“Crack seal” vein geometry in eclogitic rocks

AbstractFluid-deposited veins occur in eclogite facies rocks in the Mont Viso meta-ophiolitic complex, Western Alps. These veins contain omphacite, garnet, rutile and apatite implying that veining took place at near to peak metamorphic conditions. Two types of omphacitic vein structure are described, (a) Cross-fibre veins which are charactenzed by omphacite fibres which long axis is parallel to the stretching direction of the country rocks. The fibres are cut accross by linear arrays of solid/fluid inclusions oriented normal to the long axes of the fibres, which are interpreted as crack-seal phenomena. (b) Massive, coarse-grained randomly oriented concentric and sector zoned omphacite with garnet, rutile and apatite in " hydrostatic " veins suggests nucleation in open fractures during eclogite facies metamorphic conditions.Formation of syn-kinematic veins under conditions of at least P > 12 kbar and 500 < T < 550 °C indicates that fracturing and solution transfer processes occurred during subduction event.