Prograde lawsonite during the flow of continental crust in the Alpine subduction: Strain vs. metamorphism partitioning, a field-analysis approach to infer tectonometamorphic evolutions (Sesia-Lanzo Zone, Western Italian Alps)

Detailed mapping of superposed fabrics and their mineral support allows for reconstruction of the tectonometamorphic evolution of the Ivozio Complex, within the inner portion of the Sesia-Lanzo Zone (Western Italian Alps). The resulting evolution is characterized by a multi-stage structural and metamorphic re-equilibration during Alpine subduction, starting from the pre-Alpine igneous association (Amp 0 + Cpx 0). The prograde associations begin with S 1a marked by Amp I + Zo I which pre-date the growth of Grt I (S 1b); successive increase in pressure stabilizes a second generation of Amp + Grt (S 1c Amp II + ZoI + Grt II). The growth of prograde lawsonite and omphacite occur during S 1d (Omp I + Lws + Grt II + Amp II) within lawsonite-bearing eclogites, while S 1e is associated with the break-down of lawsonite, producing the association Omp I + Ky + Zo II + Grt II + Amp II ( lws-bearing eclogites); S 1d-e stages are associated with Amp II + ZoI + Grt II + Omp I in eclogites. The second generation of penetrative foliation (S 2), describing the retrograde evolution, is divided into S 2a (Amp II + GrtII + Pg + Zo II) and S 2b (Chl + AmpIII + Pg + Ab). The comparison between the reconstructed evolution of the Ivozio Complex and P–T paths inferred in the Southern Sesia-Lanzo Zone suggests a non-uniqueness of the Sesia-Lanzo Zone continental crust, during the Alpine subduction. ► Detailed mapping of superposed fabrics and their mineral support are used to reconstruct the tectonometamorphic evolution of the Ivozio Complex, within the inner portion of the Sesia-Lanzo Zone (Western Italian Alps). ► The clockwise P-T-t-d evolution of the Ivozio lawsonite-bearing eclogites (prograde lawsonite) is compared with the P-T evolutions of other two lawsonite-bearing localities from the southern Sesia-Lanzo Zone, where the path is counter-clockwise (retrograde or peak lawsonite). ► The reconstructed evolution suggests a non-uniqueness of the Sesia-Lanzo Zone continental crust during the burial-exhumation-related path of the Alpine subduction.