10:45 AM - 12:15 PM
[SVC31-P09] Magma Activity of Southeastern Part Izu-Oshima
Keywords:Izu-Oshima , plagioclase control, satellite cone, phreatomagmatic eruption
Izu-Oshima is a volcanic island located at the northernmost part of the Izu-Bonin-Mariana arc, is one of the most active volcanoes in Japan. Oshima volcano is based on Pre-Oshima volcanoes such as Okata, Gyojanoiwaya and Fudeshima volcano (Nakamura, 1964). Oshima-volcano is divided into Senzu-Group, Older-Oshima-Group (OOG) and Younger-Oshima-Group(YOG)(Nakamura, 1964).
In this study, we researched for The southeastern part of Izu-Oshima. Sea cliff in this area consists of Fudeshima volcano of Pre-Oshima volcanoes, Ryuozaki Lava, Kakihara L3・L2・L1 Lava, and Shimobara Lava of the OOG, and Habu Lava and Imasaki Lava of YOG, addition to pyroclastic rock layers such as lapilli tuff and breccia layers that were formed by phreatomagmatic and phreatic explosions OOG to YOG. Volcanic products of YOG clearly show the existence of two type of magmas. Nakano and Yamamoto (1991) divided into plagioclase controlled magma (Pl-magma) and differentiated magma(D-magma).Pl-magma of the bulk chemistry of magma is controlled by plagioclase addition or removal, D-magma is controlled by fractionation of plagioclase, orthopyroxene, clinopyroxene and titanomagnetite(Nakano and Yamamoto,1991). We classified into Pl-magma and the D-magma for the lava and lithic fragments in pyroclastic rocks layers in the southeastern part of Izu-Oshima. The classification was based mainly on the correlation between the amount of plagioclase phenocryst measured by modal measurements and the values of Al2O3 by whole-rock chemical analysis.
Lava of Fudeshima volcano, Ryuozaki lava, and Kakihara L2・L1 lava, which showed positive correlation (more than 0.4) are classified to Pl-magma. Kakihara L3 lava, Shimobara lava, Habu lava, and Imasaki lava, which showed no positive correlation (less than 0.4), are classified to D-magma. Pl-magma is not observed in YOG. and is not limited to the southeastern part but is a general phenomenon in the lava of the lateral eruptions of the YOG in Izu-Oshima (JMA, 2008). This is due to a change in the magma plumbing system of Oshima volcano caused by the formation of the caldera (JMA, 2008).
In the case of Pl-magma, the liquid composition can be estimated by subtracting the compositions of plagioclase phenocryst from whole rock composition (Nakano and Yamamoto, (1991)). Based on this method, we compared the major element compositions of the groundmass. We found that the series of the Pre-Oshima volcano and the series of Oshima Volcano are clearly differentiated.
The fragments of pyroclastic layers are mostly composed of lava exposed in the southeastern part. Some of the fragments have plagioclase phenocryst of more than 20 vol.%. Among the lava exposed in the southeastern part of the volcano, only the lava from Fudeshima volcano has plagioclase phenocryst exceeding 20 vol.%. However, some of the above-mentioned phyric fragments show different petrographical features from Fudeshima volcano. Since these rock fragments were positively correlated with the amount of plagioclase phenocryst and Al2O3 values, they were classified as Pl-magma, and their liquid compositions were determined, which showed the liquid compositions of the lava from Oshima volcano. The liquid compositions of the phyric fragments are like those of Oshima volcanic lava, suggesting that phyric basalt of Oshima volcano lie beneath the southeastern part of Oshima. The origin of these rocks are considered to be phyric basalt from the lower part of OOG exposed in the northeastern part of Izu-Oshima, or cumulate plagioclase phenocryst concentrated in a magma reservoir of the Pl-magma lava in the southeastern part of Izu-Oshima.
Rererences
Nakamura (1964) Bull. Earthq. Res. Inst., Univ. Tokyo, 42, 649-728.
Japan Meteorological Agency (2008) 41p
Nakano and Yamamoto (1991) Bull Volcanology, 53, 112-120
In this study, we researched for The southeastern part of Izu-Oshima. Sea cliff in this area consists of Fudeshima volcano of Pre-Oshima volcanoes, Ryuozaki Lava, Kakihara L3・L2・L1 Lava, and Shimobara Lava of the OOG, and Habu Lava and Imasaki Lava of YOG, addition to pyroclastic rock layers such as lapilli tuff and breccia layers that were formed by phreatomagmatic and phreatic explosions OOG to YOG. Volcanic products of YOG clearly show the existence of two type of magmas. Nakano and Yamamoto (1991) divided into plagioclase controlled magma (Pl-magma) and differentiated magma(D-magma).Pl-magma of the bulk chemistry of magma is controlled by plagioclase addition or removal, D-magma is controlled by fractionation of plagioclase, orthopyroxene, clinopyroxene and titanomagnetite(Nakano and Yamamoto,1991). We classified into Pl-magma and the D-magma for the lava and lithic fragments in pyroclastic rocks layers in the southeastern part of Izu-Oshima. The classification was based mainly on the correlation between the amount of plagioclase phenocryst measured by modal measurements and the values of Al2O3 by whole-rock chemical analysis.
Lava of Fudeshima volcano, Ryuozaki lava, and Kakihara L2・L1 lava, which showed positive correlation (more than 0.4) are classified to Pl-magma. Kakihara L3 lava, Shimobara lava, Habu lava, and Imasaki lava, which showed no positive correlation (less than 0.4), are classified to D-magma. Pl-magma is not observed in YOG. and is not limited to the southeastern part but is a general phenomenon in the lava of the lateral eruptions of the YOG in Izu-Oshima (JMA, 2008). This is due to a change in the magma plumbing system of Oshima volcano caused by the formation of the caldera (JMA, 2008).
In the case of Pl-magma, the liquid composition can be estimated by subtracting the compositions of plagioclase phenocryst from whole rock composition (Nakano and Yamamoto, (1991)). Based on this method, we compared the major element compositions of the groundmass. We found that the series of the Pre-Oshima volcano and the series of Oshima Volcano are clearly differentiated.
The fragments of pyroclastic layers are mostly composed of lava exposed in the southeastern part. Some of the fragments have plagioclase phenocryst of more than 20 vol.%. Among the lava exposed in the southeastern part of the volcano, only the lava from Fudeshima volcano has plagioclase phenocryst exceeding 20 vol.%. However, some of the above-mentioned phyric fragments show different petrographical features from Fudeshima volcano. Since these rock fragments were positively correlated with the amount of plagioclase phenocryst and Al2O3 values, they were classified as Pl-magma, and their liquid compositions were determined, which showed the liquid compositions of the lava from Oshima volcano. The liquid compositions of the phyric fragments are like those of Oshima volcanic lava, suggesting that phyric basalt of Oshima volcano lie beneath the southeastern part of Oshima. The origin of these rocks are considered to be phyric basalt from the lower part of OOG exposed in the northeastern part of Izu-Oshima, or cumulate plagioclase phenocryst concentrated in a magma reservoir of the Pl-magma lava in the southeastern part of Izu-Oshima.
Rererences
Nakamura (1964) Bull. Earthq. Res. Inst., Univ. Tokyo, 42, 649-728.
Japan Meteorological Agency (2008) 41p
Nakano and Yamamoto (1991) Bull Volcanology, 53, 112-120