The metamorphic history preserved in the UHP Snieznik eclogites (Sudetes, NE Bohemian Massif)

The Orlica-Snieznik Dome (OSD) is located in the northeastern part of the Bohemian Massif and is interpreted as a fragment of the Moldanubian zone within the Variscan orogen, representing part of the Variscan orogenic root. The dome is composed primarily of orthogneisses interspersed with metamorphosed volcano-sedimentary sequences. In the Snieznik Massif, which forms the eastern segment of the OSD, lenses of high- and ultrahigh-pressure (UHP) rocks, including granulites and eclogites, are embedded within the orthogneisses. This study investigates the metamorphic evolution of eclogites exposed in two specific areas of the Snieznik Massif: Nowa Wies and Bielice.

We distinguish two varieties among the examined eclogites: Ph-bearing and Ph-free eclogite. Both exhibit a typical metamorphic trajectory for UHP rocks, encompassing a UHP metamorphic event followed by isothermal decompression and subsequent retrogression under amphibolite-facies conditions. The samples are characterized by steeply dipping, subvertical foliation, defined by alternating garnet- and omphacite-rich layers and the parallel alignment of elongated grains of kyanite, rutile ± phengite. Evidence of isothermal decompression is observed in the form of small amphibole grains and diopside-amphibole-plagioclase symplectite, which occur locally along grain boundaries. The final metamorphic stage is marked by amphibole+plagioclase+zoisite/clinozoisite±margarite±tytanite, found within fractures that crosscut the primary foliation. This stage is associated with retrogression under amphibolite-facies conditions.

The UHP metamorphic event in the studied samples is reconstructed based on the results of thermodynamic modelling and the presence of coesite, identified as tiny (~10–20 µm) inclusions within omphacite and garnet. The well-preserved mineral assemblage indicative of UHP conditions includes garnet + omphacite + kyanite + rutile + coesite ± phengite. Phase diagram modeling combined with isopleth geothermobarometry indicates peak metamorphic conditions of approximately 3.0 GPa and 750°C. These findings are consistent with results from conventional geothermobarometry (Grt-Cpx-Ph-Ky-Coe geothermobarometer) and Zr-in-rutile thermometry. The onset of isothermal decompression is marked by the formation of small amphibole grains, indicating conditions of around 2.3 GPa at 750°C, within the stability field of amphibole.

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101005611 for Transnational Access conducted at Earth Sciences Department, University of Cambridge. The project was also supported by the Polish National Science Centre (UMO-2022/47/I/ST10/02504) and the Deutsche Forschunggemeinschaft (project Nr. 535198529).