The Japanese Islands have grown through the formation of igneous rocks and accretionary prism caused by subduction of oceanic plates. However, the timing of the initiation of the subduction is not well defined. The South Kitakami Belt (SKB) in NE Japan is the best field to solve this problem. Here, basement igneous rocks are covered by successions of Ordovician to Early Cretaceous beds (450-100 Ma) . We obtained LA-ICP-MS U-Pb zircon ages from the following localities. Along the Yakushigawa-Valley section in the northeastern part of the SKB, we examined the ages of (1) trondhjemite of the Kagura Complex of basement igneous rocks, (2) felsic tuff of the Koguro Formation conformably covering the Kagura Complex, and (3) Yakushigawa Formation covering the Koguro Formation and lying under the Silurian Odagoe Formation. In the Ohasama area in the northwestern part of the SKB, we examined the age of (4) Nameirizawa Formation, which probably lies below the Silurian Orikabetoge Formation. Moreover, we examined the ages of four samples from the Hikami Granite body in the central part of the SKB. The ages of trondhjemite (08331-5: 466±6 Ma) of the Kagura Complex and felsic tuff (08331-4b: 457±10 Ma) of the Koguro Formation indicate that the subduction of an oceanic plate had already started at 466 Ma, and that the Koguro Formation is the oldest age-known formation of the SKB. The tuffaceous sandstone of the Yakushigawa Formation (08331-3) has detrital zircons with the youngest age cluster of around 425 Ma, and is probably a Silurian formation. The tuffaceous sandstone of the Nameirizawa Formation (08331-9) has detrital zircons with the youngest age cluster of around 430 Ma. The formation is probably a Silurian formation and is correlated with the Yakushigawa Formation. Precise ages of around 412 Ma were obtained from the Hikamiyama body of the Hikami Granitic Rocks (08330-1,-3,-4) , clearly suggesting that at least some parts of the body are not pre-Silurian basement.
Using the double-dating approach, we have conducted fission-track (FT) and U-Pb age dating on the same zircon grain to evaluate the reliability of the resulting age data. The zircon grains (OD-) used for the investigation were collected from the Mihara Body of the Kawamoto Granodiorite, Shimane Prefecture, Japan. The zircon crystals yielded a FT age of Ma, with a spontaneous FT density of -cm . The U-Pb age data were obtained using inductively coupled plasma-mass spectrometry (ICP-MS) combined with a frequency quintupled Nd-YAG (λ = nm) laser ablation (LA) sample introduction system. The U-Pb age determinations on the zircon samples were performed after chemical leaching using % HF for hrs at room temperature, or after FT etching using a KOH-NaOH eutectic solution for hrs at °C. The resulting U-Pb age of Ma was concordant with the FT age data, suggesting that there was no significant loss of Pb during etching or chemical preparation, thus indicating the feasibility of FT and U-Pb double dating of single zircon grains. The results indicate that the zircon grains (OD-) from the Kawamoto Granodiorite (Mihara Body) can become a new working standard for precise U-Pb age determinations of young zircons.
In order to understand compositional characteristics of zircon crystals with various appearance and to relate them with evolution of Stype, felsic magmatism of the Kumano Acidic Rocks in the Outer Zone of southwest Japan, U-Pb ages and trace element compositions of colorless and reddish zircons were measured using LA-ICPMS technique.
The Hitoegane Formation, with the Middle Ordovician conodont, in the Hida marginal belt, Southwest Japan, represents the oldest sedimentary unit in Japan. In order to date the lowest part of the formation, U-Pb age of igneous zircon from tuff beds was measured by LA-ICP-MS. A felsic tuff bed from the lower part of the Hitoegane Fm was newly dated 472 Ma (latest Early Ordovician) that marks the oldest age for sedimentary unit hitherto reported from Japan. This result suggests that the basement of proto-Japan was probably older than the Ordovician. The Iwatsubodani Formation, an apparently underlying unit below the Hitoegane Fm, however, yielded U-Pb zircon age of 280 Ma (Early Permian) . Thus this unit is too young to be the sedimentary basement of the Hitoegane Fm. Newly added also were some pieces of information on the U-Pb ages and stratigraphy of the Paleozoic-Mesozoic sedimentary units of the Hida marginal belt.
This paper reviews the geology, petrology, and tectonics of the Proto-Izu-Mariana arc against the Honshu arc, Japan, since 15 Ma, to formulate current topics related to the Pacific-type orogeny.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.