[S-CG44] 岩石・鉱物・資源

[SCG44-P02] Whole-rock chemical composition of the Nojima granodiorite in northern part of Awaji Island, southwestern Japan

*石原 彰博¹、壷井 基裕¹ (1.関西学院大学)

キーワード：花崗閃緑岩、XRF、ICP-MS、希土類元素、淡路島

The Late Cretaceous to Paleogene granitic rocks in the inner belt of southwestern Japan is divided into three zones from north to south: the Sanin belt, the Sanyo belt, and the Ryoke belt (1). Granitic rocks in Awaji Island in Hyogo Prefecture belongs to the Ryoke or Sanyo belt. The granitoids of Awaji Island are divided into three groups (granitoids I, II, and III) and eleven rock types according to the intrusion sequences (2). The Nojima granodiorite is a medium-grained hornblende biotite granodiorite to granite that is classified as granitoids Ⅱ. The chemical compositions of whole-rocks and minerals of the granitoids of Awaji Island have been reported (3), however, the chemical compositions of rare earth elements are rarely reported. In this study, chemical composition of the Nojima granodiorite including rare earth elements were analyzed and the formation process and petrogenesis were investigated. Major and trace elements were measured by using wavelength-dispersive X-ray fluorescence spectrometer (XRF). Rare earth elements were determined by inductively coupled plasma mass spectrometer (ICP-MS). SiO₂ content is in the range of 58.7-72.8 wt%. Fe₂O₃, MnO, TiO₂, CaO, P₂O₅ and MgO decreases and K₂O increases with increasing SiO₂. Al₂O₃ and Na₂O shows almost constant. Alumina saturation index (Al₂O₃/(CaO+Na₂O+K₂O)) ranges from 0.87 to 1.10. Alumina saturation index increases with increasing SiO₂. On the Chondrite normalized REE pattern diagram, the light rare earth elements (La, Ce, Pr, Nd, Sm, Eu) are more abundant than the heavy rare earth elements (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). This may be due to the relationships between ionic radii of REE and differentiation process of parental magma. It shows negative Eu anomalies for three samples and positive Eu anomalies for two samples among six samples. Two samples show negative Ce anomalies among six samples.

(1)Ishihara, S. (1971) Modal and chemical compositions of the granitic rocks related to the major molybdenum and tungsten deposits in the Inner Zone of Southwest Japan. Jour. Geol. Soc. Japan, 77, 441-452.

(2)Takahashi, Y. and Hattori H. (1992) Granitic rocks in Awaji Island-with an emphasis on the foliated granites-. Bull. Geol. Surv. Japan, 43, 335-357.

(3)Takahashi, Y. (1995) Major element geochemistry and mineral chemistry of granitic rocks in Awaji Island-Implications for the zonal distribution of Cretaceous granitic rocks, Inner Zone of Southwest Japan-. Bull. Geol. Surv. Japan, 46, 23-40.