西日本における三郡-中国帯の基盤岩類の地下構造の推定 : 大山-青野火山群中の捕獲岩の再検討によせて  [in Japanese] An inferred structure of the basement rocks under the Sangun-Chugoku belt in West Japan : with reference to re-examination of xenoliths in the Daisen-Aono volcanic group  [in Japanese]

Xenoliths in hornblende- and biotite-hornblende andesite of the Daisen-Aono volcanic group, especially those found from Mitake-zan (Mt. Mitake), were re-examined. In the light of our knowledge of xenoliths found from Cenozoic volcanic rocks in West Japan, two groups of the basement rocks were inferred to be laid under the upper Palaeozoic terrane of the Sangun-Chugoku belt. The one is called A-group and the other B-group. The A-group is characterized by association of granulite with norite and/or noritic gabbro, and the B-group is chracterized by association of gneissic rocks with biotite or hornblende-biotite gabbro. The A-group is perhaps lacking in granite, while the B-group is accompanied with granitic rocks. The areas onto which the calc-alkalic basalt of the San-in series and the calc-alkalic andesite of the Setouchi series were erupted belong to the A-group. The whole of the Ryoke granites terrane would belong to the A-group area, because the Ryoke granites are often closely accompanied with norite or noritic gabbro which would correspond to the gabbroic rocks of the A-group. On the other hand, the 300-380 Ma metamorphic and gabbroic rocks which are often associated with the Sangun crystalline schist and/or the Yakuno complex have characters of rocks of the B-group. The Yakuno complex itself belong probably to the B-group. Thus, the rocks of the Sangun-Chugoku belt and probably of the Tamba belt seem to be chiefly laid on the B-group besement rocks; especially the Sangun crystalline schists appear within limit of the area on the B-group. The Daisen-Aono volcanic group was erupted on the B-group area. An inferred distribution of the basement rocks is shown in Fig. 7. SASAKI et al. (1970) showed two models of crustal structure of East Chugoku, Model 1 and Model 2, which were given by the explosion seismologic observation in the Kurayoshi-Hanabusa area. If Model 1 is more likely than Model 2 and is applied for the whole West Japan, the crust of West Japan would be composed of 3-layers defined by velocity of the P-waves: 5.5 km/s, 6.1 km/s and 6.7 km/s. The boundary between the 6.1 km/s and 6.7 km/s layers wds found about 11 km in depth. The 6.1 km/s layer would be mainly composed of the batholithic Cretaceous-Tertiary granites with sub-ordinate amount of the basement rocks of the A- and B-groups. The upper part of the 6.7 km/s layer would be occupied by the basement rocks of both groups, and the lower part of the layer generally by basic and ultrabasic igneous rocks. The basement rocks of the A-group would be juxtaposed with those of the B-group.