Southern Chichibu Research Articles

Differences in geochemical features between Permian and Triassic cherts from the Southern Chichibu terrane, southwest Japan: REE abundances, major element compositions and Sr isotopic ratios

REE abundances, Sr isotopic data and major element compositions are reported for the Permian and Triassic cherts from the Kuma and Saiki areas in the Southern Chichibu (Sambosan) terrane, the Outer Zone of southwest Japan, in order to elucidate the nature of sources, depositional environment and any changes that occurred from the Permian to the Triassic. A higher contribution of detrital components in the Triassic Sambosan cherts than in the Permian ones is suggested by the Ce-anomaly seen in the REE pattern, MnO/TiO 2 and Al 2O 3/(Al 2O 3 + Fe 2O 3) ratios and initial 87Sr/ 86Sr ratio of these cherts. Small negative Ce-anomalies (Ce/Ce * = 0.75–0.95) are observed in the Permian cherts from both Kuma and Saiki areas, in contrast to small positive Ce-anomalies (Ce/Ce * = 1.1–1.3) for the Triassic cherts from both areas. It should be noted that the Sambosan cherts show neither large negative Ce-anomalies (Ce/Ce * = 0.2–0.5) commonly observed in deep-sea cherts nor large variation in Ce-anomalies (Ce/Ce * = 0.3–1.5) observed for cherts from the Franciscan terrane of California. Al 2O 3/(Al 2O 3 + Fe 2O 3) ratios are 0.5–0.8 for all Sambosan cherts. MnO/TiO 2 ratios are larger than 0.5 for most of the Permian Kuma cherts and the Early Permian Saiki cherts, while the ratios are smaller than 0.5 for the Triassic Kuma cherts and Late Permian and Triassic cherts from the Saiki area. Initial 87Sr/ 86Sr ratios for the Permian Saiki cherts are 0.706–0.707 and are slightly lower than those for the Triassic Kuma cherts (0.707–0.709). These 87Sr/ 86Sr ratios for the Permian and Triassic Sambosan cherts are close to the coeval marine 87Sr/ 86Sr ratios of Burke et al. (1982)and are clearly lower than that for cherts from the Mino terrane (0.713), Inner Zone of southwest Japan ( Shibata and Mizutani, 1982). Our geochemical and isotopic data of the Sambosan cherts suggest a change from ocean-basin floor deposition for the Permian to hemipelagic for the Triassic cherts. However, geochemical features showing deposition on a typical deep-ocean floor are not observed even for the Permian cherts. Some difference in depositional location between the Kuma and Saiki cherts in the Sambosan terrane is also implied from the difference in temporal variation of geochemical features between these two cherts (e.g., increase of REE content with age for the Kuma cherts, in contrast to a decrease for the Saiki cherts). In addition, it is suggested that the Permian and Triassic Sambosan cherts were deposited in a more open-ocean environment than the Triassic cherts in the Mino terrane. This study demonstrates that geochemical and isotopic data could be used to distinguish differences of depositional environment of cherts and those data from the Permian to the Triassic cherts suggest increasing proximity to land in their depositional environments.

98/01390 Fluid inclusion study of the Kirishima geothermal system, Japan: Sawaki, T. et al. Geothermics, 1997, 26, (3), 305–327

