Prograde LWS-KY Transition During Subduction Of The Alpine Continental Crust Of The Sesia-Lanzo Zone: The Ivozio Complex

Michele Zucali, Maria Iole Spalla, Guido Gosso, Sonia Racchetti, and Fabio Zulbati

The first occurrence of the lawsonite-kyanite transition is described in the Ivozio complex eclogites of the central Sesia-Lanzo Zone (Western Austroalpine Domain, Italian Alps). The transition from prograde lawsonite to kyanite-bearing eclogites was recorded during a clockwise subduction-exhumation P-T-t-d path. The P-T-t-d evolution of the Ivozio complex is characterized by an Alpine multistage structural and metamorphic re-equilibration: D1 deformation, represented by a penetrative foliation, is the relic of a prograde low-T history, which took place under the epidote-blueschists facies conditions (T = 350 - 500°C and P ≤ 1.2 GPa). Post-D1a re-equilibration stage is marked by the growth of Omp and Lws in eclogites: during this stage T was 520 ± 30 °C at P = 1.4 to 2.2 GPa (eclogite facies conditions). During post-D1b stage the stable association of Omp + Ky + Ep (in lawsonite-eclogites) developed at T 610 ± 20°C and P ≥ 2.0 GPa. This assemblage coincided with Tmax-PTmax conditions. During D2 a penetrative foliation marked by Omp + phengitic/paragonitic white mica + Amp + Ep in eclogites and amphibole-schists was imprinted. In ultramafics (including serpentinites) S2 is widespread as a planar fabric, marked by serpentine + chlorite + amphibole ± ilmenite ± clinopyroxene ± carbonate ± talc . This stage is characterized by temperatures spanning from 500 to 600°C at P ≤ 2.0 GPa. D3 deformation, developed under greenschists facies conditions, is associated with a crenulation cleavage or discrete shear bands.

The early stages of this subduction-exhumation cycle mainly occurred under a low-T regime (i.e. lawsonite-bearing conditions); subsequently the temperature increased (kyanite-eclogite conditions) before the exhumation of the Ivozio complex, marked by the transition to paragonite-eclogite conditions under a steady state thermal regime.