VARISCAN SM-ND AND AR-AR AGES OF ECLOGITE-FACIES ROCKS FROM THE ERZGEBIRGE, BOHEMIAN MASSIF

The Erzgebirge Crystalline Complex (ECC) is a rare example where both 'crustal' eclogites and mantle-derived garnet-bearing ultramafic rocks (GBUs) occur in the same tectonic unit. Thus, the ECC represents a ke y complex for studying tectonic processes such as crustal thickening o r incorporation of mantle-derived material into the continental crust. This study provides the first evidence that high-pressure metamorphis m in the ECC is of Variscan age. Sm-Nd isochrons define ages of 333 +/ - 6 (Grt-WR), 337 +/- 5 (Grt-WR), 360 +/- 7 (Grt-Cpx-WR) (eclogites) a nd 353 +/- 7 Ma (Grt-WR) (garnet-pyroxenite). Ar-40/Ar-39 spectra of p hengite from two eclogite samples give plateau ages of 348 +/- 2 and 3 55 +/- 2 Ma. The overlap of ages from isotopic systems with blocking t emperatures that differ by about 300 degrees C indicates extremely fas t tectonic uplift rates. Minimum cooling rates were about 50 degrees C Myr(-1). As a consequence, the closure temperature of the specific iso topic system is of minor importance, and the ages correspond to the ti me of high-pressure metamorphism. Despite textural equilibrium and met amorphic temperatures in excess of 800 degrees C, clinopyroxene, garne t and whole rock do not define a three-point isochron in three of four samples. The metamorphic clinopyroxenes seem to have inherited their isotopic signature from magmatic precursors. Rapid tectonic burial and uplift within only a few million years might be the reason for the ob served Sm-Nd disequilibrium. The epsilon(Nd) values of the eclogites ( +4.4 to +6.9) suggest the protoliths were derived from a long-term dep leted mantle, probably a MORE source, whereas the isotopically enriche d garnet-pyroxenite (epsilon(Nd) - 2.9) might represent subcontinental mantle material, emplaced into the crust prior to or during collision . The similarity of ages of the two different rock types suggests a sh ared metamorphic history.