Introduction
The Antrona ophiolite consists of serpentinized ultramafites, metagabbros and mafic rocks (metavolcanics) covered by calcschists (Colombi and Pfeifer, 1986; Pfeifer et al., 1989; Carrupt and Schulp, 1998; Turco and Tartarotti, 2006). The internal setting and stratigraphy of the Antrona ophiolite are still not well constrained, probably due to the small extent and pervasive metamorphic retrogression. Consequently, these ophiolites have been often neglected and poorly considered for the paleogeographic reconstruction of the Western Tethys. The ultramafites of the Antrona ophiolite have recently been investigated (Tartarotti et al., 2011). The microstructural and microchemical features of these rocks have led these Authors to interpret this ophiolite portion as deriving from original mantle harzburgites and/or lherzolites associated with dunite pods and (possibly) pyroxenite layers. These rocks partly retain either the original chemistry or the effect of oceanic hydrothermal alteration (Tartarotti et al., 2011). The Antrona ophiolite thus represents remnants of the oceanic lithosphere of the Mesozoic western Tethys, now inserted as tectonic slices in the Penninic nappe pile of the western Central Alps. The Alpine tectono-metamorphic evolution of the ophiolite is characterized by a subduction-related blueschist prograde path, followed by high-pressure (eclogitic) metamorphic peak, and subsequent retrograde exhumation dominated by epidote-amphibolite to amphibolite facies conditions (Colombi and Pfeifer, 1986; Pfeifer et al., 1989; Carrupt and Schlup, 1998; Turco and Tartarotti, 2006). In this paper, we have addressed our investigations to the microstructural features of mantle olivine and spinel still preserved in less serpentinized peridotite samples. The microstructural study is complemented by a quantitative texture analysis by neutron diffraction on selected olivine-rich samples, to investigate the lattice preferred orientations (LPO) of relict fresh olivine. Quantitative study of LPO in natural rocks is of great interest if one wants to infer the conditions of deformation of crystalline aggregates or unravel multiple stages of recrystallization producing distorted lattices (Gatta et al. 2009). The resulting olivine LPO will be compared with experimental and natural observations on lattice orientations of olivine (Karato, 2008 and reference therein). We use the quantitative LPO study of relict olivine to constraint the deformation mechanisms active during the development of fabrics predating the S2 foliation and relate them to the pre-Alpine evolution, from the mantle to oceanic stages.