1. Zircon and Garnet Geochronology of Eclogites from the Moldanubian Zone of the Black Forest, Germany.
- Author
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Chen, Fukun, Todt, Wolfgang, and Hann, Horst Peter
- Subjects
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ECLOGITE , *GEOLOGICAL time scales - Abstract
The Moldanubian zone of the Black Forest, Germany, part of the internal zone of the central European Variscan belt, exposes high-pressure metamorphic rocks as small eclogite lenses in host gneisses. We report zircon U-Pb and PbPb ages and garnet Sm-Nd and Pb-Pb ages for five eclogites. The less mobile trace element contents of these eclogites are similar to those of mid-ocean ridge basalts but have moderate to highly fractionated concentrations of the rare earth elements, enrichments in the incompatible elements, and negative anomalies in the high-field strength elements. Initial ∈[sub Nd] values range from -3 to +1.8, and initial[sup 87]Sr/[sup 86]Sr ratios range from 0.7040 to 0.7062. These characteristics of the eclogites suggest an origin from mantle-derived magmas contaminated either by evolved crustal material during intrusion or altered by fluid before or during eclogitization. A combination of zircon U-Pb and Pb-Pb evaporation dates of the eclogites constrains a magmatic episode between ∼486 and ∼445 Ma. This episode was contemporary with the early Paleozoic mafic to ultramafic magmatism in the eastern part of the Variscan belt and probably represents an early Paleozoic rift-stage product. Garnets subjected to the stepwise acid-leaching method from three eclogites yield Sm-Nd isochron ages of about 348-340 Ma, similar to the dates previously reported. The same garnets give less precise Devonian to Early Carboniferous Pb-Pb isochron ages of 406 ± 31 Ma, 386 ± 21 Ma, and 353 ± 89 Ma, which probably date to an earlier stage of the Variscan convergence. [ABSTRACT FROM AUTHOR]
- Published
- 2003
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