著者

中田 節也 鎌田 浩毅

出版者

特定非営利活動法人 日本火山学会

雑誌

火山.第2集 (ISSN:04534360)

巻号頁・発行日

vol.33, no.4, pp.273-289, 1988

被引用文献数

8

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Volcanism and deposition of volcaniclastic materials within a graben in the Shimabara area, western Kyushu, started in early Pliocene and have continued up to the present. The volcanic products comprise alkalic to high-alkali tholeiitic basalts in addition to calc-alkalic andesites. The basalts are of a member of magmas erupted extensively in NW Kyushu from late Miocene to Recent. Their incompatible-element patterns in spider gram, in which the abundances of Rb, Ba, K, Nb, Zr and Y are normalized to those of MORB, are uniform independently of their eruption-ages, and show positive Ba and Nb anomalies strongly indicative of the marked affinity with ocean-island basalts. These patterns are clearly different from those of island-arc basalts which have negative Nb anomaly, as represented by the Quaternary high-alumina basalts in central Kyushu, and from those of back-arc basin basalts usually showing the same anomaly. It is concluded that basaltic magmas in NW Kyushu were derived from partial melt of fertile mantle with high Nb/Zr. Most of the andesites in the Shimabara area have phenocryst assemblages showing co-existence of magmas with different compositions. This fact and the spidergram pattern without Nb anomaly of the andesites lead us to a magmatic model; the andesite magma originated from mixing of positive-Nb-anomaly basaltic melt with negative-Nb-anomaly acidic partial melt of lower-crust materials which were presumably heated by the former melt. The graben in the Shimabara area seems to have developed in southern periphery of the region under which fertile mantle materials have been rising up from the depth and the subducted oceanic slab has not reached, like the "hot region".