Early Caledonian tectonothermal evolution in outboard terranes, central Scandinavian Caledonides: new constraints from U-Pb zircon dates

Early Caledonian deformation in an outboard terrane (Storfjallet) in the central Scandinavian Caledonides is constrained to the time-period Arenig to Caradoc on thebasis of U-Pb zircon age determinations. A trondhjemite clast in a conglomerate belonging to a supracrustal sequence that was affected by the early deformational event yields an age of 489 +10 –5 Ma. A granite intruded after this event defines an age of 445 +24 –6 Ma. The clast is similar in age to a trondhjemite intrusion in a contiguous terrane (Gjersvik) dated at 483 +5 –3 Ma and the granite age is in agreement with a Rb–Sr whole-rock isochron of 438 ± 6 Ma determined in a previous study. The three new ages provide further evidence for the separation of igneous events in outboard terranes in the Scandinavian Caledonides into older (Tremadoc to Arenig) and younger (Late Ordovician to Early Silurian) episodes. More critically, they argue against simple correlation of early Caledonian deformational events in outboard terranes with the Late Cambrian to Early Ordovician tectonometamorphic history along the margin of the continent Baltica (the so-called Finnmarkian orogenic phase). This age disparity provides support for significant spatial separation of the Storfjallet terrane from Baltica during the early–mid Ordovician and an influence of the Taconic orogenic event on this outboard terrane is proposed.