東中国における後期中生代〜古第三紀酸性岩類の南北変化の概要

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As is well known, the Japanese Islands are one of the sites of violent acid igneous activity in the circum-Pacific region during the Mesozoic to early Tertiaryage. The products of this activity, plutonics as well as volcanics are distributed broadly with an E-W to NE-SW extension and constitute three zones, Ryoke, San'yo and San'in from south to north, in the inner side of Southwest Japan. Recent works on plutonic rocks clarified the variation in several characters such as isotopic age, whole rock and constituent mineral chemistry, type of opaque mineral, oxygen and hydrogen isotoperatios, and so on, amongthethreezones. However, as each of these works was done independently not only in the manner of investigation but also in the field, without any synthesized collaboration in one field across the zones, they were not sufficient to make clear the longitudinal variation. For this reason, we tried to investigate one elongated area traversing the three zones in eastern Chugoku, Southwest Japan (Fig. 1), to elucidate the features and genesis of these zones geologically, petrographically and geochemically. The outline of the result obtained is briefly described below: I. Rock association. Mesozoic to Paleogene igneous rocks of this area are classified into five groups of plutonics and four groups of volcanics as shown in the following table. [Table] Plutonic rocks: Main components of the plutonics are granodiorite to adamellite accompanied by small amounts of quartz diorite to quartz gabbro which occur as small stocks and dikes. Porphyritic texture is not uncommon in the Namariyama and Imbi Intrusives, especially prominent in the former characteristically accompanying granophyric rocks. The Ryoke Intrusives occurring in close association with the amphibolite facies regional metamorphic rocks usually have weak, partly strong, foliation. Relatively large outcrops of granitized diabasic and gabbroic rocks are also known in this zone. These facts suggest the deepest formation of the Ryoke Intrusives among the five groups of plutonic rocks. The Shirotori Granites resemble the Hiroshima Intrusives in their component rock type (largely coarse-to medium-grained adamellite) and dike rocks cutting the granites, although they crop out within the Ryoke zone on the southern most side of the investigated area. Volcanic rocks: The Paleogenevolcanic rocks consist of andesite to rhyolitic pyroclastics with minor quantities of lava. They overlie the Imbi Intrusives and are intruded by the Namariyama Intrusives. The field relation between Cretaceous volcanics (II) and (I) is uncertain, but there is the divergency in rock components. That is, the rhyolitic rocks are more predominant over andesitic rocks in (I) than in (II). Meta-acid pyroclastics occurring in the Ryoke zone may be the oldest volcanic rocks in the area under investigation. They suffered recrystallzation, although their relation with the Ryoke metamorphic rocks has not been made clear. 2. Radiometric age. The following results have been obtained up to date. Namariyama Intrusives………………37-39 m.y. Imbi Intrusives………………51-70 m.y., including two older data (78 & 81 my). Hiroshima Intrusives………………74-88 m.y. Ryoke Intrusives………………80 m.y. Shirotori Granites………………77-87 m.y. 3. Modal composition. The larger parts of the component rocks are granodiorite to adamellite on the Q-A-P triangular diagram, although a part of the Hiroshima Intrusives belongs to granite according to the nomenclature of BATEMAN and DODGE (1970). The granodiorite and tonalite surpass the adamellite in the Ryoke Intrusives, the Namariyama Intrusives and the earlier stage of the Imbi Intrusives. 4. Whole rock chemistry. As discussed by ISHIHARA (1971), ARAMAKI & NOZAWA (1974) and MURAKAMI (1974), the most evident difference in whole rock chemistry among the three zones of plutonic rocks is the Fe_2O_3/FeO ratio which is highest in the granites of the San'in zone, while lowest in those of th
日本地質学会の論文
1979-09-20

東中国における後期中生代〜古第三紀酸性岩類の南北変化の概要

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