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.

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The middle part of the volcanosedimentary Zaonega Formation of the Ludikovian Suprahorizon (approximately 2.0 Ga) includes large carbonates concretions and lenses in shungite layers. Carbonate lenses and concretions are primarily elongated and flattened, and their thickness varies from tens of centimeters to a few meters. Some lenses retain relicts of lamination. Concretions are composed of calcite or dolomite. They contain abundant organic matter, as well as mica, talc, chlorite, quartz, and pyrite crystals. The calcite con cretions contain some dolomite admixture (Mg/Ca = 0.011-0.045) and differ from sedimentary limestones by a low Fe/Mn value (0.3-2.1). The Sr content is as much as 385-505 μg/g in most samples and is low (86 μg/g) only in one sample. The Rb-Sr systematics of carbonate concretions was studied with the stepwise dissolution procedure, which included processing with the ammonium acetate solution (AMA fraction) to partially remove the secondary carbonate material, with dissolution of the residue in acetic acid (ACA frac tion). In individual calcite samples, discrepancy between the measured 87 Sr/ 86 Sr values in the AMA and ACA calcite fractions shows a variation range of 0.0008-0.0033. The initial 87 Sr/ 86 Sr ratio in the ACA fractions of the studied samples varies from 0.7053 to 0.7162. The ratio shows a positive correlation with Mg/Ca and the proportion of siliciclastic admixture and negative correlation with the Mn content. The concretions were formed when the sediments subsided, probably, during the transition from a zone with "mild" reductive con ditions to zones with active sulfate reduction and methanogenesis. In the sulfate reduction zone, where most pyrite bearing concretions were formed, the sediment was not geochemically exchaged with the bottom water and was evolved into a closed or semiclosed system. Processes of diagenesis in this zone promoted the release of the radiogenic 87 Sr from the associated siliciclastic minerals, resulting in growth of the initial 87 Sr/ 86 Sr in concretions up to 0.7108-0.7162. Some calcite concretions, which lacked pyrite (or contained its minimal amount) were likely formed in a thin surficial sediment layer located above the sulfate reduction zone. There fore, they precipitated Sr in isotope equilibrium with Sr of the bottom water. However, large concretions and carbonate lenses with an insignificant siliciclastic admixture could retain the signature of early diagenesis or even sedimentation. The initial 87 Sr/ 86 Sr ratio in one of such samples with the siliciclastic admixture of 6.2% makes it possible to estimate the maximal value of this ratio (0.7053) in the Ludikovian paleobasin.