U-Pb zircon geochronology of coesite-bearing eclogites from the Orlica-Snieznik Dome (SW Poland) 

The Orlica-Śnieżnik Dome is located in the NE part of the Bohemian Massif within the Sudetes, forming part of the European Variscan Belt. It comprises mostly orthogneisses, that host lenses of ultrahigh-pressure (UHP) eclogites and granulites. Peak-pressure metamorphic conditions recorded by these rocks have previously been estimated at 2.9–3.2 GPa and 750°C–830°C, based on thermodynamic modelling, conventional geothermobarometry, and Zr-in-rutile thermometry.In this contribution, we present preliminary U-Pb zircon ages and trace element characteristics from two localities, Bielice and Nowa Wieś. The analysed zircons are mostly isometric or slightly elongated. Their internal structure was documented using back-scattered electron (BSE) and cathodoluminescence (CL) imaging prior to analysis. Zircons from Nowa Wieś predominantly display fir-tree and sector zoning patterns, whereas those from Bielice are mainly characterized by irregular and diffuse (aurora-light) zoning. Some grains from both localities contain possible inherited cores. Zircons from both localities show flat HREE patterns, a slight positive Ce anomaly, and no Eu anomaly. These features support the interpretation that zircon growth occurred under HP condition, contemporaneous with the eclogite-facies assemblage. Metamorphic zircon ages from Bielice cluster at 338.3 ± 3.4 Ma, whereas those from Nowa Wieś yield an age of 341.5 ± 3.4 Ma.This project was supported by the Polish National Science Centre (UMO-2024/53/N/ST10/03586).