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

Abstract

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.