Geological study of bedded chert in Japan

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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