The Japanese Islands have evolved as a subduction-related orogenic belt since the Cambrian, as recorded by lower Paleozoic granitoids and blueschists. These older rocks occur as disrupted tectonic blocks within serpentinite mélange zones (e.g., the Kurosegawa belt in southwest Japan), which form a discontinuous belt less than a few kilometers wide. Using LA-ICP-MS, this study reports in-situ zircon U-Pb ages for high-pressure metagabbro and granitoids from the Sakamoto area in Tokyo. Zircon grains from the metagabbro yield an age of ca. 490 Ma, which is coeval with similar rocks from the Kurosegawa belt reported in the islands of Kyushu and Shikoku. Zircon grains from the granitoids yield an igneous age of ca. 440 Ma, whereas xenocrystic or inherited zircon grains range in age from 2400 to 500 Ma. Similar zircon ages have been reported from granitoids of the Kurosegawa belt in Kyushu, Shikoku, and the Kii Peninsula. Results from the present study confirm that the dated igneous and metamorphic rocks in the study area represent some of the oldest components of the Kurosegawa belt, and that the belt extended discontinuously to the eastern Kanto Mountains. The ca. 490 Ma upper Cambrian (Furongian)-Lower Ordovician metagabbro (ophiolites) and the ca. 440 Ma Upper Ordovician-lower Silurian (Llandovery) granitoids near Tokyo metropolis thus indicate development of a >800 km arc-trench system across the Japanese area from the early Paleozoic.
Four ammonoid species, Olcostephanus atherstoni, O. cf. stephanophorus, Busnardoites aff. campylotoxus and Neocomites? sp. are described from the uppermost part of the Futawatashi Shale Member of the Ayukawa Formation distributed in Ajishima Island, Oshika Area, South Kitakami Belt, Northeast Japan. The age of this ammonoid fauna likely corresponds to the middle part (Neocomites neocomiensiformis Zone) of the early Valanginian, Early Cretaceous. According to our taxonomic re-examination of ammonoids previously reported from the Kobitawatashi Sandstone and Shale Member, and from the lower part of the Futawatashi Shale Member, the boundary between the Kobitawatashi and Futawatashi members correlates approximately with the Berriasian/Valanginian boundary. The starting time of the intense volcanic activity in the Oshika Area, represented by the deposition of the Yamadori Formation overlying unconformably the Ayukawa Formation, is considered to be late Valanginian to early Hauterivian.
Central Tokyo is underlain by the Pleistocene Tokyo Formation, which is considered to have been deposited during Marine Isotope Stage (MIS) 5e based on stratigraphic examination of the core type section (Yoyogi Park core). In this study, we investigated the Kita-ku Central Park core from the eastern Musahino Upland to re-examine the stratigraphy of the Tokyo Formation in this area. Two depositional cycles, each of which is composed of a basal gravel bed and overlying shallow marine sandy and muddy sediments, are observed at depths shallower than 33 m in the core, based on sedimentary facies and diatom analyses. Tephrostratigraphy indicated that the upper cycle was deposited during MIS 7e, and the lower depositional cycle may have accumulated during MIS 9 or during an earlier stage. The upper depositional cycle is considered to represent the Tokyo Formation. These results suggest that the Tokyo Formation includes a number of sedimentary cycles that not only formed during MIS 5e but also during earlier MIS periods. The distribution of the Tokyo Formation across central Tokyo should be re-examined from a stratigraphic perspective in the future.
In this study, we carried out zircon U-Pb dating of greenish tuff deposits from the eastern base of Yotei Volcano, southwest Hokkaido, Japan, to investigate the Miocene ignimbrite volcanism associated with the opening of the Japan Sea. This tuff may be correlated with the Pepenai-gawa green tuff member of the Wakikata Formation. Zircon grains from the tuff yield an eruption age of 5.7±0.2 and 5.6±0.2 Ma based on the youngest five zircon grains from a total of twenty analyses and using a weighted mean value and the Tera-Wasserburg method, respectively. The tuff also includes zircon grains with ages of 8-25 Ma. These zircons may represent xenocrysts that formed in the crust beneath Hokkaido due to intermittent heat flux during opening of the Japan Sea.
The Pankehoronai Unit occupies the lowest part of the Cretaceous Kamuikotan high-P metamorphic rocks in central Hokkaido, Northeast Japan, and consists mainly of pelitic and mafic schists. Detrital zircon grains from the pelitic schist of the Pankehoronai Unit along the Kamuidaiyonsengawa River, in the west of Asahikawa, have the youngest age cluster with the weighted mean of the 206Pb/238U ages of 81.45±0.99 Ma (95% confidence; Campanian). This result indicates that the pelitic protolith of the Pankehoronai Unit was deposited in the Campanian (latest Cretaceous) or later, which is the youngest record of the depositional age from the Kamuikotan metamorphic rocks.