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--

主要研究成果如下：1)楚国地区火成岩的岩石学、块体地球化学和锶-钕同位素特征在白垩纪早期至晚古近纪、晚古近纪至中新世早期发生了较大的变化。这种变化是由于日本海的开放，在构造环境由活动大陆边缘向岛弧过渡的过程中，上地幔和下地壳的化学和物理变化所致。2)白垩纪和早古近纪长英质火成岩根据其锶同位素初始比值(高比值和低比值)至少划分为两个地体。在麦祖鲁带北端两个锶同位素地体之间的边界带上分布着各种火山岩组合，包括玄武岩和高镁安山岩。这些组合由离散的不同地体组成。3)在大量的白垩纪长英质岩浆活动之前，出现了包括高镁安山岩在内的基性岩浆活动，表明基性岩浆活动对长英质岩浆的形成起着重要的触发作用。4)在桑西Masuda地区晚新生代玄武岩中发现了包括蓝晶石和硅线石在内的片麻岩包体。具有碱性流纹岩成分的片麻岩在玄武岩岩浆注入之前就已经发生了部分熔融。这显示了晚新生代在西萨宁地区地下产生长英质岩浆的可能性。5)根据包体的岩石学和矿物化学特征，俯冲沉积经历了角闪岩-麻粒岩相变质和糜棱岩变形，部分熔融发生在岛曼托增生棱柱的深部。包体和花岗岩类的地球化学和锶-钕同位素特征表明，S型花岗闪长岩是高镁安山岩岩浆与混合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.