Geochemical approach to characterization and source identification of the protoliths of metasedimentary rocks : an example from the Southern Alps

We compare here the geochemical characters of metasediments from two main units of the Southern Alps (Serie dei Laghi and Orobic Basement). Concerning the protoliths of the paragneisses from Serie dei Laghi, the Th/Sc vs Zr/Sc correlations indicate that they are mainly controlled by the composition of their sources (likely diorites to granodiorites); their REE patterns are well comparable to the average composition of continental arc wackes. The suggested environment is a dissected continental arc on an active margin. For the Serie dei Laghi metapelites (Scisti dei Laghi) the large compositional variations and the wide spread in some parameters (Th 5-15 ppm, Th/U 1.4-4.8, Th/Sc 0.35-0.92, La/Th 3.3-7.8, REEtot 149-248) suggest mixing in different proportions of sediments from geochemically “undifferentiated” and from differentiated igneous sources, with a variable contribution of recycled sediments enriched in heavy minerals. The metasediments from the Orobic basement have a more pelitic character and show the influence of old upper crustal materials (Th/U= 3.6-6.7). Their main geochemical characters are compatible with a passive margin environment.