Generation of andesitic–dacitic magmas by open-system progressive melting of the middle crust in Rishiri Volcano, northern Japan

Understanding the genesis of andesite and dacite magmas in subduction zones is important because their compositions are similar to the average composition of the continental crust and they have higher potential for violent eruptions than mafic magmas. In this study, we conducted a petrological and geochemical analysis of Rishiri-Wankonosawa (Rs-Wn) pumices that erupted explosively from Rishiri Volcano, located at the rear of the Kuril Arc, to understand the origin of the voluminous andesitic–dacitic magmas. The main volume of pumices has relatively homogeneous whole-rock compositions (66.0 to 67.7 wt% SiO₂); however, those ejected in the later stage of the eruption are less differentiated and show wide compositional variations (59.0 to 64.5 wt% SiO₂). The Sr, Nd, and Pb isotopic ratios of the pumices vary systematically with the SiO₂ content. We demonstrate that the tightly curved whole-rock compositional trends of the Rs-Wn pumices in geochemical variation diagrams cannot have been produced by assimilation and fractional crystallization of mafic magmas; rather, the compositional variations were established by open-system melting of the middle crust, that is, progressive partial melting of the crustal source into which there was a continuous influx of mafic melts. Partial melting of the middle crust was caused by heating due to the emplacement of hot mafic melts, and some of the mafic melts were supplied to the melting region. The partial melts then accumulated to form a zoned magma reservoir at a depth of ∼400 MPa, from which heterogeneous magmas ascended and erupted explosively. At Rishiri, andesite–dacite suites with different geochemical features have occurred at least three times since 50 ka. Through a comparison of the generation processes for these suites, it is suggested that the origin of the Rs-Wn magmas resulted essentially from the emplacement of hot mafic magmas in the thermally mature middle crust.