Temporal and spatial variations of lower crustal material in an island arc --an approach from Mesozoic and Cenozoic igneous rocks and xenoliths in the Inner Zone of Southwest Japan--

研究结果表明：1)在白垩纪-早古近纪-晚古近纪和晚古近纪-中新世早期，Chugoku地区火成岩的岩石学、整体地球化学和Sr-Nd同位素特征发生了剧烈变化。2)白垩系和早古近纪长英质火成岩的Sr同位素初始比值（高比值和低比值）可划分为至少两个地体。迈祖鲁带北端沿两个sr同位素地体的交界带分布着玄武岩和高镁安山岩等不同组合的火山岩。这些组合包括离散的和不同的地形。3)在白垩纪大量长英质火成岩活动之前，存在包括高镁安山岩在内的基性火成岩活动，表明基性火成岩活动对长英质岩浆的形成起着重要的触发作用。4)在西三印地区增田地区晚新生代玄武岩中发现了蓝晶石、硅线石等片麻岩捕虏体。在玄武岩岩浆注入之前，含碱性流纹岩成分的片麻岩已发生部分熔融。这说明了晚新生代西三印地区具有长英质岩浆的生成潜力。5)根据捕虏体的岩石学和矿物化学特征，俯冲沉积物经历了角闪岩-麻粒岩相变质作用和糜棱岩变形作用，在石曼托增生棱柱的深部发生了部分熔融作用。包体和花岗岩体的地球化学特征和Sr-Nd同位素表明，s型花岗闪长岩是由高镁安山岩岩浆与混染岩部分熔融相混合形成的。

Research results are summarized as follows : 1)Petrographic and bulk geochemical and Sr-Nd isotopes features of igneous rocks in the Chugoku District change drastically between the Cretaceousearly Paleogene to late Paleogene, and the late Paleogene to early Miocene. The changes result from chemical and physical changes of the upper mantle and lower crust during a transition in tectonic setting from active continental margina to island arc, due to opening of the Japan Sea. 2)Cretaceous and early Paleogene felsic igneous rocks are divided into at least two terranes on the basis of their Sr isotope initial ratios (high ratio and low ratio). Varied assemblages of volcanic rocks, including basalts and high Mg andesites, are distributed along the boundary zone between the two Sr-isotope terranes in the northern end of the Maizuru Belt. These assemblages comprise discrete and different terranes. 3)Prior to voluminous Cretaceous felsic igneous activity, basic igneous activity including high Mg andesite occurred, suggesting that the basic igneous activity playd an importanta role as the trigger for genesis of felsic magmas. 4)Gneiss xenoliths including kyanite and sillimanite have been found in late Cenozoic basalt in the Masuda area, west San'in region. Partial melting of gneiss with alkaline rhyolitic composition had already taken place prior to injection of basalt magma. This demonstrates the potential for generation of felsic magma beneath the west San'in region in late Cenozoic time. 5)On the basis of petrography and mineral chemistry of xenoliths, subducted sediments underwent amphibolite to granulite facies metamorphism and mylonitic deformation, and partial melting occurred in the deeper part of the Shimanto accretionary prism. Geochemistry and Sr-Nd isotopes of xenoliths and granitoids indicate that S-type granodiorite was formed by mixing of high Mg andesite magma and partial melt phases of migmatites.