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Geological study of bedded chert in Japan

日本层状燧石地质研究

基本信息

批准号：
02640644
负责人：
SAITO Yasuji
金额：
$ 0.13万
依托单位：
National Science Museum, Tokyo
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1990
资助国家：
日本
起止时间：
1990 至 1991
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-02640644/
关键词：
Bedded chert Radiolaria Chaotic block-in-matrix units Subduction zone Active convergent margin チャ-ト付加体

项目摘要

Paleozoic and Mesozoic bedded cherts in Japan consist mainly of radiolarian remains and siliceous sponge spicules with subordinate clay material of aeolian origin and Fe-Mn minerals precipitated from sea water. They are pelagic sediments fon-ned under deep-sea floor and accreted in subduction zone. Geological data from the nonhem zone of Chichibu belt, Kanto Mountains, suggest that the zone could have been formed under a low-temperature condition with high pore-water pressure, possibly at an active convergent margin. The zone includes chaotic block-in-matrix units as well as stratally disrupted units. The blocks in fissile to scaly pelitic matrix are composed not only of sandstone but also of chert, greenstone and limestone. Middle to Late Triassic conodonts and Permian fusulinids occur in the chert and limestone, respectively, whereas Middle to Late Jurassic radiolarians are found from the pelitic matrix. Most of the chert blocks break up into smaller fragments. Their outer parts are … More furthermore brecciated and invaded by mudstone along a network of fracture. In the brecciated chert are retained low-temperature deformation features such as undulatory extinction, extension microfracture and cleavage, riedel shear, veins, stylolite, etc. Microscopic fabric of the brecciated chert is anastomosing fracture with tectonic stylolite. Although tension shear fractures are frequently filled with mudstone injections, closely packed fine-grained fragments of the chert have sutured contacts as being the result of pressure solution. Such meso- and microscopic fabrics of the chert blocks and chert breccia are conceived as deformation feature in subduction melange formed in accretionary wedge. In addition to the blocks mentioned above, a peculiar rock with unusual mineral assemblage is also found as blocks in pelitic matrix. Clilorite, magnesio-riebeckite and stilpnomelane are involved as metamorphic products therein, but phlogopite, apatite, clinopyroxene, kaersutite, and magnetite are present as relict minerals. K-Ar ages of phlogopite are 258<plus-minus>13 and 242<plus-minus>12 Ma. The survival of assemblage means that throughout the time of deformation the temperature condition had been kept relatively Less

日本的古生代和中生代层状燧石主要由放射虫遗骸和硅质海绵骨针组成，其次是风成粘土物质和海水中沉淀的铁锰矿物。它们是深海海底形成并在俯冲带增生的中上层沉积物。关东山脉秩父带非边缘地带的地质数据表明，该地带可能是在高孔隙水压力的低温条件下形成的，可能位于活跃的汇聚边缘。该区域包括混沌矩阵块单元以及地层扰乱单元。裂变至鳞片状泥质基质中的块体不仅由砂岩组成，还由燧石、绿岩和石灰岩组成。中至晚三叠世牙形刺和二叠纪 fusulinids 分别出现在燧石和石灰岩中，而中至晚侏罗世放射虫则在泥质基质中发现。大多数燧石块分解成更小的碎片。它们的外部还被角砾化并被泥岩沿着裂缝网络侵入。角砾状燧石中保留有波状消光、伸展微裂隙和解理、里德尔剪切、脉纹、缝合线等低温变形特征。角砾状燧石的微观结构是与构造缝合线的吻合断裂。尽管张拉剪切裂缝经常被泥岩注入物填充，但紧密堆积的细粒燧石碎片由于压力溶解而缝合了接触面。燧石块和燧石角砾岩的这种细观和微观结构被认为是增生楔中形成的俯冲混杂岩的变形特征。除了上述块体之外，在泥质基质中还发现了一种具有不寻常矿物组合的奇特岩石。绿泥石、菱镁矿和硅镁石作为变质产物存在其中，但金云母、磷灰石、单斜辉石、钙苏石和磁铁矿作为残余矿物存在。金云母的K-Ar年龄为258±13和242±12 Ma。组合体的幸存意味着在整个变形过程中温度条件一直保持相对较小