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

Abstract

 K–Ar and Rb–Sr age determinations on muscovites and biotites and K–Ar age determinations on amphiboles are presented for leucocratic gneisses, amphibolites and (meta-)igneous rocks from the Zone of Erbendorf Vohenstrauss (ZEV) and the KTB boreholes located in the northeastern ZEV and leucocratic gneisses from the Erbendorf Greenschist zone (EGZ). The investigations were carried out to contribute to the knowledge of the tectonothermal evolution of the area and to the response of the isotope systems to retrograde processes including recent elevated in- situ temperatures in the boreholes. A memory of an early Ordovician igneous event is given by Rb–Sr ages of 480±5–475±5 Ma obtained on very coarse-grained relic igneous muscovites of metapegmatites from the ZEV. This memory is missing in micas from peg matoids of the drill site. Coarse-grained muscovites of gneisses from ZEV and EGZ surface outcrops and of gneisses and pegmatoids from the boreholes yielded maximum K–Ar ages of 377±3–371±3 Ma related to the end of the Devonian regional metamorphism in both units. Consistent K–Ar and Rb–Sr apparent ages of some muscovites from surface outcrops point to a fast cooling of the metamorphic rocks due to rapid tectonic uplift. The lack of a depth-related gradient in model ages of coarse-grained KTB muscovites is explained by post-Variscan stacking processes. For minerals from intermediate to greater depths of the KTB, a strong age dependence on grain size and disturbances of the isotope systems due to various late- to post-Variscan retrograde processes can be demonstrated. A strong decrease in K–Ar model ages with increasing depth as obtained for biotites from the deepest section of the HB borehole probably indicates time-integrated response of the isotope system to elevated in situ temperatures.