Global nature of the Paleoproterozoic Lomagundi carbon isotope excursion: A review of occurrences in Brazil, India, and Uruguay

摘要

Positive carbon isotope excursion is reported from Paleoproterozoic carbonates of the Aravalli Supergroup (northwestern India), the Minas Supergroup (Brazil), and new sections of the Paso Severino Formation (Uruguay). The 2.42Ga Gandarela Formation, Minas Gerais, Brazil, contains red carbonate-facies BIF grading into dolostones and limestones and yielding δ~(13)C values ranging from -1.6 to +0.4‰ V-PDB. The positive C-isotope excursion (up to + 11‰ V-PDB) in marine shallow-water carbonates in India and Brazil (Jhamarkotra Formation in northwestern India, and Cercadinho and Fecho do Funil formations in Minas Gerais State, Brazil) is comparable to that observed in 2.22-2.1Ga carbonate successions worldwide that were deposited during the Lomagundi excursion. In Uruguay, δ~(13)C values up to +11.6‰ V-PDB in the deep-water Paso Severino Formation of the Piedra Alta Terrane are compatible with deposition at ca. 2.15Ga, as indicated by the 2146±7Ma U-Pb age of dacites occurring at the top of the unit. Negative δ~(13)C values are also present in carbonates of the Paso Severino Formation, but an origin related to organic-matter remineralization cannot be ruled out. Thin carbonate beds in the Rio Itapicuru greenstone belt, Bahia State, Brazil, are associated, as in the Paso Severino Formation, with deep-water black shales and have carbon isotope values up to +9‰ V-PDB. High metamorphic grade carbonates of the Jacurici terrane in the Medrado-Ipueira area, Bahia, Brazil, have carbon isotope values up to +6.9‰ V-PDB, consistent with their minimum age of 2085±5Ma inferred from the intrusive contact with and the age of the Medrado norite. No evidence was found in India, Brazil, or Uruguay for Paleoproterozoic glacial events recognized in the 2.45-2.22Ga sedimentary successions worldwide. Unconformities between the Gandarela and Cercadinho formations in Brazil and the banded gneissic Complex and the Lower Aravalli Supergroup in India might explain the absence of glacial record. Compositional and isotopic data presented here for studied Paleoproterozoic carbonate successions allow their integration into the global record of the Paleoproterozoic evolution as well as correlation with other successions of similar age. The study highlights the global nature of the Lomagundi excursion. Furthermore, it indicates that the Lomagundi excursion is recorded in both shallow-water (Aravalli and Minas supergroups) and deep-water carbonates (Paso Severino Formation and Rio Itapicuru greenstone belt) negating a significant impact of stromatolite productivity and hypersaline conditions on carbon isotope values of carbonates deposited in shallow-water, open-marine and isolated basins.