 parts per thousand) at the top, this plunge correlated with the Sturtian glacial phase. In contrast to C-isotopes, only few samples may have retained their near-primary Sr-87/Sr-86 values of 0.70614-0.70734. The lowest Sr-87/Sr-86 value (0.70614), from fibrous cement in the upper part of the formation, correlates with the negative VC shift and is also consistent with the Sr-isotope signature proposed for the glacial Sturtian seawater. The delta S-34 values, obtained from sulfates trapped in carbonates, range between 10.8 and 16.9 parts per thousand with a jump to 21.3 parts per thousand in the overlying formation. All isotope signals are within the range suggested for the early Neoproterozoic seawater, but the Precambrian baseline is poorly known and additional work is required to confirm this tentative assignment. (C) 2001 Elsevier Science B.V. All rights reserved.