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

The oldest rocks of the Bergstrasser Odenwald are Variscan paragneisses. Mo st of them contain biotite and hornblende, but occasionally also muscovite- bearing varieties occur. Published K-Ar hornblende and biotite ages for the se paragneisses are between 370 and 330 Ma. Muscovite from a sillimanite-mu scovite gneiss (formerly called metamorphic schist) within the Heidelberg i ntrusive complex close to Weinheim was dated by the Ar-40/Ar-39 step degass ing 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 Bergstrass er Odenwald. No influence from the nearby Early Permian volcanism (Wachenbe rg rhyolite) is visible in the Ar-40/Ar-39 spectra. The dating results do n ot constrain the early regional metamorphism but indicate Late-Early Carbon iferous contact-metamorphism by the granite intrusions.