Pb-Sr-O-C isotope compositions of metacarbonate rocks of the Derbina Formation (East Sayan): Chemostratigraphic and geochronological significance

The Pb-Sr-O-C isotope compositions of calcite marbles of the Derbina Formation, exposed in the northwestern part of the Derbina block of the East Sayan, were studied. Rocks of the Derbina Formation were metamorphosed under high-temperature amphibolite facies conditions. The carbonate constituent of marbles contains (ppm) 15–130 Mn, 130–160 Fe, 0.008–0.039 Rb, 645–2190 Sr, 0.565–0.894 U, and 0.288–1.42 Pb. These concentrations are similar to those in modern carbonate sediments. The values of δ¹³C in marbles of the Derbina Formation range from–0.6 to +1.4‰ PDB; the values of δ¹⁸O range from 21.5 to 28.6‰ SMOW. The ⁸⁷Sr/⁸⁶Sr ratio values in the two least altered rocks, which meet geochemical criteria of the Rb-Sr system preservation in high-grade carbonate rocks, are 0.70804 and 0.70829. The protolith ages of marbles determined using Sr and C chemostratigraphy lie within the interval of 560–530 Ma, which is regarded as the period of carbonate sedimentation. The slope of the straight line on the ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb diagram (n = 9, MSWD = 19) constructed on the basis of the data points of bulk carbonate constituents of all samples studied and those representing leachate steps of one of them in 0.5N HBr yields Late Vendian age (556 ± 31 (2σ) Ma. Taking into account the data on Sr and C isotope systematics of Derbina marbles, this age is regarded as the age of early diagenesis of carbonate sediments close to the age of sedimentation. Thus, metacarbonate rocks of the Derbina Formation preserved the pre-metamorphic chemostratigraphic and isotope-geochronological information. The age obtained testifies that formation of the carbonate cover of the Derbina block occurred in the Late Vendian. At the end of the Cambrian, carbonate deposits were metamorphosed during the Early Caledonian tectonic event in the southeastern part of the Central Asian Fold Belt.