Metamorphic timescales for the Kamieniec Metamorphic Belt (NE Bohemian Massif) based on garnet diffusion modelling

The Kamieniec Metamorphic Belt (KMB), exposed in the Sudetes and forming part of the northeastern margin of the Bohemian Massif, represents one of the outermost exposures of the crystalline basement of the Variscan belt in Europe. It experienced an initial episode of high pressure low temperature (HP-LT) metamorphism reaching conditions of around 15 to 18 kbar and 470 to 570 °C, followed by a low pressure middle temperature (LP-MT) metamorphic event at pressures of 3 to 8 kbar and temperatures of 500 to 600°C (Szczepański et al., 2022a; Szczepański and Goleń, 2022). These metamorphic episodes were investigated using phase equilibria modelling, Raman barometry, and conventional geothermobarometry. Although, metamorphic ages of ca. 347 to 337 Ma have been reported from neighbouring units (Jastrzębski et al., 2020; Szczepański et al., 2022b), precise timing constraints for the metamorphic events within the KMB are still lacking.

In this study, we analysed zoned garnets from mica schists of the KMB. We used the GDIFF software (Moulas, 2023) to simulate compositional diffusion profiles and estimate the corresponding timescales related to the last thermal event experienced by the rocks. Our diffusion approach was supplemented by the use of Hamiltonian Monte Carlo (HMC) to rigorously estimate the statistical uncertainties of the inferred timescales. We successfully calculated the compositional profiles and were able to provide quantitative data on the time evolution of the KMB, which fit the overall known geodynamical history.

 

References

Jastrzębski, M., Żelaźniewicz, A., Budzyń, B., Sláma, J., and Konečny, P.: Age constraints on the Pre-Variscan and Variscan thermal events in the Kamieniec Ząbkowicki Metamorphic belt (the Fore-Sudetic Block, SW Poland), Ann. Soc. Geol. Pol., 90, 27–49, https://doi.org/10.14241/asgp.2020.05, 2020.

Moulas, E.: GDIFF: a Finite Difference code for the calculation of multicomponent diffusion in garnet, Zenodo[code], https://doi.org/10.5281/zenodo.8224137, 2023.

Szczepański, J. and Goleń, M.: Tracing exhumation record in high-pressure micaschists: A new tectonometamorphic model of the evolution of the eastern part of the Fore Sudetic Block, Kamieniec Metamorphic Belt, NE Bohemian Massif, SW Poland, Geochemistry, 82, 125859, https://doi.org/10.1016/j.chemer.2021.125859, 2022.

Szczepański, J., Zhong, X., Dąbrowski, M., Wang, H., and Goleń, M.: Combined phase diagram modelling and quartz‐in‐garnet barometry of HP metapelites from the Kamieniec Metamorphic Belt (NE Bohemian Massif), J. Metamorph. Geol., 40, 3–37, https://doi.org/10.1111/jmg.12608, 2022a.

Szczepański, J., Anczkiewicz, R., and Marciniak, D.: P-T conditions and chronology of the Variscan collision in the easternmost part of the Saxothuringian crust (Bohemian Massif, Fore-Sudetic Block, Poland), Mineralogia, Special Papers 50, 88, 2022b.