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

Prograde dehydration reactions within metabasalts at the blueschisteclogite 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 Al~2~O~3~ and SiO~2~ 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 phenomena and not as fluid dehydration channels of the eo-Alpine subduction zone. Calculations have been performed for a stagnant vein formation model that considers the rate of the dehydration reactions (k) responsible for providing the H~2~O 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.