Gabbro-derived granulites from External liguride units (northern Apennine, Italy): implications for the rifting processes in the western Tethys

Abstract

In Santonian-Early Campanian sedimentary melanges of the External Liguride units (northern Apennine), slide blocks of subcontinental mantle and MOR basalts are associated with lithologies derived from the continental crust. One of these sedimentary melanges, the Mt. Ragola complex, is characterized by the close association of mantle ultramafic, mafic and quartzo-feldspathic granulites. Mafic granulites show a wide compositional range. They generally display a marked metamorphic layering, but undeformed rocks which preserve a gabbroic fabric are found locally. The most frequent lithologies are Al-spinel gabbronorites, generally containing minor olivine, and Fe-Ti oxidebearing gabbronorites. Troctolites, olivine gabbronorites and anorthosites were also recovered. Relics of primary textures as well as mineral and bulk-rock compositional variations indicate a comagmatic intrusive origin for the protoliths of the mafic granulites. This intrusive mafic complex underwent a subsolidus reequilibration under granulite facies conditions, at 0.6–0.9 GPa and 810–920°C, and was derived from crystallization at intermediate levels of tholeiite-derived liquids, possibly affected by crustal contamination. Its primary features are similar to those of the upper zone of the Ivrea layered complex. The gabbroic protolith for the granulites of External Liguride units were probably crystallized into the extending Adria lithosphere in relation to the initial stages of the opening of the western Tethys.