A muscovite age of a contact metamorphic gneiss from the SW Bergsträsser Odenwald/Germany

The oldest rocks of the Bergstrasser Odenwald are Variscan paragneisses. Most of them contain biotite and hornblende, but occasionally also muscovite-bearing varieties occur. Published K-Ar hornblende and biotite ages for these paragneisses are between 370 and 330 Ma. Muscovite from a sillimanite-muscovite gneiss (formerly called metamorphic schist) within the Heidelberg intrusive complex close to Weinheim was dated by the 40Ar/39Ar step degassing technique and yielded well-developed age plateaus. The total-argon ages of 327.5 ± 1.6 Ma correspond with the K-Ar biotite age of the neighbouring Heidelberg granite and other magmatic rocks of the southwestern Bergstrasser Odenwald. No influence from the nearby Early Permian volcanism (Wachenberg rhyolite) is visible in the 40Ar/39Ar spectra. The dating results do not constrain the early regional metamorphism but indicate Late-Early Carboniferous contact-metamorphism by the granite intrusions.