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

Abstract

Fragments of Jurassic oceanic crust exposed in the Northern Apennine (Italy) are either associated with continental 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 continental 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 analyzed 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 similarities 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 ~161Ma, thereby arguing against previous geochronological investigations suggesting a period of ~26Ma for the formation of the Ligurian gabbroic crust. The time interval intervened from onset of gabbroic crust formation to configuration of a “slow spreading ridge type” crust was most likely ≤5Ma. New insights into the opening mechanisms of the fossil, slow seafloor spreading basin are provided.