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

Christopher J Barnes,Sabine Rousku,Jarosław Majka,Michał Bukała,Erika Nääs

doi:10.7494/geol.2021.47.1.21

Abstract

The Tsäkkok Lens of the Seve Nappe Complex in the Scandinavian Caledonides comprises eclogite bodies hosted within metasedimentary rocks. These rocks are thought to be derived from the outermost margin of Baltica along the periphery of the Iapetus Ocean, but detrital records from the sedimentary rocks are lacking.Many metasedimentary outcrops within the lens expose both well-foliated metapelitic rocks and massive calc-silicates. The contacts between these two lithologies are irregular and are observed to trend at all angles to the high-pressure foliation in the metapelites. Where folding is present in the metapelites, the calc-silicate rocks are also locally folded. These relationships suggest metasomatism of the metapelites during the Caledonian orogenesis. Zircon U-Pb geochronology was conducted on sixty-one zircon grains from a calc-silicate sample to investigate if they recorded the metasomatic event and to assess the detrital zircon populations. Zircon grains predominantly show oscillatory zoning, sometimes with thin, homogeneous rims that have embayed contacts with the oscillatory-zoned cores. The zircon cores yielded prominent early Stenian, Calymmian, and Statherian populations with a subordinate number of Tonian grains. The zircon rims exhibit dissolution-reprecipitation of the cores or new growth and provide ages that span similar time frames, indicating overprinting of successive tectonic events. Altogether, the zircon record of the calc-silicate suggests that the Tsäkkok Lens may be correlated to Neoproterozoic basins that are preserved in allochthonous positions within the northern extents of the Caledonian Orogen.

Highlights

The architecture of the Scandinavian Caledonides provides an excellent record of Wilson-cycle tectonics from ocean basin formation, closure, and subsequent continental collision (e.g., Gee et al 2008)
Exhumed remnants of the subducted Baltican crust are represented by the Seve Nappe Complex (SNC), preserving multiple localities of ultra-high pressure (UHP) to high pressure (HP) metamorphic rocks (e.g., Gee et al 2013, 2020)
In the Tsäkkok Lens, some of the eclogite protoliths are recognized as pillow basalts (Kullerud et al 1990), whereas the eclogite protoliths for the Vaimok Lens are interpreted to be mafic dykes (Andréasson & Albrecht 1995), and all are interpreted to be related to rifting and Iapetus Ocean formation (e.g., Andréasson & Albrecht 1995, Root & Corfu 2012, Gee et al 2013, Gee et al 2020)

Summary

Introduction

The architecture of the Scandinavian Caledonides provides an excellent record of Wilson-cycle tectonics from ocean basin formation, closure, and subsequent continental collision (e.g., Gee et al 2008). During Caledonian orogenesis, the continental crust of the Baltican margin was subducted to mantle depths beneath an Iapetus oceanic arc complex (e.g., Gee et al 2020, Stephens et al 2020). Exhumed remnants of the subducted Baltican crust are represented by the Seve Nappe Complex (SNC), preserving multiple localities of ultra-high pressure (UHP) to high pressure (HP) metamorphic rocks (e.g., Gee et al 2013, 2020). In southern Norrbotten, the Tsäkkok Lens (the focus of this study; Fig. 1) and the tectonically underlying Vaimok Lens (e.g., Zachrisson & Stephens 1984, Andréasson 2020) both comprise siliciclastic and calcareous metasedimentary rocks that host eclogites (Kullerud et al 1990, Andréasson & Albrecht 1995, Andréasson 2020). The metasedimentary lithologies are generally similar, consisting of quartzites, metapsammites, metapelites, calc-silicates, and marbles (e.g., Andréasson 2020) but the pelitic and calcareous lithologies are much more prevalent in the Tsäkkok Lens compared to the Vaimok Lens (Kullerud et al 1990, Albrecht 2000)

Discussion

Conclusion