Anatectic Granitic Pegmatites from the Eastern Alps: A Case of Variable Rare Metal Enrichment During High-Grade Regional Metamorphism. III: Fluid Inclusions as Potential Indicators for Anatectic Pegmatite Parent Melt Formation

Abstract

ABSTRACT Fluid inclusions trapped in quartz, beryl, and cassiterite from a granitic pegmatite of the Austroalpine basement of the Eastern Alps in Northern Italy were analyzed using a cooling/heating stage and micro-Raman spectroscopy with the following aims: (1) to characterize the range in fluid compositions and fluid densities, (2) to calculate minimum pressures of fluid entrapment from density data and temperature estimates for pegmatite crystallization based on pegmatite mineral assemblages, and (3) to assess possible relations between the compositions of the fluids, in particular their N contents, and potential sources of pegmatite parent melts. The pegmatite intruded into Al-rich metapelitic gneisses during the Permian tectonometamorphic event (∼290 Ma) and was overprinted at amphibolite-eclogite-facies P-T conditions during Eoalpine (∼80–90 Ma) metamorphism. It is characterized by a major phase assemblage quartz + muscovite + paragonite + albite with minor garnet + beryl and accessory columbite-group phases + cassiterite + ixiolite/wodginite + chrysoberyl + cordierite + staurolite + apatite + wyllieite-group phases + zircon. Two types of fluids are preserved in the phases investigated: (1) complex CO2±N2±CH4-H2O-NaCl±MgCl2±CaCl2 fluids with highly variable proportions of the constituent species trapped in all three host phases. The compositions of these fluids range from CO2-rich (∼72–92 mol.%) to N2-rich (∼25–66 mol.%) with additional minor (∼3–8 mol.%) CH4 contents; and (2) H2O-rich fluids with variable salinity (∼3–23 mass%) trapped in beryl and quartz. Trapping of CO2-N2-CH4 fluids in cassiterite along interfaces between the cassiterite host and columbite-group phase inclusions indicates that these fluids were present during joint cassiterite-columbite-group phase crystallization when the pegmatite solidified. In the absence of clear textural evidence for a successive trapping of the two types of fluids, a possible explanation for their simultaneous presence is fluid immiscibility. Isochores for fluid inclusions with the highest densities observed in cassiterite and beryl yield trapping pressures of ∼9–10 kbar for 650–700 °C. By comparison, temperatures estimated for pegmatite crystallization based on the assemblage beryl + chrysoberyl + gahnite and the stability of albite + muscovite + quartz lie in the range ∼650–700 °C. As a potential source of N for the N2-rich fluids, biotite and muscovite from the metapelites of the pegmatite host rocks were identified. Infrared analysis yields NH4 contents of 1430–1772 μg/g in biotite and of 297–489 μg/g in muscovite from two representative gneiss samples. This finding provides further evidence for an anatectic origin of most, if not all, Permian pegmatites found throughout the Austroalpine basement of the Eastern Alps.