5:15 PM - 6:45 PM
[SCG54-P05] Geochemical characteristics and magma dynamics of rhyolitic volcanism in the southern Izu intra-arc rift zone
Keywords:Southern Izu arc, intra-arc rift zone, rhyolitic volcanism, pumice raft, across-arc geochemical variation
On the west side of the volcanic front of the Izu arc, a normal fault structure has developed in the direction of extension of the island arc, forming a rift zone. Fault activity not only forms back-arc depressions of various sizes, but also brings about volcanic activity, and many groups of small hills arranged in a straight line, which are thought to be related to the fault structure, have been observed. In early October 2024, an earthquake swarm and tsunami with an epicenter in the same area were observed, and drifting pumice was discovered on October 20, drawing renewed attention to volcanic activity in the rift zone.
In 1995, the R/V Moana Wave, a research vessel from the University of Hawaii, conducted a volcanic rock sampling survey in the same area, from the volcanic front from Aogashima to Torishima to the Karimuki seamount chain, and conducted many dredges in the back-arc rift zone. taken. These volcanic rocks exhibit a "bimodal" compositional variation dominated by basalt and rhyolite. Basalt shows changes in chemical composition across the island arc, and the contributions of magma-derived materials and subducting slabs have been considered (e.g. Kimura et al., 2010). Rhyolite also shows changes in chemical composition across the island arc, but it also shows characteristics different from basalt, in that the middle crust, which was formed by island arc activity before the expansion of the Shikoku Basin, is formed by rhyolites due to decompression partial melting caused by rifting activity. It was thought that rock magma was generated (Haraguchi et al., 2017). In response to the discovery of drifting pumice thought to originate from the rift zone, we would like to reconsider magma activity in the rift zone using chemical data from previous researches.
In 1995, the R/V Moana Wave, a research vessel from the University of Hawaii, conducted a volcanic rock sampling survey in the same area, from the volcanic front from Aogashima to Torishima to the Karimuki seamount chain, and conducted many dredges in the back-arc rift zone. taken. These volcanic rocks exhibit a "bimodal" compositional variation dominated by basalt and rhyolite. Basalt shows changes in chemical composition across the island arc, and the contributions of magma-derived materials and subducting slabs have been considered (e.g. Kimura et al., 2010). Rhyolite also shows changes in chemical composition across the island arc, but it also shows characteristics different from basalt, in that the middle crust, which was formed by island arc activity before the expansion of the Shikoku Basin, is formed by rhyolites due to decompression partial melting caused by rifting activity. It was thought that rock magma was generated (Haraguchi et al., 2017). In response to the discovery of drifting pumice thought to originate from the rift zone, we would like to reconsider magma activity in the rift zone using chemical data from previous researches.