Diachronous collision in the Seve Nappe Complex: Evidence from Lu–Hf geochronology of eclogites (Norrbotten, North Sweden)

Abstract

The collision of Baltica and Laurentia during the Caledonian Orogeny happened at c. 400–420 Ma. However, subduction and collision processes also took place before this main collisional phase and the tectonic history of these is still not fully resolved. The Seve Nappe Complex in Sweden has recorded these earlier phases. The Seve Nappe Complex in Norrbotten (North Swedish Caledonides) comprises four superimposed nappes emplaced by eastward thrusting (from base to top according to the conventional structural interpretation): Lower Seve Nappe, Vaimok, Sarek, and Tsäkkok Lenses. Eclogites occur in the Vaimok and Tsäkkok Lenses. The Vaimok Lens represents rocks of the Baltican continental margin intruded by Neoproterozoic dolerite dikes which were later eclogitized and boudinaged. By contrast, eclogites of the Tsäkkok Lens are former oceanic basalts associated with calcschists, possibly representing the ocean–continent transition between Baltica and Iapetus. Previous age determinations for eclogitization yielded various ages between c. 500 and 480 Ma, in contrast to younger (460–450 Ma) ages of ultra high‐P metamorphism in the Seve Nappe Complex further south in Jämtland. Eclogites from the Vaimok (one sample) and Tsäkkok (three samples) lenses were dated using Lu–Hf garnet geochronology. Garnet from all samples shows prograde zoning of major element and Lu contents and yielded well‐defined isochrons of the following ages: 480.4 ± 1.2 Ma (Vaimok); 487.7 ± 4.6 Ma, 486.2 ± 3.2, 484.6 ± 4.6 Ma (Tsäkkok). The ages from Tsäkkok are interpreted to date the burial of the Iapetus–Baltica ocean–continent transition in a west‐dipping subduction zone around c. 485 Ma, and the age from the structurally deeper Vaimok Nappe the following subduction of the continental margin. Previously reported ages of 500 Ma and older are not supported by this study. The age difference between eclogites in the Seve Nappe Complex in Jämtland (c. 460–450 Ma) and Norrbotten (c. 488–480 Ma) may reflect the collision of an island arc with an irregularly shaped passive continental margin of Baltica or alternatively the collision of a straight margin with a microcontinent (Sarek Lens) accreted to the upper plate.

Detrital zircon U-Pb geochronology of a metasomatic calc-silicate in the Tsäkkok Lens, Scandinavian Caledonides

“…The timing of eclogite-facies metamorphism is similar for both lenses at c. 482 Ma (Root & Corfu 2012, Fassmer et al 2021) but the metamorphic conditions for the Tsäkkok Lens (~2.5 GPa and ~620°C; Bukała et al 2020a, Fassmer et al 2021 are lower than the conditions for the Vaimok Lens (~3.0 GPa and ~720°C; Bukała et al 2018). Kullerud et al 1990, Barnes et al 2020b.…”

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.…”

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

“…It is divided into several (U)HP terranes > 1000 km along strike of the Scandinavian Caledonides, and are often grouped as the northern, central and west-central SNC terranes. The record of (U)HP metamorphism has often been discussed to be older for the northern terranes (late Cambrian/early Ordovician metamorphism; Root and Corfu 2012; Barnes et al 2021a;Fassmer et al 2021) versus the central and west-central SNC (Middle Ordovician metamorphism; Brueckner and Van Roermund 2007;Fassmer et al 2017). The apparently different temporal (U)HP records have led to tectonic models involving diachronous subduction of the Baltican margin, either progressing from north to the south through time, or reflecting a promontory in the north that was subducted prior to the southern portions of the margin Bukała et al 2018;Fassmer et al 2021).…”

“…The record of (U)HP metamorphism has often been discussed to be older for the northern terranes (late Cambrian/early Ordovician metamorphism; Root and Corfu 2012; Barnes et al 2021a;Fassmer et al 2021) versus the central and west-central SNC (Middle Ordovician metamorphism; Brueckner and Van Roermund 2007;Fassmer et al 2017). The apparently different temporal (U)HP records have led to tectonic models involving diachronous subduction of the Baltican margin, either progressing from north to the south through time, or reflecting a promontory in the north that was subducted prior to the southern portions of the margin Bukała et al 2018;Fassmer et al 2021). Further understanding and evaluation of these models is critical for unraveling the evolution of the Scandinavian Caledonides and Iapetus Ocean closure.…”