Fluid evolution during high-pressure partial melting in the Austroalpine Ulten Zone, northern Italy

Abstract

Fluid inclusions in high-grade metamorphic rocks of the Ulten Zone (northern Italy) have been investigated by microthermometry and Raman microspectrometry. Two different types of fluid inclusions can be distinguished: (1) high- to low density CO2±N2-CH4 inclusions in kyanite 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 event (M1) under fluid oversaturation. Kyanite and quartz of the garnet-kyanite felses contain a CO2-dominated fluid, which may result from a higher solubility of H2O in the melt. A saline, H2O-rich fluid occurs as early inclusions forming small clusters in quartz of the leucosome from migmatitic orthogneisses and as late trail-bound inclusions of lower density in quartz of metasediments 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-equilibration of paragneisses, metapelites and metabasites under amphibolite-facies conditions (M2) was enabled.