日本地球惑星科学連合2019年大会

講演情報

[E] ポスター発表

セッション記号 S (固体地球科学) » S-MP 岩石学・鉱物学

[S-MP29] Oceanic and Continental Subduction Processes

2019年5月28日(火) 10:45 〜 12:15 ポスター会場 (幕張メッセ国際展示場 8ホール)

コンビーナ:Hafiz Ur REHMAN(Department of Earth and Environmental Sciences, Graduate School of Science and Engineering, Kagoshima University)、辻森 樹(東北大学)、Chin-Ho Tsai(Department of Natural Resources and Environmental Studies, National Dong Hwa University)、岡本 和明(埼玉大学教育学部地学)

[SMP29-P02] Late Permian to Early Triassic back-arc type volcanism in the southern Mongolia volcano-plutonic belt of the Central Asian Orogenic Belt: Implication for timing of the final closure of the Palaeo-Asian Ocean

*今山 武志1國府 陽一郎2青木 一勝3實吉 玄貴4八木 公史5青木 翔吾4澤田 順弘6猪川 千晶5寺田 智也4石垣 忍4豊田 新3Tsogtbaatar Khishigjav7Mainbayar Buuvei7 (1.岡山理科大学自然科学研究所、2.北海道大学大学院環境科学院、3.岡山理科大学理学部、4.岡山理科大学生物地球学部、5.蒜山地質年代学研究所、6.島根大学名誉教授、7.モンゴル科学アカデミー古生物学地質学研究所)

キーワード:古アジア海、南モンゴル、中央アジア造山帯、背弧型火山活動

The timing of final subduction and closure of the Palaeo-Asian Ocean is still controversial in the eastern segments of the Central Asian Orogenic Belt (CAOB). A more detailed understanding is needed with respect to the formation of volcanic rocks and sediments associated with the final closure of the Palaeo-Asian Ocean in southern Mongolia. We investigated alkali series volcanic rocks newly discovered in the southern Mongolia volcano-plutonic belt of the CAOB and classified as basalt, basaltic trachyandesite, andesite, trachyandesite, and trachyte, mainly showing andesitic-dacitic compositions. In Harker diagrams, the TiO2, FeO, MnO, Al2O3, CaO, and MgO contents generally decrease with increasing SiO2, representing a series of differentiated volcanic rocks. The primitive mantle-normalized trace element spider diagram is characterized by incompatible element-rich patterns and negative Nb anomalies. The tectonic discrimination diagrams show that the volcanic rocks were formed by arc-related volcanism rather than within-plate volcanism. K-Ar dating of feldspars from six volcanic rocks yields ages ranging from 270 to 239 Ma, indicating that andesitic and dacitic magmatism occurred during this period. These geochemical and geochronological data indicate that the differentiated volcanic rocks intruding middle to late Palaeozoic volcanic sediments were formed by back-arc type subduction ca. 270–240 Ma. Based on detrital zircon U-Pb ages, the terrigenous sandstones underlain by the volcanic sediments record a maximum depositional age of Late Triassic (ca. 220–200 Ma) and include a main cluster of Early Devonian ages (ca. 410–390 Ma), possibly derived from the Devonian-Carboniferous arc complex observed surrounding this study area. These results from the volcanic and sedimentary rocks in this study, combined with investigations in previous studies, imply that subduction beneath the eastern segments of the CAOB to consume the Palaeo-Asian Ocean continued until Late Permian to Early Triassic.