Geochemical, O-H-Sr Isotopic, and Thermodynamic Insights into the Metasomatic and Metamorphic Evolution of the Sax- othuringian Crust: The Case of the Kowadło Ultramafic Suite (SW Poland)

The Orlica-Śnieżnik Dome in the Bohemian Massif represents a key window into the tectonic and geochemical evolution of the Saxothuringian continental crust, exemplified by the stratified Kowadło ultramafic suite. The Kowadło ultramafic exposures are found in gneisses of the Central Sudetes, comprising a lithological sequence from harzburgite to hornblendite, with transitional garnet-bearing varieties. Petrographic, geochemical, mineral chemical, and Sr-O-H isotopic analyses reveal a complex, multistage (stages I–V) evolution involving both metasomatic and metamorphic processes. Initial harzburgite crystallized from a high-Mg MORB-like basaltic melt, as indicated by Cr-spinel compositions (Cr# = 0.27–0.44) and low initial ⁸⁷Sr/⁸⁶Sr ratios (0.70274–0.70285). Subsequent infiltration of hydrous, Al-rich tholeiitic melts triggered amphibole formation and the growth of metasomatic spinel (pleonaste-hercynite) and garnet. Progressive metamorphism led to amphibole overprinting, garnet recrystallization, and the development of corundum through the oxidation of spinel. Stable isotopic compositions (δ¹⁸O = 5.64–5.72‰; δD = –89.1 to –68.2‰) suggest interaction with melts from metasomatized mantle domains and later hydrothermal fluids. Thermodynamic modeling constrains these processes to pressures of 7–13 kbar and temperatures up to 1250°C, followed by retrograde and prograde metamorphism. These results provide new insights into crust–mantle interactions, fluid–rock metasomatism, and the geodynamic evolution of the Saxothuringian domain during the Variscan orogeny.