STRUCTURAL EVOLUTION OF PERIADRIATIC PLUTONS AND ITS IMPLICATIONS ON SOLID-STATE DEFORMATION OF GRANITOID ROCKS

The study of the structural evolution of cooling granitoid plutons allow us to decipher the deformation-controlling processes that usually act at different crustal levels. Moreover, they provide a snapshot of the regional stresses and kinematics in time. Field surveying and microstructural investigations has been carried out on selected Periadriatic plutons. The granodioritic Rieserferner Pluton (Eastern Alps) develops in sequence: high-temperature (quartz) mylonites exploiting earlier joints; brittleductile anastomosing fault planes associated with fracture folding; purely brittle cohesive cataclasite and pseudotachylites, followed by fault gouges and mirror-like surfaces. First investigations demonstrate that deformation in pure quartz mylonites could be controlled by crystallographic orientation of former vein crystals, evolving from homogeneous deformation, to localized recrystallization bands, to a fine grained ultramylonite. Qualitative palaeostress inversion analysis suggests a clockwise rotation of the shortening direction during cooling. Coexisting strikeand dip-slip kinematics are consistent with the regional Oligocene-Miocene orogen-perpendicular compression and orogen-parallel extension. Introduction Continental lithosphere is in many cases made of polymetamorphic terranes with a tangled deformation history. Therefore, is quite complex to define the deformation-controlling mechanisms at both brittle and ductile conditions. Granitoid rocks, which are usually taken as the most common and representative lithotype of the continental lithosphere, are the perfect natural laboratory for studying deformation processes. They are simple homogeneous bodies characterized by discrete mechanical and compositional anisotropies, and their composition are very similar to the average whole crust. In addition, granitoid plutons usually record a single event of deformation during their cooling. Therefore, structures are not obliterated by other events, and especially in the ductile field, even pristine structures are preserved. During their down-temeperature path they encounter all the conditions that usually characterize different crustal levels. They thus become a compendium of deformation structures, and thus the study of the structural evolution of these bodies is a huge tool for the understanding of nucleation and development of strain (Pennacchioni, 2005; Pennacchioni et alii, 2010; Pennacchioni & Zucchi, 2013). In this PhD project we are focusing on the deformation of Periadriatic plutons (alpine oligocenic magmatism; Rosenberg, 2004) with a twofold aim: (i) Definition of the role of controlling parameters (temperature and pressure, chemical environment and lithological composition, geometry of structures) on ductile strain localization and on the brittle-ductile transition; assessment of the P-T-X condition at which deformation occurs; (ii) Reconstruction of the structural evolution during cooling and exhumation of plutonic bodies; palaeostress inversion, structural and kinematic analysis will allow us to retrace the geodynamic evolution of the region. During my first year of PhD I was focused on the study of the structural evolution of the Rieserferner pluton (Rein in Taufers, Bozen). I had also carried out a first in-depth investigations on quartz mylonitic shear zones from the same area. For a first attempt of comparison, I have done some field work on the Bergell plutonic complex (Val dei Ratti, Sondrio) and some petrographical investigations of mylonitic quartz veins outcropping in the area. Periadriatic Plutons, of which Adamello, Bergell and Rieserferner are the most important, are the result of the Tertiary calc-alkaline magmatism of the Alps (42-28 Ma; 30±3 Ma: Rieserferner; 33-30 Ma: Bergell; Rosenberg, 2004). They are emplaced mainly close to the Periadriatic Lineament (PL), at different depths (between 35 and 5 km; Rosenberg, 2004). The Rieserferner plutonic complex (Eastern Alps) is composed by three mutually cross-cutting units: coarse grained Grt-bearing tonalite; mediumto fine-grained tonalite-granodiorite; fine-grained leuco-granodiorite (Bellieni et al., 1981; Cesare et al. 2004). It is