日本列島の古地理学―砕屑性ジルコン年代頻度分布と造山帯後背地の変遷―

Provenance analysis using zircon U-Pb dating for sandstone enables us to reconstruct the paleogeographic history of the Japanese Islands. We determined the U-Pb ages of detrital zircons (50-200 grains) in 14 Phanerozoic sandstones (Silurian, Permian, Triassic, Jurassic, Cretaceous, and Neogene) and two modern river-sands collected from 13 areas in Japan. The measurements clarified two groups of Precambrian clusters (2500-1500 Ma and 900-800 Ma) and five distinct peaks of the Phanerozoic (520-400 Ma, 280-210 Ma, 190-160 Ma, 110-90 Ma, and 80-60 Ma). The Precambrian clusters from Paleozoic-Mesozoic sandstones suggest that proto-Japan was located close to the Yangtze (South China) craton, and the interaction with the Sino-Korean (North China) craton started sometime in the middle Triassic. The dominance of 520-400 Ma detrital zircons and scarcity of Precambrian ones in Paleozoic sandstone suggest that proto-Japan is likely to have formed an intra-oceanic island arc system isolated from large continental blocks. The strong peaks of 520-400 Ma, 280-210 Ma, and 190-160 Ma show a clear contrast with current small exposures of corresponding granites in Japan, indicating that all these granitic bodies may have been eroded away probably by tectonic erosion. The detrital zircon age spectra of the Miocene sandstone clarified that the final separation of the Japanese Island arc system from the Asian continental margin occurred around 16 Ma.