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Geological and petrological recovery of Paleo-Mesozoic seamounts in the southwest Japan

日本西南部古中生代海山的地质和岩石学恢复

基本信息

批准号：
03640646
负责人：
TAZAKI Koichi
金额：
$ 1.02万
依托单位：
EHIME UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1991
资助国家：
日本
起止时间：
1991 至 1992
项目状态：
已结题

项目摘要

Geochemistry of Paleo-Mesozoic greenstones give petrological and geological constraints on geotectonic settings of Japanese Islands. This project deals with the greenstones collected from basement greenstones of Akiyoshi, Taishaku and Atetsu of the Inner belt, Shikoku Karst, Nakatsu Myojin of the Outer belt of southwest Japan. Ages of the greenstones are younger than overlain limestones which are biostratigraphically estimated from upper Carboniferous to Permian by including fusulinid.Abundance of whole REE in massive basalt lava from Taishaku limestone plateau is exceedingly higher in alkaline metabasalt and enriched in light REE than that of tholeiite. In alkaline basalts Ce rangews from 70 to 120 ppm and La content is from 34 to 57 ppm, whereas in tholeiite, abundances of these elements are low from 28 to 38 and 12 to 15ppm respectively. Heavy REE contents are nearly the same to both rock types. Yb concentration is 1 to 2 ppm and does not depend on rock type. Basalt lava from Shikoku Karst characterized by Ti and Al rich purple colored augite phenocryst have lower concentration of REE than Taishaku metabasalts.Chondrite normarized REE patterns of the Taishaku greenstone exhibit LREE enriched steep inclined pattern. The LREE enriched inclined pattern is characteristic of recent intra-plate ocean island or seamount basalts. Alkaline ocean island basalts which bear total REE more than 250 ppm and chondrite normarized value of La more than 200 as revealed from Taishaku greenstones suggest partial melting in the large ionic lithophile elements enriched mantle.

古中生代绿岩的地球化学特征对日本列岛的大地构造背景提供了岩石学和地质学的制约。本项目涉及从日本西南部内带的Akiyoshi，Taishaku和Atetsu，四国喀斯特，中津Myojin的基底绿岩中收集的绿岩。大沙库灰岩台地块状玄武岩的全稀土元素丰度在碱性变玄武岩中明显高于拉斑玄武岩，且轻稀土元素丰度明显高于拉斑玄武岩。在碱性玄武岩中，Ce的含量为70 ~ 120 ppm，La的含量为34 ~ 57 ppm，而在拉斑玄武岩中，这些元素的丰度较低，分别为28 ~ 38和12 ~ 15 ppm。两种岩石重稀土元素含量基本相同。Yb浓度为1至2 ppm，与岩石类型无关。四国喀斯特玄武岩以富含Ti和Al的紫色辉石斑晶为特征，稀土元素含量低于大社变质玄武岩，大社绿岩球粒陨石标准化稀土配分模式表现为LREE富集的陡倾配分模式。LREE倾斜富集模式是现代板内洋岛或海山玄武岩的特征。碱性洋岛玄武岩的稀土总量大于250 ppm，球粒陨石的La标准化值大于200，表明大规模的亲石元素富集地幔发生了部分熔融。