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Volume 115, Issue 8
Displaying 1-6 of 6 articles from this issue
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Articles
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Takeshi Hasegawa, Hiroshi Kishimoto, Mitsuhiro Nakagawa, Jun-ichi Itoh ...2009 Volume 115 Issue 8 Pages 369-390
Published: 2009
Released on J-STAGE: December 08, 2009
JOURNAL FREE ACCESSThe eruptive history of post-caldera volcanoes of Kutcharo caldera (Atosanupuri, Nakajima, and Mashu), eastern Hokkaido, Japan, was re-examined based on tephrostratigraphy above the layer KpI (35 ka), which is the youngest caldera-forming ignimbrite produced from the caldera. The studied areas are located to the east (Konsen) and north (Shari) of Kutcharo caldera. Between KpI and Ma-l (12 ka; the first eruption of the caldera-forming stage of Mashu volcano), at least 24 explosive eruptions Ml-a to (M1-d, Ch-a to Ch-d, and Nu-a to Nu-r, in descending order) are recognized in the Konsen area, and 10 (KoP, NaP, WTfl-2, KgSc, WTfl-1, KuSc, TyP, HkP, Ds-Oh, and YmP) are recognized in the Shari area. In addition, more than 20 thin scoria layers were found. Eight tephra layers in the Shari area can be correlated with those in the Konsen area, based on stratigraphy and petrology (e.g., glass chemistry and the mineralogy of juvenile materials) . The main phenocryst phases in these tephras are plagioclase, two pyroxenes, and Fe-Ti oxides, although some layers contain olivine and Nu-q includes hornblende. These tephras are divided into the following three groups based on the K2O content of glass: Low-K (K2O=0.4-0.9 wt.%), Medium-K (1.6-2.5 wt.%), and High-K (4.2-4.9 wt.%) . Nu-q, characterized by the presence of hornblende and a High-K composition, was possibly produced by the Daisetsu Ohachidaira caldera of central Hokkaido. The Medium-K group consists of 10 porphyritic (>11% phenocrysts), white-colored tephra layers. All other tephras, including the thin scoria layers, are assigned to the Low-K group. Based on geological and petrological data, and correlations with proximal deposits and edifices of each post-caldera volcano, we infer that the source volcanoes of the Medium-K and Low-K groups were the Atosanupuri/ Nakajima volcanoes and Mashu volcano, respectively. These results indicate that 10 explosive eruptions occurred at the Atosanupuri/Nakajima volcanoes over the period from 25 to 13 ka, with a total tephra volume of 16 km3. Mashu volcano produced more than 50 plinian eruptions (total tephra volume >90 km3) over the period from 35 to 1 ka, with no dormant periods exceeding several thousand years in duration.View full abstractDownload PDF (1811K) -
Tomohiko Sato, Yukio Isozaki, Motoyuki Matsuo2009 Volume 115 Issue 8 Pages 391-399
Published: 2009
Released on J-STAGE: December 08, 2009
JOURNAL FREE ACCESSThe deep oceans were anoxic throughout the Precambrian until the first oxidation in the late Neoproterozoic; however, the precise timing and magnitude of this oxidation event are unknown. To examine the oxidation levels of the late Neoproterozoic-Early Paleozoic deep oceans, we investigated the chemical states of iron in deep-sea pelagic cherts of this age, based on 57Fe Mösbauer spectroscopy. The analyzed cherts occur as exotic blocks within accretionary complexes in Anglesey, Wales, UK (late Neoproterozoic), the Kurai area of the Gorny Altai Mountains, Russia (Ediacaran-Cambrian), Port au Port Bay, Newfoundland, Canada (Ordovician), and the Bayanhongor area of Mongolia (Devonian). The Mösbauer spectroscopy clarified that all the samples contain hematite as the main iron mineral, without pyrite. The common occurrence of hematite suggests that these deep-sea pelagic cherts were deposited under oxidizing conditions with a redox potential at least higher than that of the Fe(III)/Fe(II) transition (boundary between iron-hydroxide and dissolved Fe2+). The present results suggest that the late Neoproterozoic deep ocean was already ventilated up to the level required for the deposition of ferric iron.View full abstractDownload PDF (1198K) -
Masashi Mukai, Yasuyuki Miyake, Tomoyoshi Kosaka2009 Volume 115 Issue 8 Pages 400-422
Published: 2009
Released on J-STAGE: December 08, 2009
JOURNAL FREE ACCESSThe upper Pliocene to lower Pleistocene Enrei Volcanic Rocks occur upon the Utsukushigahara Plateau and adjacent areas on the northwestern margin of the Chushin Highland area, Central Japan. Upon the Utsukushigahara Plateau, the Enrei Volcanic Rocks unconformably overlie the Pliocene Kotakiyama Group, and comprise the following five units: the Wadabokujo, Karasawagawa, Utsukushigahara, Mitsumine, and Wadatoge volcanics. Previous geochronological data and geological data of the present study reveal that the volcanic activity that produced the Enrei Volcanic Rocks started at around 2.1 Ma. From this time, basaltic andesite to andesite, andesite, dacite to rhyolite, and rhyolite volcanism occurred independently of each other until 1.3 Ma, forming several volcanic bodies upon the Utsukushigahara Plateau, and in the Mitsumine and Wada Pass areas.
