Multi-scale strain analysis using allanite single crystals: the case of the Lago della Vecchia metaintrusive (Sesia-Lanzo zone, Western Alps)

The Lago della Vecchia metaintrusive is part of the Eclogitic Micaschists Complex (EMC) of the Sesia-Lanzo Zone within the Austroalpine of the Western Alps (Compagnoni et al., 1977). It spectacularly preserves large volumes of igneous textures and minerals at metre to ten-metre scales only partly affected by alpine eclogite-blueschist metamorphism. The deformation partitioning of the Lago della Vecchia metaintrusives is investigated by multiscale (km- to atomic-scale) and multidisciplinary approach (optical microscopy, wavelength dispersive Xray microanalysis, and single-crystal X-ray diffraction). Mesoscale structural analysis aims recognizing strain gradients, classified in terms of coronitic (i.e. low-strain), tectonitic (i.e. intermediate-strain), and mylonitic (i.e. high-strain) fabric domains (Gosso et al., 2015). Therefore, the evaluation of degree of fabric evolution of each sample is categorised in three fabric domains intervals (Fig.1): 0-20% coronitic; 21-60% tectonitic; 61-100% mylonitic (Corti et al., 2017). Microstructural analysis reveals that in all strain domains allanite occurs in mm-sized single crystals characterized by high relief and wrapped by high-pressure metamorphic assemblage. In low-strain domains the crystal are well-preserved with different coronae as a function of which microdomains are in contact with. In intermediate-strain domains, they are wrapped by foliations showing only allanite cores preserved and rim replacement in clinozoisite. In mylonitic domains, most of allanite crystals are boudinage into pervasive foliation (Fig.1). Moreover, the mineral chemistry analysis display that allanite crystals maintain their chemical compositions in all fabric domains (Fig.2), showing homogeneous chemical variation from core to rim (Ca and Al increase respect the decrease of REE content). The single-crystal X-ray diffraction investigation of the allanite crystals is in progress and it is aimed to show: 1) the presence of potential textured patterns on some specific crystallographic planes and 2) any potential difference between the structural parameters refined in this study with respect to those of previously published unstrained gem-quality crystals. Previous similar experience on plastically deformed kyanite crystals showed how, for example, the anisotropic displacement ellipsoids of the atomic sites were significantly larger than those previously described for unstrained crystals, and systematically oriented (Gatta et al, 2009). The larger ellipsoids would reflect the displacement of the centre of gravity of the electron distribution, rather than an anomalous atomic thermal motion.