The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists
Online ISSN : 1883-0765
Print ISSN : 0021-4825
ISSN-L : 0021-4825
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Volume 71, Issue 2
Displaying 1-3 of 3 articles from this issue
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  • PETROGRAPHIC FEATURES OF THE MANNARI-TYPE GRANITE AND THE RHYOLITE COMPLEX ON THE SOUTHEAST OF HIROSHIMA PREFECTURE AND THEIR RELATION
    Satoshi Tomonari
    1976 Volume 71 Issue 2 Pages 37-43
    Published: February 05, 1976
    Released on J-STAGE: August 07, 2008
    DOIhttps://doi.org/10.2465/ganko1941.71.37
    JOURNAL FREE ACCESS
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    On the southeast of Hiroshima Prefecture, several kinds of granitic rocks and rhyolites belonging to the Upper Mesozoic are widely distributed.
    The writer has classified the granities into four types according to their petrological features and geological relations, and has distinguished three bodies of rhyolites, namely,
    granites: (1) the Mannari type (2) the Ashida type (3) the Hirotani type (4) the Mikawa type
    rhyolites: (1) the Toyosaka rhyolite (2) the Joge rhyolite (3) the porphylitic rhyolite
    Among these bodies, it has been made clear that the Mannari type granite is the oldest one and older than the rhyolites as well. This fact indicates the presence of granitic activity before the eruption of at least some members of the so-called “Takata rhyolite”.
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  • PSEUDO-PILLOW LAVAS IN THE ASO CALDERA, KYUSHU, JAPAN
    KAZUNORI WATANABE, YOSHIO KATSUI
    1976 Volume 71 Issue 2 Pages 44-49
    Published: February 05, 1976
    Released on J-STAGE: August 07, 2008
    DOIhttps://doi.org/10.2465/ganko1941.71.44
    JOURNAL FREE ACCESS
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    Subaqueous dacite lavas were found in three older central cones in the northern part of the Aso caldera basin, Kyushu. The upper limit of distribution of the subaqueous lavas is about 510m above sea level, above which subaerial lavas are developed. This evidence suggests that the water level of the Palaeo-Aso Lake had reached to this elevation at the time of eruption.
    The subaqueous lavas consist of numerous dacite blocks and their comminuted fragments. Some of the larger blocks have a pillow-like appearance with columnar joints. These blocks are named “pseudo-pillow lava” because of difference in many respects from basaltic pillow lava. The mechanism of formation of the pseudo-pillow lavas can be interpreted as follows: - The viscous lava is cracked with curved or spheroidal fissures during movement and cooling in water. Water penetrates into the fissures, and cooling contraction produces columnar joints perpendicular to the fissure. Then, pillow-like blocks split away, and the outer rim of the blocks cuts the flow layers obliquely. This is an important feature that distinguishes the acidic pseudo-pillows from basaltic pillows.
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  • PETROLOGICAL STUDY OF THE VOLCANIC ROCKS IN THE AKIMA, AIMA-GAWA AND KENNOMINE FORMATIONS OF THE SOUTH WESTERN PART OF GUNMA PREFECTURE, CENTRAL JAPAN.
    Motokazu Isogai, Kenji Shuto, Keigo Togashi
    1976 Volume 71 Issue 2 Pages 50-59
    Published: February 05, 1976
    Released on J-STAGE: August 07, 2008
    DOIhttps://doi.org/10.2465/ganko1941.71.50
    JOURNAL FREE ACCESS
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    The Akima, Aima-gawa and Kennomine formations of the Neogene system are distributed between the Quaternary Asama and Haruna volcanoes, which occupy the south-western part of Gunma Prefecture. The Pliocene Kennomine formation overlies unconformably the Akima and Aima-gawa formations of the Upper Miocene time (Akima Collaborative Research Group, 1975).
    These formations mainly consist of pyroclastic rocks and volcanic rocks which occur in the form of lava flows and intrusive masses. The main part of these volcanic rocks is made of two pyroxene andesites of hypersthenic rock series, while a little amount of siliceous basaltic rocks of the same rock series are also observed in the early stage of volcanism in this district.
    Twenty representative andesites and basalts are chemically analysed and 87Sr/86Sr ratios are determined for five andesites. They are characterized by low alkali contents, especially low potassium contents. The AFM diagram, (FeO+0.9×Fe2O3)/MgO-SiO2 diagram and (Na2O+ K2O)-SiO2 diagram show that andesites and basalts correspond to those of calc-alkalic type with low alkali contents and those of low alkali tholeiitic type respectively. These volcanic rocks have low K2O/Na2O ratios resembling to those of volcanics of pigeonitic rock series from Izu-Hakone region.
    Five andesites have 87Sr/86Sr ratios ranging from 0.7041 to 0.7043 and the average is 0.7042. These ratios are similar to those of volcanic rocks of the L zone recently proposed by Shuto (1974a, 1974b) and Shuto and Kagami (1975).
    Considering from the results of the present work, it may be concluded that the genesis of volcanic rocks in the above formations can not be ascribed to the contamination with granitic material in magma and that andesites are derived by fractional crystallization of certain basic andesitic (basaltic andesitic) magma probably originated from the upper mantle material presumably of peridotitic composition.
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