Sr and Pb Isotopic Compositions in Dolostones of the Lower Riphean Billyakh Group, Anabar Uplift: Step-Leaching Technique in Chemostratigraphy and Geochronology

— This paper presents new age estimates and results obtained by the chemostratigraphic study of dolostones of the Billyakh Group, which consists of the Kotuikan and the Yusmastakh formations. The Billyakh Group forms the upper part of the Riphean section of the Anabar uplift in northern Siberia. The stepwise dissolution technique was used for the first time to determine the 87 Sr/ 86 Sr, 206 Pb/ 204 Pb, and 207 Pb/ 204 Pb isotopic ratios in dolostones. The Rb‒Sr systematics was studied by the chemical removal of about a third of the crushed sample (fraction L1) by the preliminary acid leaching in 0.2 N CH 3 COOH and the subsequent partial dissolution (fraction L2) of the remaining part of the sample in CH 3 COOH with the same concentration. The Pb‒Pb isotope systematics of dolostones was studied by the six-step dissolution of crushed samples in 0.5 N HBr. These procedures led to the removal of secondary carbonate material and greatly improved the quality of Sr chemostratigraphic and geochronological information. The initial 87 Sr/ 86 Sr ratios of the least altered carbonate material (fraction L2) of the Billyakh Group dolostones are 0.70502 ± 0.00029 in the Kotuikan Formation, 0.70519 ± 0.00026 in the lower subformation of the Yusmastakh Formation, and 0.70511 ± 0.00018 in the upper subformation of the Yusmastakh Formation. The Pb‒Pb age of early diagenesis of Kotuikan and Yusmastakh dolostones (1519 ± 18 Ma at MSWD = 1.8) was calculated from the results obtained for carbonate fractions L3–L6. Secondary carbonate fractions L1‒L2 are characterized by a Pb‒Pb age of 1466 ± 54 Ma at MSWD = 0.6. The δ 13 C values vary from ‒1 to ‒0.4‰ in dolostones of the Kotuikan Formation and from ‒0.4 to +0.8‰ in those of the Yusmastakh Formation (from ‒0.1 to +0.4‰ in the lower subformation and from ‒0.4 to +0.8‰ in the upper subformation). Comparison of these variations, as well as variations in the initial 87 Sr/ 86 Sr ratios in dolostones of the Kotuikan Formation and the Lower and Upper Yusmastakh subformations (0.70460‒0.70499, 0.70450‒0.70525, and 0.70462‒0.70523, respectively), does not make it possible to distinguish these units on the basis of chemostratigraphic characteristics.