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

Rb-Sr dating of clay subfractions from three sedimentary formations of Late Riphean to Vendian age from Varanger Peninsula, northern Norway, has helpe d to reline the geochronology of the Late Neoproterozoic-Cambrian time inte rval in this region. Eighteen subfractions in six narrow size-ranges from 1 -2 mum to <0.1 <mu>m were separated from shales of the Stangenes, Nyborg an d Stappogiedde formations. The coarser subfractions include some 2M(1) illi te 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 subfracti ons. As the clay particle size decreases from 1-2 to 0.1-0.2 mum: (1) the p roportion of chlorite, where present, is reduced; (2) the Crystallinity, In dex Standard (CIS) increases; (3) the Rb-87/Sr-86 ratio in the residues inc reases; and (4) the apparent Rb-Sr age decreases. In addition, the data poi nts of the residues are aligned linearly in both the Rb-87/Sr-86-Sr-87/Sr-8 6 and the 1/Sr-Sr-87/Sr-86 diagrams. A similar pattern in the Rb-87/Sr-86-S r-87/Sr-86 coordinates is observed for the data points of leachates. This s uggests that mixtures of at least two, non-cogenetic, illite generations ar e present in all the shales, and that these illites crystallized in environ ments with dissimilar Sr-87/Sr-86 ratios. The minimum Rb-Sr ages of early b urial diagenesis are c. 650 Ma for the Stangenes, and 560-530 Ma for the Ny borg 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 Formatio n and post-tillite Stappogiedde Formation may result from subsidence and se dimentation associated with the Late Vendian Timanian (Baikalian) deformati on in adjacent areas; and (4) the ages of the finer, authigenic illite subt ractions range from 440 to 390 Ma and thus appear to reflect phases of Scan dian deformation and uplift.