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

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

Basement lithostratigraphy of the Adula nappe: implications for Palaeozoic evolution and Alpine kinematics

The Adula nappe belongs to the Lower Penni- nic domain of the Central Swiss Alps. It consists mostly of pre-Triassic basement lithologies occurring as strongly folded and sheared gneisses of various types with mafic boudins. We propose a new lithostratigraphy for the northern Adula nappe basement that is supported by detailed field investigations, U-Pb zircon geochronology, and whole-rock geochemistry. The following units have been identified: Cambrian clastic metasediments with abundant carbonate lenses and minor bimodal magmatism (Salahorn Formation); Ordovician metapelites associated with amphibolite boudins with abundant eclogite relicts representing oceanic metabasalts (Trescolmen Formation); Ordovician peraluminous metagranites of calc-alkaline affinity ascribed to subduction-related magmatism (Ga- renstock Augengneiss); Ordovician metamorphic volcanic- sedimentary deposits (Heinisch Stafel Formation); Early Permian post-collisional granites recording only Alpine orogenic events (Zervreila orthogneiss). All basement lithologies except the Permian granites record a Vari- scan ? Alpine polyorogenic metamorphic history. They document a complex Paleozoic geotectonic evolution consistent with the broader picture given by the pre- Mesozoic basement framework in the Alps. The internal consistency of the Adula basement lithologies and the stratigraphic coherence of the overlying Triassic sediments suggest that most tectonic contacts within the Adula nappe are pre-Alpine in age. Consequently, me ́lange models for the Tertiary emplacement of the Adula nappe are not consistent and must be rejected. The present-day structural complexity of the Adula nappe is the result of the intense Alpine ductile deformation of a pre-structured entity