QUANTITATIVE ANALYSIS OF FABRIC EVOLUTION AND METAMORPHIC TRANSFORMATION IN CRYSTALLINE BASEMENTS
Creators: name: CORTI, LUCA
Publisher: name: Universit� degli Studi di Milano
Year: 2020

description: The aim of this thesis is to study the relationships between the degree of fabric evolution and the degree of metamorphic reactions progress in crystalline basements, integrating multiscale structural analysis with a multidisciplinary approach. The knowledge of the dynamics of the earth's crust and mantle is mainly based on the study of the elements of the fabric, the related metamorphic assemblages and of their relationships within rock volumes forged during subduction and collision processes. Deformation and metamorphic gradients are responsible for the development of coronitic domains, in which metamorphic reactions occur without the development of a new oriented fabric. On the contrary, where deformation is accumulated, tectonitic and mylonitic domains develop and metamorphic assemblages mark planar/linear tectonites and mylonites. The heterogeneities, which accompany the evolution of the fabric, also influence the progress of the metamorphic reactions that is useful to reconstruct the structural and metamorphic evolution of the rock volumes in crystalline basements. The resulting distribution of the superposed fabric elements and metamorphic assemblages allows the evaluation of the percentage of volumes that are mechanically and chemically involving during the subsequent tectonic-metamorphic stages. The correlation between the degree of fabric evolution and the degree of metamorphic reaction progress can therefore be used to quantify the size of the rock volumes affected by these structural and metamorphic heterogeneities within crustal portions involved in convergent dynamics. In this context, multiscale structural analysis (from the atomic- to the km-scale) combined with a multidisciplinary approach (fieldwork, GIS database, use of software based on Python scripts, optical and electronic microscopy, thermobarometric estimates, thermodynamic modelling, geochronology, SPO analysis, high resolution optical scanning, quantitative analysis of X-ray maps, micro-CT, 3D geological modelling and X-ray single crystals diffraction) is used to quantify the structural and chemical heterogeneities developed during the deformation and metamorphism partitioning. This quantitative analysis turns out to be an excellent method for: (i) defining tectonic-metamorphic evolutions; (ii) provide rheological constraints for numerical modelling on the dynamics of subduction processes; (iii) defining the physical and chemical parameters that control the evolution of the fabrics, the metamorphic reactions, and their relationships within the subduction-collision systems. This developed methodology has been applied in three different rock volumes of the Western Alps which are characterized by different thermal and structural evolutions: HP-LT (Sesia-Lanzo Zone) and IP-HT (Argentera massif). The analysis of the parameters that control the relationships between the degree of fabric evolution and the metamorphic reaction progress was carried out once the PT conditions are estimated for the subsequent tectonic-metamorphic stages. The tectonic-metamorphic evolution has been defined for two continental crust volume: i) the metagranitoids and metapelites of the Lago della Vecchia area surfacing within the Sesia-Lanzo Zone which are characterized by a dominant fabric developed under blueschist facies conditions; ii) migmatites and granulites outcropping within the Argentera Massif, which are characterized by a dominant metamorphic imprint developed under amphibolitic facies conditions. The modelling of volumes showing homogeneous degree of fabric and metamorphism allowed to define the kilometric pervasiveness of a single tectonic-metamorphic stage in 2D and 3D. The quantitative analysis of the distribution of physical and chemical heterogeneities at granular scale was carried out to explore the structural parameters, chemical variations between allanite crystals and Lago della Vecchia metagranitoids rock matrix from. The modal percentage, the mineral chemical variation, the grainsize distribution, the grains orientations, and the shapes of grain boundaries of mineralogical phases marking the superimposed fabrics were quantified through a multidisciplinary approach that combines detailed microstructural analysis, micro-computed tomography and chemical X-ray maps. This methodology allows a quantitatively analysis of chemical and deformation gradients in relation with the heterogeneous development of metamorphic reactions and deformation.