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-An70-90 and low-An30-50 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.