[S-VC34] Volcanic and igneous activities, and these long-term forecasting

[SVC34-P19] Sr Isotopic Ratios for the Volcanic Rocks from Kinbo Volcano, Central Kyushu, Japan

*Taro Shinmura¹, Yoji Arakawa² (1.Faculty of Economics, Kumamoto Gakuen University, 2.Earth Evolution Sciences, Graduate School of Life and Environmental Sciences, University of Tsukuba)

Keywords:Kinbo Volcano, Sr isotopic ratio

Kinbo Volcano is a Quaternary volcano located on the east side of the Ariake Sea in central Kyushu. The volcanic rocks, which are mainly andesite, are distributed over an area of about 10 km from east to west and 13 km from north to south, forming a mountain range consisting of several peaks, the highest of which is Ichinotake (Kinbo-san) at 665.2 m above sea level. The most recent work on the classification and dating of the active stages of Kinbo volcano was done by Toshida et al. (2006). They conducted K-Ar dating of 14 samples systematically collected to cover the entire active period of Kinbo volcano, and reported that the ages of 6 samples from the Matsuo, Old Kinbo, and Ishigamiyama volcanic rocks in the Older stage ranged from 1.38 to 1.15 Ma, and those of 6 samples from the Ninotake and Sannotake volcanic rocks in the Middle stage ranged from 0.58-0.50 Ma and those of 2 samples from the Ichinotake volcanic rocks were 0.2 Ma. In addition, the active periods were redefined and the distributions of each unit were slightly revised.
Geochemical data including Sr isotopic ratios (⁸⁷Sr/⁸⁶Sr) (hereafter described as Sr ratio) for Kinbo volcano were reported by Kurasawa and Takahashi (1963) and Yanagi et al. (1988). Kurasawa and Takahashi (1963) reported Sr ratios of 0.7036 and 0.7049 for basalt and andesite samples, respectively, in the southern part of Kinbo volcano, and Yanagi et al. (1988) reported a total of 36 andesite and basalt samples with ratios ranged from 0.7038 to 0.7054. Yanagi et al. (1988) discussed magmatic processes based on Sr ratios and whole rock chemical composition data. Because these data were based on the classification before the redefinition of the activity period by Toshida et al. (2006) and the specific sampling points were not specified, it was difficult to understand the characteristics of volcanic products from Kinbo volcano and their temporal changes according to the activity period. Therefore, in this study, we measured the Sr ratios of 9 volcanic rocks collected from or near the sampling points where the ages reported by Toshida et al. (2006) were obtained, and the results were examined by correlating the results with the ages and classifications of Toshida et al. (2006) (Fig. 1). The values of 7 samples collected from volcanic rocks corresponding to the Older and Middle stages (0.7036 to 0.7054) were within the range of variation reported by Yanagi et al. The values of two samples collected from the Ichinotake volcanic rocks of the Younger stage activity were almost consistent with the value of 0.7049 reported by Kurasawa and Takahashi (1963), and did not show a large variation as in other stages. Based on these data, assuming that the Sr ratios of the magma source materials were similar throughout the entire Older and Middle stages of Kinbo volcano, the magma may have mixed with materials with a certain Sr ratio in various degrees or may have mixed with materials with various Sr ratios, furthermore, magma with relatively high Sr ratios within the range may have formed the Ichinotake lava dome in the Younger stage. Yanagi et al. (1988) attributed the large range in Sr ratios between the Older and Middle stage (equivalent volcanic rocks) to the influence of fluids introduced from granulitic materials in the lower crust. On the other hand, Yokose and Yamamoto (1996) reported that volcanic rocks from Ishigamiyama and Araoyama, which belong to the Older stage, contain crustal xenoliths and cognate xenolith. So, it was possible that these materials were mixed into the magma and affected the Sr ratio in these stages.