Pluton construction and deformation in the Sveconorwegian crust of SW Norway: Magnetic fabric and U-Pb geochronology of the Kleivan and Sjelset granitic complexes

Abstract

The Kleivan and Sjelset granitic complexes are two composite plutons, containing both orthopyroxene and biotite (± hornblende) facies, emplaced in the Sveconorwegian (Grenvillian) high-grade basement of SW Norway. A structural study of these two plutons, based on the anisotropy of magnetic susceptibility (AMS) technique and combined with high-precision U-Th-Pb zircon dating, is presented here. Geochronological data demonstrate a rapid emplacement of successive magmatic pulses in the Kleivan complex (from 936.94 ± 0.42 Ma to 935.62 ± 0.67 Ma) and, on the contrary, a non-negligible rest period (∼3.2 Ma) in the construction of the Sjelset complex that was formed by two pulses emplaced, respectively, at 935.67 ± 0.37 Ma and 932.43 ± 0.75 Ma. Locally discordant magnetic fabrics in the latter pluton confirm this rather protracted construction time. Thermomagnetic and hysteresis measurements supporting the AMS data indicate a magnetic mineralogy dominated by a multidomain, Ti-poor titanomagnetite, except in samples having a very low magnetic susceptibility. The susceptibility magnitudes, paramagnetic to ferromagnetic in agreement with the rock magnetic data, rely on the petrographic rock-types and on the alteration degree. Image analysis confirms that the magnetic fabric is usually coaxial with the shape fabric in both complexes, supporting the use of AMS as a proxy for the petrofabric orientation. Combined with micro- and macrostructural data, the magnetic fabric demonstrates that the Kleivan and Sjelset granitic complexes have their internal fabrics largely dominated by tectonic strain. Models of synfolding emplacement coeval with the last stage of Sveconorwegian contraction recorded in the area are proposed for the two plutons.