Unravelling the polycyclic evolution of basement units through multiscale and multidisciplinary analysis of pre-Alpine metamorphic remnants: Insights from the Tambo and Suretta nappes (Central Alps)

Structural and metamorphic relics in crystalline basement units allow reconstruction of the tectonometamorphic evolution of complex polycyclic basements. In the Central Alps, the Tambo and Suretta nappes (Briançonnais terrane, Middle Penninic Domain) experienced intense Alpine overprint, which extensively obliterated the record of the pre-Alpine evolution of this portion of the paleo-European crust. Recent detailed fieldwork carried out in the framework of the Italian Geological Mapping Project (sheets 021-022 “Passo dello Spluga and Madesimo” and 037-038 “Bodengo and Chiavenna”; Tartarotti et al., 2026) has allowed a renewed and comprehensive field characterization of the central-southern portions of these nappes. Despite pervasive Alpine reworking, several sectors preserve domains in which pre-Alpine structural fabrics and/or metamorphic mineral assemblages are still recognizable. These include: 1) highgrade metapelites with garnet-kyanite-staurolite assemblages and locally preserved migmatite textures; 2) garnet-bearing amphibolites, with evidence of a pre-alpine, eclogite-facies event (Nussbaum et al., 1998); 3) meta-aplite and meta-pegmatite bodies, possibly associated to the widespread evidence of Carboniferous-Permian magmatism at the nappe scale and coherent with regional‑scale evidence from the Central Alps (e.g., Schaltegger, 1997). Such domains provide a valuable record of pre-Alpine tectono-metamorphic events and magmatic processes, potentially linked to different stages of the Variscan orogeny. A multi-scale and multidisciplinary approach combining detailed fieldwork, petrographic and microstructural analyses, geochemistry and geochronology is applied to characterize these remnants and to disentangle Alpine from Pre-Alpine signatures. Preliminary results indicate that, even within strongly reworked basement sectors, older metamorphic and magmatic records are locally preserved and could be used to reconstruct meaningful P-T-t paths for the Variscan evolution of the Tambo and Suretta basement units. The geodynamic implications of the provided constraints are evaluated and discussed within the regional framework of the Central Alps.