5:15 PM - 7:15 PM
[SMP28-P06] Metamorphic characterization and geochronology of Komori metamorphic rocks from the Northern Subzone of the Maizuru Terrane
Keywords:Maizuru Terrane, Northern Subzone, Komori-Kuwagai metamorphic rocks, U–Pb age of zircon, Th–U–Total Pb age of monazite
The Maizuru Terrane, distributed in southwest Japan, is divided into the northern subzone, central subzone, and southern subzone based on lithofacies (Kano et al., 1959, Journal of Geological Society of Japan). The southern and central subzones exhibit island arc-trench characteristics, while the northern subzone is dominated by granite, with amphibolite and garnet-biotite schist (Ikeda & Hayasaka, 1994, journal of mineralogy, petrology and economic geology; Fujii et al., 2008, Isl. Arc). The lithofacies of the northern subzone resemble continental crust, which is similar to the continental blocks of northeastern Asia. To do this, age and chemistry of granitic rocks are relied upon so far. However, metamorphic rocks are more important to fix the tectonic evolution of rocks with precise geochronological data for comparing the Maizuru Terrane with northeastern Asia. To obtain this aim, we targeted the metamorphic rocks for understanding metamorphic processes over time that facilitate comparisons with similar processes in northeastern Asia for paleogeographic reconstructions.
The present study includes a field survey of the Komori-Kuwagai region in the northern subzone of the Maizuru Terrane focusing on amphibolite and garnet-biotite schist. Microscopic observations revealed that both rocks show a foliation defined by the parallel alignment of major minerals. In amphibolite, plagioclase and hornblende exhibited variation on chemical composition, while in garnet-biotite schist, zoning was observed in garnet. The compositions of core-core and rim-rim pairs of plagioclase-hornblende and garnet-biotite were used to estimate temperature and pressure using the plagioclase-hornblende and garnet-biotite geothermometers. Additionally, LA-ICP-MS zircon U–Pb dating and EPMA monazite Th–U–Total Pb chemical dating were conducted to estimate the metamorphic age, temperature, and pressure history.
Temperature and pressure conditions of ~0.5 GPa and 630℃ were obtained from the core of plagioclase and hornblende, while the core of garnet and biotite indicated ~ 600℃. The rim showed 0.38 GPa and 480℃ for plagioclase and hornblende, and 515℃ for garnet and biotite. These results suggest that the rocks underwent peak metamorphism at least at 600–630℃ and 0.5 GPa, followed by cooling to around 480–515℃ and 0.38 GPa. Zircon U–Pb dating of the garnet-biotite schist showed a wide range of ages from approximately 1800 to 280 Ma, with 16% pre-Cambrian zircons. Core ages were concentrated around 470 Ma and 300 Ma, corresponding to the peaks of inherited zircon ages. The weighted average age of the rim was 281.6 ± 5.1 Ma (n = 8, MSWD = 0.42). Rim ages were younger than core ages, and the Th/U ratio was significantly lower, suggesting that rim ages likely reflect the timing of metamorphism. Monazite chemical ages ranged from 500 to 300 Ma, with no older ages detected, unlike zircon. The youngest monazite age cluster was around 300 Ma, coinciding with the zircon rim ages within error. Both amphibolite and garnet-biotite schist exhibited brittle deformation, with chlorite filling fractures in hornblende.
In summary, the metamorphic rocks in the Komori-Kuwagai region underwent high-temperature, low-pressure metamorphism at approximately 282 Ma. Subsequently, during the uplift process, the rocks experienced brittle deformation and chloritization. The metamorphic conditions and ages of the Komori rocks, presented for the first time in this study, provide important insights into the tectonics of the northern subzone of the Maizuru Terrane.
The present study includes a field survey of the Komori-Kuwagai region in the northern subzone of the Maizuru Terrane focusing on amphibolite and garnet-biotite schist. Microscopic observations revealed that both rocks show a foliation defined by the parallel alignment of major minerals. In amphibolite, plagioclase and hornblende exhibited variation on chemical composition, while in garnet-biotite schist, zoning was observed in garnet. The compositions of core-core and rim-rim pairs of plagioclase-hornblende and garnet-biotite were used to estimate temperature and pressure using the plagioclase-hornblende and garnet-biotite geothermometers. Additionally, LA-ICP-MS zircon U–Pb dating and EPMA monazite Th–U–Total Pb chemical dating were conducted to estimate the metamorphic age, temperature, and pressure history.
Temperature and pressure conditions of ~0.5 GPa and 630℃ were obtained from the core of plagioclase and hornblende, while the core of garnet and biotite indicated ~ 600℃. The rim showed 0.38 GPa and 480℃ for plagioclase and hornblende, and 515℃ for garnet and biotite. These results suggest that the rocks underwent peak metamorphism at least at 600–630℃ and 0.5 GPa, followed by cooling to around 480–515℃ and 0.38 GPa. Zircon U–Pb dating of the garnet-biotite schist showed a wide range of ages from approximately 1800 to 280 Ma, with 16% pre-Cambrian zircons. Core ages were concentrated around 470 Ma and 300 Ma, corresponding to the peaks of inherited zircon ages. The weighted average age of the rim was 281.6 ± 5.1 Ma (n = 8, MSWD = 0.42). Rim ages were younger than core ages, and the Th/U ratio was significantly lower, suggesting that rim ages likely reflect the timing of metamorphism. Monazite chemical ages ranged from 500 to 300 Ma, with no older ages detected, unlike zircon. The youngest monazite age cluster was around 300 Ma, coinciding with the zircon rim ages within error. Both amphibolite and garnet-biotite schist exhibited brittle deformation, with chlorite filling fractures in hornblende.
In summary, the metamorphic rocks in the Komori-Kuwagai region underwent high-temperature, low-pressure metamorphism at approximately 282 Ma. Subsequently, during the uplift process, the rocks experienced brittle deformation and chloritization. The metamorphic conditions and ages of the Komori rocks, presented for the first time in this study, provide important insights into the tectonics of the northern subzone of the Maizuru Terrane.