MULTI-SCALE APPROACH TO THE PERMIAN TECTONOMETAMORPHIC EVOLUTION OF THE DENT-BLANCHE UNIT (AUSTROALPINE DOMAIN, WESTERN ITALIAN ALPS)

The Dent-Blanche Unit (Austroalpine domain, Western Italian Alps) is constituted by Valpelline and Arolla units, respectively composed by high-grade pre-Alpine basement rocks and Permian intrusive with high-grade pre-Permian xenoliths. The main structural imprint of the high-grade rocks is characterized by amphibolite to granulite facies conditions, commonly associated with large melt-bearing parageneses. During this melt-producing stage the meso- and microstructural evolution is polyphase and produce decametric isoclinal locally rootless folds. The multilayer is defined by alternate garnet-clinopyroxene bearing amphibolite, marble and melt-bearing gneisses (i.e. migmatites). Locally metre-scale orthopyroxene-clinopyroxene basic granulite boudins are enclosed in migmatites. Melt-bearing associations differs with respect to bulk chemical compositions, leading to cordierite + melt, orthopyroxene + melt and garnet + melt associations. Absolute geochronological data for the pre-Alpine history of the Valpelline unit, until now reported as pre-Permian, show that the main metamorphic and structural imprint of the Valpelline unit is Permian, corresponding to the main granulite imprint, characterized by pegmatite bodies and by the partial melting of metapelites. These results highlight the occurrence of an important tectonometamorphic Permian event in this portion of Austroalpine basement, likely reflecting a lithospheric scale thermal perturbation in the whole African continental lithosphere (SPALLA AND MAROTTA, 2007; PERESSINI et al., 2007).