Abstract
Eruptions of calc-alkaline magma developed the main body of the Adatara volcano in recent 250,000 years. Geology and stratigraphy are reexamined to distinguish 37 eruptives. Based on geological affinity, the eruptives are grouped into 7 volcanic units; eastern side No1 and No2 ( ES1, ES2), Minowasan (MI), summit area (SA), northern and southern rims of Numanotaira (NN, SN), and Numajiri (NU) units, respectively. Chronological data revealed that the ES1 and MI units and SA unit were developed 0.25-0.20Ma and around 0.12Ma, respectively. Bulk chemically, these are classified into 4groups: Group 1 ( stage 3-1a; ES1, ES2 ) ejecta are rich in TiO2 and FeO*, poor in MgO and K2O, and FeO*/MgO = 2.2-2.7. Also, they show low Rb, Ba and Zr, high Sc, V and Co contents. Rb/Ba, Nb/Zr and Rb/Zr are consistently around 0.12, 0.04 and 0.25-0.3, respectively. Group 2 (stage 3-1b; MI) is rich in Al2O3, poor in CaO and K2O. These show similar Rb/Ba, Nb/Zr and Rb/Zr to those of the group1, but display distinctive trends for Sr, Nb, Sc and V. Composition of the group 3 (stage 3-2; SA) is scattered, showing no distinct trend. Group 4 (stage 3-3; NN, SN, NU) shows contrastive chemical characteristics to the group 1. Also, Rb/Ba, Nb/Zr and Rb/Zr (0.14, 0.035, 0.3-0.4, respectively) are distinctive. If the compositional difference between the groups 1 and 4 reflects the transition of the plumbing system, the scattering in the group 3 may represent various products from different systems. Within a single pyroclastic fall deposit, three essential materials (pumice, scoria 1 and scoria 2) are recognized. None of the three types can be derived from other types, having been isolated prior to the eruption. Further, correlations of geochemistry and stratigraphy indicate that the low-Mg andesitic magma has been replaced by more mafic magma during the eruption. There are observations that support that the abrupt replacement from the scoria 1 to the scoria 2 reflects the major change of the plumbing system in recent 250,000 years. Compositions of the scoria 1 correspond to those of the primitive members of ES1, and those of scoria 2 come close to the low SiO2 extension of the trends for the SN. Also, these correlations accord with their relevant chemical characteristics such as Rb/Ba, Nb/Zr and Rb/Zr. Change of the magma plumbing system is assumed as follows: About 0.25 to 0.2 Ma, magma system 1 (stage 3-1a + scoria 1) and the system 2 (stage 3-1b) had been developed. During 80,000 years of dormancy, primitive magma had been preserved only in the system 1. Around 0.12 Ma, magma system 3 (stage 3-3 + scoria 2) came under the system 1. At the beginning of the eruption, felsic and two andesitic magmas co-existed, but system 1 exhausted before coming to the end of eruption. The magma system 3 has been preserved and active after the 0.12 Ma eruption.
