Sr isotope composition in paleoproterozoic carbonates extremely enriched in 13C: Kaniapiskau supergroup, the Labrador trough of the Canadian shield

Abstract

Paleoproterozoic dolostones of the Dunphy, Portage, Alder, and Uve formations representing lower horizons of the Kaniapiskau Supergroup in the Labrador trough of the Canadian Shield are remarkable, as is known, owing to their extremely high ¹³C values ranging from +5.3 to +15.4‰ PDB. Petrographic characteristics of dolostones suggest limy composition of original sediments, which were dolomitized immediately after formation of the early diagenetic calcite cement in nonconsolidated deposits. Dolostones with Mg/Ca > 0.62 and with low Mn/Sr and Fe/Sr ratios are products of the early diagenesis, which display minimal signs of secondary alteration. Another group of rocks (Mg/Ca < 0.62) is represented by dolostones with relics of mosaic drusy calcite, which could be formed after sediments containing aragonite at the time of the early dolomitization. Prior to investigation of R-Sr systematics, carbonate samples were leached in 1N solution of ammonium acetate in order to remove late diagenetic carbonate phases for a more reliable assessment of ⁸⁷Sr/⁸⁶Sr ratio in sedimentation medium. The Rb–Sr systems of dolostones from lower formations of the Kaniapiskau Supergroup have been reset in response to their partial dedolomitisation that took place likely at the time of Hudsonian regional metamorphism 1.8 Ga ago. The least altered sample from the Alder Formation (Mn/Sr = 2.7, Fe/Sr= 11.8) with ⁸⁷Sr/⁸⁶Sr = 0.70479 characterizes the admissible maximum of this ratio in seawater 2.15 Ga ago. The obtained low value of ⁸⁷Sr/⁸⁶Sr ratio excludes a possibility that a high rate of continental erosion could be responsible for intensified burial of organic matter and for concurrent precipitation of carbonates enriched in ¹³C.