Re-Os dating of the Makimine and Shimokawa VMS deposits for new age constraints on ridge subduction beneath Japanese Islands.

Ridge subduction is a trigger of thermal metamorphism and hydrothermal activity; thus, it is an important process for understanding geological history of accretionary complexes. However, determining the timing of ridge subduction is often challenging owing to metamorphism and poor microfossil preservation. Some Besshi-type volcanogenic massive sulphide (VMS) deposits associated with in situ greenstone originated by hydrothermal mineralisation on a sediment-covered mid-ocean ridge (MOR); thus, their depositional ages constrain the timing of ridge subduction. Here, we report Re-Os isochron ages of the Makimine and Shimokawa VMS deposits in southwest and northeast Japan. The Re-Os isotope ratios exhibit well-defined linearity and their isochron ages are 89.4 ± 1.2 and 48.2 ± 0.9 Ma. Considering (1) the almost same depositional ages of the VMS deposits with surrounding sedimentary rocks; (2) their close association with in situ greenstone and absence of chert; (3) their radiogenic Pb isotope composition; (4) their high sulphur isotope (δS) composition with a wide variation; and (5) high thermal gradient in the Makimine area, we inferred the depositional setting of both VMS deposits to be a sediment-covered MOR in a shelf sea. Thus, the VMS deposits were formed just before Izanagi-Pacific Ridge subduction beneath the paleo-Japanese Islands.