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Volume 76, Issue 12
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PART 2, PETROLOGY OF THE MARGINAL FACIES AND THE ORIGIN OF VARIOUS ROCK FACIESHISATADA AKAHANE1981 Volume 76 Issue 12 Pages 377-385
Published: December 05, 1981
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSThe marginal facies of the Ogawa granodiorite mass is observed not only at the margin of the mass but as dykes in the surrounding rocks.
The marginal facies in question show particular porphyritic structure which is distinguished from the petrographic fabrics ubiquitously observed in the granodiorite mass. Characteristic features of the marginal facies are as follows; 1) The phenocrysts comprise plagioclase, quartz, hornblende and biotite. It is noteworthy that hornblende phenocryst is also generally observed. 2) Quartz and other phenocrysts are corroded and enclose the groundmass minerals at their margins. 3) K-feldspar is almost absent as phenocryst.
The chemistry of the marginal facies corresponds to an average of the chemical compositions of the three rock facies (medium-grained, porphyritic and fine-grained facies) which form the main part of the granodiorite mass.
From these petrographic and chemical features, the author was led to the conclusion that the magma which formed the Ogawa granodiorite mass had the chemical composition corresponding to the marginal facies in question with about 71% silica and that after the consolidation of chilled marginal facies, the medium-grained facies formed first, followed by the formation of porphyritic facies and finally fine-grained facies as the results of the crystallization differentiation in situ.View full abstractDownload PDF (2809K) -
Takeo Bamba1981 Volume 76 Issue 12 Pages 386-394
Published: December 05, 1981
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSThe Tokoro belt has been known as a tectonic belt characterized by preponderance of Jurassic pillow lavas, hyaloclastites and of radiolarian cherts from superficial look. The thick pile of the effusive rocks is called Nikoro Group being regarded as a middle formation of the Jurassic System. There is however a fact that troctolite and gabbro are recently found in some places of the Tokoro belt but the quantity is extremely scant. For the estimation of the hypabyssal level of the Tokoro belt, the preceding fact is regarded as important on this subject.
In this paper, a troctolite from the Tokoro belt was petrographically described as cumulate, and was compared with the gabbro to know the genetic relation of each other. Especially, opaque mineral petrology for the chromite-ilmenite-maghemite paragenesis in the troctolite and the ilmenite-magnetite association in the gabbro made the formative process of these two rocks clear.
Consequently, it was forecasted that the gabbro must be derived from the intercumulus liquid of the troctolite. Besides, widely spread effusive rocks of the Tokoro belt, e.g. tholeiitic pillow lavas have been reinvestigated from the view point of its origin and it was concluded that the effusive rocks may be a product of a magmatism related to the formation of the troctolite and the gabbro in question.
Judging from the newly obtained geologic data, the presence of Steinmann trinity (ophiolite sequence) is expected at hypabyssal level of the Tokoro belt. As a result, the geologic situation of the Yubetsu Group which has been regarded as a substratum of the Nikoro Group must be reinvestigated.View full abstractDownload PDF (3028K) -
Takashi Miki1981 Volume 76 Issue 12 Pages 395-402
Published: December 05, 1981
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSDegree of diagenesis of the Tertiary sedimentary rocks distributed in the Fukuoka City area and its environs, Fukuoka Prefecture, was examined on the basis of the mineralogical and coal petrographical data.
The Tertiary formations in the present areas contain authigenic minerals such as clinoptilolite and heulandite besides montmorillonite. The level of organic metamorphism (LOM) which obtained by coal ranks varies from about 5.5 at the upper horizon to 9.5 at the lowest one. These experimental results indicate that the degrees of diagenesis of the Tertiary formations in the areas studied are remarkably lower than those in other districts in Kyushu already reported (Miki, 1980, and others).
The low grade diagenetic alteration of sediments and small decreasing rate of coal rank from the upper to the lower horizon support the field evidence that the development of sedimentary basin in the present area was small-scale and tectonically stable through the Tertiary age.
Further continuous investigations of the equivalent formations in other adjacent districts are necessary to examine the detailed areal variation of the diagenetic conditions.View full abstractDownload PDF (652K) -
GREEN BIOTITE IN SANBAGAWA BASIC SCHISTS IN THE KANTO MOUNTAINS, JAPANCHIHIRO SAKAI1981 Volume 76 Issue 12 Pages 403-411
Published: December 05, 1981
Released on J-STAGE: August 07, 2008
JOURNAL FREE ACCESSIn the northeastern part of the Kanto Mountains, central Japan, in basic schists, green biotite widely occurs in a lower-grade part where garnet is common in pelitic and psammitic schists, while brown biotite is present in a higher-grade part where brown biotite commonly occurs in them.
Comparing with brown biotite from higher-grade part of the Sanbagawa metamorphic terrain in this area and central Shioku, green biotite in lower-grade part of this area contains lower Al2O3 and higher Fe2O3, and slightly lower TiO2.
On the basis of the paragenetic relation of the green biotite-bearing assemblages, the occurrence of green biotite is explained by high Fe2O3, (FeO+MgO), and low Al2O3 contents of the host rocks. Brown biotite in basic schists in a higher-grade part is formed by the following reaction; green biotite+ (Fe, Mg-rich) chlorite+epidote+albite+quartz=brown bitite+hornblende+(Al-rich) chlorite+magnetite+H2O. This reaction enlarges the stability field of biotite, and in a higher-grade part, brown biotite-bearing assemblages widely occurs in the common basic schists.View full abstractDownload PDF (624K)
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