5:15 PM - 6:45 PM
[SCG54-P01] Cryptic silicic magma beneath Izu-Tobu volcanoes: Constraints from trace element compositions of plagioclase phenocrysts
Keywords:Izu-Tobu volcanic field, monogenetic volcano, trace element composition, plagioclase, magma plumbing system, silicic magma
The Higashi-Izu Monogenetic Volcanic Field (HIMVF) has been active since ~0.1 Ma. The erupted magma in HIMVF had been mafic with < 60 wt% SiO2 until Kawagodaira rhyolitic eruption occurred, and dacitic-rhyolitic magma has also begun to erupt since Kawagodaira rhyolitic eruption ~3.1 ka. However, Hatada et al. (2020) found rhyolitic glass inclusion and their host low An# plagioclase with An# [=100Ca/(Ca+Na)] of ~30-50 in the basaltic andesite of Omuroyama erupted ~4ka and discussed that cryptic rhyolitic magma existed ~900 years earlier than Kawagodaira rhyolitic eruption. Nozawa et al. (2023VSJ abstract) analyzed trace element compositions of the rhyolitic glass inclusions and their host low-An# plagioclase of Omuroyama using LA-ICP-MS (Laser Ablation ICP-MS). They showed that they are in equilibrium and that the trace element composition of rhyolitic melts of Omuroyama and Kawagodaira are almost identical. There are monogenetic volcanoes (Kadono, Uchino Chano, Komuroyama, Akakubo, Ioyama) that erupted at 2.7-100 ka around Omuroyama, Izu-Kogen. Nozawa et al. (2022VSJ abstract) show that low-An# plagioclase phenocrysts with similar major element composition with that of Omuroyama are included up to 3.1 vol% in magmas of all of these six volcanoes. Are these low-An# plagioclases the same origin as the plagioclase in equilibrium with the rhyolite melt of Omuroyama? To clarify this, we analyzed the trace element compositions of the low-An# plagioclases from the 6 volcanoes of Izu-Kogen and compared them with the low-An# plagioclases of Omuroyama. In addition, we discuss about about the magma plumbing system beneath HIMVF (Izu-Kogen).
Major element composition of the rhyolite melt inclusion of Omuroyama is similar to those of the residual melt of the partial melting experiment of the basalt from HIMVF under H2O-rich, high-P-T condition (0.5 GPa and 875ºC: Kawamoto, 1996). Mass balance calculation using the modal abundance of the experimental run product and the trace element compositions of constituent minerals from Omuroyama and Kawagodaira lavas indicates that the calculated trace element composition of the residual melt is almost similar to the composition of the rhyolite of Omuroyama analyzed using LA-ICP-MS. The results suggest that the rhyolitic melt could be formed by partial melting/crystallization differentiation of hydrous basalt (magma).
The trace element compositions of low-An# plagioclases from the six volcanoes of Izu-Kogen well coincide with that of Omuroyama. In addition, the trace element compositions of the coexisting melt calculated from the low-An# plagioclases are well consistent with those of rhyolitic glass inclusions from Omuroyama and also Ioyama. The results suggest that rhyolitic melt had been (Probably intermittently) beneath Izu-Kogen since 100 ka.
Mafic magmas of Izu-Kogen show a linear trend toward the rhyolites of Omuroyama and Kawagodaira and do not deviate toward plagioclase in Harker diagrams. With considering that all magmas of the six volcanoes have low-An# plagioclases, the compositional variation is attributed to the mixing between the basaltic and the rhyolitic magmas. In addition, we estimated the abundance of low-An# plagioclases in the end-member rhyolite to be < 10vol% at the timing of magma mixing. Present results suggest that rhyolitic magma had formed before the monogenetic volcanic eruptions, and the deep-derived basaltic magmas had mixed with the rhyolitic magma with various ratios and then erupted; this process has occurred since at least 100ka.
Major element composition of the rhyolite melt inclusion of Omuroyama is similar to those of the residual melt of the partial melting experiment of the basalt from HIMVF under H2O-rich, high-P-T condition (0.5 GPa and 875ºC: Kawamoto, 1996). Mass balance calculation using the modal abundance of the experimental run product and the trace element compositions of constituent minerals from Omuroyama and Kawagodaira lavas indicates that the calculated trace element composition of the residual melt is almost similar to the composition of the rhyolite of Omuroyama analyzed using LA-ICP-MS. The results suggest that the rhyolitic melt could be formed by partial melting/crystallization differentiation of hydrous basalt (magma).
The trace element compositions of low-An# plagioclases from the six volcanoes of Izu-Kogen well coincide with that of Omuroyama. In addition, the trace element compositions of the coexisting melt calculated from the low-An# plagioclases are well consistent with those of rhyolitic glass inclusions from Omuroyama and also Ioyama. The results suggest that rhyolitic melt had been (Probably intermittently) beneath Izu-Kogen since 100 ka.
Mafic magmas of Izu-Kogen show a linear trend toward the rhyolites of Omuroyama and Kawagodaira and do not deviate toward plagioclase in Harker diagrams. With considering that all magmas of the six volcanoes have low-An# plagioclases, the compositional variation is attributed to the mixing between the basaltic and the rhyolitic magmas. In addition, we estimated the abundance of low-An# plagioclases in the end-member rhyolite to be < 10vol% at the timing of magma mixing. Present results suggest that rhyolitic magma had formed before the monogenetic volcanic eruptions, and the deep-derived basaltic magmas had mixed with the rhyolitic magma with various ratios and then erupted; this process has occurred since at least 100ka.