BASAL SEQUENCES OF THE BARATAL GROUP OF GORNY ALTAI: GEOCHEMICAL AND ISOTOPE FEATURES, AGE, AND POST-SEDIMENTARY ALTERATIONS
B.B. Kochnev1,2, N.I. Vetrova2, E.V. Vetrov3, G.A. Karlova1
1Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk, Russia
2Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk,Russia
3PJSC Polyus, Moscow, Russia
Keywords: Vendian, Cambrian, Ediacaran, Gorny Altai, Baratal Group, carbonate sedimentary rocks, geochemistry, carbon, oxygen and strontium isotopes, metasomatosis
Abstract
The chemical and isotope (C, O, Sr) composition of carbonate deposits in three sections of the lower Baratal Group in the southeast of the Altai Mountains, which are considered as one of the oldest known fragments of the carbonate cover of oceanic uplifts preserved in the structure of the Central Asian folded belt, was studied. The contents of Fe, Mn and Sr and their ratios to each other, and to isotope parameters indicate a post-sedimentary alteration which 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, which 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 observed, which are often accompanied by a decrease in δ18O values and an increase in Mn content. These unusual characteristics of carbonates are proposed to be the result of influence of the metasomatic fluids tied with underlying basalts and spatially related to Kuvai regional fault zone. In the «Kurai» section, skeletal small-shelly fossils Cambrotubulus were found, which are not known to be older than 550 Ma, and confirms the transitional terminal Precambrian – earliest Phanerozoic age of the basal sequences of Baratal Group.
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