Recent insights into the metamorphic evolution of high-pressure rocks from Svalbard

Metamorphic rocks exposed on Svalbard document a complex tectonothermal history associated with the early stages of the Caledonian orogeny. Particularly high-pressure (HP) rocks are key-targets for reconstructing the geodynamic evolution of the Arctic. Two HP units crop out on Svalbard, namely the Richarddalen and Vestgötabreen complexes. Although the rocks have been recognized since the 1960s, the age of the HP metamorphism was not unequivocally resolved. Here, we present new petrochronological data for both units.

The Richarddalen Complex comprises HP orthogneisses, eclogites, and metagabbros. Peak conditions for the eclogite reached 2.4–2.5 GPa and 720–740°C, followed by decompression to ~1.2 GPa (Elvevold et al. 2013, GSL, Spec Pub). The prograde conditions estimated using quartz in garnet and Zr in rutile thermometry yield 1.7–1.8 GPa at 700°C for eclogite and 1.2–1.4 GPa at 700°C for orthogneiss. In-situ Lu-Hf dating of garnet from augen gneiss and mylonitic orthogneiss yields Tonian ages of 967±44 Ma and 959±28, respectively. Smaller, II-generation garnet yields a poorly constrained age of 477±98 Ma. In-situ Rb-Sr dating provides Early Ordovician ages of 470±12 Ma for white mica from mylonitic orthogneiss, and 473±4 Ma for biotite from augen gneiss. A recent geochronological study constrained Neoproterozoic age of HP metamorphism based on U-Pb zircon dating (Koglin et al., 2022, JGSL), while Mazur et al. (2022, Terra Nova) presented Ar-Ar dating of white mica interpreted as cooling after HP event and further deformation and tectonic assembly with lower-P units at ca. 440–438 Ma. The latter ages together with the new geochronological data presented here, rule out the Neoproterozoic age of HP metamorphism proposed by Koglin et al. (2022). Additionally, Lu-Hf data further confirm the Tonian age of the protoliths (e.g. Pettersson et al. 2009, JGSL; Gromet&Gee 1998, GFF).

The Vestgötabreen Complex represents HP low-temperature units composed of eclogites, blueschists, schists, and serpentinites. Geothermobarometry defines three stages for eclogite: prograde at 1.6±0.3 GPa and 460±60°C, peak-P at 2.3±0.3 GPa and 507±60°C, and peak-temperature at 2.1±0.3 GPa and 553±60°C (Kośmińska et al. 2023, ConMinPet). U-Pb zircon age of 482±10 Ma records prograde growth, whereas U-Pb monazite age of 471±6 Ma is interpreted as post-peak P growth. Peak-P conditions of 2.0±0.03 GPa and 500±30 °C were estimated for blueschist. Lu-Hf garnet dating yields 471±4 Ma for blueschist. Barnes et al. (2021, Minerals) presented an extended dataset of Ar-Ar ages and interpreted age populations as: cooling after HP metamorphism at 476±2 Ma, assembling the Upper and Lower units at 454±6 Ma, and late deformation in the Lower Unit at c. 430–400 Ma. This data provides further support for an early Ordovician subduction system along the Baltican margin in the High Arctic sector of the orogen.

The recent studies are extending our understanding of the geological evolution of this part of the Arctic during the early stages of the Caledonian orogeny. However, further integrated field and analytical studies are needed to help develop the geodynamic reconstructions for the Arctic. This study was supported by the NCN projects 2021/43/D/ST10/02305 (KK) and 2019/33/B/ST10/01728 (JM).