Pb-Pb dating and Pb isotope geochemistry of Neoproterozoic carbonate rocksfrom the Sao Francisco basin, Brazil: implications for the mobility of Ph isotopes during tectonism and metamorphism

A U-Pb study was carried out on carbonate rocks from the Neoproterozoic Bam bui Group in the southern part of the Sao Francisco basin, Brazil. Pb isoto pic compositions and U and Pb concentrations were determined on more than 9 0 samples from different parts of the basin. These samples were found to co ntain four distinct types of Pb, here called Types I, TI, III and IV. Type I Pb was found in samples with low Pb concentrations and relatively high U concentrations; it represents in situ growth of radiogenic Pb and has a lar ge enough variation to define Pb/Pb isochron ages. Type II Pb is present in samples with relatively high Pb concentrations and low U concentrations; i t is nonradiogenic crustal Pb that could either represent average crustal P b at the time of deposition or, as interpreted here, at the time of deforma tion of these rocks. Type III Pb is also found in samples with high Pb conc entrations and low U concentrations, but it apparently is radiogenic crusta l Pb from the Archean or Paleoproterozoic basement that was incorporated in to the carbonates during the 600 Ma Brasiliano orogeny. Type IV Pb is inter mediate in composition between Type III and Type I Pb; it represents a mixt ure of these two types. Pb/Pb isochron ages obtained from mesoscopically undeformed carbonates cont aining Type I Pb range from 686 +/- 69 to 520 +/- 53 Ma; the older age is t he minimum depositional age for carbonate rocks from the Sete Lagoas Format ion. During the interval from 690 to 500 Ma, the Pb isotope system of carbo nates from the Sao Francisco basin was disturbed, and in some areas it was totally reset. Type III Pb compositions define a straight line; Type II Pb falls on the lower end of this line, which intercepts the Stacey and Kramer s (S & K) Pb growth curve at about 520 and 2100 Ma. This line is interprete d as representing a stage of crustal Pb evolution beginning about 2100 Ma a nd ending about 550 to 500 Ma, when variably radiogenic Pb from the basemen t was incorporated into these carbonates. The ages determined in this study are in agreement with most published ages for Brasiliano fold belts margin al to underlying Sao Francisco craton, showing that isotopic systems of Sao Francisco basin rocks were largely affected by Brasiliano tectonism. (C) 1 999 Elsevier Science B.V. All rights reserved.