Metamorphic evolution and significance of a serpentinized peridotite slice within the Eclogitic Micaschist Complex of the Sesia-Zone (Western Alps – Italy)
The petrography, mineral chemistry, and metamorphic evolution of a serpentinized peridotite slice, which occurs within the Eclogitic Micaschist Complex of the Sesia-Lanzo zone, western Alps (Italy), are described and possible tectonic implications are discussed. The ultramafics are mainly foliated serpentinites which preserve, in the central portion, relics of the peridotitic protolith. The relic peridotite includes both orthopyroxene- and olivine-rich layers and consists of olivine, diopsidic clinopyroxene, Cr-bearing chlorite and orthopyroxene porphyroclasts. The latter include olivine, amphibole, and orange-brownish Cr-Mg-Al-rich spinel aggregates, whose spatial arrangement indicates pseudomorphism after a mineral with a perfect cleavage set, most likely chlorite. Based on this interpretation, arisen from both the composition and geometric arrangement of spinel droplets, the host peridotite preserves evidence of a prograde metamorphism from the chlorite- to the spinel-peridotite field, which predates the Alpine polyphase evolution. The study of a rodingite – most likely derived from a gabbro protolith crosscutting the peridotite – which contains a zoned garnet rich in grossular in the core and in almandine in the rim, suggests that an episode of low-T metasomatic alteration occurred prior to the early-Alpine eclogite-facies metamorphism. The presence of this rodingite, which shows the same mineral zoning and evolution as those already described for the nearby metaophiolitic ultramafic Lanzo Massif, suggests that the relic peridotite slice belongs to the Piemonte zone. It is thus seen as a sliver derived from the Mesozoic Tethys Ocean that has been tectonically emplaced into the continental crust of the Sesia zone. These data partly support the interpretation of Venturini (1995) and Venturini et al. (1991, 1992, 1994) and indicate that the Eclogitic Micaschist Complex, usually considered as a coherent fragment of Variscan continental crust, is in fact a unit with a more complex internal tectonic setting.
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Document Type: Research Article
Publication date: 01 January 2003