16:30 〜 16:45
[SVC30-11] Petrogenesis of Nekodake magma in Aso volcano
キーワード：阿蘇火山、カルデラ火山、根子岳、マグマ供給系、マグマ混合
Caldera volcanoes, which cause large-volume explosive eruptions (more than tens cubic kilometers), have magma plumbing systems producing voluminous magma. Aso volcano is one of the largest caldera volcanoes in Japan, and its activities are characterized by four large-scale eruptions (Aso-1, -2, -3, and -4) and many small eruptions (e.g., Ono and Watanabe, 1985). Previous studies have shown that the Aso magmas with high K2O content to SiO2 content (high-K series) are petrologically distinctive from magmas of other quaternary volcanoes around Aso volcano (e.g., Ono and Watanabe, 1985) and are generated by melting of the lower crust (Kaneko et al., 2015).
Nekodake is a small volcano with about 3 km in diameter and about 700 m higher than its base and located near the eastern rim of the Aso caldera like the post-caldera central cones in Aso volcano. Geological study and radiometric dating indicate that the activity of Nekodake occurred during the activities of Aso volcano although the activity age is not strictly determined (before Aso-3 or near Aso-4). On the other hand, the Nekodake magma is clearly lower in K2O content than the Aso magmas. Then, it has been considered that the Nekodake magma came from a different magma plumbing system from Aso volcano. The purpose of this study is to clarify petrogenesis of the Nekodake magma. The distinctive feature of the Nekodake magma means that a different type of magma was generated adjoining to the large magma plumbing system of Aso volcano. Understanding this is important to elucidate the entire magma plumbing system of caldera volcanoes.
We carried out detailed petrological description and compositional analyses of whole-rock, groundmass, and phenocryst minerals. Important petrological features are in the following.
(1) Whole-rock compositions of the the Nekodake rocks are 53 - 60 wt.% SiO2.
(2) All the rocks are very porphyritic (47- 69 vol.% in modal composition) and commonly contain plagioclase, clinopyroxene, orthopyroxene, and opaque minerals as phenocryst. Rocks with less SiO2 content contain olivine phenocryst while with more SiO2 content hornblende phenocryst.
(3) The whole-rock compositions show a single linear trend on each of the Harker diagrams.
(4) Anorthite (An) contents of plagioclase phenocrysts are bimodal; the peak An-contents are 58 and 76, respectively. Corroded textures are observed in plagiocalse, pyroxene, and olivine phenocrysts. Part of plagioclase phenocrysts have reverse zonation in An-contents and part of pyroxene and hornblende phenocrysts in Mg/Fe ratio.
(5) Sr isotopic ratio of the Nekodake rocks is higher (0.7041-0.7047) than those of the Aso magmas (0.7040 – 0.7042)(Shinmura, 2013).
Features 3 and 4 suggest that the Nekodake magmas were formed by two-component mixing of mafic and intermediate magmas. We estimated the endmember magmas using our original calculation method on the basis of number of An-contents distribution of plagioclase phenocryst and whole-rock composition. The estimated mafic magma is 49 – 52 wt.% SiO2, which is compositionally similar to the mafic magma of Aso volcano. The estimated intermediate magma is 59 - 60 wt.% SiO2 and compositionally different from the Aso magmas, and is not generated by fractional crystallization from the estimated mafic magma. This result and Feature 5 suggest that this high Sr isotopic ratio comes from the intermediate endmember magma which was produced by melting of some shallower crust than the magma generation level of the Aso magmas. We conclude that the magma plumbing system of Nekodake is not completely different from Aso volcano but only the magma generation process of the SiO2-rich magma is different from Aso magmas. Our result shows that crustal melting occurs at the shallower level as well as in the lower crust in Aso volcano.
We thank Dr. Takehiro Koyaguchi and Mr. Hiroshi Ikemoto for their providing rock samples of Nekodake.
Nekodake is a small volcano with about 3 km in diameter and about 700 m higher than its base and located near the eastern rim of the Aso caldera like the post-caldera central cones in Aso volcano. Geological study and radiometric dating indicate that the activity of Nekodake occurred during the activities of Aso volcano although the activity age is not strictly determined (before Aso-3 or near Aso-4). On the other hand, the Nekodake magma is clearly lower in K2O content than the Aso magmas. Then, it has been considered that the Nekodake magma came from a different magma plumbing system from Aso volcano. The purpose of this study is to clarify petrogenesis of the Nekodake magma. The distinctive feature of the Nekodake magma means that a different type of magma was generated adjoining to the large magma plumbing system of Aso volcano. Understanding this is important to elucidate the entire magma plumbing system of caldera volcanoes.
We carried out detailed petrological description and compositional analyses of whole-rock, groundmass, and phenocryst minerals. Important petrological features are in the following.
(1) Whole-rock compositions of the the Nekodake rocks are 53 - 60 wt.% SiO2.
(2) All the rocks are very porphyritic (47- 69 vol.% in modal composition) and commonly contain plagioclase, clinopyroxene, orthopyroxene, and opaque minerals as phenocryst. Rocks with less SiO2 content contain olivine phenocryst while with more SiO2 content hornblende phenocryst.
(3) The whole-rock compositions show a single linear trend on each of the Harker diagrams.
(4) Anorthite (An) contents of plagioclase phenocrysts are bimodal; the peak An-contents are 58 and 76, respectively. Corroded textures are observed in plagiocalse, pyroxene, and olivine phenocrysts. Part of plagioclase phenocrysts have reverse zonation in An-contents and part of pyroxene and hornblende phenocrysts in Mg/Fe ratio.
(5) Sr isotopic ratio of the Nekodake rocks is higher (0.7041-0.7047) than those of the Aso magmas (0.7040 – 0.7042)(Shinmura, 2013).
Features 3 and 4 suggest that the Nekodake magmas were formed by two-component mixing of mafic and intermediate magmas. We estimated the endmember magmas using our original calculation method on the basis of number of An-contents distribution of plagioclase phenocryst and whole-rock composition. The estimated mafic magma is 49 – 52 wt.% SiO2, which is compositionally similar to the mafic magma of Aso volcano. The estimated intermediate magma is 59 - 60 wt.% SiO2 and compositionally different from the Aso magmas, and is not generated by fractional crystallization from the estimated mafic magma. This result and Feature 5 suggest that this high Sr isotopic ratio comes from the intermediate endmember magma which was produced by melting of some shallower crust than the magma generation level of the Aso magmas. We conclude that the magma plumbing system of Nekodake is not completely different from Aso volcano but only the magma generation process of the SiO2-rich magma is different from Aso magmas. Our result shows that crustal melting occurs at the shallower level as well as in the lower crust in Aso volcano.
We thank Dr. Takehiro Koyaguchi and Mr. Hiroshi Ikemoto for their providing rock samples of Nekodake.