Austrian Journal of Earth Sciences Vienna 2018 Volume 111/2 155 - 170 DOI: 10.17738/ajes.2018.0010 Tectonic interpretation of the metamorphic field gradient south of the Koralpe in the Eastern Alps Alexandra HERG*) & Kurt STÜWE Institute of Earth Sciences, University of Graz, Universitätsplatz 2, A-8010, Graz, Austria; *) Corresponding author: Alexandra Herg,
[email protected] KEYWORDS Pohorje; Koralpe; slab extraction; Plattengneis-Plankogel shear zone Abstract In order to constrain tectonic models for the nature of the Eoalpine high pressure belt at the eastern end of the Alps, we investigate the formation pressure of metamorphic rocks along a profile between the Koralpe and the well-known UHP rocks of the southern Pohorje mountains. Rocks from three different regions are considered: (i) the rocks of the southernmost Koralpe to the north, (ii) the rocks of the Plankogel Unit between the Plankogel detachment and the Drava valley and (iii) the rocks between the Possruck range and the southern Pohorje mountains. In the Koralpe, pelitic rocks record a formation pressure around 15 – 18 kbar, as reported in the literature. For the Plankogel Unit, we derive pres- sures between 7.1 ± 1.95 kbar and 11.5 ± 3.42 kbar at 650 °C and recognize only a single Eoalpine metamorphic event. For the high grade rocks of the Pohorje mountains, we derive peak metamorphic pressures (explored with the garnet- muscovite-kyanite-quartz assemblage) that rise from 16.2 ± 3.45 kbar (at 700°C) in the north, to 23.9 ± 2.49 kbar (at 700 °C) in the south. There, we also recognize a later lower pressure event that is derived from pressure calculations with the full equilibrium assemblage.