Pb–Pb dating and Pb isotope geochemistry of Neoproterozoic carbonate rocks from the São Francisco basin, Brazil: implications for the mobility of Pb isotopes during tectonism and metamorphism

Abstract

A U–Pb study was carried out on carbonate rocks from the Neoproterozoic Bambuı́ Group in the southern part of the São Francisco basin, Brazil. Pb isotopic compositions and U and Pb concentrations were determined on more than 90 samples from different parts of the basin. These samples were found to contain four distinct types of Pb, here called Types I, II, 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 large enough variation to define Pb/Pb isochron ages. Type II Pb is present in samples with relatively high Pb concentrations and low U concentrations; it is nonradiogenic crustal Pb that could either represent average crustal Pb at the time of deposition or, as interpreted here, at the time of deformation of these rocks. Type III Pb is also found in samples with high Pb concentrations and low U concentrations, but it apparently is radiogenic crustal Pb from the Archean or Paleoproterozoic basement that was incorporated into the carbonates during the 600 Ma Brasiliano orogeny. Type IV Pb is intermediate in composition between Type III and Type I Pb; it represents a mixture of these two types. Pb/Pb isochron ages obtained from mesoscopically undeformed carbonates containing Type I Pb range from 686±69 to 520±53 Ma; the older age is the minimum depositional age for carbonate rocks from the Sete Lagoas Formation. During the interval from 690 to 500 Ma, the Pb isotope system of carbonates from the São 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 Kramers (S&K) Pb growth curve at about 520 and 2100 Ma. This line is interpreted as representing a stage of crustal Pb evolution beginning about 2100 Ma and ending about 550 to 500 Ma, when variably radiogenic Pb from the basement was incorporated into these carbonates. The ages determined in this study are in agreement with most published ages for Brasiliano fold belts marginal to underlying São Francisco craton, showing that isotopic systems of São Francisco basin rocks were largely affected by Brasiliano tectonism.