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In the Myoko volcanoes, pyroxene andesite and hornblende andesite are very common, and basalt which belongs to the high-alumina basalt by KUNO (1960) is also accompanied in subordinate amount. Generally, the rocks tend to change from lower to upper in each group as follows: basalt→pyroxene andesite→hornblende andesite. The rocks of these volcanoes may be distinguished form the Quaternary volcanic rocks just east of MYOKO in common occurrence of hornblende phenocrysts, and from those of the Southwest Japan in absense of biotite phenocrysts. The features of the chemical composition such as high K2O and Al2O3 contents and low CaO content, and of the mineral assemblage such as common occurrence of hornblende phenocrysts, have well resemblance to those of the Chokai volcanic belt, and they form a striking contrast to those found along the Pacific Ocean side. The K2O content of the rocks of MYOKO tends to increase northward. This tendency is keeping with the increase of K2O content from the Pacific Ocean side to the Japan Sea side. The xenoliths, and inclusions assumed to be xenoliths, are commonly found in the rocks of MYOKO. They are very common in the hypersthenic series rocks, but are rather rare in pigeonitic series rocks. They have generally reacted with magma, and perfectly or partly changed their original mineral compositions and textures. Characteristic large crystals of plagioclase, 3 cm long in maximum, are discovered in the basalt of the Myoko volcano. They correspond to the so-called "large anorthite crystal" named by ISHIKAWA (1951). Their size, An-content, inclusion, zoning, and twinning are remarkably different from those of the common plagioclase phenocrysts in the host basalt. Under the microscope, a continuous transition in texture and mineral assemblage is traced from accidental xenoliths of the basal Neogene sedimentary rocks (pelitic or psammitic rocks) to the large anorthite crystals. Therefore, the large anorthite crystals may have been formed through the reaction between the xenoliths and the host magma. It is assumed that water included in the xenoliths may be one of the most important factors to form the large anorthite crystals. Many characteristic inclusions which are somewhat globular and more mafic than their host rocks are in the horblende andesites from MYOKO. They generally contain glass, and are porous. They have been called cognate inclusion, and interpreted as fragments of the rocks crystallized at an earlier stage in the same "evolutional series" as the host rocks. As the result of the microscopic observation of the rocks of the Myoko IVth stage, a continuous transition in texture, mineral assemblage, and nature of minerals is traced from the accidental xenoliths in the basalt through the inclusions in the basic andesite to the so-called cognate inclusions in the acidic hornblende andesite. Such inclusions in the hornblende andesite, therefore, are considered to have been formed through the reaction of accidental xenoliths with magma. From a view-point of field occurrence, the hornblende andesite of MYOKO is regarded to be derived from basaltic magma. The addition of water in magma from xenolith seems to be one of the essential factors for producing the hornblende andesite. |
