The Erzgebirge constitutes one of the major metamorphic terrains in the Variscan mountain belt of central Europe. It represents an antiformal structure with a core of medium to high grade mica schists and gneisses, overlain and surrounded by a phyllitic mantle.
The Gneiss-Eclogite Unit in the central Erzgebirge is a composite tectonometamorphic unit characterised by a HP-HT-imprint (830C/21 kbar in felsic granulites) (Willner et al., 1995). It comprises migmatitic para- and orthogneisses, HT-mylonites, kyanite-bearing granulites, eclogites and garnet peridotites. It is tectonically sandwiched between two further major units with distinctly lower PT histories to form a stack with mainly subhorizontal transposition foliations. The Gneiss-Eclogite Unit has a characteristic "kinked" retrograde PT-path after HP-equilibration with (1) strong initial near-isothermal decompression at high temperatures and
(2) extensive hydration and re-equilibration at medium pressures (6-8 kbar; 600-750C), followed (3) by rapid cooling during continued uplift (Willner et al., 1995). We dated single zircons (Pb-Pb-evaporation) and monazite fractions (conventional U/Pb method) from quartz-feldspar rocks of this unit, namely granitoid orthogneisses and metapelites. The orthogneisses contain euhedral, long-prismatic zircons of igneous origin, occasionally showing rounding at their terminations ascribed to metamorphic "corrosion". These zircons provided protolith ages which are 524.2±0.8 Ma, 481.7±1.1 Ma, 478±0.9 Ma and 470.0±0.9 Ma respectively (errors are 2sm).
The metapelites locally retain well-preserved granulite-facies mineral assemblages and contain spherical, multifacetted metamorphic zircons. In one kyanite granulite sample phengite and rutile is included in such zircon. The phengite equilibrated at about 15 kbar before the thermal peak of metamorphism was reached, when phengite was not stable with quartz in this rock. Hence the "ball-shaped" zircons enclosing a prograde HP-phase are of metamorphic origin and grew at or near the peak of HP/HT metamorphism. Metamorphic zircons of three samples from different localities yielded identical 207Pb/206Pb ages of 340.5±0.7 Ma, 341.2±0.5 Ma and 341.6±0.5 Ma respectively.
Monazite fractions from two metapelite samples with well-preserved granulite-facies mineral assemblages also yielded remarkably similar concordant U/Pb ages of 335±2 Ma, 336±2 Ma respectively. These ages are about 5-6 Ma younger than those of the metamorphic zircons. If we assume a relatively high closing temperature of 650C for monazite due to rapid cooling rates and small grain size (Suzuki et al., 1994), we can postulate a cooling rate in the order of 30-35 C/Ma. Taking an estimated concomitant decompression of 13 kbar and a mean crustal density of 2.9 g/cm3, the uplift rate of the Gneiss Eclogite Unit was about 7.5-9 km/Ma.
Exhumation of rocks from the lower part of the fossil, thickened Variscan continental crust (>60 km) with these or similar estimated rates is also assumed for other tectonometamorphic units of the mid-European Variscides (e.g. Black Forest, West Sudetes, southern Bohemia). This is typical for large-scale extensional tectonic unroofing following continental collision and subsequent orogenic collapse.
Suzuki, K., Adachi, M. & Kajizuka, I., Earth Planet. Sci. Lett. 128, 391-405 (1994).
Willner, A.P., Rötzler, K., Krohe, A., Maresch, W.V. & Schumacher, R., Terra Nostra 3, 104-106 (1995).