Geochemistry of Quaternary basalts from the Yatsugatake volcano, central Japan: Generation of an unusually deep Wadati-Benioff zone beneath the volcanic front by double-overlapping subduction.

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 trench side 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.

Nd and Sr isotopes and K-Ar ages of the Ulreungdo alkali volcanic rocks in the East Sea, South Korea.

“…Most alkali volcanic activities in the backarc basin in terms of the petrogenesis, chemical and isotopical characteristics and tectonic framework are closely related to the opening of the Sea of Japan and East Sea Wasserburg, 1981, 1986;Fujibayashi et al, 1988;Nohda et al, 1988;Tamaki, 1988;Morris and Kagami, 1989;Nakamura et al, 1989;Tatsumi et al, 1989;Kaneoka, 1990;Kaneoka et al, 1990;Nakamura et al, 1990;Shimazu et al, 1990b;Otofuji et al, 1991;Tatsumoto and Nakamura, 1991;Cousens and Allan, 1992;Shimazu et al, 1993;Jolivet et al, 1991Jolivet et al, , 1994Uto et al, 1994;Kaneko et al, 1996;Morris et al, 1997). Although several tens of prehistorical submarine volcanoes found in the backarc basin of the Japanese island, surface vol canoes on the sea surface cropped out extremely limited numbers such as the Ulreungdo, Dogdo and Oki islands.…”