島根県中央部邑智町周辺の白亜紀〜古第三紀火成岩類

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Acid to intermediate volcanic and plutonic rocks of Late Cretaceous to Paleogene ages are widely distributed in Chugoku district. It is generally accepted that the igneous activity shifted its site from south to north during this period. The investigated area located at the central portion of the district is occupied by the various igneous rocks of this period. On the basis of geological evidences, lithological features, and available K-Ar age dating, their activities can be divided into following four stages. [Table] The Ouchi Group composed mainly of rhyolitic welded tuffs, probably of early Late Cretaceous (Gyliakian), constitutes a part of the acid volcanics formed by the extensive volcanism on land surface in Chugoku district. This is followed by the batholithic intrusions of the granitoids of the second stage. Among them the Takahata Granite is the oldest and may be correlated to the Hiroshima Granites emplaced mainly in Urakawan age, and the others to the Imbi Granites in Paleocene or Early Eocene age. During and after this stage the investigated area has upheaved and been eroded, then the Takayama Group unconformably covered the igneous rocks of the first and second stages in Middle or Late Eocene age. The Takayama Group composed mainly of rhyolitic to andesitic pyroclastics with thin intercalations of clastic sediments is inferred to be piled mainly in a shallow lake locally formed during the upheaval movement. This volcanism is followed by the intrusion of Izimi Granite presumed to be Late Eocene or Early Oligocene. The Izimi Granite is composed of various rock types indicating the shallow facies such as micrographic granite, granophyre, quartz porphyry, granite porphyry and aplitic granite. The volcanics and plutonics correlated to the third and last stages in this area has been recently found by several authors in the neighboring areas. Then it may be suggested that the igneous activities during middle to late Paleogeneages were more extensive and intense than those have been hitherto estimated. Thus the igneous rocks from Late Cretaceous to late Paleogene ages are complicatedly distributed in this area. This may indicate that the igneous activity during the period did not so systematically shift its site from south to north.
日本地質学会の論文
1979-09-20

島根県中央部邑智町周辺の白亜紀〜古第三紀火成岩類

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