Local origin of high-pressure vein material in eclogite facies rocks of the Zermatt-Saas

ABSTRACT. Prograde dehydration reactions within metabasalts at the blueschist-eclogite facies transition produce kyanite veins, usually with quartz, sometimes with omphacite or chloritoid, of up to several meters length and 20 cm width. Bulk rock analyses show clear depletion haloes for Al2O3 and SiO2 near the veins. Mass balance calculations support the suggestion that the vein material must have been derived from the immediate surroundings. These veins are interpreted as local segregation phenom-ena and not as fluid dehydration channels of the eo-Alpine subduction zone. Calcula-tions have been performed for a stagnant vein formation model that considers the rate of the dehydration reactions (k) responsible for providing the H2O for Al and Si transport and the rate of diffusion (D) of these from wall rock to growing vein. The model is satisfied for quite a wide range of values for k/D and upper limits on the duration of diffusional vein formation of about 1 Ma are indicated. introduction Veins of high pressure minerals (notably one or two of kyanite, omphacite, garnet, glaucophane, chloritoid—commonly with quartz) are often found in eclogite facie