TRANSITION FROM ECLOGITE TO AMPHIBOLITE-FACIES METAMORPHISM IN THE AUSTROALPINE ULTEN ZONE

The Ulten Zone of the Austroalpine crystalline basement south-west of Meran (Italy) contains metapelitic schists and granoblastic paragneiss es, leucocratic orthogneisses, migmatites (in both gneiss-lithologies) , metabasites and ultramafic lenses. Metamorphic textures of the metap elitic schists and granoblastic paragneisses indicate two different me tamorphic events, characterized by two mineral assemblages, which diff er in mineral chemistry: (1) an eclogite facies mineral assemblage (M1 ) comprising Grt - Ky I - Bt - Ms - Kfs - P1 - Qtz - Rt, End (2) an am phibolite facies mineral assemblage (M2) comprising Grt - Ky II - Bt - Ms - Fl - Qtz - Ilm +/- St. For the M1 event, pressures of at least 1 5 kbar and temperatures of about 700 degrees +/- 50 degrees C can be e stimated. The later amphibolite facies overprint occurred at pressures of 6 to 8 kbar and about 600 degrees +/- 50 degrees C. The M1 and M2 assemblages belong to a continuous clockwise metamorphic evolution dur ing the Variscan orogeny. Evidence for Alpine metamorphism can only be detected by sericite rims around kyanite and reset biotite ages. The migmatites, which contribute about 15-30 vol. % of all rocks in the in vestigated area, were formed on the prograde path during the M1 event. Dissolution of H2O in the melted part of the migmatites resulted in a CO2 dominated fluid, which was trapped in primary kyanite (M1) fluid inclusions. Secondary H2O-rich fluid inclusions are found in quartz gr ains and may represent the fluid which enabled a pervasive equilibrati on during M2.