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A large database of structural, geochronological and petrological data combined with a Bouguer anomaly map is used to develop a two-stage exhumation model of deep-seated rocks in the eastern sector of the Variscan belt. An early sub-vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks. These events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18)20 kbar to depths equivalent to 8)10 kbar, and for coeval burial of upper crustal rocks to depths equivalent to 8-9 kbar. Following the folding and vertical extrusion event, sub-horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening. Fabrics that record the early vertical extrusion originated between 350 and 340 Ma, during building of an orogenic root in response to SE-directed Saxothuringian continental subduction. Fabrics that record the later subhorizontal exhumation event relate to an eastern promontory of the Brunia continent indenting into the rheologically weaker rocks of the orogenic root. Indentation initiated thrusting or flow of the orogenic crust over the Brunia continent in a north-directed sub-horizontal channel. This sub-horizontal flow operated between 330 and 325 Ma, and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re-equilibration. The erosion depth as well as the degree of reworking decreases from south to north, pointing to an outflow of lower crustal material to the surface, which was subsequently eroded and deposited in a foreland basin. Indentation by the Brunia continental promontory was highly noncoaxial with respect to the SEoriented Saxothuringian continental subduction in the Early Visean, suggesting a major switch of plate configuration during the Middle to Late Visean.

At the eastern margin of the Bohemian Massif (Variscan belt of Central Europe), large bodies of felsic granulite preserve mineral assemblages and structures developed during the early stages of exhumation of the orogenic lower continental crust within the Moldanubian orogenic root. The development of an early steep fabric is associated with east-west-oriented compression and vertical extrusion of the highgrade rocks into higher crustal levels. The high-pressure mineral assemblage Grt-Ky-Kfs-Pl-Qtz-Liq corresponds to metamorphic pressures of 18 kbar at 850°C, which are minimum estimates, whereas crystallization of biotite occurred at 13 kbar and 790°C during decompression with slight cooling. The late stages of the granulite exhumation were associated with lateral spreading of associated highgrade rocks over a middle crustal unit at 4 kbar and 700°C, as estimated from accompanying cordierite-bearing gneisses. The internal structure of a contemporaneously intruded syenite is coherent with late structures developed in felsic granulites and surrounding gneisses, and the magma only locally explored the early subvertical fabric of the felsic granulite during emplacement. Consequently, the emplacement age of the syenite provides an independent constraint on the timing of the final stages of exhumation and allows calculation of exhumation and cooling rates, which for this part of the Variscan orogenic root are 2.9-3.5 mm yr )1 and 7-9.4°C Myr )1 , respectively. The final part of the temperature evolution shows very rapid cooling, which is interpreted as the result of juxtaposition of hot high-grade rocks with a cold upper-crustal lid.

[1] High-grade orthogneisses from granulite-bearing lower crustal unit show extreme finite strains of both K-feldspar and plagioclase with respect to weakly deformed quartz aggregates. K-feldspar aggregate in the most intensely deformed sample shows interstitial grains of quartz and albite, which also mark some intragranular fractures within K-feldspar grains. Both interstitial grains and fractures are oriented mostly perpendicular to the sample stretching lineation. Quartz and albite grains within K-feldspar bands are interpreted as crystallized from interstitial melt and the petrology study shows that the melt was produced by a metamorphic reaction in plagioclase-mica bands. Thermodynamic Perple_X modeling shows that melt volume increase was negligible and melt amount was too small to generate considerable melt overpressure for calculated PT conditions. It is therefore suggested that dilation of K-feldspar aggregates and fracturing of its grains represent a final creep failure state, which resulted from the cavitation process accompanying grain boundary sliding controlled diffusion creep. The consequence of cavitation-driven dilation of K-feldspar aggregates is the local underpressure resulting in infiltration of melt from plagioclase bands. Analogy with metallurgy experiments shows that the cavitation process, exclusively developed in cryptoperthitic K-feldspar, can be attributed to its lower purity compared to more pure plagioclase. Contrasting rheological behavior of feldspars with respect to quartz prior to fracturing is attributed to different deformation mechanisms. Feldspars appear weaker due to grain boundary sliding accommodated by coupled melt-enhanced diffusion creep along grain boundaries and dislocation creep within grains, in contrast to quartz deforming via grain boundary migration accommodated dislocation creep.Citation: Závada, P., K. Schulmann, J. Konopásek, S. Ulrich, and O. Lexa (2007), Extreme ductility of feldspar aggregates-Meltenhanced grain boundary sliding and creep failure: Rheological implications for felsic lower crust,

In the central part of the Erzgebirge, the age of granulite-facies metamorphism corresponds to the age of formation of mafic eclogites exhumed at the base of the recently defined Lower Crystalline nappe. Granulite-facies conditions in acidic continental crust and eclogite-facies conditions in mafic eclogites developed at the same time of about 342 Ma, suggesting the existence of two distinct tectonic units with contrasting initial geothermal gradients. Existing petrological and geochronological data are used to present a new model for Carboniferous collision at the western margin of the Bohemian Massif. We propose that juxtaposition of rocks with contrasting thermal histories is the result of subduction of the Saxothuringian crust and its later collision with a thickened continental orogenic root existing to the SE. A short time span between the formation ages of high-pressure rocks and cooling ages of the host lithologies suggests extremely fast exhumation of a coupled subduction zone-orogenic root system during continuing Variscan collision.

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