Existence of Different Peridotite Types and of a Layered Igneous Complex in the Ivrea Zone of the Western Alps

The Ivrea zone consists of a metapelitic series and of a main basic body, with related ultramafites (peridotites and pyroxenites). The peridotites are currently considered as mantle material. On the basis of their relationships with the country rocks, bulk and phase composition and factorial analysis, it is demonstrated in this paper that the peridotites are in formed by fractionation and cumulus processes. The main basic body is generally considered as a metamorphic suite of basic granulites, that have suffered the same metamorphic event as the metapelitic series. In this paper the hypothesis that it represents a deep-seated stratiform complex is proposed. This possibility is supported by the igneous contacts between the main basic body and the metapelitic series, by the fractionation pattern of the main basic body and by its stratigraphy. Its geochemical features and dynamic characteristics suggest an emplacement into an orogenic environment, possibly related to a continental margin. The ‘granulitic’ mineral assemblages of the main basic body are attributed to re-equilibration by slow cooling in deep crust and not to a metamorphic event. This point is supported by the relationships between the temperature and pressure paths from the estimated original conditions to those of re-equilibration, by the preservation of evidence of a thermal increase in the country rock, and by the preservation of primary parageneses. The relationships between the main basic body and the metapelitic series indicate that the emplacement of the first postdates the earlier (Caledonian) metamorphism of the metapelites.