SYNTECTONIC FLUID-ROCK INTERACTION AND MINERAL REACTIONS IN AMPHIBOLITES FROM THE ADULA NAPPE (CENTRAL ALPS, SWITZERLAND)

Mineral reactions and fluid Bur calculations from the basal part of th e Adula nappe and the upper part of the Simano nappe (Central Alps, Sw itzerland) are presented. The base of the Adula nappe (hangingwall) an d the upper part of the Simano nappe (footwall) contain a series of am phibolite layers that are coherently intercalated with gneisses. The i ntercalations are subparallel to the metacarbonates of the Soja zone, which separates the two nappes. Our data indicate that during emplacem ent of the Adula onto the Simano nappe and its autochthonous cover, th e Soja zone, amphibolite layers were altered by a mixed CO2-H2O fluid. However, the alteration is restricted to amphibolites that are adjace nt to the metacarbonates. Amphibolites located further away from the S oja zone do not show any alteration by a mixed CO2-H2O fluid. Field da ta indicate that the fluid apparently was released from the metacarbon ates of the Soja zone. Compositional variations in amphibole from amph ibolites from the basal part of the Adula nappe indicate that the proc ess of fluid infiltration and associated mineral reactions occurred du ring progressive metamorphism from greenschist to epidote-amphibolite facies conditions. Two distinct mineral reactions were observed. (i) A mphibolite layers located close to the metacarbonates adjusted their m ineral assemblages and the compositions of their minerals toward an eq uilibrium with the infiltrating fluid. These rocks experienced carbona tion-dehydration reactions. (ii) Amphibolite layers located about 40 m above the basal thrust of the Adula nappe did not react with CO2 and experienced hydration reactions only. The differences in mineral react ion from one amphibolite layer to another suggest that the first layer s on either side of the metacarbonates retained CO2 from the fluid pha se. Thus, the fluid that left this layer was depleted in CO2. Thermody namic computations of phase equilibria in temperature-X(CO2) sections at isobaric conditions (7 kbar) suggest that X(CO2) in equilibrium wit h the rock was at about 0.05 +/- 0.03. This value is equal to the mole fraction of CO2 of the infiltrating fluid. Fluid Bur calculations ind icate that the basal amphibolite layers were infiltrated by variable a mounts of fluid ranging from 47 to 158 cm(3) (fluid)/cm(2) (rock).