Relics of high-pressure metamorphism from the Grossglockner region, Hohe Tauern, Austria; paragenetic evolution and PT-paths of retrogressed eclogites

Retrogressed eclogites occur embedded in mainly calcareous mica schists and greenschists of the Upper Schieferhulle. A four stage metamorphic evolution has been established. Stage I: relics of early pre-eclogite facies events (chlorite, actinolite, plagioclase and glaucophane, paragonite, clinozoisite) are preserved mainly in the cores of garnets. A peak metamorphic paragenesis (stage II) of garnet, omphacite, paragonite, (clino)zoisite, quartz and rutile + or - dolomite records conditions of approximately 17 kbar and 570 degrees C. In several instances, growth of coarse-grained barroisitic to actinolitic amphibole occurred during uplift, apparently still within the eclogite facies (stage III). The final emplacement in the present tectonic setting and co-metamorphism with the surrounding metasediments (stage IV) occurred at 5-6 kbar and 500-530 degrees C, causing severe hydration and retrograde alteration (symplectite formation), and transforming most of the eclogite bodies into garnet amphibolites and even greenschists. It is suggested that a major tectonic slice was injected into the evolving Pennine nappe stack during underthrusting of the Penninic units beneath the Austroalpine.