Magmatism in Kyushu : Geochemistry for the mantle wedge and tectonics

1.为了探索与九州南部大型破火山口建造有关的硅质岩浆的成因，在九州南部大型破火山口之一的Kikai火山口进行了野外调查和岩石学研究。岩浆活动具有双峰式(玄武岩和流纹岩)成分特征。硅质岩浆起源于底板玄武岩岩浆的部分熔融。2.为了与南九州岩浆作用进行对比，在琉球弧-冲绳海槽系统南部进行了海底调查，并进行了岩石化学分析。揭示了以123E为界的区域东西化学变化，这与冲绳海槽的发育阶段有关。3.研究了九州中北部地幔楔体化学成分的跨弧区变化。弧后岩浆的源地幔为洋岛玄武岩型，而火山前缘岩浆的源地幔为亏损的类似MORB的地幔。4.发展了一种新的铅同位素分析方法--“双尖峰”法。利用这种方法，许多火山岩都获得了准确的铅同位素数据。5.为了评价板块的性质，菲律宾海板块(西菲律宾盆地，WPB)从九州俯冲到琉球弧下，进行了包括地形和ROV潜水在内的海底调查。结果表明，WPB西部岩浆来源于富集型OIB源区地幔。

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.