In order to identify the parentage of low-grade metasediments in the Besshi unit of the Sanbagawa Metamorphic Belt in Shikoku, southwest Japan, we have studied the bulk chemistry of psammitic and pelitic rocks in the Sanbagawa Belt and in related nonmetamorphosed Middle/Late Jurassic–Cretaceous accretionary complexes (Southern Chichibu and Northern Shimanto Belts). Tectonostratigraphic relationships and paleontological evidence suggest a post-Middle Jurassic depositional age for the protoliths of the Sanbagawa psammitic/pelitic schists. Zr/TiO2 ratios of metasandstones in the Besshi unit differ from those of the Middle/Late Jurassic sandstones in the Southern Chichibu Belt. Besshi metasandstones from the Iya–Inouchidani area have chemical affinities with sandstones from the KSII unit (Coniacian–Campanian) of the Northern Shimanto Belt. The Besshi unit in the Asemi–Tachikawa area can be subdivided into northern and southern parts based on the geochemistry of the metasandstones and metapelites. The geochemistry of the metasediments suggests that the parents of the northern and southern parts are the KSII unit and KSIc unit (Late Albian–Early Coniacian), respectively. The depositional ages of the protoliths of the Besshi metasandstones and metapelites are thus inferred to be Late Albian–Early Coniacian (southern part of the Asemi–Tachikawa area), and Coniacian–Campanian (Iya–Inouchidani area and the northern part of the Asemi–Tachikawa area). These conclusions are consistent with the radiometric ages of metasediments from Asemi–Tachikawa, and imply that most of the low-grade metasediments in the Sanbagawa Belt are an underplated, deeper facies of the Cretaceous Shimanto accretionary complex.Protoliths of the Besshi unit represent muddy facies with greenstones, whereas the KSIc and KSII units in the Shimanto Belt mainly consist of sandy facies of probable trench-fan and trench-wedge origin. The lithologic contrast between the KSIc/KSII and Besshi units may be explained by selective underthrusting and underplating of hemipelagic mud deposited outboard of a trench, and as offscraping of turbidite sequences deposited within a trench.

The siliceous rock facies near the Permo-Triassic (P/T) boundary in the Southern Chichibu Terrane, Kyushu

The siliceous rock facies near the Permo-Triassic (P/T) boundary in the Southern Chichibu Terrane, Kyushu

Jurassic-Early cretaceous tectonic evolution of the Southern Chichibu terrane, southwest Japan

The Southern Chichibu terrane fringes the southern margin of the Jurassic-Early Cretaceous accretionary complex of southwest Japan. Two subterranes, the Togano subterrane in the north and Sambosan subterrane to the south, are recognized by examination of the common features of the terrane. They are juxtaposed throughout the terrane from western Kyushu to the Kanto Mountains a distance of over 1000 km. The geology of the Sakawa area, central Shikoku, is described in detail to show an example of the terrane. The geology around Tsukumi in eastern Kyushi and Okutama in the Kanto Mountains is introduced to emphasize the common features of this terrane. The Togano subterrane is composed of predominantly Triassic-Jurassic chert-clastic sequences with subordinate Jurassic olistostromal sequences and Upper Jurassic-Lower Cretaceous limestone-bearing clastic sequences. The chert-clastic sequences show an upward-coarsening sequence which reflects landward drift of the sea floor from a pelagic realm towards a trench. The olistostromal sequence contains abundant upper Paleozoic blocks. This subterrane is characterized by a south-vergent imbricate structure and south-younging polarity of the chert-clastic sequences. The Sambosan subterrane consists of mainly Upper Jurassic-Lower Cretaceous olistostromal sequences associated with Upper Jurassic-Lower Cretaceous limestone-bearing clastic sequences. The olistostromal sequence includes abundant Triassic-Jurassic chert blocks and Triassic limestone blocks accompanied by greenstones. The limestone-bearing clastic sequences of both subterranes are regarded as shelf or inner-trench slope sediments on the chert-clastic and olistostromal sequences. Two major stages of tectonic development are discriminated. The Togano subterrane was formed by successive offscrape-accretion mainly during the Middle to Late Jurassic. The Sambosan subterrane was constructed by collision-accretion of seamounts mainly during the Late Jurassic to Early Cretaceous. The difference in the tectonic processes is considered to relate to topography of subducting oceanic plate; an oceanic plate with an abyssal plain for the Togano subterrane and an oceanic plate with seamounts for the Sambosan subterrane.

Geology of the Southern Chichibu Belt in the eastern part of Shimizu-cho, Wakayama Prefecture, Southwest Japan

Geology of the Southern Chichibu Belt in the eastern part of Shimizu-cho, Wakayama Prefecture, Southwest Japan