A muscovite age of a contact metamorphic gneiss form the SW Bergstrasser Odenwald/Germany

摘要

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 ~(40)Ar/~(39)Ar 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 ~(40)Ar/~(39)Ar spectra. The dating results do not constrain the early regional metamorphism but indicate Late-Early Carboniferous contact-metamorphism by the granite intrusions.