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)

Abstract

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 mesoand 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 ~700 and 780 °C.

Structural, lithostratigraphic and thermal features of a Permian lower crust from the Western Italian Alps (Valpelline Series, Valle d’Aosta)

“…Gardien et al (1994) pointed out the presence of cordierite growing after regional foliation development. Conversely, our observations highlight that cordierite develops during the D 2 stage and is wrapped by the S 3 sillimanite-rich foliation (Caso, 2023). Due to the local isoclinal geometry of the D 3 folds, the S 3 (black lines in Fig.…”

“…[4] Ti-in-Amp of Liao et al (2021). N are samples from Nicot, (1977); *is the value from Caso (2023).…”

“…14g and h) that forms the pervasive foliation in sillimanite-gneiss could be produced in two ways: (i) destabilisation of white mica according to the reaction Ms + Qz = Sil + Kfs + melt or (ii) during retrogression in the presence of melt by back-crossing the reaction Bt + Sil + Qz + Pl = Grt + Crd + Kfs + melt (Waters, 1988; Waters, 2001; Kreigsman & Alvarez-Valero, 2010; White et al 2014). The S 3 sillimanite-rich foliation wraps peritectic mineral as garnet (Manzotti & Zucali, 2013) and cordierite (Caso, 2023), likely represents the back-reaction with melt (Spear, 1994; Kriegsman & Alvarez-Valero, 2010) involving garnet and cordierite. The intergrowths between quartz and biotite marking S 3 and wrapping cordierite or orthopyroxene (Fig.…”

“…In the Western Alps, there are pre-alpine lower crustal slices such as the II Zona Diorito-Kinzigitica (IIDK; Carraro et al 1970), the Ivrea-Verbano Zone (IVZ; Quick et al 2003;Redler et al 2012Redler et al , 2013Ewing et al 2013Ewing et al , 2015 and the Valpelline Series (VP; Gardien et al 1994;Manzotti & Zucali, 2013;Caso, 2023), which have been affected by widespread hightemperature (hereafter HT) metamorphism and are now fragmented and found in different portions of the Western Alpine belt. The HT evolution is related to the post-Variscan collisional collapse and/or Permian lithospheric extension (Lardeaux & Spalla, 1991;Marotta & Spalla, 2007;Schuster & Stüwe, 2008;Roda et al 2018;Roda et al 2023).…”

Comparison between 2D and 3D microstructures and implications for metamorphic constraints using a chloritoid–garnet-bearing mica schist

“…), since the GSD tool alone is not able to discriminate grains of different mineral phases without the chemical input (i.e. the chemical X-ray maps; see Caso, 2023, andCaso et al, 2024, for a practical example).…”