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 analyses were performed to constrain the age, origin and geodynamic significance of plagiogranites that intrude lherzolites and gabbros in the Ligurian Alps and the Northern Apennines. In addition, a host Fe-diorite was investigated. Samples from the Ligurian Alps were collected from the Voltri Group and the Sestri-Voltaggio Zone, whereas the plagiogranites from the Northern Apennines were taken in the Bracco unit. All these units have been affected by Alpine metamorphism reaching eclogite facies in the Voltri Group, blueschist degree in the Sestri Voltaggio samples, and prehnite-pumpellyite facies in the Bracco Unit, which has additionally been affected by rodingitization.

U-Pb zircon ages of 150 ± 1, 153 ± 1 and ≈ 156 Ma were obtained, respectively, 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 magmas with small amounts of roughly 1.7–2.1 Ga old continental crust material. REE patterns in both the plagiogranites and the host diorite are 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 isotopically heterogeneous. The geochemical pattern of the plagiogranites and the host Fe-diorite requires melting of a MORB-type mantle source that experienced LREE enrichment shortly before melting. The most likely explanation for such enrichment is the injection of melts derived by small degrees of melting from an adjacent mantle region. The basaltic, LREE-enriched parent magmas generated from this enriched domain have probably undergone up to about 72% of low-pressure fractional crystallization 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 floor. 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.     

Early Permian zircon uranium-lead ages for plagiogranites in the Yakuno ophiolite, Asago district, Southwest Japan

Abstract Prior work has defined a two-stage history for Yakuno ophiolite petrogenesis consisting of older oceanic basement, and younger island-arc rocks and subordinate elastic rocks. First-stage Yakuno rocks have transitional-type M orb or oceanic plateau affinities, and second-stage Yakuno rocks are more similar to immature island-arc settings. Zircon U-Pb isotopic dates of plagiogranite from the Asago district of Southwest Japan yield crystallization ages of 285 ± 2 Ma for the first-stage ophiolite component, and 282 ± 2 Ma for the second-stage component of the ophiolite. These results indicate that the two petrologi-cally distinct components of the Yakuno ophiolite in this area formed in a short time interval in the Early Permian. The zircon U-Pb crystallization ages provide a maximum age for the base of the stratigraphically overlying Maizuru Group.     

The high temperature reaction zone of the Oman ophiolite: new field data,microthermometry of fluid inclusions,PIXE analyses and oxygen isotopic ratios

Marine Geophysical Research - The present study is focused on the so-called High Temprature Reaction Zone of the Oman ophiolite, a thin zone located between the roots of the sheeted dyke complex...