新潟県米山地域の火山岩類

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タイトル別名
  • Volcanic Rocks in the Yoneyama District, Niigata Prefecture, Japan
  • ニイガタケン ヨネヤマ チイキ ノ カザンガンルイ

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説明

Pliocene volcanic rocks are widely distributed in the Yoneyama district, Niigata prefecture, central Japan. The volcanic activity is divided into four stages, in each of which volcanic rocks vary from basic to acidic. The volcanic rocks are characterized by porphyritic texture and mainly divided into the following rock types by mineral assemblage of phenocrysts: olivine-augite-basalt with or without hornblende, hypersthene-augite-andesite and hornblende-hypersthene-augite-andesite. The andesites of the Yoneyama district are characterized by the presence of rhombic pyroxene in groundmass. In the basalt, however, groundmass hypersthene is not found. Plagioclase phenocryst attains to 21-35% in modal analyses and usually forms zoned crystals. The composition of plagioclase as phenocryst ranges from 44 to 81% in An content, whereas that as ground-mass constituent from 35 to 65%. The optical properties of mafic minerals are described. One augite and four hornblende phenocrysts are analyzed by the electron microprobe. Thity-one lava, two block in pyrocrastic rocks from the Yoneyama formation, one lava from the Komanoma formation and eight intrusive rocks were chemically analyzed. Chemical composition of the Yoneyama volcanic rocks is characterized by high content of Al2O3, ranging from 17 to 21 wt %, and K2O, 1.2 to 2.7 wt%, and low content of MgO, 1.9 to 5.6 wt%. The potassium Ka X-ray image shows that potassium is concentrated in the interstitial glass and irregular-shaped feldspar of groundmass except lath-shaped plagioclase and granular pyroxene. Normative olivine and nepheline often appear in the basalt. Sr and Rb contents of forty volcanic rocks were determined. Sr and Rb contents are high and their average value is 500 and 74 ppm, respectively. K/Rb ratio is 200 on the average. The chemical composition of the Yoneyama basalt is similar to that of the Pliocene volcanic rocks in the northern part of Nagano Prefecture. The composition of the magma seems to be originally rich in Al2O3 and K2O contents.

収録刊行物

  • 地球科学

    地球科学 29 (5), 211-226b, 1975

    地学団体研究会

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