Eclogite Facies Regional Metamorphism of Hydrous Mafic Rocks in the Central Alpine Adula Nappe

The Adula Nappe is a slice of Pre-Mesozoic continental basementaffected by Early Alpine (Mesozoic or Lower Tertiary) high-pressuremetamorphism. Mineral compositions in mafic rocks containingomphacite + garnet + quartz record a continuous regional trendof increasing recrystalliza tion temperatures and pressuresthat can be ascribed to this regional high-pressure metamorphicevent. P-T estimates derived from mineral compositions gradefrom about 12 kb and 500 ?C or less in the north of the nappeto more than 20 kb/800 ?C in the south. The regional P-T trend is associated with a mineralogical transitionfrom assemblages containing additional albite and abundant amphiboles,epidote minerals, and white micas in the north (omphacite-garnetamphibolites) to kyanite eclogites containing smaller amountsof hornblende and zoi.site in the south. Textures and mineralcompositional data show that these hydrous and anhydrous silicatesassociated with omphacite + garnet + quartz arc primary partsof the high-pressure assem blages. Observed phase relationsbetween these primary silicates, theoretical Schreinemakersanalysis, and the thermobarometric results, together indicatethat the regional transition from omphacite amphibolites tokyanite eclogites can be described by two simplified reactions: alb+epi+hbl=omp+kya+qtz+par (H₂O-conserving) (15) par+epi+hbl+qtz=omp+kya+H₂O (dehydration) (12) which have the character of isograd reactions. Local variations of water activity (a_H2O) as indicated by isofacialmineral assemblages, and the H₂O character of the reaction (15),are interpreted to reflect largely H and predominantly fluid-absenthigh-pressure metamorphism within the northern part of the nappe.The omphacite amphibolites and paragonite eclogites in thisarea are thought to have formed by H₂O reactions from Pre-Mesozoichigh-grade amphibolites, i.e. from protoliths of similar bulkH₂O-countent. The second ‘isograd’ (12) is interpreted to markthe regional transition from largely fluid-absent metamorphismin the north to fluid-present metamorphism in the south, wheremetamorphic pressures and temperatures in excess of 12-15kband 500-600?C were sufficient for prograde in-situ dehydrationof similar hydrous protoliths to kyanite eclogites. The observationof abundant veins, filled with quartz+kyanite+omphacite, suggeststhat a free fluid coexisted locally with the kyanite eclogitesof the southern Adula Nappe at some time during progressivedehydration.