The central Wölzer Tauern, Styria, belong to the Austroalpine metamorphic units. In the area Hochweberspitze - Hohenwart - Hühnerkogel, 10 km north of Oberwölz, two types of polyphase micaschists can be distinguished.
    The first type is dominated by the Alpine assemblage garnet + muscovite + paragonite + biotite + quartz + rutile. Some samples contain additionally calcic amphibole in equilibrium with paragonite and muscovite. Alpine garnet overgrows texturally and chemically distinct older cores. The cores show prograde growth zoning and a nearly constant Xgrs of 0.04-0.07. Alpine garnet starts to grow with a Xgrs of ca. 0.20, which is decreasing to 0.03 at the rims. Ti-phases are ilmenite in the cores and in the inner part of the Alpine garnet, whereas the outer part of the Alpine garnet contains rutile. Garnet-biotite thermometry and rutile-ilmenite equilibria, calculated with TWQ, suggests equilibration at 590-640° C and > 9 kbar for the outer part of the Alpine garnet. This result is consistent with the paragenesis calcic amphibole + paragonite, indicating P-conditions of 9-15 kbar at 600° C (Schuster & Thöni, 1996). Sm-Nd dating of garnet cores from a similar type of micaschist at Bruderkogel, 15 km ENE of the investigated area, yielded a Permian age, whereas the garnet rims gave an Alpine age (Will, 1998).
    The second type of micaschists is characterized by large pre-Alpine porphyroblasts of garnet (with tiny radial TiO2-inclusions - „Sterngranat“) + staurolite + kyanite + biotite. These pre-Alpine garnets exhibit prograde growth zoning. Xgrs is nearly constant in the inner part (0.15-0.20) and decreases to 0.05-0.10 at the outermost part. Alpine meta-morphism resulted in recrystallization of the muscovite-biotite rich matrix and growth of a new generation of individual, homogeneous garnet blasts which only subordinate overgrow the old garnets. The grossular-content of the Alpine garnets varies strongly from sample to sample (Xgrs = 0.02-0.20). Incipient prograde staurolite breakdown to kyanite and garnet is also tentatively interpreted to be of Alpine age. Garnet-biotite thermometry and rutile-ilmenite equilibria give 600-650° C and > 8 kbar for the Alpine overprint. Calculated temperatures of the staurolite breakdown reaction range from 620-670° C at 10 kbar and aH2O = 1, but shifts to lower temperatures with decreasing water activity. The age of the „Sterngranat“ is geochronologically not constraint so far.
References:
Will, T. et al. (1998): Contrib. Mineral. Petrol. 132, 85-102.
Schuster, R. & Thöni, M. (1996): Mitt. Österr. Min. Ges. 141, 219-221.