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 (Amp0 + Cpx0). The prograde associations begin with S1a marked by AmpI + ZoI which pre-date the growth of GrtI (S1b); successive increase in pressure stabilizes a second generation of Amp + Grt (S1c AmpII + ZoI + GrtII). The growth of prograde lawsonite and omphacite occur during S1d (OmpI + Lws + GrtII + AmpII) within lawsonite-bearing eclogites, while S1e is associated with the break-down of lawsonite, producing the association OmpI + Ky + ZoII + GrtII + AmpII (lws-bearing eclogites); S1d-e stages are associated with AmpII + ZoI + GrtII + OmpI in eclogites. The second generation of penetrative foliation (S2), describing the retrograde evolution, is divided into S2a (AmpII + GrtII + Pg + ZoII) and S2b (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.