U-PB SYSTEMATICS OF PROTEROZOIC CARBONATE ROCKS - THE INTER FORMATIONOF THE UPPER RIPHEAN STRATOTYPE (SOUTHERN URALS)

U-Pb and Pb-Pb isotopic systems were studied in limestones of the Inte r Formation representing the middle part of the Karatau Group of the U rals, whose section is authorized as the Upper Riphean stratotype. Car bonate rock samples selected to study the U-Pb and Pb-Pb systematics a re apparently lacking indications of significant secondary alterations . Geochemical criteria applied to select the least altered samples wer e ratios Mn/Sr less than or equal to 0.2, Fe/Sr less than or equal to 5, and Rb/Sr less than or equal to 0.001, and also the minimum differe nce between Sr-87/Sr-86 ratios in primary and secondary carbonate phas es separated by sample treating in 1 N solution of ammonium acetate (N H4OAc). Carbonate fractions, some preliminary treated in NH4OAc to rem ove secondary carbonate components, were dissolved in 1 N HCl under th e room temperature before extracting U and Pb. Samples of the lower In ter limestones satisfying the above criteria yielded Pb-Pb age value o f 836 +/- 25 Ma. This value corresponds to the time of early diagenesi s in sediments and represents the isotopic date for the Upper Riphean stratotype best substantiated at present in methodical aspects. Diagen etic alterations in the lower Inter sediments occurred when U and Pb s ystems in the latter were open under the influence of fluids derived f rom the underlying deposits of the Katav Formation. The inferred mu va lue and Pb isotope composition in the fluid are close to these paramet ers in siliciclastic fractions of the last formation. The parallel stu dy of U-Pb and Pb-Pb systems of carbonate rocks is useful for detectin g conditions of diagenesis and for elucidating the composition and pos sible source of diagenetic fluids.