Origin of ferromanganese deposits in the Jurassic to Cretaceous accretionary complex: Implications for the deep-sea environment around ocean anoxic events

• Fe deposits in the Tokoro Belt formed at a mid-ocean ridge (MOR) at ∼ 161.5–165.3 Ma. • In contrast, Mn deposits formed at an oceanic island around 120 Ma. • MOR volcanism was already active before rapid global warming in the Oxfordian. • The deep sea in pelagic regions was oxic throughout oceanic anoxic event 1a. In the Tokoro Belt, which is a Jurassic to Cretaceous accretionary complex in Japan, Fe and Mn deposits are distributed along basaltic rocks and chert. This study proposes the origin of these Fe and Mn deposits and reconstructs marine Os isotope ratios. The samples were found to exhibit negative Ce anomalies and low transition metal contents other than Fe and Mn, which are typical of submarine hydrothermal ferromanganese oxides. The Fe deposit samples were enriched in rare-earth elements due to apatite accumulation. Several Mn deposit samples showed positive Eu anomalies, suggesting high-temperature water–rock interactions. The radiolarian ages of red cherts in previous studies and the distributions of mid-ocean ridge (MOR) and oceanic island basalts indicate that Fe deposits were formed by hydrothermal activity at an MOR in the Callovian or older. In contrast, Mn deposits were formed by hydrothermal activity on oceanic islands approximately 5 × 10 3 km from the MOR at approximately 120 Ma. The low initial Os isotope ratios of the Fe deposit samples (0.411–0.445) suggest that volcanism in the MOR was active before the Late Jurassic oceanic anoxic event (OAE). The initial Os isotope ratios of the Mn deposit samples from the western and southern sections correspond to the marine Os isotope ratios observed before and during OAE1a (∼124–119.55 Ma and 119.5–118.5 Ma, respectively). As the Mn deposits are interbedded with red chert, it can be inferred that an oxic environment was maintained in the deep-sea pelagic regions throughout OAE1a.