Résumé
The Lanzo massif is a 150 km (super 2) body of relatively fresh plagioclase peridotite located in the Piedmont Alps. Although it has been subducted to eclogite facies before fast exhumation during the Alpine orogenesis, eclogitic metamorphic equilibration is restricted to the borders of the massif and most of the massif records lithospheric thinning associated with formation of Piemont-Ligure ocean. The massif is composed of three units (the northern, the central and the southern bodies) separated by two mylonite shear zones, several hundreds of meters wide. Here we present field observations and microstructural data for the shear zone that separates the southern and the central units of the Lanzo massif, which were interpreted as a high-temperature asthenospheric mantle diapir, which rose from the garnet stability field to shallow depths, being submitted to a large degree of melt extraction, and subcontinental mantle lithosphere fragment modified by reactive melt percolation, respectively. This shear zone is marked by a sharp reorientation of the N-trending, subvertical high-temperature peridotite foliation of plagioclase peridotite in the southern body towards a SE direction with a moderate dip to the NE. This reorientation is accompanied by development of mylonitic textures in the peridotites, marked by a decrease in grain size and development of orthopyroxene stretching lineations. The latter stages of deformation are characterized by development of antigorite in the foliation. Analysis of the microstructures and of the crystallographic preferred orientation of the constituent minerals in a detailed section across the shear zone, from the high-T peridotites to the low-T serpentine-bearing mylonites allows determining the active deformation mechanisms and the thermo-mechanical evolution of the shear zone. The continuity of the structures and the decreasing metamorphic grade of the shear zone rocks indicate ductile deformation and strain localization under retrograde conditions and in presence of fluids, with the final stages of deformation within the antigorite stability field.