U-Pb zircon dating of Paleozoic volcanic rocks from the Rheno-Hercynian Zone: new age constraints for the Steinkopf formation, Lahn-Dill area, Germany

Small and volumetrically subordinate outcrops of alkali-rhyolitic to trachytic metavolcanic rocks are widespread in the Lahn-Dill area of the Rheinisches Schiefergebirge, Germany. These occurrences are either related to late Early Devonian or to Middle Devonian episodes of volcanic activity. An unambiguous age assignment for the time of eruption could not be established so far. Most volcanic rocks of the study area are not suitable for application of U-Pb geochronology. A notable exception is the Steinkopf rock suite in the SW Lahn-Dill area, but a straightforward interpretation of the LA-ICP-MS U-Pb data is hampered by complex zircon populations recording various processes including crystallization from newly formed melt, inheritance, assimilation of country rocks, variable degrees of Pb-loss and fluid-assisted Zr-mobility. A plausible evaluation of U-Pb zircon data of two metatrachytes and a metavolcaniclastic rock document a coherent age group at ca. 390-384 Ma and a probable eruption age at ca. 385 Ma, suggesting a relationship to Middle Devonian (Givetian) igneous processes. Zircon-bearing veinlets in a metatrachyte indicate post-eruptive infiltration of external hydrothermal fluids and/or internal Zr-redistribution in Late Devonian to Early Carboniferous time (364 +/- 10 Ma), but this process is of rather limited importance.