EXHUMATION HISTORY OF THE ORLICA-SNIEZNIK DOME (BOHEMIAN MASSIF) REVEALED THROUGH AR-AR THERMOCHRONOMETRY

The Orlica-Snieznik Dome (OSD), located in the northeastern part of the Bohemian Massif (Czech Republic and Poland), represents a Variscan UHP terrane consisting of predominantly high-grade, amphibolite facies gneisses and migmatites enclosing eclogite and granulite crustal-scale boudins. Previous direct Sm-Nd dating of the OSD eclogites reveal crystallization ages between 350-330 Ma, which is coeval with peak Variscan conditions. We conducted incremental step heating Ar-Ar thermochronometry on sixteen mineral separates from amphibolite facies grade gneisses, that surround these UHP rocks across the OSD in order to provide more detailed time constraints on the thermal evolution of this terrane. Ar-Ar results from eleven separates of micaceous gneisses and schists from the Snieznik Massif (eastern side of the dome) 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. One amphibolite-derived hornblende from the same area yields an integrated Ar-Ar age of ca. 400 Ma. Five mineral separates from augen gneisses of the western Orlica Massif, where exposed UHP rocks are not as prevalent, yield cooling ages between 338 ± 0.9 to 335 ± 0.5 Ma. Notably, these cooling ages are remarkably consistent across the dome, and similar to other nearby cooling ages from the orogenic belt (340-320 Ma). Moreover, recently obtained metamorphic monazite crystallization ages from this area are also between 343 and 333 Ma. These combined data represent wholesale cooling of the OSD from amphibolite facies metamorphism directly following UHP conditions. This further suggests rapid exhumation of the eclogites to shallow crustal levels, probably near isothermal decompression conditions as indicated by kyanite-sillimanite intergrowth mineral assemblages found within this portion of the Bohemian Massif. The timing of exhumation relates to late Variscan orogenic events and the nature is characteristic of orogenic collapse; the Niemcza shear zone, adjacent to the OSD, has been identified as a low-angle normal-motion detachment structure, a likely suspect responsible for tectonic denudation.