17:15 〜 18:45
[SMP24-P06] On the newly obtained zircon U–Pb ages of the Ryoke granitoids from central Mie Prefecture, SW Japan
キーワード：領家帯、花崗岩類、U-Pbジルコン年代測定、LA-ICP-MS
This study is a preliminary report of new zircon U–Pb age data for the Ryoke granitoids exposed in central Mie Prefecture, SW Japan. The area is constituted of vast extent of the Ryoke granitoids and several small lenses of metamorphic and basic rocks, bordered with the Sambagawa belt at the southern part by the Median Tectonic Line (MTL). The granitoid names in this abstract follows nomenclature of Hayama et al. [1] hereafter.
While intrusion ages and their intrusive sequences of the Ryoke granitoids were established by U–Pb zircon age dating in the Mikawa and Yanai areas [2, 3, 4], this research area still lacks U–Pb zircon ages of the granitoids, despite several K–Ar [5], Rb–Sr [6], fission track ages [7] and some preliminary U–Pb zircon ages [8] reported for a limited number of granitoid bodies.
The studied samples include one gneissose granitoid (Hatai tonalite), and one massive granitoid (Shimotakao granite). The Hatai tonalite mainly consists of plagioclase+quartz+biotite+amphibole, while the Shimotakao granite mainly consists of plagioclase+quartz+K-feldspar+biotite with a small amount of muscovite. Zircon in these granitoids is present as inclusions in biotite and plagioclase, or in the matrix, with a grain size of 100 to 300 μm.
The U–Pb dating of zircon was conducted with the LA-ICP-MS at the University of Tokyo, and the analysis points were determined based on the microstructure under CL images. The U–Pb ages reported here are given as concordia age ±2σ error, calculated from analysis points with 95-105% concordance.
Zircon from the Hatai tonalite commonly exhibited oscillatory zoning, and this zoning is rarely truncated by a homogeneous CL-moderate domain at the rim. The inner oscillatory zone yielded an age peak at 111 ± 1 Ma (n=29, MSWD=0.98), with variable Th/U ratio (0.19-1.01), while the homogeneous CL-moderate domain gave the 206Pb/238U date distribution around ca. 100-90 Ma, with the Th/U ratio of 0.04-0.22. Zircon from the Shimotakao granite shows CL-dark and CL-bright oscillatory-zoning, which resulted in single age peak at 72 ± 1 Ma (n=12, MSWD=0.74). Meanwhile, 10 zircon grains of the Shimotakao granite showed inherited 206Pb/238U dates of 1702-96 Ma from its cores. Especially, the 206Pb/238U date of ca. 100 Ma was obtained from three inherited cores with oscillatory or sector zonings. The ages of 111 ± 1 Ma from the Hatai tonalite and 72 ± 1 Ma from the Shimotakao granite were interpreted as the crystallization ages of the granitoids.
Compilation of these newly obtained ages with previously reported U–Pb zircon ages of the Ryoke granitoids [8] from the eastern Kinki district reveals that the gneissose granitoids (Hatai tonalite and Kimigano granodiorite) intruded before regional metamorphism at ca. 110-99 Ma, followed by regional metamorphism (ca. 97-90 Ma [9, 10]) and a gneissose granitoid intrusion at ca. 92-84 Ma (Joryu tonalite). These granitoids and metamorphic rocks are discordantly intruded by ca. 83-65 Ma massive granitoids. Especially, the 75-70 Ma massive granitoids are intruded from north (Aoyama area) to south (this study area) of the eastern Kinki district. Dating of granitoids exposed near the MTL is required to confirm the similarity with the trend observed in the Mikawa area [3]. This study confirmed that the oldest gneissose granitoids in the eastern Kinki district is ~10 Myr older than the age of regional metamorphism. These pieces of observation will play an important role in understanding the evolution of plutono-metamorphism of the Ryoke belt in the eastern Kinki district.
