16:00 〜 16:15
[SVC30-08] Generation processes of three silicic pyroclastic flows’ magmas at Kuju volcano and difference of magmatic features between Kuju and Aso volcanoes
キーワード：九重火山、阿蘇火山、珪長質マグマ活動、岩石学、火砕流
Kuju volcano and Aso volcano on the volcanic front in Kyushu are close to each other (about 20 km) and repeatedly erupted silicic magmas as pyroclastic flows. However, eruption volumes of the magmas are different in the two volcanoes; Kuju volcano discharged less than 10 km3 of silicic magma during one pyroclastic eruption whereas Aso volcano 100 km3 order. In order to understand difference of magma plumbing system with different size of silicic magma, we first aim to clarify the detailed petrological properties and generation processes of silicic magmas in pyroclastic eruptions at Kuju volcano, which have not been reported in detail. Furthermore, we compare features of the silicic magmas in Kuju and Aso volcanoes.
Kuju volcano is composed of many andesitic stratovolcanic cones and their skirt with pyroclastic flow deposits. Three medium-sized pyroclastic flow deposits have been recognized; they are Miyagi Pyroclastic Flow Deposit (Myg, about 150 ka, 1.00 DRE km3), Shimosakada Pyroclastic Flow Deposit (Sms, about 120 ka, 0.20 DRE km3) and Handa Pyroclastic Flow Deposit (Hnd, about 54 ka, 7.2 DRE km3) (these three pyroclastic flow deposits are referred to as the 3-pfls hereafter). We carried out chemical analyses of juvenile clasts in the 3-pfl in whole-rock, groundmass, and phenocryst composition.
All the 3-pfls have three kinds of juvenile clasts; most of them is pumice (PM), and small amounts of banded pumice (BPM) and dark enclave (DEN) are found as the juvenile clasts. We carried out petrographical description, modal analysis of phenocryst, and compositional analyses in whole-rock, groundmass, and phenocrysts for the PMs in all the 3-pfls, and additionally the same analyses for the BPM and DEN in Hnd.
Each type of the three juvenile clasts in the 3-pfls has the same petrological features. The PM is the most silicic in the three juvenile clasts (about 65 wt.% SiO2 in whole-rock; about 78 wt.% SiO2 in groundmass), amount of phenocryst is 30~40 vol.%, and An content of plagioclase phenocryst are 80~40 and about 45 in core and rim compositions, respectively. White part of the BPM is petrologically the same as the PM. Dark part of the BPM (BPM-D) are less SiO2 contents (60-64 wt.% SiO2 in whole-rock; 62-75 wt.% SiO2 in groundmass), amount of phenocryst is less (10~20 vol.%) than the PM, and composition of plagioclase phenocryst is similar to the PM. The DENs are less SiO2 content in whole-rock (55-59 wt.%) than the PM while the same in groundmass as the PM, amount of phenocryst is the most abundant (50~70 vol.%) in the three juvenile clasts, and the ratio of high An plagioclase is larger than the PM and BPM-D.
On the basis of the above petrological features of the juvenile clasts, it is inferred that magma genesis of for the juvenile clasts and processes of the magma chamber in Kuju silicic activity as follows. First, a mafic magma formed a magma chamber in the upper crust and differentiated to a silicic magma, which is the PM magma, and simultaneously made crystal mush around the silicic magma. Some of this crystal mush is corresponding to the DEN magma. Then a new mafic magma injected to the chamber and mixed with the silicic magma, which is the BPM-D magma. In all the three silicic activities, Myg, Sms, and Hnd, the magmatic processes and composition are common, indicating that supply system of the silicic magma in Kuju volcano did not change during 150 ky.
The magmas of the medium-sized silicic eruption at Kuju volcano and the large silicic eruption at Aso volcano are compared. Kuju volcano erupted clearly different magma in composition from Aso volcano, for example, in K2O content and etc. Hornblende thermobarometry and Fe-Ti oxide redox-meter shows Kuju’s magma is richer in crystal, more highly evolved, colder, and more oxidized in a shallower magma chamber than Aso volcano. These differences of magmatic features and conditions between the two volcanoes are interesting, although causes of these differences are unknown.
Kuju volcano is composed of many andesitic stratovolcanic cones and their skirt with pyroclastic flow deposits. Three medium-sized pyroclastic flow deposits have been recognized; they are Miyagi Pyroclastic Flow Deposit (Myg, about 150 ka, 1.00 DRE km3), Shimosakada Pyroclastic Flow Deposit (Sms, about 120 ka, 0.20 DRE km3) and Handa Pyroclastic Flow Deposit (Hnd, about 54 ka, 7.2 DRE km3) (these three pyroclastic flow deposits are referred to as the 3-pfls hereafter). We carried out chemical analyses of juvenile clasts in the 3-pfl in whole-rock, groundmass, and phenocryst composition.
All the 3-pfls have three kinds of juvenile clasts; most of them is pumice (PM), and small amounts of banded pumice (BPM) and dark enclave (DEN) are found as the juvenile clasts. We carried out petrographical description, modal analysis of phenocryst, and compositional analyses in whole-rock, groundmass, and phenocrysts for the PMs in all the 3-pfls, and additionally the same analyses for the BPM and DEN in Hnd.
Each type of the three juvenile clasts in the 3-pfls has the same petrological features. The PM is the most silicic in the three juvenile clasts (about 65 wt.% SiO2 in whole-rock; about 78 wt.% SiO2 in groundmass), amount of phenocryst is 30~40 vol.%, and An content of plagioclase phenocryst are 80~40 and about 45 in core and rim compositions, respectively. White part of the BPM is petrologically the same as the PM. Dark part of the BPM (BPM-D) are less SiO2 contents (60-64 wt.% SiO2 in whole-rock; 62-75 wt.% SiO2 in groundmass), amount of phenocryst is less (10~20 vol.%) than the PM, and composition of plagioclase phenocryst is similar to the PM. The DENs are less SiO2 content in whole-rock (55-59 wt.%) than the PM while the same in groundmass as the PM, amount of phenocryst is the most abundant (50~70 vol.%) in the three juvenile clasts, and the ratio of high An plagioclase is larger than the PM and BPM-D.
On the basis of the above petrological features of the juvenile clasts, it is inferred that magma genesis of for the juvenile clasts and processes of the magma chamber in Kuju silicic activity as follows. First, a mafic magma formed a magma chamber in the upper crust and differentiated to a silicic magma, which is the PM magma, and simultaneously made crystal mush around the silicic magma. Some of this crystal mush is corresponding to the DEN magma. Then a new mafic magma injected to the chamber and mixed with the silicic magma, which is the BPM-D magma. In all the three silicic activities, Myg, Sms, and Hnd, the magmatic processes and composition are common, indicating that supply system of the silicic magma in Kuju volcano did not change during 150 ky.
The magmas of the medium-sized silicic eruption at Kuju volcano and the large silicic eruption at Aso volcano are compared. Kuju volcano erupted clearly different magma in composition from Aso volcano, for example, in K2O content and etc. Hornblende thermobarometry and Fe-Ti oxide redox-meter shows Kuju’s magma is richer in crystal, more highly evolved, colder, and more oxidized in a shallower magma chamber than Aso volcano. These differences of magmatic features and conditions between the two volcanoes are interesting, although causes of these differences are unknown.