[S-CG62] 沈み込み帯へのインプットを探る：海溝海側で生じる過程の影響

[SCG62-P05] Subduction initiation at active spreading mid-ocean ridge indicated by ultrahigh temperature slab melting

*Chiou-ting Lin¹、Kuei-Chih Feng²、Masaki Enami³、Hou-Chun Liu⁴、Ji-Hao Zhu⁶、Ron Harris⁵、Wei-Dong Sun¹ (1.Center of Deep Sea Research, Institute of Oceanology, Chinese Academy of Sciences、2.Department of Mechanical Engineering, Ming Chi University of Technology、3.Institute for Space–Earth Environmental Research, Nagoya University、4.Department of Earth Science, National Cheng Kung University、5.Department of Geology, Brigham Young University、6.State Oceanic Administration, Second Institute of Oceanography, Ministry of Natural Resources)

キーワード：Subduction Initiation, Ridge Subduction, Slab Melting, Manila Trench, South China Sea, Lichi Mélange

The initiation of plate subduction is the last piece of the jigsaw puzzle of the plate tectonic theory. How the subduction of small oceanic plates commenced, and the magmatism of the subduction initiation has been particularly controversial. Here we report a new type of calc-alkaline andesite/dacite associated with initiation of subduction. The Lichi andesite/dacite (LAD) has typical geochemical features of slab melting, but abnormally displays relative enrichments in Nb-Ta and depletion in Ti and Pb. The LAD mineral assemblage consists mostly of plagioclase and two-pyroxenes with rare hornblende and biotite. No quartz or K-feldspar phenocrysts are observed. The presence of cristobalite in the matrix of the dacite and geochemical relations indicate slab melting occurs at an ultrahigh temperature and relatively low pressure environment, which is consistent with subduction initiation of an active spreading ridge along a transform fault.