Thermochronology of the west Sudetes (Bohemian Massif): Rapid and repeated eduction in the eastern Variscides, Poland and Czech Republic

Abstract

The Sudete Mountains of northeastern Bohemian Massif were amalgamated during the closure of the Rheic ocean, culminating with Variscan orogenesis, and contain occurrences of high-pressure granulite and small relict ultrahigh-pressure eclogite formed during subduction. We performed ⁴⁰Ar/³⁹Ar thermochronometry on primarily amphibolite-facies gneisses and schists from two crustal blocks within the Sudetes: Góry Sowie and Orlica-Snieznik massifs. Hornblende and mica plateau ages from the mountainous portion of the Góry Sowie reflect relatively rapid cooling between 382 ± 1 Ma and 373 ± 0.5 Ma, following peak conditions at ca. 400 Ma. Kyanite-sillimanite and cordierite-garnet mineral growth textures denote near isothermal decompression during eduction. Concordant hornblende and biotite cooling ages obtained from the eastern, topographically flat region adjacent to the Niemcza shear zone indicate markedly younger cooling at 337 ± 0.8 Ma. ⁴⁰Ar/³⁹Ar results from the Snieznik Mountains, 50 km to the south, yield plateau cooling ages for white mica and biotite between 341 ± 1 to 337 ± 0.6 Ma and 342 ± 1 to 334 ± 0.6 Ma, respectively, and are remarkably similar in age to an amphibolite-facies overprinting episode. Mica analyses from the Orlica Mountains also yield cooling ages between 338 ± 0.9 to 335 ± 0.5 Ma. Thermochronometry illustrates temporally disparate cooling histories but similar metamorphic evolutions between the Góry Sowie Block and the Orlica-Snieznik Complex. In both terranes, it took ∼35 m.y. from the time of peak high-pressure conditions to residence in the upper crust, and possibly as little as 10 m.y. between high-pressure and mid-crustal supra-Barrovian events. These results in light of regional geochronology illustrate the rapid, localized and repetitious nature of eduction/exhumation of the Sudetic orogenic root including the eclogite and HP-granulite assemblages during protracted subduction.