Magmatism in Kyushu : Geochemistry for the mantle wedge and tectonics

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1.In order to explore origin of silicic magmas related to large caldera formation in southern Kyushu, field survey and petrological investigation have been undertaken at the Kikai Caldera volcano, one of the large calderas in south Kyushu. The magmatism is characterized by bimodal (basaltic and rhyolitic) composition. The silicic magmas were originated by partial melting of underplated basaltic magmas.2.For comparison with south Kyushu magmatism, submarine survey in the southern Ryukyu arc-Okinawa Trough system have been done, and petrochemical analyses were also performed. The regional east-west chemical variation with boundary at 123E have been revealed, which is related with the development stage of the Okinawa Trough.3.Across-arc regional variation in chemistry of mantle wedge under the north-central Kyushu was investigated. The source mantle for back-arc side magmas is inferred to be Ocean island basalt (OIB) type, while mantle for volcanic front magmas is depleted MORB-like.4.New analytical technique, using ‘Double spike' method, for Pb isotopic measurement has been developed. Using this method, accurate Pb isotope data have been obtained for many volcanic rocks.5.To evaluate the nature of plate, the Philippine Sea Plate (the West Philippine Basin, WPB), subducting beneath the Kyushu to Ryukyu arc, submarine survey including topography and ROV dives have been undertaken. It is concluded that magmas at the western part of WPB were derived from enriched-OIB like source mantle.