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