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Volume 109, Issue 3
Displaying 1-6 of 6 articles from this issue
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Hiroyuki Hoshi, Hideki Iwano, Tohru Danhara, Takeyoshi Yoshida2003 Volume 109 Issue 3 Pages 139-150
Published: March 15, 2003
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSFission-track dating of volcanic, hypabyssal, and plutonic rocks of the Miocene Shionomisaki Igneous Complex exposed at the southern tip of Kii Peninsula was carried out to delimit the age of volcanic activity. Applying the external detector method to internal and external surfaces of zircon crystals (ED1 and EP2 methods respectively), ages from both methods were compared for individual samples. Three samples from felsic extrusive and hypabyssal rocks showed that the ages by the ED2 method (13-18 Ma) were older than those by ED1 (13-15 Ma). Lengths of semi-tracks on external surfaces of zircons in these samples were abnormally short, whereas confined tracks had a normal-length distribution and short tracks were lacking. Experiments with chemical solutions revealed that the older ED2 ages were very likely caused by an external effect: spontaneous tracks of uranium existing around or adhering to the external surfaces of zircon may have created the apparently older ages. This external effect may thus occur not only in plutonic rocks but also in some extrusive and hypabyssal rocks. The ED 1 ages are considered to represent cooling ages on the basis of confined-track length data, which leads to the conclusion that the Shionomisaki igneous activity commenced about 15 Ma and ceased about 13 Ma. In addition, some rhyolite samples contained colorless zircon grains as well as reddish ones. Such character is generally uncommon, but has also been noted from other Middle Miocene felsic bodies in the Kii Peninsula.View full abstractDownload PDF (1496K) -
Takashi Kudo, Mitsuru Okuno, Toshio Nakamura2003 Volume 109 Issue 3 Pages 151-165
Published: March 15, 2003
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSKita-Hakkoda volcanic group in Northeast Japan arc is an activevolcanic group comprised of several small stratovolcanoes. Weestablished the eruptive history of this volcanic group during the last 6000 years by tephra stratigraphy and AMS 14C dating. We identified five pyroclastic fall deposits, Hk-1, Hk-2, Hk-3, Hk-4, and Hk-5 in descending stratigraphic order. Hk-4 is divided into two stratigraphic units : Hk-4a and Hk-1b, and Hk-5 is divided into five stratigraphic units : Hk-5a, Hk-5b, Hk-5c, Hk-5d, and Hk-5e, in descending order. Hk-1, Hk-2, Hk-4b, and Hk-5d are mainly composed of clayey fine ash with altered andesite blocks. Hk-3, Hk-4a, Hk-5a, Hk-5b, Hk-5c, and Hk-5e are mainly composed of coarse to fine ash with essential andesite blocks and essential spongy pumices. Judging from the constituent materials of each stratigraphic unit, the former was derived from phreatic eruptions while the latter was derived from vulcanian eruptions. These tephras would erupt from the summit craters of Odake volcano, located at the central part of this volcanic group. The magnitude of each eruption ranged from 107 to 105 m3 in volume. On the basis of radiocarbon dates, the eruptive ages are estimated to be 1.5 cal ka BP for Hk-4, 2.0 cal ka BP for Hk-2, 3.1 cal ka BP for Hk-3, 4.2 cal ka BP for Hk-4, and 4.8 cal ka BP for Hk-5. The magmatic eruption rate during the last 6000 years is estimated to be 0.0008 km3 ky. This rate is much smaller than the rate over the entire activity of the volcanic group (0.04-0.06km3 ky, 400-250ka to present). Frequency of eruptions from Hk-5 to Hk-1 is once for 500-1100 years regardless of eruption style. Because 1500 years have passed since Hk-1 eruption, similar-scale vulcanian or phreatic eruption may occur at the Odake summit area in the near future.View full abstractDownload PDF (2330K) -
Michiaki Fujioka, Koji Kameo, Nobuhiro Kotake2003 Volume 109 Issue 3 Pages 166-178
Published: March 15, 2003
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSNew evidence for correlation of three key tephra layers in the Quaternary strata between the Yokohama district and the Boso Peninsula, central Japan is provided. Strong coincidence of petrographic and chemical characteristics of tephra layers clearly demonstrates that the Sg 3 tephra layer in the Ofuna Formation and the Sg 2 and Sg 3 tephra layers in the Koshiba Formation in the Yokohama district can be correlated with the Kd 38, Kd 25 and Kd 24 key tephra layers in the Kiwada Formation in the Boso Peninsula, respectively. Biostratigraphic datums of calcareous planktons in the Ofuna-Koshiba Formations also support this result. Our study indicates that previous correlation of the Quaternary strata between the Yokohama district and the Boso Peninsula should be re-considered and detailed chronological studies of them will be needed in order to reconstruct basin evolution around the Kanto region during the Quaternary.View full abstractDownload PDF (3400K) -
Hideki Iwano, Hiroyuki Hoshi, Tohru Danhara, Tetsu Yoshioka2003 Volume 109 Issue 3 Pages 179-191
Published: March 15, 2003
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSA detailed fission-track (FT) dating study was conducted on zircon crystals to elucidate the age of Miocene volcanic units exposed in the southern margin of the Asahi Mountains. Three samples of pyroclastic rock were collected from the Kitaoguni Formation (KF), Bentozawabashi Formation (BF), and Myozawagawa Tuff Member (MTM) of the Myozawabashi Formation. Dating was carried out by the external detector method, which was applied to the internal (ED1) and external surfaces (ED2) of the zircons. Grouping of zircon crystals was made not only on morphological features including color, shape, elongation, sharpness of edges, and flatness of faces, but also on the density and length distribution of FTs on external surfaces. For the KF and BF samples, FT ages of essential zircon grains were consistent between the ED1 and ED2 methods. Zircons of the MTM sample comprised four groups. ED1 ages showed a single age population, but ED 2 ages varied for each group, and were older than the ED1 ages. Based on FT thermochronologic interpretations and thermal annealing experiments, we conclude that the ED1 ages of detrital grains were completely reset at the time of the MTM eruption. Furthermore, the data suggest that the inconsistency between ED1 and ED2 ages is caused by natural etching of FTs on external surfaces of detrital zircon, as well as by the external effect. Our conclusion is that the KF, BF, and MTM were erupted around 22Ma, 16Ma, and 16 15Ma, respectively.View full abstractDownload PDF (1995K) -
Jun-ichi Uchida, Akihiko Suzuki, Masayuki Mukai2003 Volume 109 Issue 3 Pages 192-194
Published: March 15, 2003
Released on J-STAGE: April 11, 2008
JOURNAL FREE ACCESSA whole rock K-Ar dating is made for the Penkenai Lava at eastern Utanobori area, northern Hokkaido. This analysis gives the age of 5.2±0.3 Ma, older than the age assignment of Late Pliocene by the previous workers. The present result also gives the upper limit of the Penke Formation, which is the uppermost formation of the Neogene marine sequence in this area. This formation is directly overlain by the Penkenai Lava and has been regarded as the Upper Pliocene, but now it is assigned to the Upper Miocene.View full abstractDownload PDF (595K) -
Takao Miyata, Ahmad Khorsandi, Jun-ichiro Yurugi2003 Volume 109 Issue 3 Pages V-VI
Published: 2003
Released on J-STAGE: November 26, 2010
JOURNAL FREE ACCESSDownload PDF (2583K)
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