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Volume 116, Issue 4
Displaying 1-6 of 6 articles from this issue
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Articles
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Satoshi Okamura, Masahiro Yahata, Hirotsugu Nishido, Atsushi Ibusuki, ...2010 Volume 116 Issue 4 Pages 181-198
Published: 2010
Released on J-STAGE: October 13, 2010
JOURNAL FREE ACCESSThe K-Ar ages and chemical compositions of Early Miocene volcanic rocks in southern central Hokkaido were determined to clarify the tectono-magmatic evolution of the back-arc basins of the Japan and Okhotsk seas. In the Yufutsu Oil and Gas Field, andesitic tuffs of the Takinoue Formation range in age from 14.2 ± 0.4 to 19.9 ± 2.4 Ma. Andesitic lavas of the Takinoue Formation, where they occur in the Umaoi Hills, are 19.1 ± 0.5 Ma in age, and andesitic breccias of the Sakae Formation in the Sakae region yield an age of 17.3 ± 0.5 Ma, which is within error of the age of the Takinoue Formation. These ages, combined with microfossil data, indicate that volcanism of the Takinoue stage occurred from 14 to 21 Ma. The Takinoue volcanic rocks in central Hokkaido show a wide range of trace-element abundances and isotopic ratios. Volcanic rocks of the Yufutsu field are characterized by depleted isotopic ratios (low 87Sr/86Sr and high 143Nd/144Nd values) and high Zr/Nb ratios, similar to those of mid-Miocene lavas from the NE Honshu arc associated with opening of the Japan Sea, and consistent with an origin via melting of a depleted mantle source. In contrast, other Takinoue stage volcanic rocks from central Hokkaido do not show depleted isotopic ratios and have low-medium Zr/Nb ratios. The wide range of geochemical abundances in the Takinoue stage volcanic rocks suggests that central Hokkaido may record a different tectono-magmatic history to that of the NE Honshu arc.View full abstractDownload PDF (1801K) -
Chie Furukata, Mitsuhiro Nakagawa, Wataru Hirose, Yoshiko Adachi2010 Volume 116 Issue 4 Pages 199-218
Published: 2010
Released on J-STAGE: October 13, 2010
JOURNAL FREE ACCESSHokkaido is located within the region of extension along the spreading axis of the Japan Basin, and volcanism related to spreading occurred in western and central Hokkaido during the period 20-14 Ma. The volcanic rocks of central Hokkaido show a bimodal composition, with both mafic and felsic rocks. The mafic rocks are geochemically divided into three types that occur in distinct regions. The rocks of the eastern region show a weak Nb anomaly and an ocean island basalt (OIB) signature in spider diagrams (Eastern type: E-type), similar to basaltic rocks of the Japan Basin. The rocks of the Urakawa region, within the eastern region, exhibit no Nb anomaly and a strong OIB signature (Urakawa type: Ur-type). The rocks of the western region show a clear Nb anomaly and an arc-type signature (Western type: W-type) . An analysis of Sr-Nd isotope ratios reveals that E-type magma is slightly enriched relative to MORB, whereas W-type magma is strongly enriched. In addition, the Ur-type rocks have an EM1 isotopic signature. Felsic rocks, which occur in the same region as the E-type mafic rocks, can be divided into three types: high alkali, adakite, and high silica. High-silica rhyolite shows the same geochemical features as Quaternary rhyolite in southwestern Hokkaido, whereas the other two types possess distinct geochemical features: high-alkali rhyolite is characterized by high K2O and Nb contents, and adakite-type rhyolite is characterized by high Sr/Y and LREE/HREE values. We inferred that these felsic magmas were produced not by fractional crystallization or Assimilation and Fractional crystallization (AFC) processes affecting coexisting mafic magmas, but by partial melting of lower-crust materials. Thus mantle-derived mafic magmas are only means to know mantle condition during Japan Basin Opening. The Nb/Yb values of mafic rocks from central Hokkaido are higher than those of 20-14 Ma rocks in other areas in and around the Japan Sea. This observation, combined with the presence of EM1 magma, could suggest that the source rocks and conditions of partial melting beneath central Hokkaido were different from those in other areas.View full abstractDownload PDF (2153K) -
