Constraining the process of intracontinental subduction in the Austroalpine Nappes: Implications from petrology and Lu-Hf geochronology of eclogites

Abstract

High- and ultrahigh-pressure rocks occur in the Austroalpine Nappes in a ~400 km long belt from the Texel Complex in the west to the Sieggraben Unit in the east. Garnet growth during pressure increase was dated using Lu-Hf chronometry. The results range between c. 100 and 90 Ma, indicating a short-lived period of subduction. Combined with already published data, our estimates of metamorphic conditions indicate a field gradient with increasing pressure and temperature from the northwest to the southeast, where the rocks experienced ultrahigh-pressure metamorphism. The P-T conditions of the eclogites generally lie on the ‘warm’ side of the global range of subduction-zone metamorphic conditions. The oldest Cretaceous eclogites (c. 100 Ma) are found in the Saualpe-Koralpe area derived from widespread gabbros formed during Permian to Triassic rifting. In the Texel Complex garnets showing two growth phases yielded a Variscan-Eoalpine mixed age indicating re-subduction of Variscan eclogite-bearing continental crust during the Eoalpine orogeny. Jurassic blueschist-facies metamorphism at Meliata in the Western Carpathians and Cretaceous eclogite-facies metamorphism in the Austroalpine are separated by a time gap of c. 50 Ma and therefore do not represent a transition from oceanic to continental subduction but rather separate events. Thus, we propose that subduction initiation was intracontinental at the site of a Permian rift.