The Archaean to Palaeoproterozoic West Troms Basement Complex comprises various tonalitic gneisses, felsic and mafic intrusive rocks and intervening narrow NW-SE-trending belts with partly mylonitised, low- to medium-grade, metavolcanic and metasedimentary rocks. The low-grade, parautochthonous Vanna group unconformably overlies 2885 ± 20 Ma tonalitic basement on the island of Vanna. It is composed of siliciclastic sedimentary rocks and intercalated mudstones with well-preserved shallow-water sedimentary structures and internal erosional surfaces. A major diorite body intruded soon after consolidation of the sediments. The stack of sedimentary rocks is tilted and complexly folded and deformed by crustal transpression. Previous workers considered the rocks to be either part of the Neoproterozoic to Cambrian autochthonous cover or a part of the Middle Allochthon, and deformed during the Caledonian orogeny. New U-Pb ages for magmatic zircon and titanite of the diorite sill, however, yield a crystallization age of 2221 ± 3 Ma. Mafic dykes of the underlying tonalitic basement that do not truncate the basement-cover contact have been dated to 2403 ± 3 Ma, which is a maximum age for the sediments. Laser ICP-MS U-Pb analyses of detrital zircons from the Vanna group indicate that the nearby 2885 ± 20 Ma tonalitic basement was the principal source for the sediments. The new U-Pb age determinations from Vanna indicate that the diorite and its associated sedimentary rocks may be coeval with other Palaeoproterozoic siliciclastic successions in northern Fennoscandia. Directly correlatable mafic dyke swarms include 2220 Ma sills throughout northern Finland, the Nipissing diabase (2219 ± 4 to 2210 ± 4 Ma) of the Canadian Shield, and the Scourie dykes (c. 2.4-2.0 Ga) of the Lewisian Complex in Scotland. These intrusive swarms and associated sedimentary units were all formed in the waning stages of continental rifting and sedimentation following the break-up of an Archaean supercontinent. The Svecofennian (c. 1.8-1.7 Ga) orogenic events and regional metamorphism were likely the cause of the transpressive deformation of the Vanna group.
Steffen G. Bergh, Kåre Kullerud, Helge W. Johansen, Trine Pettersen & Stian Knudsen: Department of Geology, University of Tromsø, N-9037 Tromsø,
Fernando Corfu, Department of Geosciences, University of Oslo, PB 1047, Blindern, N-0316 Oslo, Norway.
Paul E.B. Armitage, Dept of Mineralogy, Natural History Museum, Cromwell Road, London, SW7 5BD, UK.
Børre Davidsen, Geological Survey of Norway, N-7491 Trondheim, Norway