[S-VC43] Volcanic and igneous activities, and these long-term forecasting

[SVC43-07] Contributions of crust-derived mafic/intermediate and felsic magmas to the Aira catastrophic caldera-forming eruption inferred from geochemical characteristics of essential clasts

*Ayumu Nishihara¹, Yoshiyuki Tatsumi^2,1, Keiko Suzuki-Kamata^2,1, Katsuya Kaneko¹, Jun-Ichi Kimura³, Qing Chang³, Hironobu Hinata⁴ (1.Department of Planetology, Graduate School of Science , Kobe University, 2.Kobe Ocean-Bottom Exploration Center, 3.Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology, 4.Kobe Municipal Tarumi Junior High School)

Keywords:Aira caldera, caldera-forming eruption, Sr isotope ratio, magma mixing

In order to understand the origin of voluminous felsic magmas causing catastrophic caldera-forming (CCF) eruptions, we examined geochemical and petrographical characteristics of essential clasts with variable compositions contained in the Ito ignimbrite erupted at ~30 ka from Aira CCF eruption. The composition of the cores of plagioclase phenocrysts in both white and dark-colored rhyolitic pumice clasts showed a bimodal distribution with peaks at ~An₈₅ and ~An₄₀, while those within andesitic scoria clasts exhibited a unimodal distribution peaked at ~An₈₀. Strontium isotopic composition of the high-An_70-90 and low-An_30-50 plagioclase cores were ⁸⁷Sr/⁸⁶Sr = 0.7068±0.0008 and 0.7059±0.0002, respectively. These observations may lead to a conclusion that the voluminous rhyolitic magma that caused Aira CCF eruption formed by mixing of the andesite and the felsic magmas which crystallized high- and low-An plagioclase phenocrysts, respectively. The andesite magma that may be derived from a less differentiated, possibly mafic magma shows lines of evidence for assimilation of the upper crustal basement rocks such as the Miocene granitic intrusions (⁸⁷Sr/⁸⁶Sr = 0.7119-0.7245) and/or sedimentary rocks of the Shimanto Group (⁸⁷Sr/⁸⁶Sr = ~0.7145). This suggests mixing of the andesite and the felsic magmas occurred at a shallow upper crustal level. The rhyolite magma has far low ⁸⁷Sr/⁸⁶Sr = ~0.7059 than that of the basement rocks whereas has a similar Sr isotope composition with the least assimilated andesite magma. Both the felsic and mafic magmas could have been derived from a same source rock, possibly the lower crust material beneath Aira caldera.