Sources and depositional environments of some Permian and Triassic cherts:Significance of Rb-Sr and Sm-Nd isotopic and REE abundance data

Rb-Sr and Sm-Nd isotopic data, rare earth element (REE) abundances, and maj or-element compositions are reported for the Triassic cherts of the Mino Te rrane in the Inner Zone of southwest Japan and for the Permian and Triassic cherts of the Sambosan Terrane in the Outer Zone of southwest Japan. Rb-Sr isotopic data of the Mino and Sambosan cherts define separate isochron lin es, and the Rb-Sr ages of ca. 210 and 240 Ma reflect the chert deposition a nd the end of the subsequent chemical diagenesis during which amorphous sil ica was transformed into quartz. It is concluded that the Rb-Sr isotopic sy stem of the cherts becomes essentially homogenized among biogenic silica, d etrital components and hydrogeneous components during deposition, and the s ubsequent chemical diagenesis before lithification due to high mobility of Rb and Sr. The Sr initial ratio (0.71363) of the Triassic Mino cherts is cl early higher than those (0.7079 and 0.7068) of the Triassic and Permian Sam bosan cherts, which are close to the estimated oceanic Sr ratios of 0.7066- 0.7081 in the Permian and Triassic. On the other hand, Sm-Nd isotopic data do not define isochron lines. This might be due to incomplete homogenizatio n of the system among biogenic silica, detrital components and hydrogeneous components during deposition, and chemical diagenesis because of low mobil ity of REE. Initial Sr and Nd isotopic ratios of the Mino cherts can be int erpreted by mixing typical continental crustal rocks, represented by aeolia n loess compositions, with a small amount of Precambrian rocks (<10%), whil e those of the Sambosan cherts can be explained by mixing continental crust al rocks with oceanic volcanic rocks such as MORB (mid-ocean ridge basalt). In the mixing model between Chinese loess and MORB, a proportion of loess is ca. 30%-60% for the Middle Permian Sambosan cherts and ca. 60%-90% for t he Triassic Sambosan cherts. Al2O3/(Al2O3 + Fe2O3) ratios and Ce anomalies in the REE patterns also show a higher contribution of continental componen ts in the Triassic Sambosan and Mino cherts than the Permian Sambosan chert s, suggesting deposition in a marginal sea or on the continental shelf and slope for the Triassic Sambosan and Mino cherts. Our study shows that the g eochemical and isotopic features of the Mino and Sambosan cherts essentiall y retain the nature of their formation and subsequent chemical diagenesis b efore lithification. Alteration or metamorphic effects during subduction an d accretion processes cannot be found in the chemical and isotopic features of these cherts. Our result requires the reexamination of the previous pal eogeographical reconstructions of the Japanese Islands as well as the relat ionship between the Mino and Sambosan Terranes in Permian and Triassic time .