Zircon age depth-profiling sheds light on the early Caledonian evolution of the Seve Nappe Complex in west-central Jämtland

Zircon and monazite reveal late Cambrian/early Ordovician partial melting of the Central Seve Nappe Complex, Scandinavian Caledonides

“…The (U)HP metamorphic terranes of the Seve Nappe Complex are generally grouped as the northern SNC (Vaimok Lens, Tsäkkok Lens, Mårma Terrane;Root and Corfu 2012;Bukała et al 2018Bukała et al , 2020aAndréasson et al 2018;Fassmer et al 2021), the central SNC (Svartsjöbäcken Schists, Marsfjället Gneiss, Avardo Gneiss, Sjouten Unit; Brueckner and Van Roermund, 2007;Majka et al 2012;Janák et al 2013;Gilio et al 2015;Grimmer et al 2015;Klonowska et al 2016;Fassmer et al 2017;Petrík et al 2019), and the west-central SNC (Åreskutan Gneiss, Tväråklumparna Gneiss; Majka et al 2012;Majka et al 2014a, b;Klonowska et al 2017;Walczak et al 2021).…”

“…The metamorphic records of the central SNC have predominantly been extracted from (U)HP mafic and ultramafic lithologies hosted within metasedimentary rocks. However, recent studies demonstrate that zircon and monazite often record older metamorphic histories than those preserved in the mafic and ultramafic lithologies (Barnes et al 2019(Barnes et al , 2021bPetrík et al 2019;Walczak et al 2021). In particular, zircon geochronology of the Tväråklumparna Gneiss in the west-central SNC (Walczak et al 2021) and monazite geochronology of the Marsfjället Gneiss in the central SNC (Petrík et al 2019) indicate late Cambrian/early Ordovician (U)HP metamorphism and partial melting of the SNC terranes that were previously thought to have only undergone Middle Ordovician (U)HP metamorphism.…”

“…However, recent studies demonstrate that zircon and monazite often record older metamorphic histories than those preserved in the mafic and ultramafic lithologies (Barnes et al 2019(Barnes et al , 2021bPetrík et al 2019;Walczak et al 2021). In particular, zircon geochronology of the Tväråklumparna Gneiss in the west-central SNC (Walczak et al 2021) and monazite geochronology of the Marsfjället Gneiss in the central SNC (Petrík et al 2019) indicate late Cambrian/early Ordovician (U)HP metamorphism and partial melting of the SNC terranes that were previously thought to have only undergone Middle Ordovician (U)HP metamorphism. Following these discoveries, in-situ zircon and monazite geochronology and trace element analysis are applied to paragneisses of the central SNC that host Middle Ordovician mafic and ultramafic (U)HP lithologies in order to expand the tectonic history of these terranes.…”

A diachronous record of metamorphism in metapelites of the Western Gneiss Region, Norway

“…460–445 Ma UHP metamorphic events that precede the main Scandian phase of subduction preserved within the Seve‐Blåhø nappe in Fjørtoft (e.g. Barnes et al, 2019, 2021; Brueckner & van Roermund, 2004; Fassmer et al, 2017, 2021; Majka et al, 2014; Root & Corfu, 2012; Walczak et al, 2020). The record of multiple subduction events within the WGR was first used by Brueckner and van Roermund (2004) and Brueckner (2006) to propose a ‘dunk tectonics’ geodynamic model, in which continental crust is repeatedly, briefly immersed in the mantle during successive subduction/eduction events.…”

“…In the northern Caledonides, metapelites have been found to retain a more complete history of the evolution of the collision involving Baltica and its ultimate amalgamation with Laurentia, with the record of older c. 500-480 Ma and c. 460-445 Ma UHP metamorphic events that precede the main Scandian phase of subduction preserved within the Seve-Blåhø nappe in Fjørtoft (e.g. Barnes et al, 2019Barnes et al, , 2021Fassmer et al, 2017Fassmer et al, , 2021Majka et al, 2014;Root & Corfu, 2012;Walczak et al, 2020). The record of multiple subduction events within the WGR was first used by and Brueckner (2006) to propose a 'dunk tectonics' geodynamic model, in which continental crust is repeatedly, briefly immersed in the mantle during successive subduction/eduction events.…”