研究業績

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 (δ34S) 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.