PETROLOGY OF MAGMATIC AND METAMORPHIC ROCKS IN THE PERMIAN-LOWER TRIASSIC HASELGEBIRGE OF THE AUSTRIAN EASTERN ALPS: GEODYNAMIC IMPLICATIONS

The evaporite mélanges of the Haselgebirge Formation exposed in the central Northern Calcareous Alps (Moosegg, Weitenau, Hallstatt) comprises a wide range of dm- to m-sized tectonic blocks of magmatic and metamorphic rocks. Some metagabbro clasts contain a partly altered magmatic mineral assemblage including plagioclase, clinopyroxene and titanomagnetite. The rims are partly replaced by a fine-grained mixture of chemically unresolvable amphiboles, opaques and other minerals. The metamorphic assemblage of other pseudomorphs contains phengitic white mica (Early Variscan Ar-Ar ages) and Na-rich amphiboles (winchite). Due to a lack of foliation, the assemblage is interpreted as locean floor metamorphism. Other samples of meta-biotite-diorite/-gabbro contain plagioclase, kaersutite, and Ti-rich biotite as primary minerals, and actinolite, chlorite and epidote in a metamorphic assemblage. Biotite ArAr ages range from 270 to 248 Ma. In some of these rocks, we also found a magnesio-hornblende/actinolite associated with opaque minerals in the core of kaersutite, which possibly represents a pseudomorph after clinopyroxene. Idiomorphic glaucophane with small phengite-rich white mica, resp. rims of phengitic white mica around biotite indicative for static high-pressure conditions during formation were also observed. Na-rich amphiboles (mainly magnesio-riebeckite) are particularly important in many metamorphic rocks as well as infill of extensional gashes, particularly in dolomite lenses. As a working hypothesis, we postulate formation of Na-rich amphiboles by interaction between a brine and rock at an elevated temperature.

The new data indicate a magmatic suite in the Alpine Haselgebirge, which is dominated by mildly alkaline rocks. The phengitic white mica and glaucophane of meta-gabbro suggest a phase of post-Permian blueschist metamorphism. We speculate that part of the blocks preserve remnants of a Mesozoic subduction zone of a rift-related continental margin sequence.

The work was founded by FWF-project P22,728 Polyhalit.