Asahidake is the youngest composite cone in the Taisetsu volcanic group, central Hokkaido, with total eruption products of ca. 2.6 km³. Its activity (stage-1 to stage-3) started about 20 ka, and the latest eruption occurred 250 years ago. Variously mixed andesitic to dacitic magmas were almost erupted during the stage-1 (2.4 km³) and stage-2 (0.2 km³). This study elaborated the mixing conditions and chemical compositions of three end-member magmas of Asahidake through petrographic, mineralogical and geochemical analyses of the eruption products. Disequilibrium petrological features such as coexistence of An-rich (type-A) and An-poor (type-B) plagioclase phenocrysts were observed. Type-A plagioclase phenocrysts are further classified into two subtypes on the basis of MgO contents in the core; type-A1 (MgO > 0.05 wt%) and type-A2 (MgO < 0.05 wt%). Type-A1 and type-A2 plagioclase phenocrysts are probably derived from two high temperature mafic end-member magmas (Asa-A1 magma at 1160 °C and Asa-A2 magma at 1040 °C), whereas type-B plagioclase phenocrysts are derived from a low temperature dacitic end-member magma (Asa-B magma at 820 °C). We estimated the compositions of the end-member magmas using the correlation between modal compositions of the plagioclase phenocrysts and whole-rock compositions. The obtained SiO₂ contents of Asa-A1, Asa-A2, and Asa-B end-member magmas are 52 wt%, 59 wt% and 66 wt%, respectively. In the Asahidake, most of the end-member magmas were expended in the stage-1, and relatively high ratio of mafic magma was mixed with a small amount of Asa-B magma in the stage-2.

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