U–Pb zircon geochronology of the Ligurian ophiolites (Northern Apennine, Italy) : implications for continental breakup to slow seafloor spreading

Fragments of Jurassic oceanic crust exposed in the Northern Apennine (Italy) are either associated with continen- tal lithosphere material (External Ligurian ophiolites), or bear structural and compositional resemblances to slow spreading ridge crust (Internal Ligurian ophiolites). To acquire new information about the transition from conti- nental breakup to slow seafloor spreading, we carried out a U–Pb geochronological study of zircons from gabbro bodies of both External and Internal Ligurian ophiolites. Zircons were separated from seven samples and ana- lyzed for U–Pb isotopes by laser ablation ICPMS and isotope dilution TIMS. The zircons were also investigated for morphology, internal structures, inclusions and chemistry. These characteristics reveal remarkable similari- ties to zircons collected from modern oceanic crust. Taken as a whole, the new U–Pb zircon dates obtained for the Ligurian ophiolites range from ~165 to ~161 Ma, thereby arguing against previous geochronological investi- gations suggesting a period of ~26 Ma for the formation of the Ligurian gabbroic crust. The time interval inter- vened from onset of gabbroic crust formation to configuration of a “slow spreading ridge type” crust was most likely ≤ 5 Ma. New insights into the opening mechanisms of the fossil, slow seafloor spreading basin are provided.