Rb–Sr dating of diagenetic illite in Neoproterozoic shales, Varanger Peninsula, northern Norway

Abstract

Rb–Sr dating of clay subfractions from three sedimentary formations of Late Riphean to Vendian age from Varanger Peninsula, northern Norway, has helped to refine the geochronology of the Late Neoproterozoic–Cambrian time interval in this region. Eighteen subfractions in six narrow size-ranges from 1–2 μm to <0.1 μm were separated from shales of the Stangenes, Nyborg and Stappogiedde formations. The coarser subfractions include some 2M₁ illite as well as the 1M polymorph. The finer subfractions contain only the 1Md illite. After leaching with ammonium acetate, Rb–Sr ‘leachochrons’ for the untreated subfraction, leachate and residue were derived for all subfractions. As the clay particle size decreases from 1–2 to 0.1–0.2 μm: (1) the proportion of chlorite, where present, is reduced; (2) the Crystallinity Index Standard (CIS) increases; (3) the ⁸⁷Rb/86_SR ratio in the residues increases; and (4) the apparent Rb–Sr age decreases. In addition, the data points of the residues are aligned linearly in both the ⁸⁷Rb/⁸⁶Sr–⁸⁷Sr/⁸⁶Sr and the 1/Sr–⁸⁷Sr/⁸⁶Sr diagrams. A similar pattern in the ⁸⁷Rb/⁸⁶Sr–⁸⁷Sr/⁸⁶Sr coordinates is observed for the data points of leachates. This suggests that mixtures of at least two, non-cogenetic, illite generations are present in all the shales, and that these illites crystallized in environments with dissimilar ⁸⁷Sr/⁸⁶Sr ratios. The minimum Rb–Sr ages of early burial diagenesis are c. 650 Ma for the Stangenes, and 560–530 Ma for the Nyborg and Stappogiedde formations. These results indicate that: (1) the age of the Riphean–Vendian boundary is <630 Ma; (2) the age of the Varangerian glaciation on the Varanger Peninsula is bracketed between 630 and 560 Ma; (3) a c. 560 Ma burial diagenesis age for the intra-tillite Nyborg Formation and post-tillite Stappogiedde Formation may result from subsidence and sedimentation associated with the Late Vendian Timanian (Baikalian) deformation in adjacent areas; and (4) the ages of the finer, authigenic illite subfractions range from 440 to 390 Ma and thus appear to reflect phases of Scandian deformation and uplift.