Interplay between deformation and metamorphism during eclogitization of amphibolites in the Sesia-Lanzo Zone of the Western Alps

Interactions of fabric evolution and chemical parameters driving reaction progress during the amphibolite-to-eclogite transition were investigated in eclogitized amphibolites of the Western Alps. In the Sesia\u2013Lanzo Zone (SLZ), mafic rocks ranging from eclogitized hornblendites to true eclogites occur as layers and boudins within micaschists and are characterized by different modal amounts of amphibole, omphacite, zoisite, garnet, and phengite, constituting the Alpine HP assemblage. Across narrow zones, this array of lithologies displays gradients in planar fabrics characterized by coronitic, S-tectonitic, and mylonitic textures with different extents of eclogitization. Eclogitic parageneses are controlled not only by the bulk rock composition of the protoliths but also by the degree of fabric evolution. This is the case for omphacite occurrence, which is constrained by plagioclase composition and modal amount (NK parameter) in the protoliths, whereas the increase in modal omphacite and the concomitant decrease in modal amphibole in rocks with high NK are controlled by the strain rate. Protoliths with a low NK content develop the amphibole\u2009+\u2009garnet\u2009+\u2009epidote assemblage in eclogitized hornblendites. In protoliths with higher NK values, the co-existence of amphibole with garnet, omphacite, and epidote occurs only for low-to-medium strain textures (e.g. coronites and S-tectonites), whereas an amphibole-free assemblage defines a mylonitic foliation; in this case, amphibole relics are present exclusively as armoured inclusions in garnets and omphacite porphyroclasts. Thus, amphibole persists in the eclogitic assemblage at pressures exceeding the experimentally determined amphibole stability field. Values of confining pressure under which Sesia\u2013Lanzo mafic rocks re-equilibrated during Alpine subduction were estimated, through equilibrium assemblage modelling, at 2.2\u20132.7 GPa. The amphibole-bearing eclogites of the SLZ show that large volumes of amphibole-bearing rocks can be exhumed from a depth exceeding 75 km without dehydration reactions running to completion. Petrological estimates in orogenic zones may help constrain geological and tectonic conclusions when selection of laboratory samples is assisted even by simple microstructural evaluation of planar fabric development