Constraining the metamorphic evolution of the Niemcza Shear Zone, NE Bohemian Massif (SW Poland) using thermobarometry, Raman spectroscopy, and tourmaline zoning

The Niemcza Shear Zone (NSZ) represents one of the key structural elements of the Sudetic segment of the Variscan Belt, yet its tectonic significance remains under discussion. Several models have interpreted the NSZ as a major crustal boundary separating peri-Gondwanan domains. Here we present new insights into the metamorphic evolution of the NSZ based on a comparative analysis of two lithologies from its central domain: the strongly deformed Strach mylonite and the weakly deformed Buk graphitic quartzite.

An integrated approach combining white-mica and garnet–biotite geothermobarometry, Zr-in-rutile thermometry, Raman spectroscopy of carbonaceous material, and major- and trace-element zoning in tourmaline was applied. Both lithologies record similar medium-pressure, high-temperature metamorphic conditions within the greenschist- to amphibolite-facies conditions. Peak metamorphism is estimated at ~6–10 kbar and 630–710 °C, corresponding to the sillimanite stability field.

Tourmaline compositional zoning reveals two stages of prograde growth under predominantly internally buffered conditions, followed by retrograde rim formation linked to deformation-enhanced fluid infiltration, particularly within the mylonitic rocks.

Our results support interpretation of the NSZ as a long-lived, lithospheric-scale shear zone that focused deformation, magmatism and metamorphic re-equilibration during terrane amalgamation.