NEOPROTEROZOIC CARBONATE SEQUENCES OF THE UNA GROUP, IRECE BASIN, BRAZIL - CHEMOSTRATIGRAPHY, AGE AND CORRELATIONS

The Una Group, Irec (e) over cap Basin (Bahia state) is correlative wi th the better known Bambui Group of the S (a) over tilde o Francisco B asin (Minas Gerais state); these basins are remnants of widespread car bonate platform that covers > 300000 km(2) of the S (a) over tilde o F rancisco Craton in Brazil. The Irec (e) over cap area contains a well- controlled stratigraphic succession hosting economic phosphate deposit s and sulfide mineralization. Una Group strata comprise a basal silici clastic sequence, mostly composed of glacial diamictites (Bebedouro Fo rmation), followed by a carbonate succession that is several hundred m eters thick, the Salitre Formation. An angular unconformity separates the two units. The carbonates of the Salitre Formation are characteriz ed by Sr-87/Sr-86 ratios ranging between 0.70745 and 0.71776, and delt a(13)C values ranging between -4.4 and +9.4 parts per thousand PDB. Th e carbonates also contain beds and nodules of barite and gypsum with h ighly positive delta(34)S values (+25.2 to +32.8 parts per thousand CD T). The 'best preserved' carbonate samples selected on the basis of th eir Mn and Sr concentrations as well as their ratios, yield Sr-87/Sr-8 6 values of 0.70745-0.70765. Comparison of these values with the secul ar strontium isotopic curve for the Neoproterozoic seawater suggests a 600 (to 670) Ma age for the Salitre Formation and for its correlative units in the S (a) over tilde o Francisco Basin. (C) 1998 Elsevier Sc ience B.V.