Early Silurian mafic–ultramafic and granitic plutonism in contemporaneous flysch, Magerøy, northern Norway: U–Pb ages and regional significance

The Caledonian Magerøy sequence comprises Earliest Silurian clastic metasedimentary rocks, which underwent folding and regional metamorphism and were intruded by a mafic–ultramafic complex and various granitic plutons. U–Pb ages of 438.2 ± 0.7 Ma for gabbro in the Honningsvåg Igneous Complex, 438.4 ± 0.9 Ma for an associated granite and 437.7 ± 1.6 Ma for the Finnvik granite coincide within error with the age of deposition inferred from fossils demonstrating a very rapid geological evolution. Plutons of peraluminous affinity are somewhat younger at 436.0 ± 1.0 Ma (Skarsvåg granite) and 434.5 ± 1.5 Ma (a granitic dyke in the Skarsvåg Nappe). The association of flysch deposition, folding and mafic–felsic magmatism suggests formation at a trench–ridge intersection. These Early Silurian events are the expression of a period of major magmatism in a variety of settings all along the Caledonides, probably reflecting rapid convergence and subduction during closure of the Iapetus Ocean. Re-evaluation of the palaeomagnetic signature of the Honningsvåg Igneous Complex indicates that at the time of formation the suite was located in an equatorial position, probably close to the Laurentian margin, and was subsequently translated southward by some 1350 km prior to and during emplacement onto the Baltic margin.