Multiscalar structural study of the ultramafic rocks of the Antrona Ophiolite (Pennine Alps)

In the Alpine nappe stack, the Antrona Ophiolite (Italian side of the Western Central Alps) is sandwiched between the overlying continental Monte Rosa Nappe (upper Penninic domain) and the underlying Camughera-Moncucco continental Unit (middle Penninic). The ophiolite sequence includes ultramafic rocks, metagabbros and mafic rocks covered by calcschists. Ultramafites constitute a huge body of serpentised peridotites including interbedded layers of gabbros, clinopyroxene-rich and amphibole-rich rocks, and chloriteschist. In spite of the Alpine tectonic and metamorphic reworking, the ultramafic portion of the Antrona Ophiolite still preserves relict textures and minerals that can be referred to the pre-Alpine or early Alpine evolution. A detailed microstructural analysis performed at polarised microscope and SEM on less serpentinised, olivine-rich samples is here presented. It is integrated with a quantitative textural analysis of Lattice Preferred Orientation (LPO) by neutron diffraction acquired on selected samples of olivine-rich samples. The results allow to infer a mantle origin for the ultramafic rocks, suggesting T conditions > 800°C for the activation of slip systems in olivine.