The timing and geneses of ilmenite-series and magnetite-series granitic magmatism in the north-central Hokkaido, Japan
		On some species of Sharpeiceras (Ammonoidea) from the Cretaceous of Hokkaido, North Japan (Studies of the Cretaceous ammonites from Hokkaido and Sakhalin-LXXXIV)
		A note on newly found ore fields in Govi-Altai area, southern Mongolia
		Chemical change of underground water in soil and weatherd mantle
		Abstracts for Geological Survey Seminar, no. 255

The timing and geneses of ilmenite-series and magnetite-series granitic magmatism in the north-central Hokkaido, Japan

Shunso ISHIHARA(Counselor, GSJ), Yukihiro MATSUHISA(Geochemistry Department, GSJ), Ryoji TANAKA(Institute for Study of the Earth's Interior, Okayama Univ.), Hiroki IHARA(Research Dept., Engineer School, JGSDF.), Akinori NAGASAKA(Graduate School, University of Tokyo), Toru KOIKE(Earth System Science Corporation) and Ken SHIBATA(Nagoya Bunri College, Inazawa City)

1998

vol. 49 (12) P. 605-620

7 figs., 5 tables

Keywords: Hokkaido, Uttsudake, Granitoids, Gabbroids, K-Ar age, Tertiary, Magnetic susceptibility, Kamishiyubetsu tectonic zone

Abstract: K-Ar age determination was made on twelve rocks and minerals of the Cenozoic granitoids in north-central Hokkaido where the plutonic rocks are most widely seen in the east-west direction, and petrological studies were done on representative Eocene and Miocene granitoids at Uttsudake, Nisshotoge and Kamishiyubetsu intrusive bodies. Eocene granitoids were found in two additional bodies, and the whole distribution trend is a northeast. The Eocene granitoid and gabbroid are the first magmas generated since the change of subduction direction of the Pacific Plate. Tonalitic rocks along the Kamishiyubetsu tectonic zone are ilmenite series or intermediate series until 11 Ma, the plutonism was then converted to magnetite series at 9 Ma of Pliocene.
　The ilmenite-series granitoids are least potassic in the Eocene granitoids and are most potassic in the middle Miocene ones. These granitoids have negative δ³⁴S values and are high in the δ¹⁸O values, indicating some contribution of the basement sedimentary rocks to the granitic magmas. A highest degree of 40 percent of the sediments contribution was calculated on the δ¹⁸O values of middle Miocene granitoids. Late Cenozoic volcanic rocks were also started with ilmenite-series magmatism, and converted to magnetite series at 13-11 Ma, then up to the Quaternary magnetite-series andesitic volcanoes. It is suggested that C-bearing sedimentary and igneous continental crust was first heated up to generate felsic ilmenite-series magmas by intrusion of gabbroic magmas and volatiles from the upper mantle, and later with increasing heat, C-free mafic materials became to be melt to form magnetite-series tonalitic intrusion or andesitic eruption.

On some species of Sharpeiceras (Ammonoidea) from the Cretaceous of Hokkaido, North Japan
(Studies of the Cretaceous ammonites from Hokkaido and Sakhalin-LXXXIV)

Tatsuro MATSUMOTO(c/o Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University) and Seiichi TOSHIMITSU(Geological Museum, GSJ)

1998

vol. 49 (12) P. 621-631

2 figs., 2 tables, 3 plates

Keywords: Cenomanian, Cretaceous, correlation, distribution, Hokkaido, North Japan, Sharpeiceras, Ammonoidea

Abstract: Two ammonite species, Sharpeiceras florencae Spath and Sharpeiceras mocambiquense (Choffat) are described as the first records of their occurrences in the Cretaceous of Hokkaido. The former, which has been known as fairly widespread, is now extended its distribution further to Hokkaido of the North Pacific region and can be called a cosmoplitan in the early Cenomanian time. Sharpeiceras mocambiquense, which was solely recorded from Mozambique, is now recognized to occur in the Lower Cenomanian of much distant Hokkaido. Further investigations would improve our knowledge on the taxonomy and distribution of various species of Sharpeiceras so that they would become good indices of the Lower Cenomanian for the global correlation.

