INTO THE STRUCTURE OF THE PERMIAN DEEP CONTINENTAL CRUST: A MULTISCALE APPROACH TO RECONSTRUCT MIGMATIZATION IN THE VALPELLINE SERIES (DENT BLANCHE TECTONIC SYSTEM, WESTERN ITALIAN ALPS)

Granulite and migmatite are widespread across different tectonic settings, such as those associated with collisional crustal thickening during orogenic events and post-collisional or back-arc lithospheric thinning. Studying these rocks provides an exceptional opportunity to observe the deep portions of the continental lithosphere and infer insights into the processes that control its evolution. In extensional settings, orogenic root delamination and lithospheric thinning cause asthenospheric upwelling, increasing heat flow and enhancing partial melting of the lithospheric mantle and deep crust. During the Permian, the Variscan lithosphere was affected by extension due to the Pangea break up followed by Tethys opening. This Thesis investigates the evolution of the Valpelline Series in the Dent-Blanche Tectonic System (Western Italian Alps) which represents an exceptionally preserved exposure of Permian deep continental crust, despite being involved in the subsequent Alpine-age reworking. It shows migmatitic metapelite with different mineral assemblages (i.e., garnet-, cordierite- and orthopyroxene-bearing), amphibolite and marble, intruded by aplite and pegmatite. Melt production in metapelite and locally in amphibolite occurred during the development of the regional foliation. Quantitative microstructural and mineral-chemical analyses allow identifying three tectono-metamorphic stages. The first (D1) is associated with an early foliation (S1) preserved within mafic boudins enclosed in migmatitic gneiss. The second (D2) is related to the dominant foliation in migmatitic gneiss (S2), coeval with the regional scale anatexis and growth of garnet, orthopyroxene and cordierite. The third (D3) is related to the late folding of S2 and the development of a sillimanite-rich axial plane foliation (S3) which wraps around garnet and cordierite. Meso- and microstructural analysis helped to identify melt migration pathways in the deep continental crust. These pathways, corresponding to high-strain zones such as the S3 sillimanite-rich foliation, exhibit distinct mineralogical and structural features indicative of melt-rock interaction processes during deformation. P–T conditions of migmatization in the Valpelline Series, recorded in different lithologies, are contrasting. Black amphibolite and orthopyroxene-migmatitic gneiss show peak conditions comprised between 800–900 °C and 0.8–1.0 GPa, while cordierite-migmatitic gneiss registered peak P–T conditions at 750–800 °C and ~0.5 GPa. These contrasting P–T conditions are associated with different ages. Multi-method geochronology revealed that migmatization peaked during the D2 stage at ~290 Ma and continued until ~285–280 Ma over a period of at least 5 Ma, as suggested by garnet diffusion modelling. The reconstructed tectono-metamorphic history reflects an evolution that originated from Variscan post-orogenic collapse and lithospheric extension during the late Carboniferous-early Permian.