Abstract <p>The Salair Terrane is located in the northwestern part of the Central Asian Folded Belt and comprises mainly Cambrian volcanic, terrigenous, and carbonate rocks. This terrane is a fragment of Early Paleozoic crust that formed as a result of subduction, accompanied by carbonate and terrigenous sedimentation. However, the Salair Terrane also includes a number of Precambrian tectonic blocks of ambiguous origin. To reconstruct the history of the geological development of the Paleo-Asian Ocean during the Late Proterozoic‒Early Paleozoic at the southwestern margin of the Siberian continent, we aimed to determine the age and tectonic conditions of the formation of sedimentary rocks of the Mishikha Formation in Southeastern Salair. These rocks compose the main part of the Precambrian tectonic blocks. The Ce/Ce*, Eu/Eu*, and ⁸⁷Sr/⁸⁶Sr ratios are 0.72 ± 0.04, 1.88 ± 0.70, and 0.70857–0.70860, respectively. The normal δ¹⁸O_(SMOW) values range from 20.9 to 27.1‰, and the positive δ¹³C values, from +0.5 to +4.1‰. Based on geochemical and isotope-geochemical (Sr, O, C) data, it was established that limestones of the Mishikha Formation formed in an open ocean basin far from the passive margin of the Siberian continent in a suboxygen environment. Based on the Sr- and C isotope chemostratigraphy and U‒Pb dating of zircons, we have constrained the age of the carbonate deposits in the Mishikha formation to 555–545 Ma ago.</p>