11:15 AM - 11:30 AM
[SMP37-08] Estimation of the Metamorphic P-T Condition and Age of the Oki Gneisses: Implication for the Paleoproterozoic High-Temperature Metamorphism
Keywords:Oki, metamorphic rock, CHIME, geothermobarometer
On the gneisses collected from Togo River and Choshi River in Oki-Dogo Island, polarization microscope observation, quantitative chemical composition analysis of each mineral, and mapping of garnet and monazite composition were carried out. The metamorphic P-T condition was estimated using garnet-biotite-plagioclase-quartz geothermobarometer (Perchuk, 1985; Hoisch, 1990). The CHIME age was measured at Nagoya University based on the mapping and texture of monazite.
The Mg concentration of garnet core in the Oki gneisses is higher than those of garnet rim, indicating that the Oki gneisses were suffered retrograde metamorphism after peak metamorphism. The peak and retrograde metamorphic P-T conditions from samples along the Togo River were estimated using the compositions of garnet core and rim, resulting in 718-758 ℃, 6.4-8.8 kbar and 508-654 ℃, 2.1-6.6 kbar, respectively. Also, the peak and retrograde metamorphic P-T conditions from samples along the Choshi River are 803-829 ℃, 9.0-10.3 kbar and 638-695 ℃, 4.4-5.9 kbar, respectively. These results indicate that the metamorphic grade of the gneisses along the Choshi River were higher than those of the gneisses along the Togo River.
The Y2O3 concentrations of monazites included in the garnet core of the sample along the Togo River are 1.7-2.2 wt%, and they yield the CHIME age of 1999-2089 Ma. In contrast, monazites included in the garnet rim contain the age of ca. 259 Ma in addition to the ages of 1893-2084Ma, and the younger one shows a relatively low Y2O3 concentration. The monazites in the peripheral part of the garnet yield the ages of ca. 232-250 Ma and ca. 162-168 Ma, and the latter one shows a relatively high Y2O3 concentration. These results indicate that the Oki gneisses were metamorphosed by not only the Permian-Triassic metamorphism, by but also the Paleoproterozoic high-temperature metamorphism (ca. 720-760℃, 6-9kbar). The existence of the Paleoproterozoic metamorphism suggests that the protolith ages of Oki gneisses are remarkably older than generally thought (i.e. early Paleozoic). In the future, we will clarify the relationship between the Paleoproterozoic and the Permian-Triassic metamorphism in the Oki belt to compare those in the North/South China massifs and the Gyeonggi/Yeongnam massifs in Korea.