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

Abstract

Retrogressed eclogites occur embedded in mostly calcareous micaschists and greenschists of the Upper Schieferhulle in the Grossglockner region of the central Tauern Window, east of the Eclogite Zone. A four-stage meta-morphic evolution has been derived from textural and mineral chemical observations: Relics of early pre-eclogitefacies events (stage I: chlorite, actinolite, plagioclase and glaucophane, paragonite, clinozoisite) are preserved mainly in the cores of gamets. The peak-metamorphic paragenesis (stage II) of garnet, omphacite, paragonite, glaucophane, (clino)zoisite, quartz and rutile ± phengite and dolomite records conditions of around 17 kbar and 570°C, slightly below those reported from the Eclogite Zone (around 600°C, 20 kbar). In several instances, growth of coarse-grained barroisitic to actinolitic amphibole occurred during uplift, apparently still within the eclogite facies (stage III). As it is impossible to reconcile the observed amphibole growth textures at the expense of omphacite, glaucophane, garnet, paragonite and quartz with a closed-system reaction, metasomatic interaction must have played a role in the formation of these rocks. The final emplacement in the present tectonic setting and co-metamorphism with the surrounding metasediments (stage IV) occurred under conditions of about 5-6 kbar and 500-530°C. It caused severe hydration and retrograde alteration (symplectite formation), transformed most of the eclogite bodies into garnet amphibolites and even green-schists, with a new, strong stage-IV foliation parallel to that of the country rocks, and erased most evidence of the earlier PT-path. Evidence of eclogite-facies metamorphism of the country rocks has been found only in the westernmost eclogite occurrence. Since the lower tectonic units, the gneiss domes and the Lower Schieferhulle, have not yet produced any evidence of Alpine eclogite-facies metamorphism either, we prefer the interpretation that one major tectonic slice -the Eclogite Zone—and several minor ones -those found within the Upper Schieferhulle nappes today—have been tectonically injected into the evolving Penninic nappe stack during underthrusting of the Penninic units underneath the Austroalpine.