Geological study of bedded chert in Japan

日本古生代和中生代层状硅质岩主要由放射虫遗骸和硅质海绵骨针组成，其次为风成粘土物质和海水沉淀的铁锰矿物。它们是形成于深海底下并在俯冲带中增生的远洋沉积物。从关东山脉秩父带的nonhem带的地质资料表明，该带可能是在低温高孔隙水压力的条件下形成的，可能是在一个积极的收敛边缘。该区域包括混乱的块矩阵单元以及分层中断单元。在裂变到鳞片状泥质基质中的块体不仅由砂岩组成，而且由燧石、绿岩和石灰岩组成。中至晚三叠世牙形刺和二叠纪fusulinids分别出现在硅质岩和石灰岩中，而中至晚侏罗世放射虫则出现在泥质基质中。大多数燧石块破碎成更小的碎片。它们的外部是 ...更多信息 并沿沿着裂隙网络被泥岩侵入角砾化。角砾硅质岩中保留了波状消光、伸展微裂隙和劈理、里德尔剪切、脉状、缝合线等低温变形特征，显微组构为构造缝合线与闭合裂隙。尽管张剪裂缝经常被泥岩注入物充填，但紧密堆积的细粒燧石碎片具有缝合接触，这是压力溶解的结果。硅质岩块和硅质角砾岩的这种中、微观组构被认为是在增生楔中形成的俯冲混杂岩的变形特征。除上述岩块外，在泥质基质中还发现了一种具有异常矿物组合的特殊岩石。变质产物中有镁橄榄石、镁钠闪石和辉黑锰矿，残留矿物有金云母、磷灰石、单斜辉石、钾霞石和磁铁矿。金云母的K-Ar年龄分别为<plus-minus>25813和<plus-minus>24212 Ma。组合体的存活意味着在整个变形过程中温度条件保持得相对较少

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