Early to Middle Carboniferous hornblende Ar-40/Ar-39 ages of amphibolites and gabbros from the Bergstrasser Odenwald

Pre- to Early Variscan metamorphic rocks and intrusive calc-alkaline rocks are constituting the Odenwald accretionary complex. The predominant regiona l metamorphic formation of the amphibolite is postdated by the intrusion of the Frankenstein gabbro complex in the NW Odenwald at about 362 Ma. Thus i ndications of older metamorphic events might be preserved in the amphibolit es. However, for the central and southern Odenwald zircon data revealed a t hermal peak of regional metamorphism at about 336 Ma. Nine amphibole separa tes from two gabbroic intrusives and from seven amphibolites were dated by the K-Ar method, mainly using the Ar-40/Ar-39 technique. The two gabbro amp hibole separates yielded ages of 335 and 329 Ma; i.e. they are about 30 Ma younger than the Frankenstein gabbro. The total argon ages of the amphiboli te hornblendes are at about 359 Ma in the north and about 330 to 320 Ma in the south (early and late stages of the Early Carboniferous). The ages of t he three metamorphic amphiboles from the north are in the age range of the Frankenstein gabbro intrusion, however, the four from the south slightly po stdate the regional metamorphic climax. Two of the older amphibole separate s of the north and two of the younger ones from the south yielded plateau a ges slightly older than the total argon ages (about 358 Ma and 329 Ma, resp ectively). The southernmost amphibolite with a total argon age of 320 Ma sh ows an irregular spectrum with a maximum age step for 40% of its argon corr esponding to an age of 344 Ma. It possibly has partly preserved an older ar gon component during the younger phase of regional metamorphism. Together w ith published data the results show that hornblende of amphibolites from th e Odenwald offer the chance to unravel the metamorphic veil of the Mid-Carb oniferous regional metamorphism.