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

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. The development of collisional orogens commonly takes several tens of million years, and is punctuated by short-lived periods of intensive magmatic, deformational and metamorphic activity. These reflect specific stages of subduction, rotation, collision and/or extension controlled externally by large-scale plate motions and mantle convection, and internally, by progressive changes in the structure of the crust and lithosphere of the orogenic welt. Such major events are intercalated by periods of relative quiescence, or alternatively, by intense but only locally significant magmatic activity. Charting of these events and their variable expressions across an orogen provides important clues for understanding the overall process of crustal growth. In the Scandinavian Caledonides Palaeozoic magmatism started about 500–480 Ma with the development of ophiolites and island arcs (Dunning & Pedersen 1988). Between 480 and 445 Ma magmatic activity occurred in continental arc settings of the Uppermost Allochthon and some mature island arcs of the Upper Allochthon (e.g. Nordgulen et al. 1993; Yoshinobu et al. 2002; Meyer et al. 2003; Tucker et al. 2004). A period of widespread magmatism between 445 and 435 Ma, mainly in the Upper Allochthon, coincided with the beginning of Scandian accretion and collisional processes (Fig. 1 and references therein). Subsequent high-pressure metamorphism, local formation of granite, and intense deformation during the main collisional stages at 425–400 Ma were followed by extension and exhumation concluding the orogeny between 400 and 370 Ma (Andersen 1998; Tucker et al. 2004). The event that occurred near the Ordovician–Silurian boundary (445–435 Ma; Fig. 1) is of special significance because it marks the last major episode of mantle-controlled magmatism during the closure of the oceanic basins. Thereafter, predominantly crustal processes accompanied the main collisional phase. In the Scandinavian Caledonides the events at the transition into the Silurian are recorded all along the orogen and formed such prominent rock suites as the Solund–Stavfjord ophiolite, the Røros volcanogenic massive sulphide deposit, the equally mineralized Sulitjelma and Råna layered intrusions and the Halti Igneous Complex, as well as felsic plutons such as the 447 ! 7 to 430 ! 7 Ma Bindal Batholith, the Hitra–Smøla and related intrusive rocks, the Bremanger Granitoid Complex, and the granites of Sunnhordland (Andersen & Jansen 1987; Dunning & Pedersen 1988; Fossen & Austrheim 1988; Nordgulen et al. 1993; Hansen et al. 2002; Andréasson et al. 2003; Tucker et al. 2004). With few potential exceptions (e.g. the Halti Complex) these plutons are intruded into rocks assigned to the Upper and Uppermost Allochthons. Pedersen et al. (1992) pointed to the Laurentian affinity of faunas and detrital zircon populations, which imply a development of the Late Ordovician–Early Silurian spreading complexes on the Laurentian margin of the narrowing Iapetus ocean. Roberts (2003), however, suggested an origin related to Baltica, and Andréasson et al. (2003) interpreted the Halti Complex to have formed on Baltic crust above a temporarily east-dipping subduction zone. Grenne et al. (1999) suggested that such complexes may have been formed on both margins during a transcurrent event accompanying the process of suturing of the two continental masses. In this paper we contribute to the discussion with new critical evidence from the island of Magerøy in northernmost Norway. Our new precise ages, together with existing geological, palaeontological and palaeomagnetic evidence, help to constrain the tectonic processes and contribute further arguments in favour of a Laurentian connection of these Early Silurian assemblages.