SmNd and UPb dating of eclogites and granulites from the Oberpfalz, NE Bavaria, Germany

Abstract

The formation of eclogites and felsic granulites of the Moldanubian basement of the Oberpfalz (area of Winklarn, NE Bavaria) has been dated at 424 Ma using the SmNd method on minerals (garnet, zircon, rutile) and whole rock. Only estimates on the age of the tholeiitic protoliths of the eclogite boudins are possible from the scattered U-Pb zircon data and from SmNd systematics. Both methods suggest protolith ages around 1 Ga and indicate, together with the major-, trace- and rare-earth element data, extraction of the tholeiitic melts from a suboceanic mantle with an initial ϵ Nd of + 7.5. Based on the geochronological data of three typical Moldanubian cordierite-bearing paragneisses which surround the Winklarn area in distances of 30–60 km, tectonic contacts to the low-pressure country rocks of the eclogites are probable as these paragneisses yielded Ordovician ages for their amphibolite-facies metamorphism and post-Pan-African ages for the deposition of their sedimentary precursors. Low-pressure-high-temperature metamorphism at 323 Ma fully reset monazites in the granulites. This widespread and synchronous Carboniferous overprinting of the Moldanubian basement is probably also responsible for the variable opening of UPb zircon, RbSr whole-rock and SmNd mineral systems. A terrane or microcontinent model still appears most suitable to explain the Ordovician, Silurian, Devonian and Carboniferous high-pressure (subduction) events detected all over the European Variscan belt. In this model, oceanic basins between Gondwana-derived microcontinents were successively closed since the Ordovician, causing differently old subduction zone-related high-pressure metamorphisms.