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/86SR 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.