Unraveling the Metamorphic Evolution of the Orlica-Śnieżnik Dome (Sudetes, NE Bohemian Massif) through Phase Diagram Modeling and Quartz-in-Garnet Barometry

We investigated paragneisses from the Wyszki and Młynowiec Formations and mica schists of the Stronie Formation from the volcano-sedimentary successions of the Orlica-Śnieżnik Dome (OSD) in the Central Sudetes. Phase equilibria modeling and quartz-in-garnet elastic barometry were employed to directly compare the metamorphic histories of the western and eastern parts of the OSD using consistent methodologies. Our study is based on detailed analyses of nine samples, including three paragneisses and six mica schists.

The studied successions experienced three distinct metamorphic events, identified as M1, M2, and M3. Evidence of the earliest M1 event is fragmentary, including preserved rutile grains, albitic plagioclase, and phengitic white mica. According to pseudosections calculated for unfractionated bulk rock compositions, the mineral assemblage of the M1 event likely occurred under similar pressures in both lithologies, at approximately 13–16 kbar. The associated temperatures ranged from 440 to 470°C in the mica schists and approximately 510–530°C in the paragneisses.

In contrast, the M2 event is better preserved and characterized by garnet, oligoclase, muscovite, biotite, and, in some samples, staurolite, rutile, and ilmenite. The P-T history of this event was reconstructed using thermodynamic modeling of garnet zoning (accounting for variations in rock chemical composition due to garnet fractionation) combined with quartz-in-garnet elastic barometry. For the paragneisses, reconstructed P-T paths indicate an increase in pressure and temperature from approximately 9.5 kbar to 12 kbar and 540°C to 590°C. In contrast, the M2 event in the mica schists is characterized by significantly lower pressures (5–8 kbar) and temperatures of 510–570°C.

According to pseudosections calculated for fractionated bulk rock compositions for the end of garnet growth, the mineral assemblage stable during the final M3 event underwent at approximately 3.5 kbar and 530°C in all analyzed samples. These findings suggest that only the M3 event yielded consistent P-T conditions across the investigated samples.

Our findings suggest that the mica schists and paragneisses of the OSD may represent fragments of distinct tectonic units with contrasting metamorphic histories.

Acknowledgements: The study was financed by the NCN research grant UMO-2022/47/I/ST10/02504.