Abstract
Akagi Volcano, a large composite volcano located on the volcanic front of NE Japan arc, consists of older and younger ones. The latter is subdivided into the volcanic-cone-building stage (ca. 220-150 ka) erupting voluminous andesite lava flows, the pumice-eruption stage (ca. 150-44 ka) ejecting many andesite-dacite pumice falls and pumice flows, and the post-caldera stage (44-30 ka) effusing rhyolite lava domes. Geochemical correlations between pumice flow and pumice fall deposits have improved our understanding of the eruptive history of the pumice eruption stage, using a new magma-discharge time-step diagram. After the effusive volcanic-cone-building stage, the magma-discharge rate had temporally declined until the vigorous pumice-eruption sub-stage (ca. 60-44 ka). Geochemical features of trace element abundance suggest that the products of Akagi Volcano were generated by interactions between mantle-derived magma and lower-crustal materials, along with amphibole fractionation. The contributions of the crustal melt to the generation of felsic magma was grater during the pumice-eruption stage than during the volcanic-cone-building stage. Increases in the magma-discharge rate appear to have corresponded to the injection of voluminous mantle-derived magma into the lower crust.
