Introduction
Multi-scale structural and petrographic analysis allows reconstructions, in the crustal rocks, of the tectonic evolution of orogenic belts, giving insights on the development of deformation events and the evolution of thermal history through time (e.g. Spalla et al., 2000; Spalla et al., 2011; Spalla et al., 2005). The reconstruction of structural and metamorphic evolutions in polycyclic metamorphic terrains is performed since long time through a multidisciplinary approach based on detailed correlation of superposed fabric elements, microstructural analysis and recognition of fabric gradients (Hobbs et al., 1976; Johnson & Vernon, 1995; Park, 1969; Passchier et al., 1990; Salvi et al., 2010; Spalla, 1993; Spalla et al., 2000; Turner & Weiss, 1963; Williams, 1985; Zucali, 2002).
Rocks belonging to a single tectono-metamorphic unit generally record heterogeneously the succession of tectonic imprints and the related metamorphic re-equilibrations: in many cases this has been interpreted as due to the catalyzing effect of deformation on metamorphic reaction progress (see Hobbs et al., 2010 and refs therein). At the end of the deformation history, the result is a patchy distribution of different fabrics and their supporting mineral assemblages; estimate of differently re-equilibrated volumes has given fundamental insights into the accomplishment of structural and/or metamorphic re-equilibrations at different structural levels, along active plate margins (e.g.: Salvi et al., 2010; Spalla et al., 2005). Such kind of evaluation could highlight, for example, the influence of different deformation mechanisms, phase transitions and, consequently, density and viscosity variations affecting the mechanical behaviour of the lithosphere in subduction zones: all these parameters play a fundamental role in quantitative geodynamic modelling. The Sesia-Lanzo Zone, which was deformed at high pressure under a very-low thermal regime in the Alpine time, represents an interesting subject to infer the deformation-metamorphism interactions in the continental crust during subduction (Roda et al., 2012; Zucali & Spalla, 2011). In this paper, we infer the deformation-metamorphism relationships in the Mt. Mucrone area, where Permian intrusives are exposed with their country rocks, both belonging to the Eclogitic Micaschists Complex, to evaluate the degree of fabric evolution during successive deformation stages together with the progress of syn-kinematic metamorphic reactions. Maps were performed to detail the heterogeneous distribution of textural and metamorphic transformations, taking into account the degree of mechanical and mineral–chemical transformation of different rocks at micro- to km-scale. In these maps, low- and high-strain domains are represented for each deformation phase, and supported by information on the volumes occupied by newly formed, or relict, mineral assemblages. These maps (maps of deformation partitioning and metamorphic imprints) may facilitate the perception of the influence that the heterogeneous distribution of deformation and metamorphism can play on the lithostratigraphy variations during a polyphase tectonic evolution.