Sources and Depositional Environments of Some Permian and Triassic Cherts: Significance of Rb‐Sr and Sm‐Nd Isotopic and REE Abundance Data

Abstract

Rb‐Sr and Sm‐Nd isotopic data, rare earth element (REE ) abundances, and major‐element compositions are reported for the Triassic cherts of the Mino Terrane 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 lines, and the Rb‐Sr ages of ca. 210 and 240 Ma reflect the chert deposition and the end of the subsequent chemical diagenesis during which amorphous silica was transformed into quartz. It is concluded that the Rb‐Sr isotopic system of the cherts becomes essentially homogenized among biogenic silica, detrital components and hydrogeneous components during deposition, and the subsequent chemical diagenesis before lithification due to high mobility of Rb and Sr. The Sr initial ratio (0.71363) of the Triassic Mino cherts is clearly higher than those (0.7079 and 0.7068) of the Triassic and Permian Sambosan 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 homogenization of the system among biogenic silica, detrital components and hydrogeneous components during deposition, and chemical diagenesis because of low mobility of REE. Initial Sr and Nd isotopic ratios of the Mino cherts can be interpreted by mixing typical continental crustal rocks, represented by aeolian loess compositions, with a small amount of Precambrian rocks (<10%), while those of the Sambosan cherts can be explained by mixing continental crustal 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 the Triassic Sambosan cherts. Al2O3/(Al2O3 + Fe 2O3) ratios and Ce anomalies in the REE patterns also show a higher contribution of continental components in the Triassic Sambosan and Mino cherts than the Permian Sambosan cherts, 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 geochemical and isotopic features of the Mino and Sambosan cherts essentially retain the nature of their formation and subsequent chemical diagenesis before lithification. Alteration or metamorphic effects during subduction and accretion processes cannot be found in the chemical and isotopic features of these cherts. Our result requires the reexamination of the previous paleogeographical reconstructions of the Japanese Islands as well as the relationship between the Mino and Sambosan Terranes in Permian and Triassic time.