Geological history of the Yabashira volcanoes, central Japan

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The Yabashira volcanoes, located in the Suwa–Yatsugatake Volcanic Province, Central Japan, consist of five volcanic bodies (Tokin-no-iwa, Ryugamine, Yabashirayama, Kengamine, and Byobuiwa) aligned WNW–ESE, that have produced a total eruption volume of ca. 172 km3. Based on detailed volcanological, chronological, and petrological studies, we reconstructed the geological history of the Yabashira volcanoes as follows. Volcanic activity started at ca. 1.2 Ma, forming the Tokin-no-iwa volcanic body, which is a large volcanic body with relatively gentle surface slopes, consisting mainly of thin flows of basaltic and basaltic andesitic lava, and pyroclastic rocks. At ca. 1.1 Ma, volcanic activity (basalt and basaltic andesite, with minor andesite) extended to the ESE, resulting in the formation of two large volcanic bodies (Yabashirayama and Kengamine) with relatively gentle surface slopes. Thick andesitic lavas effused from the western slope of the Tokin-no-iwa volcanic body to form the Ryugamine volcanic body. Activity ceased at ca. 0.95 Ma at the Tokin-no-iwa and Ryugamine volcanic bodies, and at ca. 0.9 Ma at the Yabashirayama volcanic body. Thick andesitic to dacitic lavas effused from the southeastern slope of the Tokin-no-iwa volcanic body at ca. 0.9 Ma, forming the Byobuiwa volcanic body. Overall, activity at the Yabashira volcanoes ceased at ca. 0.8 Ma. Geochemical data indicates that basaltic to basaltic–andesitic activity was dominant at the Yabashira volcanoes. Whole-rock geochemical data indicate that the dominant magma was not derived from the fractional crystallization of a single parental magma. We infer that the geochemical variation of the magma erupted at the Yabashira volcanoes was generated by various processes, including varying degrees of partial melting and crustal assimilation.

Geological history of the Yabashira volcanoes, central Japan

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