U-PB SYSTEMATICS OF PROTEROZOIC CARBONATE ROCKS: THE INZER FORMATION OF THE UPPER RIPHEAN STRATOTYPE (SOUTHERN URALS)

Abstract

U-Pb and Pb-Pb isotopic systems were studied in limestones of the Inzer Formation representing the middle part of the Karatau Group of the Urals, whose section is authorized as the Upper Riphean stratotype. Carbonate rock samples selected to study the U-Pb and Pb-Pb systematics are apparently lacking indications of significant secondary alterations. Geochemical criteria applied to select the least altered samples were ratios Mn/Sr ≤ 0.2, Fe/Sr ≤ 5, and Rb/Sr ≤ 0.001, and also the minimum difference between 87Sr/ 86Sr ratios in primary and secondary carbonate phases separated by sample treating in 1 N solution of ammonium acetate (NH 4OAc). Carbonate fractions, some preliminary treated in NH 4OAc to remove secondary carbonate components, were dissolved in 1 N HCl under the room temperature before extracting U and Pb. Samples of the lower Inzer limestones satisfying the above criteria yielded Pb-Pb age value of 836 ± 25 Ma. This value corresponds to the time of early diagenesis in sediments and represents the isotopic date for the Upper Riphean stratotype best substantiated at present in methodical aspects. Diagenetic alterations in the lower Inzer sediments occurred when U and Pb systems in the latter were open under the influence of fluids derived from the underlying deposits of the Katav Formation. The inferred μ value and Pb isotope composition in the fluid are close to these parameters in siliciclastic fractions of the last formation. The parallel study of U-Pb and Pb-Pb systems of carbonate rocks is useful for detecting conditions of diagenesis and for elucidating the composition and possible source of diagenetic fluids.