ABSTRACT

The Yatsugatake volcano, located on the volcanic front in central Japan, formed above an unusually deep Wadati-Benioff zone (ca. 160 km) as a trench-side volcano. In northeast Japan, incompatible element compositions and ⁸⁷Sr/⁸⁶Sr of basalts in the area above the Wadati-Benioff zone where it is ca. 110–170 km deep are clearly different from those where the Wadati-Benioff zone is more than ca. 180 km deep. These differences are inferred to have resulted from the differences in the dehydration reaction at the base of the mantle wedge dragged by the subduction and the contribution from a high ⁸⁷Sr/⁸⁶Sr reservoir (probably subcontinental upper mantle): the former zone; amphibole and chlorite dehydration and higher contribution from the high ⁸⁷Sr/⁸⁶Sr reservoir, the latter zone; phlogopite dehydration and lower contribution from that. The depth to the Wadati-Benioff zone beneath the Yatsugatake volcano is somewhat shallower than that of phlogopite dehydration, and the basalts show similar ratios between incompatible elements and ⁸⁷Sr/⁸⁶Sr to those from the amphibole and chlorite dehydration zone of northeast Japan. These observations show that the Yatsugatake magmas are produced by addition of amphibole and chlorite fluids, and the edifice is situated on the furthest position from the trench above those dehydration zone at the base of mantle wedge. It means that in central Japan there are no Quaternary volcanoes corresponding to the volcanoes in northeast Japan overlying the Wadati-Benioff zone less than ca. 160 km depth, though the amphibole and chlorite at the base of the mantle wedge exceed decomposition pressure (ca. 110 km) beneath the trenchside of the Yatsugatake volcano. This can be explained by the effects of subduction of the Philippine Sea plate, over the previously subducting Pacific plate beneath this area. Beneath the forearc-side of the Yatsugatake volcano, through the subduction of the Philippine Sea plate, the temperature of the mantle wedge has been lowered to suppress the generation of mantle diapirs. However, beneath the Yatsugatake volcano, such thermal disturbance has not taken place due to a possible splitting in the subducting Philippine Sea plate, sub-parallel to the subduction direction (NNW) in Miocene to Pliocene. The splitting might have been caused by tearing the Philippine Sea plate into the eastern and western parts in subducting around the Izu-Peninsula. Around the Yatsugatake volcano, the migration of the volcanic front toward the backarc-side is observed during mid-Pliocene (before the changing of the subduction direction of the Philippine Sea plate from NNW to NW), which might have been caused by the process of formation of this double-overlapping subduction zone.