Zircon and Garnet Geochronology of Eclogites from the Moldanubian Zone of the Black Forest, Germany

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 Pb‐Pb 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 ϵNd values range from $$-3$$ to $$+1.8$$ , and initial 87Sr/86Sr 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 $$406\pm 31$$ Ma, $$386\pm 21$$ Ma, and $$353\pm 89$$ Ma, which probably date to an earlier stage of the Variscan convergence.