Geological study of bedded chert in Japan

日本古生代和中生代层状燧石岩主要由放射虫残体和硅质海绵针状体组成，次级为风成粘土物质和海水沉淀的铁锰矿物。它们是深海底形成并在俯冲带增生的中上层沉积物。关东山赤父带nonhem带的地质资料表明，该带可能是在低温、高孔隙水压力条件下形成的，可能处于活动辐合边缘。该区域包括混沌块阵单元和分层破坏单元。可裂变成鳞状泥质基质中的块体除由砂岩组成外，还由燧石、绿岩和石灰岩组成。中-晚三叠世牙形石和二叠世镰刀虫分别产于燧石和石灰岩中，而中-晚侏罗世放射虫则产于泥质基质中。大多数燧石块会分裂成更小的碎片。此外，它们的外部被泥岩沿断裂网破碎和侵入。角化燧石中保留了起伏消光、伸展微断裂解理、里德尔剪切、脉体、柱面岩等低温变形特征。角化燧石显微构造与构造柱面岩相吻合。尽管张剪裂缝经常充满泥岩注入，但紧密堆积的细粒燧石碎片由于压力溶解而具有缝合接触。这种燧石块体和燧石角砾岩的细观和细观组构被认为是增生楔形成的俯冲杂岩的变形特征。除上述块状外，在泥质基质中还发现了一种矿物组合不寻常的奇特岩石块状。其中变质产物有辉钼矿、镁辉钼矿、辉钼矿等，残余矿物有绿云母、磷灰石、斜辉石矿、克尔辉石矿、磁铁矿等。辉云母的K-Ar年龄分别为258<正负>13和242<正负>12 Ma。组合的存活意味着在整个变形过程中，温度条件保持的相对较短

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