Tectono-Metamorphic Evolution of Serpentinites from Lanzo Valleys Subduction Complex (Piemonte—Sesia-Lanzo Zone Boundary, Western Italian Alps)

Abstract

In the upper Tesso Valley the folded contact between Piemonte Zone ophiolites and Sesia-Lanzo Zone continental crust is exposed. Here serpentinites, metabasites, calcschists and fine-grained gneisses are deformed by four ductile superposed groups of structures, associated with different mineral assemblages. Different serpentinite lithologies have been recognized and studied in detail. Mylonitic D2 structures are pervasive and mineral assemblages point to re-equilibration at T of 450 ± 50 ∘C and P of 0.8 ± 0.3 GPa, under blueschist/epidote amphibolite-facies conditions. Pre-D2 structures and mineral assemblages are relics within S2 and indicate a re-equilibration under eclogite-facies conditions, at T of 570 ± 50 ∘C and P > 1.8 GPa. Post-D2 occurs under greenschist-facies conditions. Numerical modeling of a subduction zone allows exploration of the geodynamic context in which such PT path could have developed, and to make hypotheses about the possible timing of such a scenario, in agreement with the timing generally proposed for the Alpine subduction and collision. Model predictions indicate that pre-D2 mineral assemblages may have developed during Paleocene at 60–90 km depth and 115–145 km from the trench, or, alternatively, during lower Eocene at ca. 70–90 km depth, and 135–160 km from the trench.

Metamorphic Facies and Deformation Fabrics Diagnostic of Subduction: Insights From 2D Numerical Models

“…In the Alps, rocks deeply involved in the subduction complex and recording P‐T conditions compatible with blueschist and eclogite metamorphic facies can be observed in the axial zone of the chain, and generally show heterogeneous deformation with dominant tectonitic or mylonitic fabrics (e.g., Gasco et al., 2011; Lardeaux, 2014a; Malatesta et al., 2012; Oberhänsli & Goffé, 2004; Roda et al., 2021; Zucali et al., 2020). These rocks are often associated with serpentinites (e.g., Assanelli et al., 2020; Gasco et al., 2011; Lardeaux, 2014a; Malatesta et al., 2012; Manzotti et al., 2014; Roda et al., 2020), which have a crucial role in exhumation mechanisms (Gerya et al., 2002; Guillot et al., 2009; Luoni et al., 2020; Meda et al., 2010; Roda et al., 2012; Schwartz et al., 2001; Tamblyn et al., 2020; Warren et al., 2008; Yamato et al., 2007). Differently, Alpine HP metamorphism has not been recorded by rocks from the external domains of the Alps, which generally show a lower amount of finite deformation under P‐T conditions compatible with zeolite, prehnite‐pumpellyite, greenschist and epidote‐amphibolite metamorphic facies (e.g., Handy & Oberhänsli, 2004; Lardeaux, 2014a).…”