Generation of calc-alkaline andesite magma through crustal melting induced by emplacement of mantle-derived water-rich primary magma: Evidence from Rishiri Volcano, southern Kuril Arc

Abstract

Calc-alkaline andesite is a typical rock series in many island and continental arcs, and it commonly coexists with tholeiitic series rocks in individual volcanoes. In this study, the main controlling factors on the occurrence of calc-alkaline magmatism are examined for andesitic lavas from Rishiri Volcano, northern Japan. The whole-rock SiO2 contents of lavas range from 57.9 wt% to 61.9 wt%, and they are classified as calc-alkaline series. The lavas have petrographical and mineralogical features that show evidence of magma mixing. The whole-rock compositional variations of the lavas indicate that three end-member magmas were involved in their generation, a primitive mafic end-member magma, an evolved mafic end-member magma, and a felsic end-member magma. We suggest that the primitive mafic end-member magma represents a near primary magma generated in the upper mantle, the evolved mafic end-member magma was derived from the primitive magma by olivine fractionation, and the felsic end-member magma was produced primarily by partial melting of the crust. The primitive end-member magma is estimated to contain ~5 wt% water, which is much higher than those of the primary basaltic magmas of other volcanic stages of Rishiri that do not include calc-alkaline magmas. Based on this observation, we suggest that the water content of the primary magma was the main factor controlling the genesis of calc-alkaline magmatism at Rishiri. The more hydrous the primary magma and its derivative magma, the more extensive the melting of the crust due to the higher water flux from the solidifying magma chamber.