Alpine olivine- and titanian clinohumite-bearing assemblages in the Erro-Tobbio peridotite (Voltri Massif, NW Italy)

The microstructures in the Erro-Tobbio peridotite indicate several stages of recrystallization of olivine + titanian clinohumite-bearing assemblages. The development of these assemblages is closely associated with serpentinite mylonites, in which they occur in shear bands and foliations and are inferred to have grown synkinematically, in veins, and as post-kinematic radial aggregates. In the peridotite wall-rock adjacent to these mylonites, the same assemblages have recrystallized statically at the expense of original olivine and pyroxenes, mesh-textured chrysolite and antigorite veins. In addition, the olivine-bearing assemblage occurs in widespread vein systems. The brittle deformation of the peridotite resulting in the development of these vein systems is closely related to ductile deformation of metagabbroic dykes in the peridotite. Although early metasomatism resulted in extensive rodingitization of the gabbros, some dykes show an eclogitic assemblage of Na-clinopyroxene + garnet + chloritoid + chlorite ± talc. These observations, the microstructures and the mineral chemistry all suggest that the assemblages in the ultramafic rocks and metagabbros developed during a prograde evolution towards high pressures (>13–16 kbar, 450–550° C), and during subsequent decompression. This metamorphic evolution is considered to be related to Late Cretaceous intraoceanic subduction in the Alps-Apennine system and closure of the Piedmont-Ligurian basin.