Origin and Evolution of the Post-Caldera Magmatic System at Towada Volcano, Japan: Implications for the Current Stage of Progression towards the Next Possible Caldera Eruption

Abstract Towada Volcano, located at the volcanic front of the Northeast Japan Arc, experienced multiple caldera-forming eruptions at 61, 36, and 15.7 ka; post-caldera volcanic activity has occurred intermittently since 15.7 ka with the latest volcanic eruptions with a volcanic explosivity index of 5 in 915 AD. We conducted a petrogeochemical study on the volcanic products from the post-caldera stage to investigate the origin and evolution of the magmatic system, with the aim of understanding the current progress of magmatism towards the next possible caldera-forming eruptions. The volcanic products show a wide compositional variation, ranging from 52 to 75 wt.% SiO2. After the latest caldera-forming felsic magma eruptions at 15.7 ka, mafic volcanic activity commenced. Subsequently, the composition of the erupted materials evolved from generally andesitic to rhyolitic, with an increase in the whole-rock 87Sr/86Sr and 208Pb/206Pb ratios. The storage depths of the magmas that erupted during the Holocene, as proxied by pressure, decreased over time systematically from 400–600, through 300–500, to 100–300 MPa. We suggest that mantle-derived hot magmas were emplaced at the Moho boundary, which resulted in mafic magmatism in the early post-caldera stage. Primitive magmas subsequently caused partial melting of the lower crust, and the resulting partial melts that mixed with the primitive magmas were supplied to the main magma reservoir in the lower–middle crust, where the magmas evolved through assimilation and fractional crystallisation. Intermediate and felsic magmatism following the mafic magmatism resulted from intermittent magmatic discharges from the main evolving magma reservoir and subsequent fractional crystallisation in shallower reservoirs at variable depths. The storage pressures of 100–300 MPa for the magmas of the latest eruptions in 915 AD overlapped with the optimal pressures for the formation of large magma reservoirs that lead to caldera-forming eruptions at Towada. This observation suggests that the recent magmatic system has already entered the ‘maturation phase’ of the caldera cycle. As the shallow-level magma reservoir at 100–300 MPa grows due to continuous magma supply from the main magma reservoir, the magmatic system is expected to transition into the next ‘fermentation phase’. Upon entering this phase, the next caldera-forming eruption could occur following a period of low volcanic activity lasting several thousand to over ten thousand years.