Salt-rich aqueous fluids formed during eclogitization of metabasites in the Alpine continental crust (Austroalpine Mt. Emilius unit, Italian western Alps)

Abstract

The metabasite bodies of the Mt. Emilius continental unit (western Italian Alps) underwent a stage of Alpine eclogite-facies metamorphism (1.1–1.3 GPa and 450–550°C) accompanied by polyphase deformation and recrystallization. The metabasites consist of two main rock types: (1) eclogites (omphacite+garnet+glaucophane+epidote+phengite/paragonite) preserving no relics of their precursors; (2) eclogitized granulites, i.e. rocks whose incomplete eclogitic recrystallization (clinopyroxene II+garnet II+epidote+amphibole+chlorite) allowed survival of textural and mineralogical relics of pre-Alpine granulitic assemblages (clinopyroxene I+garnet I+plagioclase+amphibole). In this latter case the pre-Alpine granulites were converted to eclogites as the result of infiltration of aqueous fluids during eclogitization. In both eclogites and eclogitized granulites hydrated high-pressure foliations are cut by eclogitic metamorphic veins. The bulk rock chemistry of the metabasites influenced the compositions of both the vein- and rock-forming clinopyroxenes and the compositional correlation between the vein- and rock-forming clinopyroxenes indicates that the syn-eclogitic fluids have re-equilibrated with the metabasite hosts. The predominant vein minerals (omphacite, epidote and garnet) contain primary high-salinity fluid inclusions. The fluids consist of two-phase (liquid+vapour) and of multiphase (liquid+vapour+salt+additional quartz) salty aqueous inclusions containing NaCl, CaCl 2 and MgCl 2 as the main chloride species. The vein inclusions show a salinity range from 17 to 45 wt.% salts in eclogites and from 20 to 50 wt.% salts in eclogitized granulites. In contrast, fluid inclusions in matrix minerals of the eclogitized granulites contain primary two-phase inclusions displaying a salinity range between 10 and 25 wt.% salts. The marked difference in fluid salinities recorded by the inclusions in the eclogitic veins and those in the partially re-equilibrated eclogitized granulites is interpreted in terms of progressive hydration during eclogitization of granulite-facies rocks, which caused an increase in the salt content of the residual inclusion fluids.