40Ar/39Ar geochronology of the Seve Nappe Complex in central Scandinavian Caledonides: Insights into exhumation processes

The Seve Nappe Complex (SNC) is an exhumed high-to-ultra high pressure (HP-UHP) metamorphic unit exposed for >1000 km along the strike of the Scandinavian Caledonides. In the Åre region in Sweden, the SNC is subdivided into the Middle and Lower Seve nappes divided by a shear zone. The Middle Seve is dominated by migmatitic paragneisses metamorphosed in the UHP diamond stability field at c. 455 Ma, and overprinted in granulite facies conditions at c. 442-435 Ma (Gee et al. 2020, Geol. Soc. Lond. Mem. 50, 517-548 and references therein). The Lower Seve is dominated by metasedimentary rocks with minor orthogneisses and amphibolites. Garnet mica schists experienced peak-pressure metamorphism and a subsequent mylonitic overprint in amphibolite facies conditions (Jeanneret et al. 2022, JMG), dated to c. 460-430 Ma (Giuntoli et al. 2020; Tectonics 39, e2020TC006267). Lower Seve shearing is dated to c. 423-417 Ma, similar to the dividing shear zone at c. 424 Ma (e.g. Majka et al. 2012, J. Geosci. 57, 3-23; Giuntoli et al. 2020; Jeanneret et al. 2022).  

In-situ laser ablation and step-heating ⁴⁰Ar/³⁹Ar geochronology was conducted on white mica and biotite in paragneisses and mylonites from Åreskutan Mt (Middle Seve), as well as orthogneisses and deformed metasediments from the Collisional Orogeny in the Scandinavian Caledonides (COSC-1) deep borehole in the Lower Seve to resolve the timing of exhumation and possible earlier metamorphic event(s).

In the Middle Seve, in-situ laser ablation of biotite included in garnet, located between HP phases, replacing garnet, and within kyanite-sillimanite-biotite lenses produced c. 451 Ma in the UHP gneisses, and c. 453 Ma in both the migmatite and mylonite. Biotite defining the main foliations of these rocks provided c. 440, 437, and 438 Ma, respectively, with the youngest date of c. 428 Ma resulting from deformed biotite. Phengitic white mica defining the foliation in the migmatite provides a date of c. 443 Ma and a range of 430-422 Ma. Step-heating results are overall younger, with biotite plateau dates of c. 430, 420 and 413 Ma from the UHP gneiss, and a white mica date of c. 404 Ma from a migmatite.

In the Lower Seve rocks, the in-situ dates from deformed and undeformed white mica and biotite are more consistent, ranging from 434 to 424 Ma. Only biotite from one metasediment preserved older dates of 441-436 Ma. Similar to the Middle Seve, the step-heating results are younger with biotite yielding plateau ages of c. 414 Ma and 408 Ma, and white mica providing c. 418 Ma, and 407-404 Ma in all rocks.

Altogether, the oldest biotite dates likely inherited records of the Ordovician-Silurian UHP-HT subduction-exhumation events in the K-rich Middle Seve gneisses. In the other rocks from both Middle and Lower Seve nappes, both deformed and undeformed biotite and white mica resolve the timing of Silurian thrusting and exhumation of the nappes, followed by a second Devonian exhumation event, which is primarily recorded by white mica plateau dates.

This work is financially supported by the National Science Centre (Poland) research project no. 2018/29/B/ST10/02315.