[SCG55-P07] Whole-rock chemical composition analysis of granitic rocks in the Goto Islands, southwestern Japan
キーワード：花崗岩類、五島列島、全岩化学組成
The Goto Islands in Nagasaki Prefecture are located on the western end of the Kyushu district, and on the southern end of the Sea of Japan. The Goto Islands consists of five main islands, Nakadori, Wakamatsu, Naru, Hisaka and Fukue islands from northeast to southwest. Some small islands distribute around main islands. Geology of the Goto islands consists of sedimentary rocks of the Goto group, tuffs, Goto granitic rocks, rhyolite and basalt (Kawada et al., 1994). Goto granitic rocks consist of granodiorite, granodiorite porphyry, and granophyre, and the ages of these rocks are reported c. 15 Ma (Ishikawa & Tagami, 1991). This age is related to the time of the opening of the Sea of Japan, and there is a possibility that the magmatism of these granitic rocks is related to the opening tectonics of the Sea of Japan. In this study, we analyzed whole-rock chemical composition of granitic rocks in the middle eastern part of Fukue Island, the northern part of Hisaka Island, and the northern part of Nakadori Island, and discuss their petrogenesis.
Granodiorite porphyry, granodiorite and granite occur in the middle eastern part of Fukue Island. The northern part of Hisaka Island is mainly composed of granodiorite and granite, and there are some outcrops where the leucocratic rocks and melanocratic rocks sharply border each other. Basic intrusive rocks are also identified. Granodiorite porphyry, granodiorite and granite with intermediate enclaves are identified in the northern part of Nakadori Island. There are also some outcrops where the melanocratic granitic rocks are included into the leucocratic granitic rocks. Granodiorite porphyry, granodiorite and granite from Fukue Island, granodiorite, granite and basic intrusive rocks from Hisaka Island, and granodiorite porphyry, granodiorite, granite and intermediate enclaves from Nakadori Island were sampled. The major elements and trace elements were analyzed with X-ray fluorescence spectrometry, and rare earth elements (REE) were analyzed with inductively coupled plasma mass spectrometry.
The content of SiO2 ranges from 65.3 to 72.7 wt.% for granodiorite porphyry, from 62.4 to 75.9 wt.% for granodiorite and granite, 48.8 and 48.9 wt.% for basic intrusive rocks, and from 58.2 to 64.5 wt.% for intermediate enclaves. Onthe granodiorite porphyry, the content of Fe2O3, MnO, TiO2 and P2O5 decreases, and K2O and Al2O3 content increases with increasing SiO2 content. On the granodiorite and granite, the content of Fe2O3, MnO, TiO2, CaO, P2O5, Al2O3 and MgO decreases, and K2O content increases with increasing SiO2 content. On the SiO2−FeO*/MgO diagram, the granodiorite porphyry shows calc-alkaline series composition. The other granitic rocks are divided into the high−FeO*/MgO group which shows extremely high FeO*/MgO ratio, and the low−FeO*/MgO group which shows tholeiitic series composition.
The geochemical discrimination diagram of trace elements (Pearce et al., 1984) shows that granodiorite porphyry and low-FeO*/MgO group granitic rocks have the characteristics of volcanic arc granite (VAG), and high-FeO*/MgO group granitic rocks have the characteristics of volcanic arc granite (VAG) to within-plate granite (WPG).
The REE patterns show that the samples are rich in light-REE and poor in heavy-REE. Both positive and negative Eu anomalies are observed in the REE patterns of the granitic rocks. Some samples show Ce anomalies, probably due tothe weathering.
The chemical composition analysis reveals that the granitic rocks in the Goto Islands may have multiple origins.
Reference
Ishikawa, N. and Tagami, T., 1991, J. Geomag. Geoelectr., 43, 229−253.
Kawada, K., Kamada, Y., Matsui, K., 1994, Geology of Fukue district. With Geological Sheet Map at 1: 50,000, Geol. Surv. Japan, 47p.
Pearce, J. A., Harris, N. B. W., Tindle, A. G., 1984, Jour. Petrol., 25, 956–983.
Granodiorite porphyry, granodiorite and granite occur in the middle eastern part of Fukue Island. The northern part of Hisaka Island is mainly composed of granodiorite and granite, and there are some outcrops where the leucocratic rocks and melanocratic rocks sharply border each other. Basic intrusive rocks are also identified. Granodiorite porphyry, granodiorite and granite with intermediate enclaves are identified in the northern part of Nakadori Island. There are also some outcrops where the melanocratic granitic rocks are included into the leucocratic granitic rocks. Granodiorite porphyry, granodiorite and granite from Fukue Island, granodiorite, granite and basic intrusive rocks from Hisaka Island, and granodiorite porphyry, granodiorite, granite and intermediate enclaves from Nakadori Island were sampled. The major elements and trace elements were analyzed with X-ray fluorescence spectrometry, and rare earth elements (REE) were analyzed with inductively coupled plasma mass spectrometry.
The content of SiO2 ranges from 65.3 to 72.7 wt.% for granodiorite porphyry, from 62.4 to 75.9 wt.% for granodiorite and granite, 48.8 and 48.9 wt.% for basic intrusive rocks, and from 58.2 to 64.5 wt.% for intermediate enclaves. Onthe granodiorite porphyry, the content of Fe2O3, MnO, TiO2 and P2O5 decreases, and K2O and Al2O3 content increases with increasing SiO2 content. On the granodiorite and granite, the content of Fe2O3, MnO, TiO2, CaO, P2O5, Al2O3 and MgO decreases, and K2O content increases with increasing SiO2 content. On the SiO2−FeO*/MgO diagram, the granodiorite porphyry shows calc-alkaline series composition. The other granitic rocks are divided into the high−FeO*/MgO group which shows extremely high FeO*/MgO ratio, and the low−FeO*/MgO group which shows tholeiitic series composition.
The geochemical discrimination diagram of trace elements (Pearce et al., 1984) shows that granodiorite porphyry and low-FeO*/MgO group granitic rocks have the characteristics of volcanic arc granite (VAG), and high-FeO*/MgO group granitic rocks have the characteristics of volcanic arc granite (VAG) to within-plate granite (WPG).
The REE patterns show that the samples are rich in light-REE and poor in heavy-REE. Both positive and negative Eu anomalies are observed in the REE patterns of the granitic rocks. Some samples show Ce anomalies, probably due tothe weathering.
The chemical composition analysis reveals that the granitic rocks in the Goto Islands may have multiple origins.
Reference
Ishikawa, N. and Tagami, T., 1991, J. Geomag. Geoelectr., 43, 229−253.
Kawada, K., Kamada, Y., Matsui, K., 1994, Geology of Fukue district. With Geological Sheet Map at 1: 50,000, Geol. Surv. Japan, 47p.
Pearce, J. A., Harris, N. B. W., Tindle, A. G., 1984, Jour. Petrol., 25, 956–983.