新生代造山運動と火山性鉱床のテクトニクス
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概要
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The Pacific plate began to subduct from the Japan trench beneath the Northeast Japan arc 42 m.y. ago. No volcanic activity, nevertheless, has been observed until 28 m.y. B.P. It is assumed that the time between 42 and 28 m.y. B.P. was spent to accumulate volatile elements from the subducted plate in the low-velocity layer. Then after, it was ready for the generation of magma. It is designated the Cryptal stage of late Cenozoic Mizuho orogeny. The orogeny in the ordinary sense started with explosive volcanism 28 m.y. ago. The Initial stage is from 28 to 17 m.y. B.P. Andesitic magma is characteristic of the Initial stage. The volcanic production rate is very high because of the tensional tectonics which was resulted from the opening of the Sea of Japan. The definite magmatic zoning did not exist probably as in the present tensional volcanic area such as the mid-oceanic ridge and marginal sea. Hydrothermal solution well circulated through the tensional crust which is very high in porosity. It was probably very high in the heavy metal contents. The model proposed harmonizes such a matter that the present metalliferous brines are mainly described in the tensional volcanic area. The period between 17 and 13 m.y. B.P. is designated the Transitional stage roughly correlated with the Nishikurozawa stage. Tectonics in the Northeast Japan arc became gradually compressional since about 17 m.y. B.P. The hydrothermal solution was forced to ascend with the advance of compressional environment, because the porosity of crust decreased gradually. Most of Kuroko-type and probably vein-type deposits were formed in the final stage of a single eruptive cycle in the Transitional stage. It is a reason why the most of mineral deposits in the Mizuho orogenic belt were formed about 13 m.y. ago. It seems likely that mineral deposits in the other orogenic belts were also formed in the transitional period from the tensional tectonics to the compressional one. It should be commented, however, that there are some exceptional cases. For example, the deposits in the Red Sea were formed under the tensional environment. The writer's hypothesis is applicable only to the ore formation in the common crust. The different metallogeny should be considered, if the orogenic belt is intercalated with a lot of soluble beds such as evaporite and limestone. Furthermore, the writer mentioned the metallogeny of mineral deposits related genetically to volcanism, but not to plutonism. After 13 m.y. B.P. the Northeast Japan arc has entered the Stationary stage and has become a typical island arc. The compressional tectonics, as a whole, has prevailed through the Northeast Japan arc. The magmatic zoning from tholeiite through high-alumina basalt to alkali olivine basalt has been established. The volcanic production rate declined markedly.
- 特定非営利活動法人日本火山学会の論文
- 1975-12-25