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

Abstract:

The Caledonian Mageray following comprises Earliest Silurian clastic metasedimentary stones which underwent folding and regional metamorphism and were intruded by the agency of a mafic-ultramafic complex and various granitic plutons. U-Pb ages of 4382 ?± 07 Ma for gabbro in the Honningsv??g Igneous compound 438.4 ?± 0.9 Ma for an associated granite and 4377 ?± 16 Ma for the Finnvik granite coincide within error with the age of deposition inferred from fossils demonstrating a actual rapid geological evolution. Plutons of peraluminous affinity are somewhat younger at 4360 ?± 10 Ma (Skarsv??g granite) and 4345 ?± 15 Ma (a granitic dyke in the Skarsv??g Nappe). The association of flysch deposition, folding and mafic-felsic magmatism put in mind ofs formation at a trench-ridge intersection. These Early Silurian incidents are the expression of a period of major magmatism in a variety of settings all along the Caledonides, probably reflecting rapid gradual approach and subduction during closure of the Iapetus Ocean. Re-evaluation of the palaeomagnetic signature of the Honningsv??g Igneous compound indicates that at the time of formation the suite was located in an equatorial position, probably shut to the Laurentian margin, and was subsequently translated southward by dint of some 1350 km prior to and during emplacement onto the Baltic margin.

The exhibition of collisional orogens commonly takes several ten of million years, and is punctuated by dint of short-lived periods of intensive magmatic, deformational and metamorphic activity. These mirror specific stages of subduction, rotation, collision and/or extension controll externally by dint of large-scale plate motions and mantle convection, and internally, by the agency of progressive changes in the make of the crust and lithosphere of the orogenic welt. similar major events are intercalated by dint of periods of relative quiescence, or alternatively, by means of intense but only locally significant magmatic activity. Charting of these incidents and their variable expressions across an orogen provides important ball of threads for understanding the overall proces of crustal extension

In the Scandinavian Caledonides Palaeozoic magmatism started about 500-480 Ma with the unravelling of ophiolites and island arcs (Dunning & Pedersen 1988) Between 480 and 445 Ma magmatic activity occurr in continental arc settings of the highest Allochthon and some mature island arcs of the Upper Allochthon (eg 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 processe (Fig. 1 and concerns therein). Subsequent high-pressure metamorphism, local formation of granite, and intense deformation during the main collisional stages at 425-400 Ma were followed through extension and exhumation concluding the orogeny between 400 and 370 Ma (Andersen 1998; Tucker et al. 2004) The incident 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 processe accompanied the main collisional phase. In the Scandinavian Caledonides the circumstances at the transition into the Silurian are recorded all along the orogen and formed of the like kind prominent rock suites as the Solund-Stavfjord ophiolite, the R??ro volcanogenic massive sulphide deposit, the equally mineralized Sulitjelma and R??na layered intrusions and the Haiti Igneous composed of several elements as well as felsic plutons of that kind as the 447 ?± 7 to 430 ?± 7 Ma Bindal Batholith, the Hitra-Sm??la and related intrusive stones 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; Andr?©asson et al. 2003; Tucker et al. 2004) With small in number potential exceptions (e.g. the Haiti Complex) these plutons are intruded into stones assigned to the Upper and supreme Allochthons. Pedersen et al. (1992) pointed to the Laurentian affinity of faunas and detrital zircon populations, which imply a unfolding of the Late Ordovician-Early Silurian spreading complexe upon the Laurentian margin of the narrowing Iapetus ocean. Roberts (2003) however, glance ated an origin related to Baltica, and Andr?©asson et al. (2003) interpreted the Haiti compound to have formed on Baltic crust above a temporarily east-dipping subduction girth Grenne et al. (1999) refer toed that such complexes may have been formed upon both margins during a transcurrent incident accompanying the process of suturing of the sum of two units continental masses.

In this paper we contribute to the discussion with fresh critical evidence from the island of Mager??y in northernmost Norway. Our of recent origin precise ages, together with existing geological, palaeontological and palaeomagnetic evidence, help to constrain the tectonic processe and contribute further arguments in favour of a Laurentian connection of these Early Silurian assemblages.