GABBRO-DERIVED GRANULITES FROM EXTERNAL LIGURIDE UNITS (NORTHERN APENNINE, ITALY) - IMPLICATIONS FOR THE RIFTING PROCESSES IN THE WESTERN TETHYS

In Santonian-Early Campanian sedimentary melanges of the External Ligu ride units (northern Apennine), slide blocks of subcontinental mantle and MOR basalts are associated with lithologies derived from the conti nental crust, One of these sedimentary melanges, the Mt. Ragola comple x, is characterized by the close association of mantle ultramafic, maf ic and quartzo-feldspathic granulites. Mafic granulites show a wide co mpositional range. They generally display a marked metamorphic layerin g, but undeformed rocks which preserve a gabbroic fabric are found loc ally. The most frequent lithologies are Al-spinel gabbronorites, gener ally containing minor olivine, and Fe-Ti oxide-bearing gabbronorites. Troctolites, olivine gabbronorites and anorthosites were also recovere d. Relics of primary textures as well as mineral and bulk-rock composi tional variations indicate a comagmatic intrusive origin for the proto liths 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 degrees C, and was derived from crystallization a t 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 f or the granulites of External Liguride units were probably crystallize d into the extending Adria lithosphere in relation to the initial stag es of the opening of the western Tethys.