The Adula Nappe is a slice of Pre-Mesozoic continental basement affected by Early Alpine (Mesozoic or Lower Tertiary) high-pressure metamorphism. Mineral compositions in mafic rocks containing omphacite + garnet + quartz record a continuous regional trend of increasing recrystalliza tion temperatures and pressures that can be ascribed to this regional high-pressure metamorphic event. P-T estimates derived from mineral compositions grade from about 12 kb and 500 °C or less in the north of the nappe to more than 20 kb/800 °C in the south. The regional P-T trend is associated with a mineralogical transition from assemblages containing additional albite and abundant amphiboles, epidote minerals, and white micas in the north (omphacite-garnet amphibolites) to kyanite eclogites containing smaller amounts of hornblende and zoi.site in the south. Textures and mineral compositional data show that these hydrous and anhydrous silicates associated with omphacite + garnet + quartz arc primary parts of the high-pressure assem blages. Observed phase relations between these primary silicates, theoretical Schreinemakers analysis, and the thermobarometric results, together indicate that the regional transition from omphacite amphibolites to kyanite 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 (aH2O) as indicated by isofacial mineral assemblages, and the H2O character of the reaction (15), are interpreted to reflect largely H and predominantly fluid-absent high-pressure metamorphism within the northern part of the nappe. The omphacite amphibolites and paragonite eclogites in this area are thought to have formed by H2O reactions from Pre-Mesozoic high-grade amphibolites, i.e. from protoliths of similar bulk H2O-countent. The second ‘isograd' (12) is interpreted to mark the regional transition from largely fluid-absent metamorphism in the north to fluid-present metamorphism in the south, where metamorphic pressures and temperatures in excess of 12-15kb and 500-600°C were sufficient for prograde in-situ dehydration of similar hydrous protoliths to kyanite eclogites. The observation of abundant veins, filled with quartz+kyanite+omphacite, suggests that a free fluid coexisted locally with the kyanite eclogites of the southern Adula Nappe at some time during progressive dehydratio
