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Comparative study on the exhumation processes of high-pressure metamorphic belts, western Pacific

西太平洋高压变质带折返过程对比研究

基本信息

批准号：
10640464
负责人：
MAEKAWA Hirokazu
金额：
$ 2.18万
依托单位：
Osaka Prefecture University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
1998
资助国家：
日本
起止时间：
1998 至 1999
项目状态：
已结题

项目摘要

To understand the tectonic processes of the high-pressure metamorphic belts in the western margins of Pacific Ocean. I studied the metamorphism and deformation of the rocks in the Sanbagawa metamorphic belt and the Haast schists of New Zealand, and compared them with those in the Kamuikotan and Franciscan belts. Predominant direction of stretching lineation is E-W in the Sanbagawa metamorphic belt at Nushima. An analysis of microfabrics of stretching lineation indicates that the pull-apart fabrics have been formed with increase of temperature at the final stage of metamorhism. I studies the Haast schists in term of the rock samples taken in 1997. The Haast schists along the Lake Hawea route can be divided into two zones ; the southern chlorite zone and the northern garnet zone. Plagioclase is albite (<AnィイD25ィエD2) in the chlorite zone, whereas that show two phases (AnィイD20-5ィエD2 and AnィイD220-30ィエD2) near the garnet isograd in the garner zone, and is AnィイD25-35ィエD2 in the further north … More higher-grade regions. Sudden decrease of modal compositions of epidote at the region where Ca-rich plagioclase appears suggests that the decomposition of epidote related to the Ca-rich plagioclase froming reaciton. The lack of biotite zone in the study area suggests that the metamorphic P-T trends of the Haast schist terrane is situated at the intermediate pressure condition between the Sanbagawa and the Scotich Highland just below the intersection of biotite- and garnet-forming curves.A large number of serpentinite seamounts in the western Pacific Izu-Ogasawara-Mariana forearcs indicate the serpentinite diapirism under the tensional stress field in the forearc regions. The presence of blueschist-facies rocks in the serpentinite seamounts provides evidence that the serpentinites generated from the hanging-wall peridotites have penetrated the high-pressure metamorphic region at depth in the accretionary prism. The high-pressure metamorphic terranes in the circum-Pacific regions, the Franciscan Complex, California, and the Kamuikotan metamorphic belt, Japan, are accompanied with serpentinite melange with deeper high-grade metamorphic rocks, whereas the Sanbagawa metamorphic belt, Japan, does not contain any serpentinite melange. It is highly probable that the former were formed under tensional stress regime, and the latter under compressional one. The rare occurrence of serpentinites and lack of serpentinite melange in the Sanbagawa belt may have been caused by the compressional stress environment in the trench-arc region during the metamorphism. Under such compressional stress environments, scraping off hanging-wall peridotites seems to be only a possibility to incorporate mantle materials into the accretionary prism which develops along the trench. The model can explain the limited occurrence of serpentinites to the high-grade pelitic schists in the Sanbagawa belt. Less

了解太平洋西缘高压变质带的构造过程。本文研究了三坝川变质带和新西兰哈斯特片岩的变质变形作用，并与Kamuikotan带和Franciscan带进行了对比。努马山三坝川变质带的拉伸线理以东西向为主。拉伸线理显微组构分析表明，拉分组构是在变质作用末期随温度升高而形成的。本文根据1997年的岩石样品对哈斯特片岩进行了研究。沿着哈威亚湖路线的哈斯特片岩可分为两个带;南部石榴石带和北方石榴石带。斜长石在榴辉岩带中为钠长石（<An D25 D2），而在Garner带中，在石榴石等值线附近显示为两个相（An D20-5 D2和An D220-30 D2），在更北的区域为An D25-35 D2 ...更多信息更高等级的区域。在富钙斜长石出现的区域，绿帘石的模态成分突然减少，表明绿帘石的分解与富钙斜长石的形成反应有关。研究区缺乏黑云母带，表明哈斯特片岩的变质作用P-T走向处于黑云母-石榴石成矿曲线交汇点下方的三坝川高地和斯科蒂希高地之间的中压环境;西太平洋伊豆-小笠原-马里亚纳前弧区大量的蛇纹岩海山，表明前弧区存在张性应力场作用下的蛇纹岩底辟作用。蛇纹岩海山中蓝片岩相岩石的存在提供了证据，蛇纹岩从上盘橄榄岩产生的高压变质区在加积棱镜的深度穿透。环太平洋地区、加州方济各杂岩和日本鸭湖滩变质带中的高压变质杂岩中，都含有蛇纹岩混杂岩和较深的高级变质岩，而日本三坝川变质带中不含蛇纹岩混杂岩。前者很可能是在张应力状态下形成的，后者则是在挤压应力状态下形成的。三坝川带蛇纹岩的罕见产出和蛇纹岩混杂岩的缺乏可能是沟弧区变质作用时的挤压应力环境造成的。在这样的挤压应力环境下，剥落挂壁橄榄岩似乎只是一种可能性，以纳入地幔物质的增生棱柱体，沿着的沟槽发展。该模式可以解释三坝川带中蛇纹岩在高级泥质片岩中的局限性。少