Unusual glaucophane and winchite-bearing veins in mafic eclogites of the Sesia-Lanzo Zone (Italian Northwestern Alps): Are they due to hydrofracturing or a paleoseismic event?

The mafic eclogites of the Sesia-Lanzo Zone exposed in the Gressoney valley (Northwestern Alps, Italy) are cut across by unusual dark blue veins, which can be interpreted as due to frictional melting and/or hydrofracturing in the Alpine subduction zone. We combine optical and electron microscopy, EBSD analysis, microprobe and bulk-rock chemical analyses, as well as pseudosection modelling to study these veins, in order to constrain their textural characteristics and P-T evolution together with those of the host eclogite. The eclogite is composed of omphacite, garnet, quartz, white mica, and rutile that equilibrated at P-T conditions of ca. 25 kbar and 570 °C. The veins are cm thick to capillary and up to few metres long; they are filled with very fine-grained fibrous blue amphiboles (glaucophane, winchite, katophorite) in the centre, and blue-green Ca and Ca-Na amphiboles + albite at the margins. Pseudosection modelling suggests that the latters froze at a higher temperature than the f...