Shinjiro Mizutani

Progressive ordering of cristobalitic silica in the early stage of diagenesis

Examination of hydrothermally transformed silica from controlled experiments reveals that amorphous silica changes to quartz through an intermediate phase of opal-CT and that the d(101) spacing of cristobalite progressively decreases from 4.10 Å to 4.05 Å. The rate of spacing decrease is definitely dependent on the reaction temperature, being faster at higher temperatures. This spacing change represents ordering of opal-CT crystals with the passage of time.The relationship between thermal history and degree of ordering suggests that stratigraphic boundaries are usually parallel to isopleths of d (101) spacings, but do not always coincide with them. The isopleths should be more or less discordant to the stratigraphic boundaries where the strata have been folded. This discordancy can be ascribed to the difference of ordering, chiefly controlled by the thermal history during the burial and folding process.

Mesozoic radiolarian biostratigraphy of Japan and collage tectonics along the eastern continental margin of Asia

Abstract Jurassic accretionary complexes mainly composed of Carboniferous to Permian limestone associated with greenstone, Triassic bedded chert, Jurassic siliceous shale and clastic rocks form the basement rocks of Japan. The stratigraphy of these complexes has recently been analysed utilizing radiolarian fossils, resulting in the discovery that Japan comprises a collage of disrupted terranes. International co-operative works reveal that lithologically and biostratigraphically similar terranes are distributed in northeast China (Nadanhada terrane) and Sikhote-Alin, USSR (Khabarovsk terrane). Paleomagnetic studies demonstrate that prior to opening of the Sea of Japan the Japanese Islands were located much closer to the eastern margin of the Asian continent where the Nadanhada and Khaborovsk terranes are now exposed. Features of the Mino terrane in central Japan are characteristic of these terranes which originally formed along the continental margin of East Asia. Seamounts covered by fossiliferous limestone formed during the Carboniferous to Permian at low latitudes. The seamounts drifted towards a continental margin together with upper Paleozoic sediments, Triassic bedded chert and Lower Jurassic siliceous shale which accumulated around them. Upper Paleozoic, Triassic and Lower Jurassic formations were accreted to the eastern continental margin, which was a large tectonic collage developed as the Chinese mainland during the Late Triassic. Enormous amounts of clastic detritus were deposited in sedimentary basins where jumbling and telescoping of pelagic sediments took place in a complicated fashion producing a melange. The provenance of clastic detritus within the Mino terrane is interpreted as a platform on which Permian and Carboniferous calcareous sediments containing diagenetic lutecite and orthoquartzite formations were widespread. These formations covered a Precambrian metamorphic and granodioritic basement similar to that seen in the South China region. Accretion culminated in the earliest Cretaceous and the large disrupted terrane which had developed was transpressed northwards along the eastern margin of the continent. During this period of dispersal the original terrane was sheared, fragmented and separated into many smaller terranes some of which were transported to the Sikhote-Alin region by the Late Cretaceous. The most recent dispersal occurred during the opening of the Sea of Japan, which is closely related to the latest movement of the Pacific plate. Studies of radiolarian micropaleontology and the significance of these fossils to the resolution of the biostratigraphy and the tectonic history of Japan are reviewed. The results of biostratigraphic analyses are discussed in relation to the Mesozoic tectonics of East Asia.

Late Paleozoic geosynclinal basalt and tectonism in the Japanese Islands

Abstract In the Japanese Late Paleozoic group basaltic volcanics are commonly found whose chemical natures are very similar to the abyssal basalt of the present mid-oceanic ridge, characterized, above all, by low K 2 O, low Fe 3+ /Fe 2+ and high K/Rb. Two types of the basaltic rocks are distinguished, especially in their distribution pattern of rare-earth elements, the one displaying the logarithmically rectilinear relationship of chondrite-normalized pattern and the other with the inflection point. Recently, Precambrian metamorphic cobbles dated at 1,700-1,500 m.y. have been found in Permian conglomerates of central Japan. Paleocurrent data indicate lateral filling of clastic materials derived from the north, where a vast expanse of Precambrian complex is supposed to have occurred. Stratigraphy and structure of the Paleozoic terrain of central Japan suggest that the Japanese Paleozoic geosyncline was grown on the Precambrian continental crust. From geochemical properties and the distribution of the geosynclinal basalt in the Japanese Islands, it is concluded that the basaltic rocks were derived from the upper mantle, the original magma having been created by partial melting of the mantle and moved upward through a rifting zone in the Precambrian crust. The early stage of tectonism of the geosynclinal belt was thought to be under a tensional condition rather than a compressional one.

A study of rock alteration process based on kinetics of hydrothermal experiment

Experimental investigation was made on hydrothermal alteration of sericite in acid solution. It is found that the sericite is changed to pyrophyllite through kaolinite at 270° C and 260° C, and the alteration is considered to be a consecutive reaction involving two steps, from sericite to kaolinite and from kaolinite to pyrophyllite. On the other hand, pyrophyllite is not formed at 190° C and the sericite is altered to kaolinite. The rate of reaction in each step of this hydrothermal alteration is dependent on reaction temperature and chemical composition of the solution. From the results of the quantitative analyses of the experimental products, kinetic constants and related parameters were obtained.By using these numerical values, an alteration process can be followed in a given model. Computed results for some cases of an alteration of sericite in acid solution are presented. Geological phenomena such as rock alteration or weathering are thought to progress in a disequilibrium state and in an open system. An attempt described in this paper is a physicochemical approach to treat an alteration process accompanied with material transfer occurring in an open system from a kientic viewpoint.