TECTONIC SIGNIFICANCE OF THE UPPER PERMIAN TO LOWER TRIASSIC HASELGEBIRGE EVAPORITIC MÉLANGE AND OF ITS MAGMATIC SUITES: EVIDENCE FROM THE MOOSEGG QUARRY IN THE CENTRAL NORTHERN CALCAREOUS ALPS (AUSTRIA)

The Permian-Lower Triassic Haselgebirge Fm. plays a key role in (1) the origin of the Haselgebirge bearing nappes, (2) the potentially primary or tectonic inclusion of magmatic and metamorphic rocks revealing tectonic processes not preserved in other units, and (3) the debated mode of gravity-driven or tectonic emplacement.

According to fluid inclusion studies by Spötl et al. (1998, Sedimentology, v. 45, p. 1019 – 1044), the Haselgebirge sulphates experienced a relatively high temperature > 240 °C. A large variety of sulphatic fabrics is preserved within the Moosegg quarry. Dominant gypsum/anhydrite bodies are tectonically mixed with subordinate lenses of dark dolomite, dark-grey and red shales, pelagic limestones, marls and dm - to m - sized tectonic clasts and boudins of plutonic rocks as well as metamophic and altered volcanic rocks. We found widespread biotite-bearing meta-gabbro, various types of epidote-rich and commonly kaersutite-bearing meta-diorite, meta-monzonite, rare ultramafic rocks as well as meta-dolerite and rare schistose metamorphites. The ⁴⁰Ar/³⁹Ar biotite ages from nine meta-gabbros and meta-diorites 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 crystallization. The chemical composition of the epidote-diorites, meta-monzonite and biotite-gabbros indicate an alkaline trend. We interpret the plutonic and volcanic rocks to represent a rift-related magmatic suite. The meta-dolerites show a MORB-origin. These, together with a high variety of Permian to Jurassic sedimentary rocks, were incorporated during nappe emplacement forming the sulphatic Haselgebirge mélange. ⁴⁰Ar/³⁹Ar white mica ages from two schistose metamorphites yield ages of 349 Ma to 378 Ma and prove the Variscan age of pressure-dominated metamorphism. They are in the range of detrital white mica ages reported from the underlying Rossfeld Fm. (von Eynatten et al., 1996, Geology, v. 24, p. 691–694) indicating a close source-sink relationship.

According to our new data, the Haselgebirge-bearing nappe was transported over the Lower Cretaceous Rossfeld Fm., which includes many clasts derived from the Haselgebirge Fm. and its exotic blocks deposited in front of the incoming nappe comprising the Haselgebirge Fm.