11:45 AM - 12:00 PM
[R6-07] U-Pb zircon age of granitoids in the Aoyama area, Ryoke belt, SW Japan
Keywords:granite, zircon, melt inclusion, metastable phase
The Ryoke belt shows an elongated distribution over 800 km in Southwest Japan, which is mainly composed of Late Cretaceous high-T/low-P type metamorphic rocks and plutonic rocks. Whereas the early geochronological studies report eastward younging of Rb-Sr whole rock ages of granitic rocks [e.g., 1, 2], some U-Pb zircon ages and chemical Th-U-total Pb isochron method (CHIME) monazite ages of granitoids are not consistent with the trend [3, 4]. The Aoyama area is one of the well-studied areas in the Ryoke belt, where high-grade pelitic and psammitic metamorphic rocks and granitoids are widely exposed. Since it is apparent that the U-Pb zircon ages of granitoids are not sufficiently available to understand the spatiotemporal evolution of magmatism in the Aoyama area, we performed LA-ICPMS zircon U-Pb dating for six samples out of five granitoids.
Magmatic zircon domains were selected based on microstructural constraint under CL images. Using analyses with concordance between 97-103 %, the weighted-mean U-Pb age is only calculated for one sample. As a result, crystallization timings of granitoids are constrained respectively as ca. 83-75 Ma for Kabuto granodiorite, ca. 76-65 Ma and ca. 77-70 Ma for Ao granite, ca. 92-84 Ma for Joryu tonalite, 75 ± 1 Ma for Misugi tonalite, and ca. 110-99 Ma for Kimigano granodiorite.
Zircon in the Misugi tonalite has a multi-phase inclusion composed of cristobalite, kumdykolite, apatite and glass. Mineral assemblage of cristobalite and kymdykolite is reported in nanogranite in garnet from high-T metamorphic rock, which is interpreted to be metastable phases [5]. These minerals found in this study are also considered to be metastable, suggesting rapid crystallization from the granitic melt. Further analyses for inclusion in zircon would help understand the range of crystallization timing of granitoids.
[1] Nakajima (1994) Lithos [2] Nakajima et al. (1990) CMP [3] Suzuki & Adachi (1998) JMG [4] Takatsuka et al. (2018) Island Arc [5] Ferrero et al. (2016) CMP
Magmatic zircon domains were selected based on microstructural constraint under CL images. Using analyses with concordance between 97-103 %, the weighted-mean U-Pb age is only calculated for one sample. As a result, crystallization timings of granitoids are constrained respectively as ca. 83-75 Ma for Kabuto granodiorite, ca. 76-65 Ma and ca. 77-70 Ma for Ao granite, ca. 92-84 Ma for Joryu tonalite, 75 ± 1 Ma for Misugi tonalite, and ca. 110-99 Ma for Kimigano granodiorite.
Zircon in the Misugi tonalite has a multi-phase inclusion composed of cristobalite, kumdykolite, apatite and glass. Mineral assemblage of cristobalite and kymdykolite is reported in nanogranite in garnet from high-T metamorphic rock, which is interpreted to be metastable phases [5]. These minerals found in this study are also considered to be metastable, suggesting rapid crystallization from the granitic melt. Further analyses for inclusion in zircon would help understand the range of crystallization timing of granitoids.
[1] Nakajima (1994) Lithos [2] Nakajima et al. (1990) CMP [3] Suzuki & Adachi (1998) JMG [4] Takatsuka et al. (2018) Island Arc [5] Ferrero et al. (2016) CMP