FLUID EVOLUTION DURING HIGH-PRESSURE PARTIAL MELTING IN THE AUSTROALPINE ULTEN ZONE, NORTHERN ITALY

Fluid inclusions in high-grade metamorphic rocks of the Ulten Zone (no rthern Italy) have been investigated by microthermometry and Raman mic rospectrometry. Two different types of fluid inclusions can be disting uished: (1) high- to low density CO2 +/- N-2 - CH4 inclusions in kyani te and quartz of coarse grained garnet-kyanite felses and (2) aqueous inclusions of high salinity in quartz of metapelites, paragneisses and amphibolites as well as migmatitic orthogneisses. The migmatites are thought to have formed on the prograde path to the eclogite-facies eve nt (M1) under fluid oversaturation. Kyanite and quartz of the garnet-k yanite felses contain a CO2-dominated fluid, which may result from a h igher solubility of H2O in the melt. A saline, H2O-rich fluid occurs a s early inclusions forming small clusters in quartz of the leucosome f rom migmatitic orthogneisses and as late trail-bound inclusions of low er density in quartz of meta-sediments and amphibolites. During uplift and crystallization of the melt, this fluid was released and together with external highly saline aqueous fluid added to the rocks, re-equi libration of paragneisses, metapelites and metabasites under amphiboli te-facies conditions (M2) was enabled.