11:45 〜 12:00
[SCG48-05] 島根県奥出雲町高田付近に分布する高田花崗閃緑岩の火成活動
キーワード：メタアルミナス花崗岩、山陰バソリス、西南日本弧、ジルコンU-Pb年代
Cretaceous to Paleogene granitic rocks are widely distributed in the San'in zone of Southwest Japan arc, and they form the San'in batholith around eastern Shimane to western Tottori. Granitic batholith is an essential element for continental crust growth, especially metaluminous granitoid is an important presence because they are derived from the more basic rocks, such as those that make up mafic lower crust (e. g. Wedepohl, 1995).
Some metaluminous granites occur in the San'in batholith, but their igneous activities are not well understood yet. In this study, we investigated a metaluminous granitoid, named Takada granodiorite, situated in the central part of the batholith from the viewpoints of geology, geochemistry, and zircon U-Pb dating.
The Takata granodiorite is surrounded by Paleogene granitoid (ca. 60 to 30 Ma) and has geologically been interpreted as the oldest Cretaceous suit in the batholith (Nishida et al., 2005). We confirmed geological occurrence of the Takada suit, which intruded by ca. 60Ma suit (Yokota granite) in the south and by ca. 61 Ma suit (Shimokuno granite) in the north. However, the isotopic age of the Takada suit has not previously been reported yet.
Nishida et al. (2005) reported that the Takada suit is composed of various granitoid such as quartz diorite, tonalite, and granodiorite. On the other hand, Matsuura et al. (2005) and Yakushiji et al. (2012) indicate that the quartz diorite is solidified from a different magma to the Takada magma, which is named Ogi quartz diorite, and they imply it is mingled with the tonalite and granodiorite of the Takada suit. Additionally, Yakushiji et al. (2012) showed that the Shimokuno granite is also scattered within the Takata suit and that it is partly mingled with and intrudes into the Takada tonalite and granodiorite. Our field observations and geochemical analyses supported the previous suggestions by Matsuura et al. (2005) and Yakushiji et al (2012), that is, the Takada suit is mainly composed of tonalite and granodiorite, and the Ogi and the Shimokuno suites are mingled with and/or intrude into the Takada suit. These are solidified from different magmas each other.
Yakushiji et al. (2012) also analyzed Rb-Sr whole-rock isochrons of the Takada tonalite, the Ogi quartz diorite, and the Shimokuno granite. They obtained ca. 61 Ma from the Shimokuno, but could not obtain ages from the other two suites. In this study, we analyzed the zircon U-Pb ages of the Takada and the Shimokuno suites and obtained unique broad ages ranging from 65 to 60 Ma on both granitoid. The zircon data are continuously arranged on a concordant curve and have not converged. There are two possibilities for this phenomenon; 1) very slow cooling of the magmas, and 2) hybridization and assimilation of magmas and rocks, although the assumption here is that the effect of lead loss of zircon is not significant.
There is geologically no time difference between these activities as mentioned above. Combined with the present results, the timing of these activities is interpreted to be approximately 65 to 60 Ma. Therefore, the Takada granodiorite is not a Cretaceous suit as previously reported. Instead, it is considered to be a product of the Paleogene peak igneous activity (68~53 Ma: Nishida et al., 2005, 2013) of the San’in batholith. The metaluminous granitoid in the San'in batholith is not a forerunner, but it would be relatively early-stage igneous activity involved in major batholith formation.
Some metaluminous granites occur in the San'in batholith, but their igneous activities are not well understood yet. In this study, we investigated a metaluminous granitoid, named Takada granodiorite, situated in the central part of the batholith from the viewpoints of geology, geochemistry, and zircon U-Pb dating.
The Takata granodiorite is surrounded by Paleogene granitoid (ca. 60 to 30 Ma) and has geologically been interpreted as the oldest Cretaceous suit in the batholith (Nishida et al., 2005). We confirmed geological occurrence of the Takada suit, which intruded by ca. 60Ma suit (Yokota granite) in the south and by ca. 61 Ma suit (Shimokuno granite) in the north. However, the isotopic age of the Takada suit has not previously been reported yet.
Nishida et al. (2005) reported that the Takada suit is composed of various granitoid such as quartz diorite, tonalite, and granodiorite. On the other hand, Matsuura et al. (2005) and Yakushiji et al. (2012) indicate that the quartz diorite is solidified from a different magma to the Takada magma, which is named Ogi quartz diorite, and they imply it is mingled with the tonalite and granodiorite of the Takada suit. Additionally, Yakushiji et al. (2012) showed that the Shimokuno granite is also scattered within the Takata suit and that it is partly mingled with and intrudes into the Takada tonalite and granodiorite. Our field observations and geochemical analyses supported the previous suggestions by Matsuura et al. (2005) and Yakushiji et al (2012), that is, the Takada suit is mainly composed of tonalite and granodiorite, and the Ogi and the Shimokuno suites are mingled with and/or intrude into the Takada suit. These are solidified from different magmas each other.
Yakushiji et al. (2012) also analyzed Rb-Sr whole-rock isochrons of the Takada tonalite, the Ogi quartz diorite, and the Shimokuno granite. They obtained ca. 61 Ma from the Shimokuno, but could not obtain ages from the other two suites. In this study, we analyzed the zircon U-Pb ages of the Takada and the Shimokuno suites and obtained unique broad ages ranging from 65 to 60 Ma on both granitoid. The zircon data are continuously arranged on a concordant curve and have not converged. There are two possibilities for this phenomenon; 1) very slow cooling of the magmas, and 2) hybridization and assimilation of magmas and rocks, although the assumption here is that the effect of lead loss of zircon is not significant.
There is geologically no time difference between these activities as mentioned above. Combined with the present results, the timing of these activities is interpreted to be approximately 65 to 60 Ma. Therefore, the Takada granodiorite is not a Cretaceous suit as previously reported. Instead, it is considered to be a product of the Paleogene peak igneous activity (68~53 Ma: Nishida et al., 2005, 2013) of the San’in batholith. The metaluminous granitoid in the San'in batholith is not a forerunner, but it would be relatively early-stage igneous activity involved in major batholith formation.