References
[1] Hayama et al. (1982) J. Geol. Soc. of Japan [2] Takatsuka et al. (2018a) Island Arc [3] Takatsuka et al. (2018b) Lithos [4] Skrzypek et al. (2016) Lithos [5] Takagi et al. (1989) J. Min. Petr. Econ. Geol. [6] Tainosho et al. (1999) The memoirs of the Geol. Soc. of Japan [7] Tagami et al. (1988) JGR [8] Higashino et al. (2023) Japan. Assoc. Mineral. Sci. Abstract [9] Kawakami et al. (2013) Contrib. Mineral. Petrol. [10] Kawakami et al. (2019) Lithos
While intrusion ages and their intrusive sequences of the Ryoke granitoids were established by U–Pb zircon age dating in the Mikawa and Yanai areas [2, 3, 4], this research area still lacks U–Pb zircon ages of the granitoids, despite several K–Ar [5], Rb–Sr [6], fission track ages [7] and some preliminary U–Pb zircon ages [8] reported for a limited number of granitoid bodies.
The studied samples include one gneissose granitoid (Hatai tonalite), and one massive granitoid (Shimotakao granite). The Hatai tonalite mainly consists of plagioclase+quartz+biotite+amphibole, while the Shimotakao granite mainly consists of plagioclase+quartz+K-feldspar+biotite with a small amount of muscovite. Zircon in these granitoids is present as inclusions in biotite and plagioclase, or in the matrix, with a grain size of 100 to 300 μm.
The U–Pb dating of zircon was conducted with the LA-ICP-MS at the University of Tokyo, and the analysis points were determined based on the microstructure under CL images. The U–Pb ages reported here are given as concordia age ±2σ error, calculated from analysis points with 95-105% concordance.
Zircon from the Hatai tonalite commonly exhibited oscillatory zoning, and this zoning is rarely truncated by a homogeneous CL-moderate domain at the rim. The inner oscillatory zone yielded an age peak at 111 ± 1 Ma (n=29, MSWD=0.98), with variable Th/U ratio (0.19-1.01), while the homogeneous CL-moderate domain gave the 206Pb/238U date distribution around ca. 100-90 Ma, with the Th/U ratio of 0.04-0.22. Zircon from the Shimotakao granite shows CL-dark and CL-bright oscillatory-zoning, which resulted in single age peak at 72 ± 1 Ma (n=12, MSWD=0.74). Meanwhile, 10 zircon grains of the Shimotakao granite showed inherited 206Pb/238U dates of 1702-96 Ma from its cores. Especially, the 206Pb/238U date of ca. 100 Ma was obtained from three inherited cores with oscillatory or sector zonings. The ages of 111 ± 1 Ma from the Hatai tonalite and 72 ± 1 Ma from the Shimotakao granite were interpreted as the crystallization ages of the granitoids.
Compilation of these newly obtained ages with previously reported U–Pb zircon ages of the Ryoke granitoids [8] from the eastern Kinki district reveals that the gneissose granitoids (Hatai tonalite and Kimigano granodiorite) intruded before regional metamorphism at ca. 110-99 Ma, followed by regional metamorphism (ca. 97-90 Ma [9, 10]) and a gneissose granitoid intrusion at ca. 92-84 Ma (Joryu tonalite). These granitoids and metamorphic rocks are discordantly intruded by ca. 83-65 Ma massive granitoids. Especially, the 75-70 Ma massive granitoids are intruded from north (Aoyama area) to south (this study area) of the eastern Kinki district. Dating of granitoids exposed near the MTL is required to confirm the similarity with the trend observed in the Mikawa area [3]. This study confirmed that the oldest gneissose granitoids in the eastern Kinki district is ~10 Myr older than the age of regional metamorphism. These pieces of observation will play an important role in understanding the evolution of plutono-metamorphism of the Ryoke belt in the eastern Kinki district.
References
[1] Hayama et al. (1982) J. Geol. Soc. of Japan [2] Takatsuka et al. (2018a) Island Arc [3] Takatsuka et al. (2018b) Lithos [4] Skrzypek et al. (2016) Lithos [5] Takagi et al. (1989) J. Min. Petr. Econ. Geol. [6] Tainosho et al. (1999) The memoirs of the Geol. Soc. of Japan [7] Tagami et al. (1988) JGR [8] Higashino et al. (2023) Japan. Assoc. Mineral. Sci. Abstract [9] Kawakami et al. (2013) Contrib. Mineral. Petrol. [10] Kawakami et al. (2019) Lithos