Simultaneous remagnetization and U-Pb isotope resetting in Neoproterozoic carbonates of the Sao Francisco craton, Brazil

The southern part of the Neoproterozoic Sao Francisco basin, in Minas Gerais State, Brazil, can be divided into three structural domains: (a) the central part of the basin where the rocks are undeformed; (b) the western domain where the rocks have been deformed by the 600-550 Ma Brasilia fold belt; and (c) the eastern domain where the rocks have been affected by the 600-550 Ma Aracuai fold belt. U-Pb and Pb isotopic data, rock magnetism data and paleomagnetism data from the carbonates from different domains support a close connection between a pervasive remagnetization and a large scale fluid percolation event that strongly affected the isotopic system of these rocks at 530-500 Ma, during the last stage of the Brasiliano/Pan-African orogeny. A Pb-Pb isochron age of 686 +/- 69 Ma has been determined from undeformed carbonates in the center of the basin, and it is interpreted as the minimum depositional age. However, most of the Pb-Pb and the U-Pb ages obtained from deformed as well as other undeformed carbonates fall in an interval of 550-500 Ma. Carbonates containing radiogenic crustal Pb with an isotopic signature of the Archean/Paleoproterozoic basement were found in the central portion of the basin, which was not affected by deformation, suggesting that this Pb was incorporated into the carbonates through fluids which promoted the resetting of the isotope system and severe remagnetization in the carbonates. The post-depositional character of the characteristic magnetizations is strengthened by the following: (a) the disclosed rock magnetic properties, such as wasp-waisted hysteresis loops, anomalously high hysteresis ratios and contradictory Lowrie-Fuller and Cisowski tests, are typical of remagnetized carbonates; (b) thermomagnetic analysis and scanning electron microscopy suggest authigenic magnetite as the main magnetic carrier; (c) moderate to high paleolatitudes inferred from paleomagnetic data for the study area would require a different climate pattern during the sedimentation of the wide carbonate platforms; (d) magnetization directions with a single polarity were found along the whole sedimentary sequence; (e) mean magnetization components identified in the carbonates show lower dispersion than would be expected if the secular variation of the geomagnetic field was fully averaged out; (f) paleomagnetic poles from carbonate sequences and adjacent Brasiliano metamorphic rocks are similar and coincide with high quality Gondwanan paleomagnetic poles for the 530-500 Ma interval. The similarity between paleomagnetic and isotopic results from the Bambui and the Salitre carbonates ca. 1000 km to the northeast implies a large scale fluid percolation event that simultaneously affected the whole basin. The paths of these fluids may have been along old basement faults reactivated during the last stage of the Brasiliano/Pan-African orogeny. (C) 2000 Elsevier Science B.V. All rights reserved.