The Sr isotopic characterization and Pb-Pb age of carbonate rocks from the Satka formation, the Lower Riphean Burzyan Group of the southern Urals

Abstract

The Rb-Sr and U-Pb systematics are studied in carbonate deposits of the Satka and Suran forma- tions corresponding to middle horizons of the Lower Riphean Burzyan Group in the Taratash and Yamantau anticlinoria, respectively, the southern Urals. The least altered rock samples retaining the 87 Sr/ 86 Sr ratio of sed- imentation basin have been selected for analysis using the original method of leaching the secondary carbonate phases and based on strict geochemical criteria of the retentivity (Mn/Sr < 0.2, Fe/Sr < 5 and Mg/Ca < 0.024). The stepwise dissolution in 0.5 N HBr has been used to enrich samples in the primary carbonate phase before the Pb-Pb dating. Three (L-4 to L-6) of seven consecutive carbonate fractions obtained by the step-wise leach- ing are most enriched in the primary carbonate (in terms of the U-Pb systematics). In the 206 Pb/ 204 Pb- 207 Pb/ 204 Pb diagram, data points of these fractions plot along an isochron determining age of 1550 ± 30 Ma (MSWD = 0.7) for the upper member of the Satka Formation. The initial 87 Sr/ 86 Sr ratio in the least altered lime- stones of this formation is within the range of 0.70460-0.70480. Generalization of the Sr isotopic data pub- lished for the Riphean carbonates from different continents showed that 1650-1350 Ma ago the 87 Sr/ 86 Sr ratio in the world ocean was low, slightly ranging from 0.70456 to 0.70494 and suggesting the prevalent impact of mantle flux. DOI: 10.1134/S0869593808020020