METAMORPHISM AND EXHUMATION OF ULTRAHIGH-GRADE TERRANES: GEOCHRONOMETRIC INVESTIGATIONS OF THE NE BOHEMIA MASSIF, POLAND AND CZECH REPUBLIC

Separated by less than 60 km, the Orlica-Snieznik Dome (OSD), a crustal pop-up structure, and the purportedly allochthonous Gory Sowie Block (GSB) are two fault-bounded high-grade terranes within the northeastern Bohemian massif that contain crustal-scale UHP/UHT lenses. Geochronometric investigations of these two terranes detail similar, but discrete, metamorphism-to-exhumation histories beginning with a well-documented initial descent phase to depths of 75-100 km. Within the western GSB, granulite-facies conditions are dated at 400 Ma, preceding peak eclogite-facies conditions in the OSD by 30 m.y. In both massifs, a stronger and more pervasive overprinting occurred during the ascent phase that was responsible for the observed structural fabric and amphibolite-facies assemblages. Isothermal decompression caused widespread midcrustal anatexis, resulting in migmatites / gneisses enveloping ultrahigh-grade blocks; partial melting lowered rock viscosity and facilitated final exhumation. Gneissic matrix from the GSB record monazite and zircon ages of ca. 385-375 Ma signaling ascent; complementary Ar-Ar hornblende and mica regional cooling ages of 382 ± 1 Ma and 373 ± 0.5 Ma, respectively, reveal the rapid nature of this event. Consistent with the 30 m.y. disparity, metamorphic ages from the migmatites of the OSD constrain the midcrustal ascent phase to 345-330 Ma coeval with rapid cooling as indicated by regional Ar-Ar white mica and biotite ages of 342 ± 1 Ma to 334 ± 0.6 Ma, respectively. Notably, the GSB east of the massif-bisecting Sudetic Margin fault preserves a tectonothermal exhumation history more similar to that of the neighboring OSD to the south (337 ± 0.8 Ma) than of the adjoining western GSB (ca. 380 Ma). Preliminary monazite analyses conducted on the crustal-scale Niemzca shear zone, adjacent to the GSB, yields a range of Th-Pb dates from 380 ± 8 Ma to 283 ± 2 Ma, illustrating the protracted nature of this high strain zone. Moreover, the common ca. 380 Ma date between all three regions suggests the GSB allochthon was most likely emplaced at that time during final closure of the Tornquist Sea. Final exhumation of the terranes was likely the result of vertical extrusion during lateral Variscan shortening coupled with rapid erosional denudation.