RB-SR AND K-AR MINERAL DATA OF THE KTB AND THE SURROUNDING AREA AND THEIR BEARING ON THE TECTONOTHERMAL EVOLUTION OF THE METAMORPHIC BASEMENT ROCKS

K-Ar and Rb-Sr age determinations on muscovites and biotites and K-Ar age determinations on amphiboles are presented for leucocratic gneisse s, amphibolites and (meta-)igneous rocks from the Zone of Erbendorf Vo henstrauss (ZEV) and the KTB boreholes located in the northeastern ZEV and leucocratic gneisses from the Erbendorf Greenschist zone (EGZ). T he investigations were carried out to contribute to the knowledge of t he tectonothermal evolution of the area and to the response of the iso tope systems to retrograde processes including recent elevated in- sit u temperatures in the boreholes. A memory of an early Ordovician igneo us event is giver? by Rb-Sr ages of 480 +/- 5-475 +/- 5 Ma obtained on very coarse-grained relic igneous muscovites of metapegmatites from t he ZEV. This memory is missing in micas from pegmatoids of the drill s ite. Coarse-grained muscovites of gneisses from ZEV and EGZ surface ou tcrops and of gneisses and pegmatoids from the boreholes yielded maxim um K-Ar ages of 377 +/- 3-371 +/- 3 Ma related to the end of the Devon ian regional metamorphism in both units. Consistent K-Ar and Rb-Sr app arent ages of some muscovites from surface outcrops point to a fast co oling of the metamorphic rocks due to rapid tectonic uplift. The lack of a depth-related gradient in model ages of coarse-grained KTB muscov ites is explained by post-Variscan stacking processes. For minerals fr om intermediate to greater depths of the KTB, a strong age dependence on grain size and disturbances of the isotope systems due to Various l ate- to post-Variscan retrograde processes can be demonstrated. A stro ng decrease in K-Ar model ages with increasing depth as obtained for b iotites from the deepest section of the HE borehole probably indicates time-integrated response of the isotope system to elevated in situ te mperatures.