Petrology and geochronology of the Handöl mélange lithologies, Köli Nappe Complex, Scandinavian Caledonides: deciphering orogenic and post-orogenic signatures

The Scandinavian Caledonides formed as a result of plate convergence, closing of the Iapetus Ocean, and continental collision between Baltica and Laurentia. The orogen is composed of allochthonous units, situated on top of the autochthonous basement of Baltica. The Köli Nappe Complex (KNC) of the Upper Allochthon and Seve Nappe Complex (SNC) of the Middle Allochthon of the Scandinavian Caledonides represent a transition between the oceanic terranes of the Iapetus Ocean and Baltica’s outer margin, respectively. Located at the interface between the KNC and the SNC is the Bunnerviken soapstone quarry in Handöl (west-central Jämtland, Sweden), interpreted as mélange-like lithology (Bergman, 1993) containing abundant lithic fragments. The suite of fragments is commonly deformed and reworked together with the host rock, showing a range of textural and compositional characteristics. Up to five groups are currently recognized, possibly reflecting different origins.

This preliminary study focuses on the characterization of the lithic fragments within the soapstone. Special emphasis is put on a single sample representing heavily altered amphibolite with an albite + amphibole + chlorite + calcite + titanite + Fe-Ti phase assemblage. Amphibole and albite are widespread throughout all zones of this lithology, suggesting they are a primary mineral assemblage. However, chemical zoning in amphibole and euhedral to subhedral titanite, dominantly associated with chlorite + calcite, indicates alteration and metamorphic record. U–Pb geochronology of titanite reveals a young, post-Caledonian lower-intercept age of 382 ±10 Ma.

The obtained age is younger than the Scandian collisional phase of the Caledonian orogeny. However, extensional, post-orogenic collapse of the orogen offers an alternative explanation. Normal faulting, thinning of the crust and the development of a post-orogenic metamorphic core complexes (Fossen et al. 2024) in the area could explain intense, prolonged heating, resulting in re-opening of the U-Pb system and the recorded post-Caledonian, Middle to Late Devonian age. The record from Bunnerviken quarry is consistent with earlier local observations by Sjöström et al. (1991) on the Röragen Detachment and may offer additional evidence for post-collisional evolution of the Caledonian allochthons.

References

Bergman, S. (1993). Geology and geochemistry of mafic-ultramafic rocks (Köli) in the Handöl area, central Scandinavian Caledonides. Norsk Geologisk Tidskrift, 73(1), 21-42.

Fossen, H., Polonio, I., Bauck, M.S., Cavalcante, C. (2024). The North Sea rift basement records extensional collapse of the Caledonian orogen. Commun Earth Environ, 5, 206. https://doi.org/10.1038/s43247-024-01374-y

Sjöström, H., Bergman, S., & Sokoutis, D. (1991). Nappe geometry, basement structure and normal faulting in the central Scandinavian Caledonides; kinematic implications. Geologiska Föreningen i Stockholm Förhandlingar, 113(2–3), 265–269. https://doi.org/10.1080/11035899109453877