THE PB-PB TRAIL DATING OF CARBONATES WITH OPEN U-PB SYSTEMS: THE MIN'YAR FORMATION OF THE UPPER RIPHEAN STRATOTYPE, SOUTHERN URALS

Abstract

Epigenetic alterations and secondary Pb-Zn mineralization in carbonate rocks of the Min'yar Formation represent a serious problem for the direct measurements of Pb-Pb ages in these upper horizons of the Upper Riphean stratotype (the Karatau Group of southern Urals). In addition to petrographic data, cathodoluminescent analysis, and strict geochemical criteria (Mn/Sr, Fe/Sr, δ18O), we paid attention, when selecting samples for Pb-Pb dating, to the minimum difference between 87Sr/86Sr ratios measured in secondary carbonate phases leached in the 1 N ammonium acetate solution and in primary phases residual after leaching. In order to remove secondary carbonates and microinclusions of Pb-Zn sulfides as carefully as possible, we subjected the preliminary selected samples to a short-term leaching in 0.5 N solution of HBr before their final dissolution in this acid. The Pb-Pb dates estimated for whole rock samples and for residual carbonate phases after the last leaching are equal to 780 ± 200 and 780 ± 110 Ma, respectively. The Pb-Pb date of 470 ± 80 Ma obtained for four carbonate phases (whole rock carbonates, two their fractions leached in 1 N NH4OAc and 0.5 N HBr solutions, and the residual phase) of one sample bearing sulfide microinclusions is considered as the age of Pb-Zn mineralization. The model age t1 = 770 ± 220 Ma is calculated for dolomites on the basis of "isochron t1-t2" assuming that t2 = 470 Ma. The weighted average age value for the Min'yar dolomites is 780 ± 85 Ma. This result well fits the formation age limits evaluated on the basis of direct data of isotopic geochronology, paleontology, and chemostratigraphy. The tested procedure of sample selection and isotopic study of carbonate rocks opens a possibility to measure Pb-Pb ages of stratigraphic importance in those cases, when U-Pb systems of rocks were reset in the course of epigenetic sulfide mineralization.