Chemical-microstructural changes in eclogite-facies shear zones (Monviso, Western Alps, north Italy) as indicators of strain history and the mechanism and scale of mass transfer

International audienceVariations of eclogite-facies mineral chemistry are related to the textural characteristics of shear zones in FeTi metagabbros from the Monviso meta-ophiolitic complex in the Italian Western Alps. Three different kinds of microstructural domains are described: (1) small lenses of low-strain rocks with preserved igneous textures; (2) larger high-strain domains where deformation caused pervasive mylonitic foliation (S1) and a stretching mineral lineation (L1); and (3) two types of eclogite-facies veins. The latter can be subdivided into veins composed of only Na-pyroxenes filling small tension gashes and dilatation fractures and veins composed of a Na-pyroxene-garnet-rutile-apatite assemblage occupying large fractures that randomly cross-cut the mylonites.In low-strain domains and mylonites the Na-pyroxenes are omphacites and the garnets are almandine rich. In some mylonite layers containing phengite and blue amphibole crosscutting S1, chloromelanite (iron-rich omphacites) exsolved from omphacite. In type-1 veins the pyroxene is either omphacite or concentric and sector-zoned omphacite-chloromelanite. In type-2 veins pyroxenes are omphacite and/or concentric and sector-zoned omphacite-chloromelanite and/or zoned kosmochlor (chromium-rich omphacite)-omphacites. Associated garnets are richer in pyrope component than those from low-and high-strain domains.Possible origins of both sector and concentric zoning and exsolution are discussed. It is emphasized that a miscibility gap between the omphacite and acmite end-members partly controlled the omphacite-chloromelanite distribution. We define a chemical vector coat constrains the strain history in eclogite-facies rocks and give indications of the scale and mechanism of mass transfer involved during foliation development and fracturing.P-T conditions are calculated for each microstructural domain. The garnet-pyroxene thermometer yields scattered temperature values. These values relate to the dependence of temperature estimates on the amount of Fe3+ in the pyroxene and possibly on non-ideal substitution in omphacite-acmite solid solution