11:00 AM - 1:00 PM
[SVC29-P07] Age and petrological characteristics of basalts distributed in Kuchinotsu area, southern Shimabara Peninsula, Nagasaki Prefecture
Keywords:Shimabara Peninsula, Basalt, petrological characteristics
Shimabara Peninsula is located on the western edge of the Beppu-Shimabara Graben and is mainly formed by Unzen Volcano, which has been active for about 500,000 years. The Minamishimabara volcanic rocks and the Kuchinotsu Group are widely distributed as older foundations. Among them, basalt in Hayasaki Peninsula, southern Shimabara Peninsula, has K-Ar age of about 4.4 Ma (Uto et.al.,2002). Otsuka et al. (1995) recognize the basalt of the Hayasaki Peninsula as the earliest volcanic activity in the Kumamoto-Shimabara region of the Beppu-Shimabara Graben. In this study, basalt with an older age was newly identified near Hisakiyama Port in Kuchinotsu area, southern Shimabara Peninsula.
The study area is a reef about 100 m offshore to the northwest of Hisakiyama Port in the Kuchinotsu region (Fig.1), and it can be identified as a rock body about 400 m long and about 280 m wide extending in the east-west direction. The rock can also be seen on the land side, and there is a possibility that it will be connected continuously. A similar basalt was found on the land side of this study area, and Otsuka et al. (1995) considered it to be at the same time as a part of the basalt of Hayasaki Peninsula. On the other hand, Terai (2018) classified this basalt as a different rock form from the basalt of Hayasaki Peninsula.Locally observed stratigraphy includes the blue-gray silt layer at the bottom, with basaltic lava flows above it, tuff including andesite above it, and gravel mainly composed of chert, metamorphic rocks, and granite above it. The tuff containing the andesite is newly called "Minamishimabara tuff" in Terai (2018) and has been identified as a key layer separating the upper and lower Kuchinotsu Formation.Therefore, it can be assumed that the basaltic lava flows in this study, which are located below the tuff, are of lower Kuchinotsu Formation or older age.A K-Ar age of 6.2 ± 0.4 Ma was obtained from the basalt samples.This indicates that the basalt was active before the formation of the Kuchinotsu Formation. Until now, the oldest record of volcanic activity in the Shimabara Peninsula was thought to be about 4.4 Ma, but it can be said that this is a volcanic rock with an even older age.
This basalt generally shows a slightly dark green color, and olivine porphyry is conspicuous. Circular to elliptical pores are remarkably observed, and some of them are filled with minerals such as zeolite. Patchy tissue containing self- to semi-selfshaped olivine phenocrysts, with the stone base representing intergranular tissue. The phenocryst mainly contained olivine containing spinel and a small amount of microphenocrysted clinopyroxene and plagioclase, and the total phenocryst content was 8 -9 vol%. Groundmass are composed mainly of plagioclase, olivine, clinopyroxene and opaque minerals and contain a small amount of glass. Total rock chemical composition analysis of the newly collected samples revealed SiO2 = 53 wt%, MgO = 6 ~ 7 wt%, and FeO */MgO = 1.3 ~ 1.4. SiO2 is almost constant with decreasing MgO content. Ni = 94 ~ 97 ppm and Cr = 325 ppm. The SiO2- (Na2O + K2O) diagram (Macdonald & Katsura,1964) is divided into subalkali series, and the Nb-Zr-Y section diagram (Meschede, 1986) is divided into intraplate tholeiitic basalts.
In Kuchinotsu area in the southern part of Shimabara Peninsula, basalt of about 6.2 Ma, which is older than the age of volcanic rock reported until now, was confirmed. As a result of chemical composition analysis of all rocks of the basalt, it was found that the basalt is classified into subalkali series. Since the results obtained this time are local, it is necessary to obtain detailed distribution and volcanic stratigraphy by increasing analysis values such as connection to land areas. Based on this, we will examine the history of volcanic rock activity, petrological characteristics, and the chemical evolution of magma in the southern part of the Shimabara Peninsula.
The study area is a reef about 100 m offshore to the northwest of Hisakiyama Port in the Kuchinotsu region (Fig.1), and it can be identified as a rock body about 400 m long and about 280 m wide extending in the east-west direction. The rock can also be seen on the land side, and there is a possibility that it will be connected continuously. A similar basalt was found on the land side of this study area, and Otsuka et al. (1995) considered it to be at the same time as a part of the basalt of Hayasaki Peninsula. On the other hand, Terai (2018) classified this basalt as a different rock form from the basalt of Hayasaki Peninsula.Locally observed stratigraphy includes the blue-gray silt layer at the bottom, with basaltic lava flows above it, tuff including andesite above it, and gravel mainly composed of chert, metamorphic rocks, and granite above it. The tuff containing the andesite is newly called "Minamishimabara tuff" in Terai (2018) and has been identified as a key layer separating the upper and lower Kuchinotsu Formation.Therefore, it can be assumed that the basaltic lava flows in this study, which are located below the tuff, are of lower Kuchinotsu Formation or older age.A K-Ar age of 6.2 ± 0.4 Ma was obtained from the basalt samples.This indicates that the basalt was active before the formation of the Kuchinotsu Formation. Until now, the oldest record of volcanic activity in the Shimabara Peninsula was thought to be about 4.4 Ma, but it can be said that this is a volcanic rock with an even older age.
This basalt generally shows a slightly dark green color, and olivine porphyry is conspicuous. Circular to elliptical pores are remarkably observed, and some of them are filled with minerals such as zeolite. Patchy tissue containing self- to semi-selfshaped olivine phenocrysts, with the stone base representing intergranular tissue. The phenocryst mainly contained olivine containing spinel and a small amount of microphenocrysted clinopyroxene and plagioclase, and the total phenocryst content was 8 -9 vol%. Groundmass are composed mainly of plagioclase, olivine, clinopyroxene and opaque minerals and contain a small amount of glass. Total rock chemical composition analysis of the newly collected samples revealed SiO2 = 53 wt%, MgO = 6 ~ 7 wt%, and FeO */MgO = 1.3 ~ 1.4. SiO2 is almost constant with decreasing MgO content. Ni = 94 ~ 97 ppm and Cr = 325 ppm. The SiO2- (Na2O + K2O) diagram (Macdonald & Katsura,1964) is divided into subalkali series, and the Nb-Zr-Y section diagram (Meschede, 1986) is divided into intraplate tholeiitic basalts.
In Kuchinotsu area in the southern part of Shimabara Peninsula, basalt of about 6.2 Ma, which is older than the age of volcanic rock reported until now, was confirmed. As a result of chemical composition analysis of all rocks of the basalt, it was found that the basalt is classified into subalkali series. Since the results obtained this time are local, it is necessary to obtain detailed distribution and volcanic stratigraphy by increasing analysis values such as connection to land areas. Based on this, we will examine the history of volcanic rock activity, petrological characteristics, and the chemical evolution of magma in the southern part of the Shimabara Peninsula.