Strontium and carbon isotope geochemistry applied to dating of carbonate sedimentation: an example from high-grade rocks of the Norwegian Caledonides

Abstract

‘Isotope stratigraphy’ has been applied to an undated, high-grade carbonate sequence from the Norwegian Caledonides in order to constrain the depositional age. Sokumfjell Group marbles of the Beiarn Nappe Complex, in the Uppermost Allochthon of the Nordland Caledonides, were subjected to several phases of deformation culminating in amphibolite-facies metamorphism. The ca. 1000 m thick marble sequence studied is composed of five different lithofacies. Lithofacies 1–4 are calcite marbles and Lithofacies 5 is dolomite marble. All the calcite marble lithofacies exhibit, throughout, rhythmic bedding and textural grading on different scales, suggesting that carbonate material was deposited in a pelagic, turbidite system. 123 samples were analysed for major and trace elements, and acid-soluble Mg and Ca, of which 42 samples of calcite marbles and four samples of dolomite marbles were selected for δ 13C, δ 18O and 87Sr/ 86Sr analyses. All selected calcite marbles have high Sr contents, averaging 2000 ppm. They are characterised by 87Sr/ 86Sr ratios ranging between 0.70642 and 0.70999, and δ 13C values ranging between +1.3 and +8.4‰ (V-PDB). δ 18O values are highly variable, 14.1–24.8‰ (V-SMOW). The ‘best preserved’ samples selected on the basis of Mn, Sr and Rb concentration, and Mn/Sr and Mg/Ca ratios yield average 87Sr/ 86Sr values of 0.70669, 0.70697, 0.70727, 0.70749 and 0.70737 for Lithofacies 1, 2, 3, 4 and 5, respectively. Post-sedimentary alterations including amphibolite-facies metamorphism resulted in only a slight increase of 87Sr/ 86Sr values in the marbles studied apparently due to aragonite precursor, high Sr concentration, and a very early and fast diagenetic alteration caused by fluids isotopically comparable with that of seawater. Projection of these values along with δ 13C measurements onto 87Sr/ 86Sr and δ 13C calibration curves for the secular variations of seawater suggests that the lithofacies studied were apparently deposited between 635 and 600 Ma.