Pliocene voluminous volcanism and tectonics in Southern Kyushu

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The Hisatsu volcanic province is a across arc belt of about 100km in length from northwest to southeast and 25km wide in Southern Kyushu. The volcanic activity, occurred in the period from late Miocene to middle Pleistocene, is divided into three stages. The Hisatsu volcanic rocks are classified into low-potassium (low-K) and high-potassium (high-K) andesites. In the first stage, the low-K andesites occurred in the west side of the province and the high-K andesites in the east side. In the second stage, the high-K andesite erupted widely all through out the area. In the third stage, the low-K andesite erupted in the west side again, and the high-K andesite in the east side. Further, the Hisatsu volcanic rocks were derived from three kinds of parental magma, high-K high-magne-sian andesite, low-K high-magnesian andesite and tholeiite magmas.The following is a plausible model for the origin of the Hisatsu Volcanism. Beneath the southern Kyushu, the fertile and hot asthenosphere has exist … More ed under the island arc type mantle from late Miocene to recent. The island arc type mantle is estimated to be composed of the fragments of the ancient subducted oceanic crusts and sediments, and the mantle wedge metasomatized by the hydrous fluid from previously subducted slabs, etc. When the asthenosphere ascends to shallow part of the mantle, it begins to melt. If the degree of the partial melting of the asthenosphere is small, within-plate type alkali basalt magma is formed. In case of large degree of partial melting, within-plate type tholeiite magma is produced. If the alkali basalt and tholeiite magmas reacts with the island arc type mantle, the high and low K_2O high-magnesian andesite magmas are formed, respectively. In this process, basalt magma acquires the subduction signature and SiO_2 in the magma increases. The fractional crystallization of the high-magnesian andesite magmas formed the voluminous two pyroxene andesites. Further, basalt magma with island arc signature seems to produce by the partial melting of the island arc type mantle.This model can also explain the characteristics of the Cenozoic volcanism in Kyushu. Less