The Haselgebirge evaporitic mélange in central Northern Calcareous Alps (Austria): Part of the Permian to Lower Triassic rift of the Meliata ocean?

For the reconstruction of Alpine tectonics of the Eastern Alps, the evaporitic Permian to Lower Triassic Haselgebirge Formation plays a key role in (1) the origin of Haselgebirge bearing nappes, (2) the inclusion of magmatic and metamorphic rocks revealing tectonic processes not preserved in other units, and (3) the debated mode of emplacement of the nappes, namely gravity-driven or tectonic. Within the Moosegg quarry of the central Northern Calcareous Alps gypsum/anhydrite bodies are tectonically mixed with lenses of sedimentary rocks and decimeter- to meter-sized tectonic clasts of plutonic and subvolcanic rocks and rare metamorphics. We examined various types of (1) widespread biotite–diorite, meta-syenite, (2) meta-dolerite and rare ultramafic rocks (serpentinite, pyroxenite) as well as (3) rare metamorphic banded meta-psammitic schists and meta-doleritic blueschists. The apparent (40)Ar/(39)Ar biotite ages from three biotite–diorite, meta-dolerite and meta-doleritic blueschist samples with variable composition and fabrics range from 248 to 270 Ma (e.g., 251.2 ± 1.1 Ma) indicating a Permian age of cooling after magma crystallisation or metamorphism. The chemical composition of biotite–diorite and meta-syenite indicates an alkaline trend interpreted to represent a rift-related magmatic suite. These, as well as Permian to Jurassic sedimentary rocks, were incorporated during Cretaceous nappe emplacement forming the sulphatic Haselgebirge mélange. The scattered (40)Ar/(39)Ar white mica ages of a meta-doleritic blueschist (of N-MORB origin) and banded meta-psammitic schist are ca. 349 and 378 Ma, respectively, proving the Variscan age of pressure-dominated metamorphism. These ages are similar to detrital white mica ages reported from the underlying Rossfeld Formations, indicating a close source–sink relationship. According to our new data, the Haselgebirge bearing nappe was transported over the Lower Cretaceous Rossfeld Formations, which include many clasts derived from the Haselgebirge Formation and its exotic blocks deposited in front of the incoming nappe comprising the Haselgebirge Formation.