Masayuki Ehiro, Kazuo Komori, Nobutaka Tsuchiya, Toshio Kawamura, Hiro ...2010 Volume 116 Issue 4 Pages 219-228
Published: 2010
Released on J-STAGE: October 13, 2010
JOURNAL FREE ACCESSLate Carboniferous (probably Moscovian) ammonoids and associated corals were recovered from limestone blocks in a Jurassic accretionary complex of the Kuzumaki-Kamaishi Sub-belt of the North Kitakami Belt, within the drainage basin of the Mabechigawa River, south of Ichinohe, Northern Kitakami Massif, Northeast Japan. The limestone blocks are associated with basalts of oceanic island origin; together, these rocks probably constitute a seamount-reef complex. The ammonoids, which are the first Paleozoic ammonoids and the first reliable Carboniferous fossils to be described from the North Kitakami Belt, indicate that the oceanic plate associated with formation of the accretionary complex in the Kuzumaki-Kamaishi Sub-belt had already formed by the Late Carboniferous.View full abstractDownload PDF (2935K)
Short Articles
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Tomohiro Kasama, Hiroyuki Yamashita, Kazutaka Mannen, Mitsuru Okuno, T ...2010 Volume 116 Issue 4 Pages 229-232
Published: 2010
Released on J-STAGE: October 13, 2010
JOURNAL FREE ACCESSThe Futagoyama lava dome, one of the post-caldera central cones of Hakone volcano (Kanagawa, Japan), is though to have formed by a single eruption at 5 ka. However, in this study, we show that the dome formed over the course of at least three eruptions. We discovered a relatively old block-and-ash flow deposit (Hakone-Futagoyama Yamazaki block-and-ash flow deposit [Hk-FtY]) that originated from the Futagoyama lava dome, as indicated by its chemical composition and its age of 20,390 ± 40 yr BP. We also re-examined the source of the Shinanoya pyroclastic flow deposit, which was previously interpreted to be of Komagatake origin and was dated at 17,920 ± 320 yr BP (reference), and concluded that this deposit also originated from the Futagoyama lava dome. The recent eruptive history of Futagoyama suggests that its eruptions may have been synchronous with those of Kamiyama. In addition, the eruptive centers of Futagoyama and Kamiyama are aligned with each other along a linear trace.View full abstractDownload PDF (717K) -
Jun-ichi Tazawa2010 Volume 116 Issue 4 Pages 233-236
Published: 2010
Released on J-STAGE: October 13, 2010
JOURNAL FREE ACCESSFour choristitid brachiopod species were found in the Upper Carboniferous (Moscovian) limestone of the Nagaiwa Formation in the Nagaiwa area, South Kitakami Belt, northeast Japan: Choristites mosquensis Fischer de Waldheim, Choristites jigulensis (Stuckenberg), Parachoristites sp. A, and Parachoristites sp. B. All these species are Boreal-type brachiopods, distributed throughout the Upper Carboniferous (Bashkirian) to lowest Permian (Asselian) of northern and western Russia, central Asia, and Northwest to Northeast China. Therefore, the occurrence of the present choristitid brachiopods suggests that the South Kitakami area was part of the continental shelf along the eastern margin of North China (Sino-Korea) during the Late Carboniferous.View full abstractDownload PDF (1174K) -
Hideko Takayanagi, Yuki Nakayama, Tsuyoshi Ishikawa, Kazuya Nagaishi, ...2010 Volume 116 Issue 4 Pages 237-240
Published: 2010
Released on J-STAGE: October 13, 2010
JOURNAL FREE ACCESSWe present strontium (Sr) isotope data for island-surface dolomites at Kita-daito-jima, northern Philippine Sea. The 87Sr/86Sr ratios of 50 samples fall within a wide range from 0.709037 to 0.709079, corresponding to 4.9-2.1 Ma. This wide range in Sr ages reflects the fact that the island-surface dolomites comprise variable mixtures of geochemically distinct, diachronous, dolomite crystal phases. Our results indicate that the Sr isotope ages of multi-generational dolomites on carbonate islands such as Niue and Little Bahama Bank, as well as Kita-daito-jima, should be re-examined employing a new method that determines the Sr isotope ratio of individual crystal phases.View full abstractDownload PDF (844K)
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