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 re ported for the Permian and Triassic cherts from the Kuma and Saiki are as in the Southern Chichibu (Sambosan) terrane, the Outer Zone of sout hwest Japan, in order to elucidate the nature of sources, depositional environment and any changes that occurred from the Permian to the Tri assic. A higher contribution of detrital components in the Triassic Sa mbosan cherts than in the Permian ones is suggested by the Ce-anomaly seen in the REE pattern, MnO/TiO2 and Al2O3/(Al2O3 + Fe2O3) ratios and initial Sr-87/Sr-86 ratio of these cherts, Small negative Ce-anomalie s (Ce/Ce = 0.75-0.95) are observed in the Permian cherts from both Ku ma 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 note d that the Sambosan cherts show neither large negative Ce-anomalies (C e/Ce = 0.2-0.5) commonly observed in deep-sea cherts nor large variat ion in Ce-anomalies (Ce/Ce = 0.3-1.5) observed for cherts from the Fr anciscan terrane of California. Al2O3/(Al2O3 + Fe2O3) ratios are 0.5-0 .8 for all Sambosan cherts. MnO/TiO2 ratios are larger than 0.5 for mo st of the Permian Kuma cherts and the Early Permian Saiki cherts, whil e the ratios are smaller than 0.5 for the Triassic Kuma cherts and Lat e Permian and Triassic cherts from the Saiki area. Initial Sr-87/Sr-86 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 Sr- 87/Sr-86 ratios for the Permian and Triassic Sambosan cherts are close to the coeval marine Sr-87/Sr-86 ratios of Burke et al. (1982) and ar e clearly lower than that for cherts from the Mine terrane (0.713), In ner Zone of southwest Japan (Shibata and Mizutani, 1982). Our geochemi cal and isotopic data of the Sambosan cherts suggest a change from oce an-basin floor deposition for the Permian to hemipelagic for the Trias sic cherts. However, geochemical features showing deposition on a typi cal deep-ocean floor are not observed even for the Permian cherts, Som e difference in depositional location between the Kuma and Saiki chert s in the Sambosan terrane is also implied from the difference in tempo ral 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-oc ean environment than the Triassic cherts in the Mine terrane. This stu dy demonstrates that geochemical and isotopic data could be used to di stinguish differences of depositional environment of cherts and those data from the Permian to the Triassic cherts suggest increasing proxim ity to land in their depositional environments. (C) 1998 Elsevier Scie nce B.V. All rights reserved.