A Rb-Sr investigation of illite-forming events in diagenesis-grade Neoproterozoic shales, Varanger Peninsula, North Norway
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In this report we present evidence for the occurrence of at least two generation of authigenic illite in diagenesis-grade Upper Riphean and Vendian shales of the Stangenes, Nyborg and Stappogiedde Formations, Varanger Peninsula, North Norway. Dating of illite subfractions by the Rb-Sr methods has yielded ages for redeposited clastic material and for the diagenetic events. As well as dating actual events, an objective of this study has been to contribute to the intense and long-lasting research and discussion on the absolute age of the Riphean - Vendian boundary, and the age ranges of the 'Varangerian glaciation' and the Late Vendian (=Ediacaran) period of time. The results suggest that the age of the Riphean - Vendian boundary is >630 Ma, and that the age of the Early Vendian 'Varangerian glaciation' is bracketed between <630 Ma and c. 560 Ma, and perhaps even between <630 and c.590 Ma. The age of burial diageneses at 560+-10Ma may be related to a regional compaction of the sedimentary succession arising from Baikalian deformation which is recognised in NW Russia. The c.530 Ma minimum age for burial disgenesis of the Stappogiedde Formation does not contradict the location of the Precambrian - Cambrian boundary at the top of this formation. The younger ages, ranging from 440 to 390 Ma, appear to reflect phases of Scandian deformation and uplift.