The Kotakiyama Group is characterized by intense folding and faulting, suggesting that the strata were deformed in a compressional stress field; however, the overlying Enrei Volcanic Rocks are undeformed. The Karasawagawa volcanics are largely of subaqueous origin, but the Utsukushigahara, Mitsumine, and Wadatoge volcanics are mainly subaerial. The Enrei Volcanic Rocks are distributed within a narrow graben-like depression that trends NW-SE (possibly an extension of the Oiwake Graben); the craters of the Yabashira Volcanoes are aligned with this graben, to the southeast. Thus, in the Yatsugatake-Chushin Highland area during late Pliocene to early Pleistocene, a large magmatic field formed along the NW-SE-trending geologic system that is continuous with the Oiwake Graben, producing about 200 km3 of volcanic ejecta.View full abstractDownload PDF (2154K)
Short Article
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Yousuke Ibaraki, Shuji Niko, Ryuji Hosaka, Jun-ichi Tazawa2009 Volume 115 Issue 8 Pages 423-426
Published: 2009
Released on J-STAGE: December 08, 2009
JOURNAL FREE ACCESSDevonian tabulate corals, Mesofavosites cf. igoi (Kamei, 1955), Pachyhelioplasma sp., Auloporoidea fam., gen. and sp. indet., are newly found from limestone float in the lower reaches of the Kotakigawa River in Senoda, Kotaki, Omi area, Niigata Prefecture, central Japan. The occurrence of Mesofavosites cf. igoi and Pachyhelioplasma sp. indicates an Early to Middle Devonian for the fauna. Based on lithology and fossil content, the limestone float probably correlates with the Fukuji Formation in the Fukuji area, Hida Gaien Belt. It is concluded that the limestone brought into the Kotaki Complex of Akiyoshi Belt was formed at the eastern margin of North China (Sino-Korea) during the Middle to Late Permian.View full abstractDownload PDF (976K)
Report
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Hideki Iwano, Tohru Danhara, Hiroyuki Hoshi2009 Volume 115 Issue 8 Pages 427-432
Published: 2009
Released on J-STAGE: December 08, 2009
JOURNAL FREE ACCESSThis paper reports fission track (FT) ages on apatite from Miocene igneous rocks of the Kii Peninsula. A total of twelve ages was determined, of which nine data passed a statistical test. Ages of around 15 Ma were obtained for the Muro Pyroclastic Flow Deposit, the Sekibutsu Tuff, and the granite porphyry of the Kumano Acidic Rocks, and were indistinguishable from the zircon FT ages we reported previously (Iwano et al., 2007, Jour. Geol. Soc. Japan, 113, 326-339), indicating instantaneous cooling. On the other hand, two plutonic bodies of the Omine Granitic Rocks yielded apatite FT ages of about 13 Ma that were significantly younger than the zircon FT ages (approximately 15 Ma), suggesting relatively slow cooling.View full abstractDownload PDF (394K)
Pictorial
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Yoshihiro Kakizaki, Akihiro Kano2009 Volume 115 Issue 8 Pages XIII-XIV
Published: August 15, 2009
Released on J-STAGE: February 23, 2013
JOURNAL FREE ACCESSDownload PDF (1572K)
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