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 Gerai s State, Brazil, can be divided into three structural domains: (a) the cent ral part of the basin where the rocks are undeformed; (b) the western domai n 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 600-55 0 Ma Aracuai fold belt. U-Pb and Pb isotopic data, rock magnetism data and paleomagnetism data from the carbonates from different domains support a cl ose 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 minimu m depositional age. However, most of the Pb-Pb and the U-Pb ages obtained f rom deformed as well as other undeformed carbonates fall in an interval of 550-500 Ma. Carbonates containing radiogenic crustal Pb with an isotopic si gnature of the Archean/Paleoproterozoic basement were found in the central portion of the basin, which was not affected by deformation, suggesting tha t this Pb was incorporated into the carbonates through fluids which promote d the resetting of the isotope system and severe remagnetization in the car bonates. The post-depositional character of the characteristic magnetizatio ns is strengthened by the following: (a) the disclosed rock magnetic proper ties, such as wasp-waisted hysteresis loops, anomalously high hysteresis ra tios and contradictory Lowrie-Fuller and Cisowski tests, are typical of rem agnetized carbonates; (b) thermomagnetic analysis and scanning electron mic roscopy suggest authigenic magnetite as the main magnetic carrier; (c) mode rate 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 po larity were found along the whole sedimentary sequence; (e) mean magnetizat ion components identified in the carbonates show lower dispersion than woul d be expected if the secular variation of the geomagnetic field was fully a veraged out; (f) paleomagnetic poles from carbonate sequences and adjacent Brasiliano metamorphic rocks are similar and coincide with high quality Gon dwanan paleomagnetic poles for the 530-500 Ma interval. The similarity betw een paleomagnetic and isotopic results from the Bambui and the Salitre carb onates ca. 1000 km to the northeast implies a large scale fluid percolation event that simultaneously affected the whole basin. The paths of these flu ids may have been along old basement faults reactivated during the last sta ge of the Brasiliano/Pan-African orogeny. (C) 2000 Elsevier Science B.V. Al l rights reserved.