Magma mixing and three end-member magmas of Asahidake in the Taisetsu volcanic group, central Hokkaido, Japan

Abstract

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.