| Abstract |
The Tertiary volcanic rocks, the Quaternary volcanic rocks and the whole granitic rocks through geological times of the inner zone of the arcuate Japan Islands, have lower alkali-lime indices than tho...se of the outer zone (Figs. 1-3). The values of Al_2O_3 /CaO are comparatively small in the outer zone, and increases towards the inner side, while the values of Na20/K_2O and FeO/MgO are comparatively large in the outer zone and decrease towards the inner side. Such a general tendency of lateral variation in chemical composition (the "general or areal variation trend") just mentioned is found in the Paleozoic slates and Tertiary silt stones and perhaps in the metamorphic rocks of the Central Japan. The great tectonic lines as large as the "Fossa magna" and "Median dislocation" lines disturb the general variation trend. The mode of disturbance of the general variation trend may be able to give useful information of geotectonics. Besides the general variation trend above mentioned, is found the special areal character of the granitic rocks, corresponding to those of the volcanic rocks; i. e. in the Seto-uchi district, some of the granitic rocks as well as some of the volcanic rocks have such a character, extraordinarily high Al_2O_3/CaO and extraordinarily low FeO/MgO and Na_2O／K_2O (Figs. 32 & 33). The similarity of granitic rocks, volcanic rocks and sedimentary rocks in the chemical character by area should be a reflection of the previous history of importance to the research of magma problem and granite problem. It schould be more recognized that the chemical compositions of the host rocks embracing magma are effectivc on the differentiation of the magma, by which the diversity of magma is derived. The material source place of sediments may not be apart far from the position of sedimentation. It seems to be true that the metamorphism must hardly change the chemical composition of the original rocks. Their lateral variation trend of the original rocks in chemical composition may be inherited through metamorphism. It would be admitted that the development of the "areal rock character" belongs to a law of developing of orogenic ring, though it is necessary to cirtify the general tendency and predominated mechanism concerning the "areal rock character" throughout the whole orogenic zones. Not a little attention should be accorded to the areal rock character on the view points of petrology and geotectonics.show more
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