Abstract:
The carbonate Malgina Formation, a constituent of the Riphean hypostratotype in Siberia, corresponds, according to paleontological evidence, to the terminal Middle Riphean horizons, and present-day geochronological data indicate that it is younger than 1300 ± 5 Ma and older than 1025 ± 40 Ma. Eight limestone samples (Mg/Ca < 0.066) selected for the Pb-Pb age determination contain less than 5.8% of silicate components and are insignificantly altered, as it is evident from geochemical criteria (Mn/Sr ≤ 0.2, Fe/Sr ≤ 5, Rb/Sr ≤ 0.001) and minimum difference between 87Sr/86Sr ratios (≤0.0007) measured in the primary and secondary carbonate phases. Secondary carbonates were removed using the step leaching of samples in HBr of variable concentration. The Pb-Pb age of 1043 ± 14 Ma (MSWD = 7.8) that is calculated for 18 data points characterizing various carbonate phases appears to be best approximating the time of early diagenesis in the Malgina limestones. With due account for stratigraphic position of studied samples, this and other Pb-Pb dates obtained for the Middle-Upper Riphean carbonate formations of the Urals and Siberia are concordant with ages measured by other methods and form altogether a regular series of five successive values. The Pb-Pb isochron dates of stratigraphic significance can be obtained only after a strict procedure of sample selection followed by the removal of secondary carbonate phases with the help of step-leaching method.
