TIMING CONSTRAINTS ON THE METAMORPHISM AND EXHUMATION OF ULTRA HIGH-GRADE TERRANES, SUDETE MOUNTAINS, NE BOHEMIAN MASSIF

The Sudete Mountains, NE Bohemian Massif (Czech Republic and Poland) preserve abundant ultra high-grade eclogitic and granulitic crustal-scale lenses, which are enveloped in a matrix of amphibolite grade gneisses, migmatites, and serpentinites. Ages and conditions of initial ultra high-grade crystallization are well established for these rocks; however, the timing and mechanism of unroofing has remained elusive due to insufficient knowledge of the exhumation (P-T-t) path required for preservation of ultra high-grade mineral assemblages. It has been postulated in other orogens that a correlation between amphibolite facies metamorphism and the exhumation of ultra high-grade terranes may be thermomechanically and temporally related. We have conducted a preliminary geochronometric and geothermometric investigation of the surrounding matrix rock in order to constrain the timing of the terrane’s metamorphic evolution related to unroofing. We utilized the ims1270 ion microprobe at UCLA to perform in situ monazite Th-Pb geochronometry on four samples from the central Sudetes; the monazite crystals were no larger than 20 mm, and analyses averaged two spots per grain. For thermometry, garnet-biotite and biotite-muscovite pairs were used on ten samples. A granulite grade metasedimentary rock collected from the northern Gory Sowie Block, that yields a temperature of 750°C, revealed a uniform age of 388 ± 3 Ma. Within the southern part of the block, where temperatures are 600°C – 700°C, a garnet-bearing protomylonite yielded two distinct metamorphic ages of 377 ± 5 Ma and 347 ± 3 Ma, most likely reflecting discrete metamorphic pulses. To the south, where the Orlica Snieznik Dome locally achieved temperatures up to 800°C, a garnet-grade augen gneiss yielded similar ages of 372 ± 8 Ma and 343 ± 7 Ma. A biotite-garnet gneiss collected in the eastern portion of the dome (conditions of 600°C) gave younger metamorphic ages of 333 ± 4 Ma and 315 ± 4 Ma. Although spatially variable, our results are consistent with previously reported dates of eclogite and granulite formation ca. 400-370 Ma, followed by Variscan metamorphic ca. 345-335 Ma. Variscan tectonothermal events resulted in isothermal decompression, prompting coeval metamorphism and rapid exhumation of the ultra high-grade rocks.