Bulletin of the Geological Survey of Japan Top Page

Bulletin of the Geological Survey of Japan Vol.53 No.4 (2002)

Cover photograph | Table of Contents | Abstract

Cover photograph

Elongated mafic enclaves in the K-feldspar porphyritic hornblende-biobite granodiorite at Kawabata, south of the Asuke township..

   This strongly foliated granodiorite in the Ryoke metamorphic belt shows high δ¹⁸O value of +12.1‰, although it belongs to I-type. Relationship between the tectonic shearing and supply of ¹⁸O-rich fluids from the crustal materials is suggested (see Text p, 421- 438).

Table of Contents

Title	Author	PDF
Article
Planktonic foraminiferal biostratigraphy of the upper Miocene Kubota Formation in the eastern Tanagura area, Northeast Japan.	Hiroki Hayashi, Tatsuhiko Yamaguchi, Masaki Takahashi and Yukio Yanagisawa (409-420)	53_04_01.pdf [13,713 KB]
Oxygen isotopic constraints on the geneses of the Cretaceous-Paleogene granitoids in the Inner Zone of Southwest Japan.	Shunso Ishihara and Yukihiro Matsuhisa (421-438)	53_04_02.pdf [6,619 KB]
Short Article
K-Ar age of the volcanic rocks, the sedimentation and the deformation in the Omine Belt, Northern Fossa Magna, central Japan.	Koji Mimura and Satoru Harayama (439-444)	53_04_03.pdf [7,619 KB]
Notes and Comments
How to Construct and Maintain Broadband Seismic Network in Indonesia.	Toshiki Ohtaki, Kenji Kanjo, Shoji Sekiguchi, Satoshi Kaneshima, Takeshi Nishimura, Yasushi Ishihara and Ibnu Purwana (445-459)	53_04_04.pdf [8,264 KB]

 

Abstract

Planktonic foraminiferal biostratigraphy of the upper Miocene Kubota Formation in the eastern Tanagura area, Northeast Japan.

Hiroki Hayashi, Tatsuhiko Yamaguchi, Masaki Takahashi and Yukio Yanagisawa

   A planktonic foraminiferal fauna consisting of 39 species was obtained from the upper Miocene Kubota Formation in the eastern Tanagura area, Northeast Japan. The general composition of the foraminiferal fauna indicates that the Kubota Formation was deposited under an open sea environment influenced by a warm current. The presence of Neogloboquadrina acostaensis, absence of Globorotalia plesiotumida, and the first occurrence of Globorotalia merotumida in the middle part of the Kubota Formation enable a correlation with Zone N.16 of Blow (1969). This foraminiferal zonal assignment is consistent with such stratigraphic data as radiometric, calcareous nannofossil, radiolarian, and diatom ages. We recognize four biohorizons defined by the first occurrences of N. acostaensis, Globorotaloides falconarae, and G. merotumida and the last occurrence of Neogloboquadrina cf. mayeri. These biohorizons will be useful for determining the age of Zone N.16 in Japan.

Oxygen isotopic constraints on the geneses of the Cretaceous-Paleogene granitoids in the Inner Zone of Southwest Japan.

