AGE, ORIGIN AND GEODYNAMIC SIGNIFICANCE OF PLAGIOGRANITES IN LHERZOLITES AND GABBROS OF THE PIEDMONT-LIGURIAN OCEAN-BASIN

U-Pb zircon dating, Sr-Nd isotope tracing and major/trace/RE element a nalyses were performed to constrain the age, origin and geodynamic sig nificance of plagiogranites that intrude lherzolites and gabbros in th e Ligurian Alps and the Northern Apennines. In addition, a host Fe-dio rite was investigated. Samples from the Ligurian Alps were collected f rom the Voltri Group and the Sestri-Voltaggio Zone, whereas the plagio granites from the Northern Apennines were taken in the Bracco unit. Al l these units have been affected by Alpine metamorphism reaching eclog ite facies in the Voltri Group, blueschist degree in the Sestri Voltag gio samples, and prehnite-pumpellyite facies in the Bracco Unit, which has additionally been affected by rodingitization. U-Pb zircon ages o f 150 +/- 1, 153 +/- 1 and approximate to 156 Ma were obtained, respec tively, for two plagiogranites and the host Fe-diorite in the Ligurian Alps, and an age of 153 +/- 1 Ma was determined for the plagiogranite in Northern Apennines. Inherited components in zircon and initial Pb in plagioclase indicate mixing of variously differentiated basaltic ma gmas with small amounts of roughly 1.7-2.1 Ga old continental crust ma terial. REE patterns in both the plagiogranites and the host diorite a re characterized by high REE abundance, and moderate LREE enrichment. Nd isotopic compositions lie in the range of N-MORB sources, yielding initial epsilon Nd values between +8.8 and +9.7, whereas Sr is isotopi cally heterogeneous. The geochemical pattern of the plagiogranites and the host Fe-diorite requires melting of a MORB-type mantle source tha t experienced LREE enrichment shortly before melting. The most likely explanation for such enrichment is the injection of melts derived by s mall degrees of melting from an adjacent mantle region. The basaltic, LREE-enriched parent magmas generated from this enriched domain have p robably undergone up to about 72% of low-pressure fractional crystalli zation prior to their emplacement into the gabbro-peridotite complex. The 156-150 Ma magmatism occurred in close relation to normal faulting , sedimentation of breccias, and detachment of the mantle complex from its overlying continental crust, followed by exposure on the ocean fl oor. This tectono-magmatic event in the Ligurian Alps and the Northern Apennines reflects rifting of the Adriatic-Iberian continental plate segment, preceding wider opening of the Piedmont-Ligurian ocean basin and pillow basalt deposition.