A note on newly found ore fields in Govi-Altai area, southern Mongolia

Tsagaanbilegiin TSEDEN(ITIT Fellow, 1991; MONGOLYNALT Co.), Satoshi MURAO(Mineral and Fuel Resources Department, GSJ), Abirmediin BAATARHUYAG(MONGOLYNALT Co.), Dambiin ALTANTSETSEG(MONGOLYNALT Co.) and Chagnaadorjiin OYUNCHIMEG(MONGOLYNALT Co.)

1998

vol. 49 (12) P. 633-638

6 figs., 1 table

Keywords: mineral occurrence, mineral deposits, Govi-Altai, Mongolia

Abstract: In the Govi-Altai area with a dimension of 3000 km², many mineral occurrences have been confirmed. Within the Erdene sum eleven hydrothermal ore fields exist and vein-type and skarn occurrences, with slight gold anomaly in some places, have been found. Representative ore fields are the Shaart Bulag, Galuutiin, and Tsetseg nuur. In the Biger sum many Cu anomalies have been detected.

Chemical change of underground water in soil and weatherd mantle

Yoji SEKI(Mineral and Fuel Resources Department, GSJ)

1998

vol. 49 (12) P. 639-667

6 figs., 5 tables

Keywords: soil, weathered mantle, undergroundwater, stream water, water quality, dissolution experiment, topography, hydrology

Abstract: Characteristics of soil and weathered mantle typically seen in forested mountainous area in Japan and chemical change of underground water taking place in the region are reviewed.
　Deep weathering of granitic rocks, wet-dry slaking of mudstone, sulfuric acid leaching due to oxidation of pyrite, and decay by formation of gypsum-rich scabs of marine sedimentary rocks are the major types of weathering in humid mountainous areas of Japan.
　Weathering products from bedrock and humus are major constituents of soil in general. In addition, volcanic ash generated by Holocene volcanic activity and long-range transported eolian dust, mostly derived from Asian continent, are the other significant components of the soil in Japan.
　Rain water is ultimate source of hydrologic system in mountainous areas. Annual mean pH value of recent precipitation over Japan is 4.7, which is relatively lower than pH 5.5 for CO₂ saturated normal rainwater. In forested mountainous areas, quality of rainwater commonly changes through interaction with vegetation. For example, rainwater under tree crown and stem flow water of beech stands are characterized by increase of Ca²⁺ and K⁺ but decrease of H⁺, while those of Japanese cypress are characterized by low pH.
　Infiltration capacity of typical forest soil in Japan commonly reaches level large enough to absorb complete ordinal rainfall so that huge amount of percolation water can be stored in the soil. Most of the percolation water subsequently outflows into stream water. The rest penetrates into the bedrocks to become underground water, having relatively long-term retention, with which deeper bedrocks interact.
　In the soil, carbonic acid is generated by dissolution of carbon dioxide mainly from biological activity. Various types of organic acids are also added to the percolation water. Where the bedrock contains pyrite, sulfuric acid generated by the oxidation of pyrite is introduced into groundwaters. The sulfuric acid containing groundwater reacts effectively with rock-forming minerals, resulting in dissolution of such minerals as carbonates and zeolites, and decomposition of feldspars, micas and mafic minerals. As this groundwater penetrates into deeper portion, neutralization by cation exchange from clay minerals may occur. Thus, the groudwater changes its water quality toward higher concentration of Na⁺, Ca²⁺ and HCO₃^- (and SO₄^2- if pyrite exists) with near neutral pH.

Abstracts for Geological Survey Seminar, no. 255
-Poster session of the newest geologic maps published by GSJ-

1998

vol. 49 (12), p. 669-677.