安達太良火山の最近25万年間における山体形成史とマグマ供給系の変遷

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Repeated eruptions of calc-alkaline magmas in recent 250000 years gave rise to the main body of the Adatara volcano. Geological and stratigraphical reexamination distinguished 37 eruptives. Based on temporal and spatial association, the eruptives were grouped into 7 volcanic units: eastern side first and second units (ES1, ES2); Minowasan unit (MI); summit area unit (SU); northern and southern rims of Numanotaira units (NN, SN); Numajiri unit (NU). Chronological data revealed that the ES1 and MI were developed 0.25-0.20 Ma, the SU was formed around 0.12 Ma, and that probably NN and SN were built up less than 0.1 Ma. Bulk chemically, 3 groups were distinguishable; group 1 (ES1, ES2) ejecta are rich in TiO2 FeO* Sc, V and Co, poor in MgO, K2O, Rb, Ba and Zr compared with group 3 (NN, SN, NU), and the group 3 has higher Rb/Ba, Zr/Nb and Rb/Zr than those of group 1. The group 2 (MI) shows similar Rb/Ba, Nb/Zr and Rb/Zr to those of the group1, but displays distinctive trends for Sr, Nb, Sc and V. Among the three essential materials (pumice and scoria 1 of the earlier eruptives and scoria 2 of the later member) in the AT2 erupted at 0.12 Ma (SU), the scoria 1 compositionally corresponds to the primitive members of the group 1, and the compositions of scoria 2 come close to the low SiO2 extension of the trends for the group 3. Further, the correlations accord with their relevant chemical characteristics such as Rb/Ba, Nb/Zr and Rb/Zr, and also compatible element trends. The inferred transition of the magma plumbing system is as follows: Two magma systems (groups 1 and 2) had been active about 0.25 to 0.2 Ma. During 80000 years of dormancy, relatively primitive magma had been preserved only in the main system (group 1). Around 0.12 Ma, the third magma system (group 3) came underneath the main system. One felsic and two andesitic magma bodies co-existed at the beginning of the eruption at 0.12Ma, but the andesitic magma of the group 1 exhausted halfway of the eruption, and was completely replaced by the third system. The renewed system has been preserved and active after the eruption of 0.12 Ma to build up SN and NN.
日本鉱物科学会の論文
2005-03-30

安達太良火山の最近25万年間における山体形成史とマグマ供給系の変遷

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