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Volume 75, Issue 2
Displaying 1-5 of 5 articles from this issue
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Akira Ono1980 Volume 75 Issue 2 Pages 31-37
Published: February 05, 1980
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSThe Ryoke-Sanbagawa paired metamorphic belt was formed by collision of the two island arcs; Hida-Sangun and Kurosegawa island arcs. The collision had proceeded by subduction of relatively narrow region consisted of thin crust and located between Sanbagawa and Ryoke belts. Remarkable tectonism such as formation of tectonic lands and thick sedimentation have taken place accompanied with the collision. After the collision and accretion of the island arcs, new thermal structure of an island arc- trench system was formed. Upper Cretaceous granitic plutons of South-west Japan were formed by this new thermal activities.View full abstractDownload PDF (426K) -
Mizuhiko Akizuki1980 Volume 75 Issue 2 Pages 38-43
Published: February 05, 1980
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSA two-dimensional atomic arrangment on growth surface will affect the degree of three-dimensional ordering produced during growth (Akizuki et al., 1978, 1979). The optical textures commonly observed in stilbite are interpreted by this mechanism.
Since the space group of stilbite is C2/m and Al/Si sites are all equivalent in the three dimensional unit cell, Al/Si is generally assumed to be disordered. However, Al and Si atoms may order on the surface of a growing face. The immediate surroundings of each Al/Si site on the surface are not equivalent with respect to the growth plane as can be seen in Fig. 7. If this non-equivalence results in different relative preferences for Al/Si on the sites then ordering of Al/Si and the reduction of symmetry will occur. The {110} growth sectors would have triclinic symmetry and the {001} and {010} growth sectors moncolinic symmetry.
The symmetry predicted in this way for the various sectors in the stilbite are consistent with the optical effect observed.View full abstractDownload PDF (2476K) -
TAKASHI NAGAO, MASATSUGU YAMAMOTO, KOSUKE ONUMA1980 Volume 75 Issue 2 Pages 44-54
Published: February 05, 1980
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSChromian spinels in the alkali basalts from the Misasa district, western Honshu, and also in the calc-alkali andesites from the Chokai volcano, northeastern Honshu, have been analysed as well as their host rocks. These spinels are the most aluminous yet reported from the island arc volacnic rocks except those from xenoliths, and the Cr/(Cr+Al) ratio ranges 0.255-0.592 and 0.173-0.668 in the Misasa spinel and the Chokai spinel, respectively. When the compositions of spinels plot in terms of Cr-Al-Fe3+ the crystallization trends proceed from Cr-Al rich portion towards Fe3+ corner associated with an increase in the Cr/(Cr+Al) ratio. On the basis of these data together with those from published papers, it is concluded that the composition of spinel is controlled by the chemistry of host magma as well as pressure, and therefore that it is rather inadequate to compare the depth of crystallization of magma at early stage in different provinces to each other in terms of the Al2O3 content of chromian spinel.View full abstractDownload PDF (445K) -
Yotaro Seki, Yasue Oki, Tomio Hirano1980 Volume 75 Issue 2 Pages 55-61
Published: February 05, 1980
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSBy X-ray diffraction analyses at dry (low humidity) condition and ethylene glycol treatment, chlorite-montmorillonite series minerals formed at the depth of Onikobe and Hakone geothermal areas can be divided into the following seven types:
I: Alkaline montmorillonite
I': Non or low alkaline montmorillonite (smectite)
I'': Mixture of I and I'
II: Mixture of I and IV
III: Mixture of I' and IV
IV: Mixed layer mineral chlorite/smectite
V: Chlorite
Chlorite-montmorillonite series minerals generally change by the order of I, I', I''
→II, III→IV→V with increasing depth and increasing temperature.
Among these types of clay minerals, I, I', IV and V types are most commonly observed and minerals of types I'', II and III are rare, indicating that most of chlorite-montmorillonite series minerals were formed at equilibrium conditions in the depth of these geothermal areas.
As far as judged from X-ray diffraction data, there seem to be gaps between type I and type I', between type I and type IV and between type I' and type IV.
The above-noted typology can well be correlated to the Kristmanndóttir's classification of clay minerals in Icelandic geothermal areas, although the mineralogic problem related to the so-called swelling chlorite still remains unsolved.View full abstractDownload PDF (262K) -
SHIGERU TERASHIMA, SHUNSO ISHIHARA1980 Volume 75 Issue 2 Pages 62-67
Published: February 05, 1980
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSMiocene granite and granodiorite of Tsushima islands contain 700 to 1, 400ppm Cl in general, which are higher in one order of magnitude than chlorine contents of common granitoids in Japan. The Miocene granitoids belong to so-called calc-alkaline series but slightly potassic, and contain no Cl-minerals. The main cause for the anomalous values seems to be chlorine trapped in fluid inclusions contained in various rock-forming minerals. Ratios of chlorine contained in fluid inclusions and rock-forming minerals appear to increase with increasing silica contents. This Cl-enrichment in aqueous phase during the magmatic fractionation as well as original high content may have played an important role for lead-zinc mineralization of the Taishu mine.View full abstractDownload PDF (223K)
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