Variscan Sm-Nd and Ar-Ar ages of eclogite facies rocks from the Erzgebirge, Bohemian Massif (original) (raw)
1995, Journal of Metamorphic Geology
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 key complex for studying tectonic processes such as crustal thickening or incorporation of mantle-derived material into the continental crust. This study provides the first evidence that highpressure metamorphism in the ECC is of Variscan age. Sm-Nd isochrons define ages of 333+6 (Grt-WR), 337 f 5 (Grt-WR), 360 f 7 (Grt-Cpx-WR) (eclogites) and 353 f 7 Ma (Grt-WR) (garnetpyroxenite). @Arf9Ar spectra of phengite from two eclogite samples give plateau ages of 348 f 2 and 355 f 2 Ma. The overlap of ages from isotopic systems with blocking temperatures that differ by about 300" C indicates extremely fast tectonic uplift rates. Minimum cooling rates were about 50" C Myr-'. As a consequence, the closure temperature of the specific isotopic system is of minor importance, and the ages correspond to the time of high-pressure metamorphism. Despite textural equilibrium and metamorphic temperatures in excess of W " C , clinopyroxene, garnet and whole rock d o 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 observed Sm-Nd disequilibrium. The cNd values of the eclogites (+4.4 to +6.9) suggest the protoliths were derived from a long-term depleted mantle, probably a MORB source, whereas the isotopically enriched garnet-pyroxenite ( E~, , -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 shared metamorphic history.
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