Chronology and Magmatic Evolution of Shiobara Caldera-Forming Eruption Deposits, Tochigi Prefecture

Abstract

We investigated the chronology and magma system of the Shiobara caldera-forming eruption at Takahara volcano in Tochigi Prefecture, using geologic investigation, chemical analyses of juvenile materials, and U-Pb dating of zircon grains. The Shiobara caldera-forming eruption deposits consist of three ignimbrite units: So-KT (KT), So-TN (TN), and So-OT (OT) (in ascending stratigraphic order). Two exotic tephra layers characteristically containing biotite are recognized in thick volcanic ash soils just below KT and TN, respectively. On the basis of their glass chemistry, Fe-Ti oxide compositions, and U-Pb ages, the lower tephra layer correlates with the Kaisho-Kamitakara Tephra (KMT: 620 ka), whereas the upper tephra layer correlates with one of the members of the Omachi APm Tephra Beds (A₁Pm to A₅Pm: 410-337 ka). In addition to these tephrochronologic results, loess chronometric calculations suggest that KT formed at ~596 ka, whereas TN and OT (OT/TN) are younger than 380 ka. Magma mixing was a common magmatic process in the production of the three ignimbrite units, with each ignimbrite having a different felsic end-member. The considerable geochemical gap between KT and OT/TN is interpreted to mean that crustal melting beneath Takahara volcano changed from wet to dry conditions after a long period of dormancy (> ca. 200 ky) following the KT eruption.