Metamorphic record of the Niemcza Shear Zone, NE Bohemian Massif (SW Poland): constraints from multiple geothermobarometers and tourmaline chemistry

The Niemcza Shear Zone (NSZ) is a major structural feature in the Sudetic segment of the Variscan Belt, yet its tectonic significance remains debated, particularly with regard to its role as an intra-orogenic terrane boundary, a reworked segment of a subduction-accretion system, or the exposed core of an oceanic suture. This study provides new constraints on the metamorphic evolution of the central NSZ based on an integrated investigation of two lithologies: the strongly sheared Strach mylonite and unsheared Buk graphitic quartzite. We combine multiple geothermobarometric approaches (muscovite, garnet–biotite and Zr-in-rutile), Raman spectroscopy of carbonaceous material, and detailed major- and trace-element zoning of tourmaline. Both lithologies record comparable medium-pressure–high-temperature metamorphism within the greenschist- to amphibolite-facies range. Peak conditions of approximately 6–10 kbar and 630–710 °C were attained within the sillimanite stability field. Neither lithology preserves mineral assemblages or pressure–temperature paths indicative of subduction-related metamorphism, such as high- or very high-pressure assemblages, or rapid decompression trajectories. Tourmaline zoning documents two stages of prograde growth under largely internally buffered conditions. Muscovite breakdown reactions near peak metamorphism, accompanied by an increase in fluid pH, likely triggered partial tourmaline resorption. Subsequent retrograde overgrowths record variable degrees of system isolation controlled by deformation: shear-enhanced fluid flow promoted open-system behavior in the mylonite, whereas the quartzite retained predominantly closed-system characteristics. The absence of preserved HP metamorphic signatures in the investigated central NSZ rocks, contrasting with high-pressure metamorphism in adjacent tectonic units, indicates pronounced tectonothermal partitioning during Variscan convergence. These observations do not support interpretation of the exposed central NSZ as the primary metamorphic core of an oceanic suture. Instead, they are consistent with the NSZ representing a long-lived, lithospheric-scale and composite terrane boundary that may include strongly reworked components of a broader subduction-accretion system, subsequently overprinted by deformation, magmatism and metamorphic re-equilibration during Variscan terrane assembly.