Detrital Zircon Age Spectra of the Upper Cretaceous Atogura and Tochiya Formations in the Northern Kanto Mountains, SW Japan:Evidence of Across-arc Link to the Hida Belt
The overall configuration of the Cretaceous subduction-related arc-trench system in Japan is preserved in the current distribution of the relevant orogenic components; i.e., the coeval set of accretionary complexes formed at the trench (Sanbosan and North Shimanto belts), high-P/T meta-ACs along the Wadati–Benioff zone (Sanbagawa and Shimanto metamorphic belts), arc batholiths (Ryoke–Sanyo and San-in belts), and fore-arc basin strata (Ryoseki–Monobegawa and Izumi groups). To document the sediment distributary pattern within the Cretaceous arctrench system and the composition of relevant provenance, detrital zircon dating was conducted for the Upper Cretaceous sandstones (Atogura and Tochiya formations) from the northern Kanto Mountains. These strata occur immediately to the south of Median Tectonic Line of SW Japan, and as klippe with unknown origin. The results of U-Pb dating by LA-ICPMS show that three sandstones have common age spectra with four major age groups; i.e., 120–150 Ma (Early Cretaceous), 170–200 Ma (Jurassic), 250–300 Ma (Permian), and 1600–2200 Ma (Paleoproterozoic), with minor amounts of much older grains up to 2900 Ma (Archean). These age spectra are unique, when compared to other coeval Cretaceous fore-arc and/or intra-arc sandstones in Japan. The Early Cretaceous grains were obviously derived from a proximal source to the depositional site, probably the Cretaceous volcanic arc of the Ryoke–Sanyo belt in SW Japan. The dominant grains of the Jurassic and Permian ages were probably derived from coeval plutonic belts in the provenance, whereas the Paleoproterozoic grains were probably derived from continental blocks in East Asia with crusts of corresponding ages, such as the North and South China blocks. Except for the Cretaceous arc source, Jurassic and Permian granitoids are extremely rare in 地学雑誌 Journal of Geography(Chigaku Zasshi) 124(4)633–656 2015 doi:10.5026/jgeography.124.633 * 東京大学大学院総合文化研究科宇宙地球科学教室 ** 日本大学文理学部地球システム科学科 *** 京都大学理学部地質学鉱物学教室 * Department of Earth Science and Astronomy, The University of Tokyo, Tokyo, 153-8902, Japan ** Department of Geosystem Sciences, College of Humanities and Sciences, Nihon University, Tokyo, 156-8550, Japan *** Department of Geology and Mineralogy, Kyoto University, Kyoto, 606-8502, Japan