16:00 〜 18:30
[R1-P-4] 山陰帯島根県雲南地域に分布する大東花崗閃緑岩の火成活動
キーワード：大東花崗閃緑岩、暁新世、山陰帯、西南日本
本研究では西南日本弧の山陰帯に産する大東花崗閃緑岩について，野外の産状，岩石記載，帯磁率測定，および全岩化学分析をもとにその火成活動を明らかとした．この岩体は，1）苦鉄質包有岩を普遍的に含むこと，2）自形性の強い普通角閃石もしくは黒雲母を含むこと，3）磁鉄鉱が苦鉄質鉱物に多く伴われること，4）高い帯磁率を有すること（5×10-3 SI単位以上）で特徴づけられる．岩相は中～粗粒普通角閃石黒雲母花崗閃緑岩から中粒黒雲母花崗岩を示し，この変化は斜長石，普通角閃石，および黒雲母の分別結晶作用によって形成されている．本研究により大東花崗閃緑岩の地質図は大幅に改訂され，その結果として貫入時期は約57 Maと判明した．既存研究も含めて火成活動を考察すると，大東花崗閃緑岩は山陰バソリスの因美新期貫入岩類（68〜53 Ma）における暁新世の火成岩体に位置づけられる．
The Daito granodiorite is a large plutonic mass in the San'in Batholith, SW Japan. The aim of this study is to clarify the igneous activity on the basis of field and petrographical descriptions, magnetic susceptibility measurements, and whole-rocks chemical analysis. We confirmed that the granodiorite is characterized by 1) ubiquitous distribution of mafic magmatic enclaves, 2) euhedral hornblende and biotite crystals, 3) abundant magnetite associated with mafic minerals, and 4) high magnetic susceptibility (more than 5 × 10-3 SI unit). The lithology varies from medium to coarse-grained hornblende-biotite granodiorite to medium-grained biotite granite. These variations were formed mainly by the fractional crystallization of plagioclase, hornblende, and biotite. The geological map of the Daito granodiorite was significantly revised, and it drew out that the timing of the intrusion was about 57 Ma. Taking into account previous studies, the Daito granodiorite is an important representative of the Paleocene activity in the Younger Inbi intrusive group (68–53 Ma) of the San'in batholith.
The Daito granodiorite is a large plutonic mass in the San'in Batholith, SW Japan. The aim of this study is to clarify the igneous activity on the basis of field and petrographical descriptions, magnetic susceptibility measurements, and whole-rocks chemical analysis. We confirmed that the granodiorite is characterized by 1) ubiquitous distribution of mafic magmatic enclaves, 2) euhedral hornblende and biotite crystals, 3) abundant magnetite associated with mafic minerals, and 4) high magnetic susceptibility (more than 5 × 10-3 SI unit). The lithology varies from medium to coarse-grained hornblende-biotite granodiorite to medium-grained biotite granite. These variations were formed mainly by the fractional crystallization of plagioclase, hornblende, and biotite. The geological map of the Daito granodiorite was significantly revised, and it drew out that the timing of the intrusion was about 57 Ma. Taking into account previous studies, the Daito granodiorite is an important representative of the Paleocene activity in the Younger Inbi intrusive group (68–53 Ma) of the San'in batholith.