Sr and C Isotope Chemostratigraphy of the Paleoproterozoic Metacarbonate Rocks of the Sortavala Group: Fennoscandian Shield, Northern Ladoga Area

Abstract

The first Sr isotope data and the new C isotope signatures have been obtained for the metacarbonate rocks of the Sortavala Group in the Fennoscandian Shield. The metacarbonate rocks of the lower carbonate horizon of this group represent the sediments of shallow partly isolated paleobasins, whereas the carbonate deposits of the upper horizon represent the sediments of the distal part of an extensive marine paleobasin. The initial 87Sr/86Sr ratios of the calcite marbles in the lower carbonate horizon vary from 0.70761 to 0.71015 (Ristiniemi area) and from 0.71274 to 0.71292 (Impilahti area), whereas the same ratios of the marbles in the upper carbonate horizon vary from 0.70482 to 0.70489 (Ruskeala calcite marble), from 0.70463 to 0.70522 (Ruskeala dolomite marble), and from 0.70483 to 0.70493 (Yuven’ Island calcite marble). Samples retaining the initial Paleoproterozoic seawater signatures have been revealed as a result of the geochemical assessment of the retentivity of the isotope systems of the metacarbonate rocks. Substantial differences in δ13С values between the marbles of the lower (from +3.8 to +7.7‰, Impilahti and Ristiniemi) and upper (from –2.2‰ to +2.1‰, Ruskeala and Yuven’) carbonate horizons of the group have been established. The isotope composition of carbon from the marbles of the Sortavala Group is consistent with the deposition of the lower carbonate horizon during the terminal stages of the Jatulian Epoch and the upper horizon during the Ludicovian Epoch. The change in the Th/U ratio from 4.3–7.1 in the metacarbonate rocks of the lower horizon to 0.1–0.32 in the upper carbonate horizon reflects transition from the weakly oxidizing sedimentation environment during the Jatulian to the reducing environment during the Ludicovian. An 87Sr/86Sr isotope signature (from 0.70463 to 0.70493) was for the first time obtained for the Svecofennian Ocean during the Ludicovian time (1.9–2.06 Ga). The increase in the 87Sr/86Sr ratio in the ocean water during the post-Jatulian period reflects the global growth of the continental crust ca. 2 Ga.