東北日本, 鳥海火山列南部, 湯殿山火山の噴出物と2種の包有物の岩石学的特徴 [in Japanese] Petrological characteristics of two types of inclusions and their host rocks from the Yudonosan volcano, southern part of the Chokai zone, northeast Japan [in Japanese]
Access this Article
Search this Article
The Yudonosan volcano is a small stratovolcano belonging to the Chokai zone in northeastern Japan. The erupted rocks are mainly calc-alkaline quartz-biotite-hornblende-dacite and a subordinate amount of quartz-pyroxene-hornblende-andesite. In these rocks, two types of inclusions can be seen. Both are composed of mainly plagioclases, pyroxenes, and hornblendes, while grain sizes of minerals tend to be coarser in type 2 than in type 1.<br> The mixing of felsic and mafic magmas, associated with the Yudonosan volcano, can be indicated by reverse zoning (An<sub>30</sub> core to An<sub>78</sub> rim) of plagioclase phenocrysts, clinopyroxene reaction rims around quartz, and dusty zones of plagioclase phenocrysts in host rocks. Most of the plagioclase, pyroxene, and hornblende phenocrysts have Na-rich, Fe-rich, and Si-rich cores, respectively. These phenocrysts were probably derived from the felsic magmas. Phenocrysts from mafic magmas were rarely found in host rocks. From pyroxene and hornblende phenocryst compositions in the host rocks, the temperature within the felsic magma chamber was estimated to have been 750-850°C, with the chamber location above the middle of the crust. Rim compositions of plagioclase phenocrysts varied, suggesting that the magma mixing was insufficient.<br> Coarser grained plagioclases, pyroxenes, and hornblendes in type 2 inclusions show the same compositions as phenocrysts in the host rocks. While, finer grained plagioclases, pyroxenes, and hornblendes in type 1 inclusions are Ca-richer, Mg-richer, and Si-poorer than those phenocrysts in the host rocks. These features suggest that the type 1 inclusions were derived from the mafic magma, and type 2 were from the felsic one. It is likely that, in the crystallizing felsic magma chamber, small amounts of mafic magmas were supplied intermittently, and caused insufficient mixings and eruptions. Mafic magmas became the type 1 inclusions, while crystal-rich parts in the felsic magmas solidified to become type 2 inclusions.
- Journal of Mineralogical and Petrological Sciences
Journal of Mineralogical and Petrological Sciences 92(6), 245-259, 1997-06-05
Japan Association of Mineralogical Sciences