Results of a comprehensive study of Paleoproterozoic carbonate rocks in Eastern Fennoscandia

We present the main results of the lithological, geochemical, and paleontological study of Paleoproterozoic carbonate rocks of the eastern part of the Fennoscandian Shield. The maximum carbonate accumulation in the east of the Fennoscandian Shield during the Paleoproterozoic occurred in the range of 2.1–2.0 billion years ago. The vast majority of carbonate rocks are confined to the Onegian (Jatulian superhorizon) and low part of the Zaonezhsky (Ludikovian superhorizon) horizons of the regional stratigraphic scale. A case study of two large paleobasins on the Karelian Craton, the Onegian in the southeast and the Pana-Kuolajarvian in the northwest, reveals that the results of the multidisciplinary geological approach do not contradict, but complement each other. The characteristic features of the Jatulian sedimentary carbonates are their redness, preserved sulfates and chlorides, as well as various pseudomorphoses after them, an abundance of various microbialites and enrichment with the heavy carbon isotope 13C. The Ludikovian carbonate rocks have gray color, contain no evaporite minerals, few microbialites, and slightly deviate in the carbon isotopic composition from the sea-average values. The distribution spectra of rare earth elements in carbonate rocks of Jatulian age from the Onegian paleobasin clearly show a negative Ce anomaly of varying intensity, indicating an oxidizing environment with variable oxygen content. The oxygen content in the Pana-Kuolajarvian paleobasin was stable, but lower. At the same time, in both the Onegian and Pana-Kuolajarvian paleobasins, the oxygen content remained transitional from dysoxic to oxic conditions. The positive Eu anomaly in rocks of both basins indicates hydrothermal material input to the sedimentation area, and entry of basic pyroclastic material is possible. Based on the whole dataset, including isotopic and geochemical data, we draw conclusions regarding the carbonate deposition environments that occurred in this area in the time range of 2.1–2.0 billion years ago. Carbonate accumulation in intracratonic basins evolved from evaporitic shallow-water conditions in the Jatulian time to a relatively deep, open marine environment with increased volcanic activity in the Ludikovian.