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1 Institut für Geowissenschaften der Universität, D-35390 Giessen, Germany wolfgang.franke{at}geo.uni-giessen.de
2 Institut für Mineralogie, Technische Universität Darmstadt, D-64287 Darmstadt, Germany
The Saxo-Thuringian Belt on the northern flank of the European Variscides resulted from SE-ward subduction of a Cambro-Ordovician rift basin under the Teplá-Barrandian (Bohemian) margin. It contains ultra-high-pressure (UHP) metamorphic rocks, which are now exposed in different tectonic settings, and were exhumed in different modes, under different thermal regimes. Eclogites of the upper allochthon (tectonic klippen of Münchberg, Wildenfels and Frankenberg) originated from early Devonian (c. 400 Ma) subduction. Cooling ages in the klippen, combined with clast spectra and ages of detrital minerals in the foreland flysch record exhumation in Famennian time. The high-pressure (HP) rocks rose in a narrow corridor along the suture zone, were retrogressed under amphibolite facies conditions, and rapidly recycled into the foreland flysch. Rocks of the lower allochthon are exposed in dome structures emerging from under the relative autochthon. The HP Saxonian Granulites were formed at c. 20 kbar/1050 °C, and the Erzgebirge contains diamond-bearing quartzo-feldspathic rocks, associated with eclogites. Peak temperatures of the lower allochthon were attained at c. 340 Ma. Immediately afterwards, these rocks were emplaced under the floor of the relative autochthon, which, at this time, was a foreland basin receiving clastic sediments from the orogen adjacent to the SE. Emplacement of the lower allochthon is most clearly documented in the Saxonian Granulite Dome, in which HP granulites are juxtaposed against low-pressure-high-temperature (LP-HT) rocks of the hanging wall. The interface is a zone of HT shear, which cuts out
60 km of crustal thickness. In more internal parts of the belt (Erzgebirge), the newly emplaced lower allochthon was subsequently reworked by thrusting and polyphase refolding. Emplacement of the hot, low-viscosity lower allochthon was probably driven by buoyancy and the hydraulic gradient between the crustal root to the SE and the lower crust of the foreland. Unlike the earlier HP rocks exposed in the upper allochthon, the 340 Ma HP rocks of the lower allochthon were thermally softened, and, instead of piercing their cover, intruded into the foreland. Therefore, the 340 Ma rocks do not appear in the clastic record of the flysch. The two contrasting mechanisms of exhumation observed in the upper and lower allochthon are apparently due to different thermal regimes.
