Detaled study of mineral compositions of rock forming minerals in the Miyamoto igneous complex in the south Abukuma mountains are presented. The Miyamoto igneous complex is mostly composed of three rock types, i.e. the Ishigami, the Arizane and the Domeki types. These three types show wide compositional variation. The rocks of the Ishigami type ranging from gabbro to tonalite contain characteristically zoned plagioclase with calcic core (up to An94) and intermediate rim (An60−45), hornblende ranging continuouslly from ferroan pargasite to ferro-hornblende with a range of the mg value (64 to 45) and biotite having narrow compositinal variation. The rocks of the Arizane type mostly composed of tonalite contain plagioclase of wide compositional variation (An53−32), hornblende of narrow compositional variation and biotite that has composition with the mg value of 42 to 28. The rocks of the Domeki type mostly composed of adamellite contain plagioclase, hornblende and biotite of narrow compositional range. Mineral chemistry suggests that the three types of the complex were derived from each unique primary magma and that wide compositional variation of each rock type was mainly caused by fractional crystallization. Mineralogical features including hornblende geobarometer indicate that the rocks of the Ishigami and Arizane types crystallized at about 7 kb, whereas those of the Domeki type at about 5 kb. The Ishigami and Arizane types that were emplaced during the early regional metamorphism intruded under higher pressure than the Domeki type that intruded during the later regional metamorphism. The rocks of the Ishigami type gave no consistent Rb-Sr whole rock age, however their petrographic features and modes of occurrence indicate that their magmas intruded just before the intrusion of the Arizane type having the Rb-Sr whole rock age of about 120 Ma. The previously estimated Sr isotopical compositions of the primary magma of the Ishigami type is very close to that of the Arizane type, so primary magmas of these two types have originated from an isotopically common source material. The Sr isotope indicates that there were two source materials of the primary magmas; one is similar to that of the neighboring Ishikawa and the Tabito igneous complex.
The Goshikigahara volcano, situated at the northern part of the Tomuraushi volcano group in Central Hokkaido, Japan, is a large Quaternary composite volcano consisting of a basal volcanic edifice (Kaundake volcano) and three overlying volcanic edifices (Ponkaundake, Chubetsudake, and Goshikidake volcanoes). Based on the K-Ar ages determined previously for the Kaundake volcano (NEDO, 1990) and the K-Ar ages determined in this study for three volcanic rocks collected from the uppermost (or near uppermost) lavas of the Ponkaundake, Chubetsudake and Goshikidake volcanoes, the formation history of the Goshikigahara volcano can be summarized as follows: (1) ca. 1.0 Ma-0.95 Ma, voluminous lava and pyroclastic eruptions to form the Kaundake stratovolcano, (2) <0.95 Ma-0.8 Ma, volcanic activity on the western flank of the Kaundake volcano to build the main part of the Ponkaundake stratovolcano, and (3) ca. 0.75 Ma, volcanic activity on the eastern flank of Kaundake, forming the two lava plateaus of Chubetudake and Goshikidake.