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To clarify the supply/deposition pattern of terrigenous clastics of Cretaceous Japan, U‒Pb and Pb‒Pb ages of detrital zircons are analyzed by laser-ablation induced coupled plasma mass spectroscopy (LA-ICPMS) of Lower Cretaceous sandstones in the Chichibu belt, SW Japan. Target rocks include Lower Cretaceous fore-arc sandstones in the Sanchu graben and the Choshi area of the southern Kanto district; e.g. Hauterivian Shiroi Formation, Barremian Ishido Fm, and Aptian‒Albian Sanyama Fm in the Sanchu graben, and Barremian Ashikajima Fm, Early Aptian Inubozaki Fm, and Aptian‒Albian Nagasakihana Fm in the Choshi area. All these fore-arc sandstones are dominated by Mesozoic (ca. 250‒100 Ma) detrital zircons with minimal amount of (300‒250 Ma) Permian grains, and Precambrian grains are extremely rare. The similar age spectra of these sandstones suggest a common provenance, despite the current along-arc separation of the Sanchu graben from the Choshi area for ca. 130 km. Hauterivian‒Barremian sandstones from both areas are characterized by the abundance of Permian, Triassic, and Jurassic zircons, where as Aptian‒Albian sandstone by monotonous dominance of Early Cretaceous grains. This stratigraphic change in zircon age spectra refl ects a secular change in the exposure /erosion conditions of older granitoids in the provenance, in remarkable accordance with that of coeval sandstones from other areas; e.g., Ryoseki Formation in Shikoku (fore-arc basin) and Kanmon Group in northern Kyushu/western Honshu (intra-arc basin). This stratigraphic change commonly detected in the Cretaceous fore-arc and intra-arc records the growth /erosion history of a new crust of the volcanic arc developed along the East Asian margin. The disappearance of older Permian, Triassic, and Jurassic zircons in sandstones during the Barremian‒Aptian interval suggests large-scale tectonic erosion along the East Asian active margin. The abundance of 地学雑誌 Journal of Geography(Chigaku Zasshi) 125(3)353‒38

chigaku zasshi/chigaku zasshi

Age Spectra of Detrital Zircons from Shallow Marine Cretaceous in Southern Kanto, SW Japan: Change in Composition of Fore-arc Sandstones in Response to the Rejuvenation of Provenance Crust