Global correlation of the Vazante Group, Sao Francisco Basin, Brazil: Re-Os and U-Pb radiometric age constraints

The Vazante Group consists of Precambrian carbonate-dominated platform deposits that extend along more than 300 km in the external zone of the Brasilia Fold Belt of the Sao Francisco Basin in east central Brazil. The sequence is about 4.8 km thick and contains a preserved glaciomarine diamictite unit (containing dropstone) at the top and a lower diamictite unit at the bottom. Previous C- and Sr-isotope profiles suggested the correlation of the upper diamictite unit with the Sturtian glacial event (ca. 750-643 Ma). However, new Re-Os isotope data from the shales associated with the upper diamictites yield radiometric age estimates between 993 +/- 46 and 1100 +/- 77 Ma. U-Pb measurements on a suite of clear euhedral zircon crystals that were separated from the same shales associate I with the upper diamictite and from the arkosic sandstone above the lower diamictite yield ages as young as 988 +/- 15 and 1000 +/- 25 Ma, respectively. Based on the Re-Os and U-Pb ages, the best age estimate of the Vazante Group is constrained to be similar to 1000-1100 Ma and thus the two diamictite units are not correlative with the Sturtian glaciation(s) but most likely are records of glacial events that occurred during the late Mesoproterozoic. (c) 2008 Elsevier B.V. All rights reserved.