Shunso Ishihara and Yukihiro Matsuhisa

   Late Cretaceous-Paleogene granitoids of the Inner Zone of Southwest Japan (SWIN) batholith were studied for oxygen isotopic ratios (δ¹⁸O/δ¹⁶O ratios) by measuring 130 whole-rock samples. The whole-rock δ¹⁸O values (relative to SMOW) higher than ca. 6 permil for magnetite series and ca. 9 permil for ilmenite series are considered unaltered values for the studied granitoids. The magnetiteseries granitoids are generally lower than the ilmenite-series granitoids in the δ¹⁸O values. The δ¹⁸O values of the magnetite series granitoids are 5.9 to 8.1 permil in the Shirakawa area of the Chubu District; 6.9 to 10.6 permil in the Okutango area to eastern Tottori Prefecture, and 6.0 to 8.2 permil in the Misasa-Kamisaihara area of the eastern Chugoku District. On the other hand, those of the ilmenite-series granitoids of the Sanyo-Naegi Belt are 9.2 to 9.8 permil for the unmineralized Toki granite and 7.4 to 8.1 permil for the W-mineralized Naegi granite in the Chubu District. The Wmineralized Otani mine stock of the Kinki District has much higher δ¹⁸O values of 11.7 to 12.0 permil. The ilmenite-series granitoids of the Sanyo-Naegi Belt of the Kinki and Chugoku Districts generally have δ¹⁸O values of 7.3 to 10.8 permil, except for 11.6 to 12.0 permil in eastern Yamaguchi Prefecture, where the granitoids may be related to W mineralization. The ilmenite-series granitoids of the Ryoke Belt in the Chubu District are generally high in δ¹⁸O values as follows: 9.9 to 10.9 permil for I-type granitoids in Zone I, 9.1 to 12.1 permil for I-type granitoids of the Zones II and III. The S-type granitoids of the Zone III have the highest δ¹⁸O values of 10.5 to 12.5 permil.
   Regional variation of the δ¹⁸O values was drawn using δ¹⁸O values normalized at 70 percent SiO₂. The δ¹⁸O contours show high-value centers in the Ryoke Belt and Sanyo-Naegi Belt, and a low-value trough along the Japan Sea coast (Fig. 7). The magnetite-series/ilmenite-series granitoids are separated at 8 permil δ¹⁸O. In the δ¹⁸O values vs. initial ⁸⁷Sr/⁸⁶Sr ratios diagram, the magnetite-series granitoids are plotted generally in igneous source rocks of the lower continental crust. Most of the ilmenite-series granitoids, on the other hand, are high in both the isotopic ratios and plotted along a zone between primitive basaltic rocks and the Ryoke metamorphic rocks, indicating that, together with the common occurrence of mafic enclaves, the ilmenite-series granitoids were formed by magmas having these two components in the source region.
   Cretaceous gabbroids appear to have a high δ¹⁸O value of 8.6 permil, implying the upper mantle at that time was enriched in δ¹⁸O, compared with Cenozoic basalts with different tectonic settings. If this gabbroic magma mingled with sedimentary protolith similar to the Ryoke metamorphic rocks having a δ¹⁸O of 15.6 permil, the maximum contribution of the sedimentary component is calculated to be 18 to 32 percent for the majority of I-type granitoids in the Ryoke Belt of the Chubu District. The two-mica granites contain no mafic enclaves, thus generated from both sedimentary and felsic igneous materials isotopically once homogenized in the lower continental crust. The δ¹⁸O-enrichment on the I-type granitoids appears to have been enhanced by regional shearing. High initial ⁸⁷Sr/⁸⁶Sr ratios in the Shirakawa, Toki and Okutango granitoids suggest the existence of an old basement at depths in these regions.

Ar age of the volcanic rocks, the sedimentation and the deformation in the Omine Belt, Northern Fossa Magna, central Japan.

Koji Mimura and Satoru Harayama

   K-Ar ages for four volcanic units were measured in the Omine Belt, Northern Fossa Magna, central Japan. Two samples of ash flow tuffs for the measurement were taken from the lower Miasa Formation, one sample of a lava from the middle Taroyama Andesite and one of an ash flow tuff from the upper Omine Formation. The K-Ar ages all resulted around 2 Ma. It means that the Omine Belt which built up more than 3,000m thick fans formed in relatively short time span in a pullapart basin around 2 Ma.

How to Construct and Maintain Broadband Seismic Network in Indonesia.

Toshiki Ohtaki, Kenji Kanjo, Shoji Sekiguchi, Satoshi Kaneshima, Takeshi Nishimura, Yasushi Ishihara and Ibnu Purwana

   We constructed a broadband digital seismic network consisting of 23 stations deployed in the western and central parts of Indonesia. In this paper, we show the process of construction, problems encountered during the maintenance of this network, and tips for constructing an overseas broadband seismic network in the future.