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.

研究结果总结如下：1）Chugoku地区火成岩岩石的岩石学和散装地球化学和SR-ND同位素特征在白垩纪的古近纪与晚期古代元之间发生了巨大变化，以及晚期古代元与早期中新世早期。由于日本海的开放，在从活跃的大陆缘到岛弧的构造环境中的上地幔和下地壳的化学和物理变化导致了这些变化。 2）白垩纪和早期的古近古代长石火成岩岩石至少根据其SR同位素初始比（高比率和低比例）分为两个地层。火山岩的各种组合，包括玄武岩和高毫克安山岩，分布在Maizuru带北端的两个SR同位素地形之间的边界区域。这些组合包括离散和不同的地形。 3）在大量白垩纪绒毛活性活性之前，发生了基本的火成活性，包括高毫克的安山岩，这表明基本的火成活性扮演着重要的角色，这是长英质岩浆起源的触发作用。 4）在西圣'地区Masuda地区的Cenozoic玄武岩晚期发现了包括Kyanite和sillimanite在内的片麻岩异型石。在注射玄武岩岩浆之前已经发生了片麻岩片与碱性流纹岩成分的部分融化。这表明了在新生代晚期，西萨恩地区下方产生长英质岩浆的潜力。 5）基于异种石的岩石学和矿物质化学，俯冲的沉积物经历了闪石的颗粒物变成颗粒状相的变质和肌膜变形，并且在Shimanto增生棱镜的更深层次中发生了部分融化。异种石和花岗岩的地球化学和SR-ND同位素表明，通过混合高Mg的Andesite Magma和Migmatites的部分熔体相，形成了S型花岗岩。