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Volume 74, Issue 6
Displaying 1-4 of 4 articles from this issue
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Takeshi Ohguchi, Keizô Yanai, Yoshio Ueda, Shirô Tamanyu1979 Volume 74 Issue 6 Pages 207-216
Published: June 05, 1979
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSThe occurrence, lithofacies and radiometric ages of the NyfldBzaki Igenous Rock, Akashima Formation, the basal part of the Monzen Group (so-called Neogene) were examined. The present study reveals that the Member was largely composed of the welded dacitic pyroclastic flow deposits, and it was unconformably overlying the pre-Tertiary granitic rocks. The K-Ar ages of 30 and 38 m. y. and the zircon fission track ages of 67.2±9.9 and 61.2±9.0m.y. were obtained from the wleded tuff of the Member.
It is possible to correlate the Nyûdôzaki Igneous Rock with the Futomiyama Group, Hokuriku Province, and the Sumikawa Formation, Asahi mountain land, based upon the radiometric ages, the lithofacies, and the stratigraphic relation to the underlying Upper Cretaceous pyroclastic flow deposits and granitic rocks.
In northeast Japan, the equivalent pyroclastic deposits of the Nyûdôzaki Igenous Rock are extending only in the region along the Japan Sea coast, and not developing in the Ôu backbone mountain range. This difference seems to suggest that the different volcanic activities in time or space existed in the former region.
K-Ar age of the welded tuff, obtained from the Nyûkawa Formation, Sado Island, which were correlated with the Nyûdôzaki Igneous Rock, and that of biotite, separated from the Shinzan Rhyolites, the upper-most part of the Monzen Group Oga, Peninsula, were also determined to compare with the age of the NyfldBzaki Igneous Rock. However, the results of the measurement; 21 and 39m.y., respectively, were not coincide with the lithological and biostratigraphical correlation of their formation.View full abstractDownload PDF (2365K) -
MUTSUMI MIYACHI, YOSHIFUMI KARAKIDA1979 Volume 74 Issue 6 Pages 217-226
Published: June 05, 1979
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSThe mass colors of seventy-seven zircon concentrates from the Cenozoic acid volcanic rocks have been observed to form a new zircon scale and to confirm the color-age relation-ship of volcanic zircons. The writers have found that the mass colors of zircons vary from pale purplish red to colorless with the lapse of time, from Oligocene to Late Pleistocene, and that there is an intimate relationship between colors and ages. There are a few problems yet to be solved. However, it is the writers' conviction that the zircon correlation method is an effective and practical mode of estimating the eruption age of volcanic rock.View full abstractDownload PDF (378K) -
Fumio Ohashi, Takashi Agata, Isamu Hattori1979 Volume 74 Issue 6 Pages 227-234
Published: June 05, 1979
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSA small separate body of ultramafic rock was found in the Paleozoic volcanic and sedimentary sequence in Nakatsubara of the Nanjo Mountains. It consists mainly of euhedral to peri-euhedral olivine and chromite, and poikilitic clinopyroxene and plagioclase, with a small amount of interstitial bronzite. Some large olivine crystals contain globular inclusions, 0.1 to 0.5mm in diameter, comprising minute crystal aggregates of kaersutite, dendritic titanaugite and chromite in the saponite mesostasis.
The bulk chemical composition of the ultramafic rock is similar to that of a picrite, relatively rich in alumina and soda. It also exhibits comparatively high Fe/Fe+Mg and presence of normative hypersthene.
The chemistry of the essential minerals is investigated. From the Mg-Fe partitioning between olivine and chromite, the temperature of their crystallization is estimated about 1, 250°C.
The texture and the mineralogy suggest that the Nakatsubara ultramafic rock was essentially formed in a magma chamber by crystals accumulating on the floor. The presence of orthopyroxene discloses this ultramafic rock coming from the magma of a tholeiitic composition.View full abstractDownload PDF (1135K) -
HISASHI TAKESHITA, YUKIO MATSUMOTO1979 Volume 74 Issue 6 Pages 235-244
Published: June 05, 1979
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSThe association of thulite-scolectei-prehnite in felsic volcanic pyroclastics of the Neogene Tertiary developed at the Nakadori Island, Goto, is reproted here as a peculiar case of the Green-Tuff alteration. The thulite is mangan-clinozoisite, (Ca2.039Mg0.034Mn0.058)2.131(Fe0.172Al2.828)3.000(Al0.023Si2.913)2.936(OH)O12. Thulite in porphyrite dikes which intrude into the pyroclastics, however, ranges from mangan-clinozoisite ((Na0.005Ca1.936Mg0.097Mn0.044)2.082(Fe0.048Al2.936)2.984Si2.972(OH)O12 to withermite ((Na0.006Ca2.033Mg0.019Mn0.038)2.095(Fe0.545Al2.385)2.930Si3.000(OH)O12-(Ca1.999Mg0.002Mn0.034)2.035(Fe0.696Al2.302)2.998Si2.893(OH)O12). Scolecite associated with thulite has the chemical composition of (K0.001Na0.012Ca0.989Mg0.004)1.006Al1.933Si3.049O10•3H2O. Prehnite associated with thulite and scolecite has the composition of (Na0.001Ca1.988Mg0.001Fe0.006Mn0.002)1.998Al1.015(Al1.036Si2.964)4.000(OH)2O10.View full abstractDownload PDF (1235K)
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