地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
論説
関東南部の浅海成白亜系の砕屑性ジルコン年代スペクトル
―後背地表層地殻の更新に伴う前弧域砂岩組成変化―
中畑 浩基磯﨑 行雄堤 之恭岩本 直哉
著者情報
ジャーナル フリー

2016 年 125 巻 3 号 p. 353-380

詳細
抄録

 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 reflects 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 Proterozoic zircons in coeval sandstones deposited at the back-arc side highlights the remarkable contrast in sedimentary flux between fore-arc/intra-arc settings and back-arc domain. The uplift of arc crust to expose new Cretaceous granitoids probably formed a great barrier to sedimentary flux from the continental interior to the fore-arc domain in East Asia.

著者関連情報
© 2016 公益社団法人 東京地学協会
前の記事 次の記事
feedback
Top