Archean-Proterozoic unconformity on the Fennoscandian Shield: Geochemistry and Sr, C and O isotope composition of Paleoproterozoic carbonate-rich regolith from Segozero Lake (Russian Karelia)

Abstract

Precambrian paleoweathering profiles provide insight into environmental changes and mark stratigraphic boundaries. However, the well-preserved and exposed stratigraphic boundaries, not affected by tectonics, are rare for the Precambrian. We report sedimentological, geochemical and Sr, C and O isotopic data for the Paleoproterozoic weathering profile in Chapanshary Island on Lake Segozero, Central Karelia, the eastern part of the Fennoscandian Shield. The Chapanshary Island weathering crust contains a unique carbonate caliche, rarely developed on the Archean granite-gneiss basement. The crust laterally grades into carbonate-cemented boulder conglomerate, which is overlain by sedimentary carbonate. The chemical index of alteration (CIA; 60–67) and plagioclase index of alteration (PIA; 53–82) values for the weathering crust correspond to weathered felsic/mafic substrate and to illite and K-rich composition of the regolith. The δ18O values of carbonate cement and sedimentary carbonate are highly negative (−22.3 to −17.6‰ V-PDB). The 87Sr/86Sr values of Fe- and Mn-rich carbonate of the regolith, boulder conglomerate cement and sedimentary carbonate are 0.7133 to 0.7242 and are exceptionally high for marine Paleoproterozoic carbonate. The high δ13C values (+3.9 to + 5.2‰ V-PDB) of carbonate cement and sedimentary carbonate indicate deposition during the ca. 2.22–2.06 Ga Lomagundi-Jatuli carbon isotope excursion of the early Paleoproterozoic. The Chapanshary Island weathering profile developed in a terrestrial setting with arid to semi-arid climate ca. 2.2–2.1 Ga ago under atmosphere with high CO2 content. The overlying sedimentary carbonates were deposited in a shallow-water setting (ephemeral ponds and lakes) evolved in the inner part of Fennoscandia.