EXHUMATION HISTORY OF THE GORY SOWIE (BOHEMIAN MASSIF) REVEALED THOUGH AR-AR THERMOCHRONOMETRY

The Gory Sowie Block (GSB), of SW Poland’s Sudete Mountains, contains numerous ultra-high temperature (UHT) granulites and small relict ultra-high pressure (UHP) eclogites. The ultra-high grade lenses of the GSB are enveloped within amphibolite-grade host gneisses and amphibolites, and exposed as a metamorphic window located in the central portion of the Sudetes. The block is bisected by the Sudetic Boundary Fault (SBF) and is bounded by serpentized gabbros, which partially underlie the GSB. Previous work suggests the GSB experienced a polyphase metamorphic history during late Caledonian to early Variscan accretionary events (440 to 330 Ma) resulting in peak metamorphic conditions of 1000°C and 25 kbar at 401.5 ± 0.9 Ma as corroborated by published Pb/Pb zircon ages on garnet peridotites. Preservation of original UHT and UHP mineral assemblages, and kyanite-sillimanite and cordierite-garnet mineral growth textures denotes near isothermal decompression during rapid unroofing. We performed incremental step heating Ar-Ar thermochronometry on twelve mineral separates from the host gneisses allowing the quantitative evaluation of exhumational processes affecting the GSB. Based on hornblende and mica plateau ages, the mountainous portion of the block, west of the SBF, experienced rapid cooling between 382 ± 1 Ma and 373 ± 0.5 Ma. Additionally, recently obtained monazite crystallization ages established the timing of amphibolite-grade metamorphism in this region to ca. 380 Ma. These early Variscan tectonothermal events and rapid exhumation probably occurred as reverse flow along a subduction zone as supported by the presence of serpentized gabbros exposed along the margins of the GSB metamorphic window. Conversely, concordant Ar-Ar cooling ages obtained from hornblende and biotite in the region east of the SBF, primarily flat and covered, and adjacent to the Niemcza Shear zone indicate markedly younger cooling at 337 ± 0.8 Ma. This time coincides well with recent cooling ages revealed in Orlica-Snieznik Dome located 50 km south of the GSB. The younger ages of the eastern portion of the block and proximity of the Niemcza Shear zone, a low-angle detachment structure, are suggestive of tectonic denudation during orogenic collapse.