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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--

岛弧下地壳物质的时空变化——来自日本西南内区中生代和新生代火成岩和捕虏体的研究——

基本信息

批准号：
05402020
负责人：
SAWADA Yoshihiro
金额：
$ 23.55万
依托单位：
Shimane University
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (A)
财政年份：
1993
资助国家：
日本
起止时间：
1993 至 1995
项目状态：
已结题

项目摘要

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.

研究结果表明：（1）中国地区火成岩的岩相学、体相地球化学和Sr-Nd同位素特征在古近纪早期-晚第三纪和晚第三纪-中新世早期发生了剧烈变化。这些变化是由于日本海的打开，在构造环境从活动大陆边缘向岛弧过渡期间，上地幔和下地壳发生化学和物理变化的结果。2)白垩纪和古近纪早期长英质火成岩根据其Sr同位素初始比值（高比值和低比值）至少可分为两个系列。舞鹤带北方端的两个锶同位素带之间的边界地带沿着分布着玄武岩和高镁安山岩等多种火山岩组合。这些组合包括离散的和不同的化石。3)在白垩纪大量长英质岩浆活动之前，曾发生过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成因起着重要的触发作用。4)在山西山阴地区增田地区晚新生代玄武岩中发现了蓝晶石和硅线石等片麻岩捕虏体。碱性流纹岩成分的片麻岩在玄武岩岩浆注入之前已经发生部分熔融。这表明在晚新生代在山阴西部地区有生成长英质岩浆的潜力。5）岩石学和捕虏体矿物化学研究表明，俯冲沉积物经历了角闪岩-麻粒岩相变质和糜棱岩变形，在四万十增生棱柱体深部发生了部分熔融。捕虏体和花岗岩类的地球化学和Sr-Nd同位素研究表明，S型花岗闪长岩是由高镁安山岩岩浆与混合岩部分熔融相混合而成。