U-Pb geochronology of the Leknes Group; an exotic early Caledonian metasedimentary assemblage stranded on Lofoten basement, northern Norway

The Leknes Group is an amphibolite-facies metasedimentary assemblage thrust onto the Palaeoproterozoic basement of the Lofoten islands west of the Caledonian chain in northern Norway. Its genesis has been enigmatic, and both Sveconorwegian and Silurian ages of metamorphism have been proposed. U-Pb dating now shows that the main metamorphic and structural overprints occurred in the Mid-Ordovician, between the 469+ or -3 Ma age of a metatonalite and the 461+ or -1 Ma age of a titanite crystal co-planar with the main fabric. According to the prevalent structural interpretation this age also dates thrusting of the Leknes Group onto the Lofoten basement. Zircon and rutile in mica schists record this as well as a younger event of zircon growth at 412+ or -2 Ma, the latter coinciding with early stages of extension and enhanced fluid activity; in southern Lofoten this is also associated with the intrusion of 410+ or -3 Ma pegmatites. The Leknes Group shows an affinity with some terranes of peri-Baltican affinity in the Upper Allochthon of the Caledonides. It probably developed in a marginal basin close to Sveconorwegian basement, as shown by detrital zircon ages and published Nd isotopic data, before being thrust onto the Lofoten basement, which appears to be itself a late Caledonian allochthonous terrane.