Kaledonische, herzynische und alpidische Ereignisse im Mittelostalpin nördlich der westlichen Hohen Tauern, abgeleitet aus petrographischen und geochronologischen Untersuchungen

Petrology and geochronology of the Schwazer Augengneis (Kellerjochgneis) and of the Steinkogelschiefer north of the western Tauern Window have been investigated. The Steinkogelschiefer are garnet-mica schists, the Schwazer Augengneis is an orthoaugengneiss. Both rock units show a prograde metamorphism of the amphibolite facies and are interbedded between the Innsbrucker Quarzphyllite and the Grauwackenzone. The Innsbrucker Quarzphyllite and the rocks of the Grauwackenzone clearly show in respect to the Schwazer Augengneis and the rocks of the Steinkogelschiefer unit a lower metamorphic grade of the greenschist facies. From combined Compston-Jeffery- and Nicolaysen-diagrams the age of the sediments from which the mica schists of the Steinkogelschiefer unit derives is 540 m. y. The age of the intrusion of the magma from which the Schwazer orthoaugengneis later originated is probably 425 m. y. A Caledonian metamorphism could not be demonstrated by radiometry. In the Schwazer Augengneis as well as in the paragneisses of the Steinkogelschiefer unit a complete Sr-homogenisation was produced by the hercynian metamorphism. An age of 322±24 m. y. and a Sr⁸⁷/Sr⁸⁶-ratio of 0.7180±0.0024 result from a Rb-Sr-Isochrone of the Schwazer Augengneis. The RbSr-Isochrone of the Steinkogelschiefer defines an age of 347±30 m. y. and a Sr⁸⁷/Sr⁸⁶-ratio of 0.7150±0.0021. From the Rb-Sr-method a cooling age of 273 and 260 m. y. is established for the phengites from the Schwazer Augengneis. K-Ar-ages of the muscovites and of the phengites are mixed ages between the hercynian and that of the eoalpine metamorphism. The eoalpine metamorphism is dated by the Rb-Sr-age of the biotites (90 m. y.). This age corresponds to the cooling of the rocks below a temperature of 300 °C before the eoalpine nappe transport. K-Ar-ages of the biotites show excess argon and are therefore from a geological point of view meaningless. From petrological and geochronological data it cannot be demonstrated that the tectonic position of the Schwazer Augengneis and of the Steinkogelschiefer unit on the top of the Innsbrucker Quarzphyllit and under the Grauwackenzone is due to the alpine nappe transport. From a tectonic point of view, the Steinkogelschiefer and the Schwazer Augengneis therefore probably belongs to the middle austroalpine unit of the Innsbrucker Quarzphyllit.