Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XIX, No.4


Petrographic Study on So-Called "Madaraishi" from Machiya Town : A Contribution to the "Koshidai-Takaboyama-Daioh'in Structural Belt", Abukuma Plateau, Japan

Shimaoka, Hiroshi

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The ultrabasic rocks in the vicinity of Machiya Town, Ibaragi Prefecture, Japan can be classified into two rock types; massive type and foliated type. The former is mainly composed of olivine-talc and anthophyllite, sometimes enstatite is accompanied. Some of these rocks have been noted as "Madaraishi" because of the presence of peculiar dappled texture. The latter is characterized by formation of pronounced foliated structure in which abundant tremolite and chlorite are present. Judging from the mode of occurrence, mineral assemblage and bulk chemical composition of the preceding rocks, the massive type including "Madaraishi" is considered to be a product which might have been converted under the heating condition, perhaps related to external system. The foliated type, on the other hand, is concluded that it has undergone shearing stress, an influence of tectonic movement after the formation of the massive type. As to the "Madaraishi", dappled texture of the rock is not considered as relict of original rock but newly formed rock facies yielded through a metamorphic process. It is conceivable that the olivine is porphyroblast being attributed to a thermo-metamorphic condition. In this paper, the origin of the "Madaraishi" is discussed in relation to the formative process of the Koshidai-Takaboyama-Daioh'in Structural Belt.