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

'Isotope stratigraphy' has been applied to an undated, high-grade carbonate sequence from the Norwegian Caledonides in order to constrain the depositi onal age. Sokumfjell Group marbles of the Beiarn Nappe Complex, in the Uppe rmost Allochthon of the Nordland Caledonides, were subjected to several pha ses of deformation culminating in amphibolite-facies metamorphism. The ca. 1000 m thick marble sequence studied is composed of five different lithofac ies. Lithofacies 1-4 are calcite marbles and Lithofacies 5 is dolomite marb le. All the calcite marble lithofacies exhibit, throughout, rhythmic beddin g and textural grading on different scales, suggesting that carbonate mater ial 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 samp les of calcite marbles and four samples of dolomite marbles were selected f or delta C-13, delta O-18 and Sr-87/Sr-86 analyses. All selected calcite ma rbles have high Sr contents, averaging 2000 ppm. They are characterised by Sr-87/Sr-86 ratios ranging between 0.70642 and 0.70999, and delta C-13 valu es ranging between + 1.3 and + 8.4% (V-PDB). delta O-18 values are highly v ariable, 14.1-24.8%(0) (V-SMOW). The 'best preserved' samples selected on t he basis of Mn, Sr and Rb concentration, and Mn;Sr and Mg/Ca ratios yield a verage Sr-87/Sr-86 values of 0.70669, 0.70697, 0.70727, 0.70749 and 0.70737 for Lithofacies 1, 2, 3, 4 and 5, respectively. Post-sedimentary alteratio ns including amphibolite-facies metamorphism resulted in only a slight incr ease of Sr-87/Sr-86 values in the marbles studied apparently due to aragoni te precursor, high Sr concentration, and a very early and fast diagenetic a lteration caused by fluids isotopically comparable with that of seawater. P rojection of these values along with delta C-13 measurements onto Sr-87/Sr- 86 and delta C-13 calibration curves for the secular variations of seawater suggests that the lithofacies studied were apparently deposited between 63 5 and 600 Ma. (C) 2001 Elsevier Science B.V. All rights reserved.