Alpine metamorphism and veining in the Zentralgneis Complex of the SW Tauern Window: a model of fluid–rock interactions based on fluid inclusions

Abstract

The granitic to tonalitic gneisses of the Zentralgneis Complex in the SW Tauern Window contain numerous discordant quartz–biotite–plagioclase veins occurring either as isolated or in parallel sets with a preferential orientation perpendicular to the main foliation of host rocks. Veins show a zonal distribution of minerals with plagioclase and biotite at the walls and quartz in the centre. Quartz contains abundant fluid inclusions (FI) mostly occurring along secondary planes; all ‘primary’ and most secondary inclusions (three-phase H2O–CO2 inclusions) formed from a dominant fluid type of aqueous-carbonic composition.FI have been characterized by microthermometry, laser Raman and laser ablation-optical emission spectroscopies, which have confirmed their similarity also with respect to composition and density. Fluids display a somewhat unusual composition, especially a wide range of rather high salinities (3–7mol%) deduced from the final melting temperature of clathrate, which covers a wide range from −4.2 to +4.0°C. The volatile phase is dominated by CO2 (3–12mol%) with small amounts of N2. The final homogenization temperature of these inclusions is generally >300°C, but independently constrained trapping temperatures of 550–600°C yield to pressure estimates of 3.5–7.5kbar. Highest pressures are in agreement with the P–T conditions reached during the Tauern metamorphism peak. Veining was triggered by hydrofracturing induced by Pl–Bt-producing devolatilization and probably decarbonation reactions in the adjacent gneisses, and quartz precipitated in response to the significant pressure and/or salinity fluctuations shown by FI during each microfracturing/healing stage.