High-Density Upper Amphibolite/Granulite Facies Fluid Inclusions in Magmatic Garnet from the Koralpe Mountains (Eastern Alps, Austria)

Abstract

Fluid and solid inclusions in magmatic garnet from Permian pegmatites of the Koralpe Mountains were investigated. On the basis of MnO/(MnO + FeO) ratios, different degrees of melt fractionation during garnet growth were linked with fluid inclusion densities and chemistries. It is shown that garnet indicating low-melt fractionation trends contained primary CO2 ± N2-rich fluid inclusions of the highest densities, up to 1.15 g/cm3, compared to garnet samples of increased fractionation trends comprising CO2-N2-rich fluid inclusions with lower densities up to 0.85 g/cm3. This fluid composition is interpreted as a part of an unmixed CO2 ± N2-H2O-rich fluid that was present during garnet crystallization. Variabilities in the nitrogen composition up to 40.83 mol% resulted from different degrees of partial melting of mica and plagioclase from the metapelitic host rock. Densities, fluid chemistries, and mineral chemical data enabled a continuous upward trend for garnet crystallization during anatexis from lower (ca. 25 km) up to middle crustal levels (12–15 km). Resulting amphibolite/granulite facies conditions of 7.6 kbar/700 °C for garnet crystallization in spodumene-free pegmatites were significantly higher than previously suggested for pegmatite formation in the Koralpe Mountains.