K-AR DATING OF AMPHIBOLE-BEARING ROCKS IN THE SCHWARZWALD, SW GERMANY.1. AR-40 AR-39 AGE CONSTRAINTS TO HERCYNIAN HT-METAMORPHISM

Throughout the crystalline basement of the Schwarzwald mountain range, SW Germany, amphibolites and gabbroic metabasites occur as small basi c rock bodies. More than two dozens of hornblende separates of both me tamorphic rock species have been subject of a Ar-40/Ar-39 study, the t otal argon age data of which cumulate between 350 and 320 Ma (Early Ca rboniferous). The results show the K-Ar systems of the amphiboles to b e closed in the course of Hercynian HT-metamorphism. The metamorphic t emperatures evidently must have reached at least 500 degrees C on a re gional scale. During this thermal episode only few metamorphic rocks e scaped from total Ar-40 (rad) loss and indicate formation or thermal o verprinting in pre-Carboniferous time. Apart from minor incorrigible e xcess Ar-40 components, scattering of age data beyond the analytical e rror of the mean value supposedly is caused by the geodynamic developm ent of the Schwarzwald massif. According to recent models, based on th e assumption of Hercynian convergence tectonics, differing age results from neighbouring rocks could be due to small-scale thrusting and imb rication. The amphibole age data show that the Schwarzwald basement do es not hold a special position within the European Hercynian belt, but has experienced an evolution similar to other Moldanubian crystalline units where essential orogenic phases are also known to be of Late-De vonian to Early-Carboniferous age.