日本地球惑星科学連合2018年大会

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[JJ] Eveningポスター発表

セッション記号 S (固体地球科学) » S-CG 固体地球科学複合領域・一般

[S-CG60] 岩石・鉱物・資源

2018年5月23日(水) 17:15 〜 18:30 ポスター会場 (幕張メッセ国際展示場 7ホール)

コンビーナ:門馬 綱一(独立行政法人国立科学博物館)、野崎 達生(海洋研究開発機構海底資源研究開発センター)、齊藤 哲(愛媛大学大学院理工学研究科、共同)、土谷 信高(岩手大学教育学部地学教室)

[SCG60-P13] Origin of low-Sr granitoids from the northeastern part of Kyushu

*村岡 やよい1大和田 正明1今岡 照喜1亀井 淳志2 (1.山口大学、2.島根大学)

Cretaceous to Paleogene granitic rocks are widely exposed on the eastern margin of Asian continent. The northern part of Kyushu, Southwest Japan, is underlain by the Cretaceous plutonic rocks. These granitoids are distinguished between low-Sr and high-Sr granitoids by their Sr contents. Low-Sr granitoids are exposed on the east side of the Kokura-Tagawa tectonic line. On the other hand, in the west side of the tectonic line, high-Sr granitoids are distributed. Izawa et al. (1990) suggests fractionation of hornblende in the lower level of continental crust and supply of periodic primitive magma into magma chamber as genesis of the high-Sr granitoids. Yada and Owada (2003) also presumed differentiation process of the Itoshima granodiorite (high-Sr granitoid in the west side) by mainly fractionation of hornblende and plagioclase from its parental magma. However, little research on genesis of the low-Sr granitoids has been done. In this study, the author discusses genesis of the low-Sr granitoids of the Hirao granodiorite.
The Hirao granodiorite occurs as a stock with 16 km N-S and 4 km E-W. The granodiorite consists mainly of plagioclase, quartz, K-feldspar, biotite, and hornblende. In addition, the granodiorite dike occurs in the study area. The granodiorite dike has mineral assemblage similar to that of the Hirao granodiorite.
Sr contents of the Hirao granodiorite and dike plot within the low-Sr granitoids field. SiO2 contents of the Hirao granodiorite range from 62 to 67 wt.%, these of dike show 73.3 wt.% and 74.0 wt.%. SiO2 contents of the Hirao granodiorite increase with decreasing modal values of hornblende, biotite and plagioclase. The granodiorite dike shows more evolved composition with decreasing of these minerals. Chondrite normalized REE patterns for the Hirao granodiorite and dike show negative Eu anomaly. These characteristics are inferred that main fractionated minerals are of hornblende, biotite and plagioclase. Results of geochemical modeling reveals that compositional variation of the Hirao granodiorite is mainly caused by fractionation of plagioclase accompanied by hornblende.
The Hirao granodiorite shows K-Ar age of 99.1 Ma. Initial isotopic ratio of the Hirao granodiorite and dike show similar value. Furthermore, these ratios resemble that of the granitoids in west side of the Kokura-Tagawa tectonic line.
The Hirao granodiorite can be originally derived from similar source rocks to the granitoid from the west side of the tectonic line because of similarity of their isotopic compositions. On the other hand, the lowest SiO2 sample in the Hirao grandiorite also show negative Eu anomaly. This feature indicates that a restite of granodiorite magma contains a large amount of plagioclase. In addition, plagioclase is main subtracted phase during crystallization of the Hirao granodiorite magma. Therefore, plagioclase is largely involved in magma processes in the low-Sr granitoids from the northeastern part of Kyushu batholiths.