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 formers, suggesting a “chilled margin” effect. The amphibole microcrystals in the vein centre mimick a fluidal texture with convoluted and turbulent flow structures, as if crystallised from a melt; in addition, amphiboles show a strong CPO of the [001] axis parallel to the vein walls. The host eclogite near the veins is strongly fractured as like as the clasts inside the veins. Other blue amphibole veins show a composite evolution with a crystallisation of blocky quartz in the core, consistent with hydrofracturing process. The crystallisation of Na- and Ca-Na amphiboles in the Lillianes veins was assisted by fluid circulation under blueschist-facies conditions, at pressures of about 8–15 kbar, during the exhumation soon after the eclogite-facies peak reached by the host rock in a paleoseismic scenario. Finally, the veins were cut across by late calcite, albite and chlorite veinlets formed under greenschist-facies conditions during exhumation.