Petrology of the 120 ka Caldera-Forming Eruption of Kutcharo Volcano, Eastern Hokkaido, Japan: Coexistence of Multiple Silicic Magmas and their Relationship with Mafic Magmas

Petrological and geochemical examination of the largest caldera-forming eruption (Kp IV) of Kutcharo volcano, eastern Hokkaido, Japan, were undertaken in order to understand the magma genesis and eruptive processes of a large silicic magma system. The eruption started with an ash and pumice fall, followed by voluminous pyroclastic flows. Juvenile materials are mainly porphyritic pumice and a small amount of heterogeneous, nearly aphyric, scoria is contained in the pyroclastic flow deposits. On the basis of mineral, whole-rock and matrix glass chemistry, two silicic magmas (rhyolitic and dacitic) and three intermediate (andesitic) magmas were identified. Temporal variations in the whole-rock and matrix glass chemistry of the juvenile materials indicate that the activity originated from a large zoned silicic magma chamber in which dacitic magma had stagnated beneath more voluminous rhyolitic magma. During the generation of pyroclastic flows, andesitic magmas were sequentially injected into the zoned chamber, resulting in the eruption of small amounts of heterogeneous products along with voluminous silicic magmas. The rhyolite-MELTS program, mass-balance calculations, and Rayleigh fractionation models cannot explain the generation of two silicic magmas by fractional crystallization of coexisting andesitic magmas. In addition, the higher 87Sr/86Sr ratio of dacitic pumice suggests that the dacitic magma was not a parent of the rhyolitic magma. Therefore, we infer that both the rhyolitic and dacitic magmas were produced by the accumulation of interstitial melts generated from crustal materials with heterogeneous Sr isotopic compositions. It may be common that large silicic magma systems that produce caldera-forming eruptions are composed of multiple silicic magmas that are produced from an extensive area of heterogeneous crust. Mg and Ti diffusion profiles in Fe–Ti oxide phenocrysts indicate that three different andesitic magmas were successively injected into the zoned chamber hours to weeks before the eruptions. Their injection may have triggered the Kp IV eruptive activities.