Quantitative combined multiscale structural and minero-chemical analysis to unravel the tectono-metamorphic evolution of cordierite-migmatite gneiss from the Valpelline Unit (Dent-Blanche Nappe, Western Italian Alps, Valle d'Aosta)

Among the Valpelline Unit migmatites, the cordierite-bearing ones represent the most fascinating type, but little is known about their P-T evolution. Since the complete understanding of these rocks results from the interplay of petrological and multiscale structural investigations, a multidisciplinary approach that combines quantitative microstructural and minero-chemical data, discriminating different generations of superimposed foliations and stages is provided. Two main deformational stages have been defined at the meso-and microscale: the first (D-1) is a solid-state deformation developing a foliation (S-1) preserved as isoclinal folds; the second (D-2) is related to the development of the main foliation (S-2), associated with cordierite and garnet growth, and melt production. The regional S-2 wraps cordierite, garnet and melt aggregates. Temperatures for the cordierite-growing stage, obtained by combining biotite calibrated maps and Ti-in-biotite thermometer, range between similar to 700 and 780 degrees C.