Identification of the source caldera for a Pliocene ash-flow tuff in Northeast Japan based on apatite trace-element compositions and zircon U-Pb ages

In tephrochronology, trace-element compositions of apatite phenocrysts are useful because they change in response to various magmatic conditions as apatite crystallizes. Moreover, because apatite tolerates burial diagenesis and welding processes without chemical changes, its trace-element compositions are a powerful tool for correlating pre-Quaternary tephra; however, this method is not extensively applied. The Hirosegawa Tuff, composed of a pair of Pliocene ash-fall and ash-flow tuffs, is a distinctive marker tephra that is used in geological mapping and for correlating sedimentary rocks in the eastern part of central NE Japan. However, the source volcano of the Hirosegawa Tuff has not been identified because many calderas coexisted in NE Japan during the Pliocene. Analysis of the trace-element compositions of apatite, major element compositions of ilmenite, and U-Pb zircon ages demonstrate that, depending on the area, the previously mapped Hirosegawa Tuff comprises different volcanic ejecta. Furthermore, this study identified that the source volcano of the Hirosegawa Tuff in the type section was the Shiroishi Caldera, located 50 km south of Sendai City.