LATE CALEDONIAN SHEARING IN SW SPITSBERGEN – NEW AGE CONSTRAINS FROM 40AR/39AR MUSCOVITE GEOCHRONOLOGY

Svalbard of the High Arctic records multiple tectonic events from Proterozoic to Paleogene. It is divided into three fault bounded provinces with contrasting tectonometamorphic histories. The Southwestern Province possesses two ductile shear zones that each sinistrially juxtaposes Neoproterozoic amphibolite facies rocks against greenschist facies rocks, yet their relationship is unclear. In southern Wedel Jarlsberg Land (WJL) it is the Vimsodden–Kosibapasset Zone (VKZ) and in northern WJL a ductile shear zone bordering the Berzeliuseggene Unit (BU). Majka et al. (2015, Canadian Journal of Earth Sciences, v. 52, p. 1–17) proposed that they might be connected, but lack of geochronological data hindered further interpretations. Here we present new age constraints on the timing of deformation obtained by ⁴⁰Ar/³⁹Ar dating of muscovite.

Tectonites from the BU evidence two high grade metamorphic events: (1) ca. 640 Ma Torellian (M₁) and (2) ca. 460 Ma Caledonian (M₂) (Majka et al. 2015). Large up to 700-500 µm mica fish (Msc-I) are overgrown by muscovite (Msc-II) in form of streaks and bundles within foliation planes. Mica fish display slight elemental zonation from rims to core and are chemically distinct from Msc-II. In contrast, upper greenschist facies schists from VKZ contain only one muscovite population that occurs as elongated ribbons internally deformed by C’-type shear bands and do not possess obvious internal zonation of major elements.

Eight muscovite grains per sample were dated via ⁴⁰Ar/³⁹Ar total fusion analysis. Ages from the BU vary from 599.8 ± 9.7 Ma to 410.2 ± 8.8 Ma with a main population at 535 – 550 Ma (n: 5). Samples from VKZ produced less dispersed ages with a variation from 400.7 ± 7.9 Ma to 434.1 ± 1.5 Ma (weighted average of 424.1 ± 5.8 Ma). A bimodal distribution of ⁴⁰Ar/³⁹Ar ages and two Ms populations from the BU reflects the late Caledonian overprint of an earlier Torellian fabric. The Late Silurian age from the VKZ is interpreted as the timing of strike–slip movements. These data provide a new line of evidence that both shear zones were active during the same event, possibly as one terrane boundary or as a set of connected high strain zones.

This work is partially funded by the NCN research project 2015/17/B/ST10/03114 and AGH research grant no 11.11.140.319.