Origin of the sequential Shirakawa ignimbrite magmas from the Aizu caldera cluster, northeast Japan: Evidence for renewal of magma system involving a crustal hot zone

The Shirakawa ignimbrites were generated by caldera-forming eruptions in the Aizu caldera cluster, northeast Japan, during the Early Pleistocene. They have a total volume of > 300 km3 dense rock equivalent and consist of the Kumado ignimbrite (1.4 Ma) from Ono caldera, the Ashino ignimbrite (1.3 Ma) and the Nagurasawa ignimbrite (1.2 Ma) from Tonohetsuri caldera, the Nishigo ignimbrite (1.1 Ma) from Narioka caldera and the Ten-ei ignimbrite (1.0 Ma) from an unknown, concealed caldera. These calderas overlap each other within a 20 × 20 km area in the eastern part of the cluster. The Shirakawa ignimbrites are relatively small in the eruption rate and have sufficient interval times that are longer than the residence times for their magma volume. The individual ignimbrites are composed of 20–40% phenocrysts set in high-silica rhyolite glass and they lack vertical chemical zoning. The SiO2 content of the Kumado, Ashino, Nagurasawa, Nishigo and Ten-ei ignimbrites are 68–70 wt.%, 67–70 wt.%, 68–69 wt.%, 72–74 wt.% and 69–70 wt.%, respectively. They plot within the medium-K field, but show distinct trends for most major elements at the same SiO2 ranges. Although they all have similar arc-related trace-elements patterns with Nb, Ta and Ti negative anomalies, high LREEs and flat HREEs, they differ in their Zr/Nb, Ba/Th and K/La ratios. Also, isotopic ratios of these pumices from each unit are distinct; 87Sr/86Sr varies from 0.7044 to 0.7047 and 143Nd/144Nd from 0.51271 to 0.51278. These results imply that the ignimbrite sources were renewed with every caldera-forming eruption. Seismic tomography and pre-eruption uplifting of the caldera cluster suggest that a substantial amount of mantle-derived magma was stored within the lower crust and contributed to the evolution of a crustal hot zone. Observed geochemical features also suggest that silicic magmas were generated by the melting of variably assimilated mafic intrusions. To explain the renewal of the Shirakawa magma system, melted materials must have been completely evacuated from a deep-seated source zone by each caldera-forming eruption and different mafic rocks subsequently subsided into the hot zone as raw materials for the next eruption.