BASAL SEQUENCES OF THE BARATAL GROUP OF GORNY ALTAI: GEOCHEMICAL AND ISOTOPE FEATURES, AGE, AND POSTSEDIMENTARY ALTERATIONS

We studied chemical and isotope (C, O, Sr) compositions of carbonate sediments in three sections of the lower Baratal Group in the southeast of Gorny Altai, which are considered to be one of the oldest known fragments of the carbonate cover of oceanic uplifts preserved in the structure of the Central Asian Orogenic Belt. The contents of Fe, Mn, Sr and their ratios to each other, and to isotope parameters indicate postsedimentary alterations varying within and between studied sections. For the least altered Akkaya section, the δ13C values are –0.4… +0.7‰, and the 87Sr/86Sr ratios vary from 0.70818 to 0.70833; this limits the age of sedimentation to the range of 550–520 Ma. In the Kurai and Chagan-Uzun sections, along with similar values, anomalously low values of 87Sr/86Sr down to 0.70662–0.70701 are documented, which are often accompanied by a decrease in δ18O values and an increase in Mn content. These unusual isotope and geochemical characteristics of carbonates are supposed to be the result of the influence of metasomatic fluids due to underlying basalt and spatially related to the Kurai regional fault zone. In the Kurai section, skeletal small shelly fossils Cambrotubulus were found, which are not known to be older than 550 Ma; this confirms the Precambrian–early Phanerozoic age of the basal sequences of the Baratal Group.