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[SMP28-05] Mesoproterozoic dike origin and Triassic anatexis of mafic granulite from the Oki-Dogo Island, Japan
Keywords:Oki metamorphic rocks, mafic dike, LIPs, Mesoproterozoic
The Oki metamorphic rocks on Oki-Dogo Island, Japan, are derived from Paleoproterozoic sediments and subordinate granitoids. These rocks experienced two distinct phases of high-grade crustal metamorphism at ~1.85 Ga and 230–220 Ma (Cho et al., 2021; Kawabata et al., 2022). The dominant lithologies are psammitic-pelitic and granitic gneisses, with minor occurrences of mafic lithologies, including mafic granulites and amphibolites. Recent studies have clarified the metamorphic history of these mafic lithologies, identifying an initial low-pressure granulite-facies stage associated with anatexis (Takahashi et al., 2023). However, the origin and timing of this high-grade metamorphism—whether Paleoproterozoic or Triassic—remain unresolved. This information is critical for understanding the tectonic evolution and correlation of the Oki metamorphic rocks.
In this study, we present results from SHRIMP zircon U-Pb dating of two mafic samples from Oki-Dogo Island. In the mafic granulite sample (1206B), zircon grains were classified as either optically clear or dusty. Cathodoluminescence (CL) imaging revealed concentric zoning in the clear grains, with darker cores (Th/U < 0.05) and brighter rims (Th/U > 0.2), while dusty grains displayed dark, irregular-shaped cores (very high Th/U). The low-Th/U cores and high-Th/U rims yielded weighted mean ages of 232.1 ± 1.9 Ma and 212.1 ± 2.2 Ma, respectively. Although U-Pb data from the dark irregular-shaped cores were largely discordant, a discordia line yielded an upper intercept age of 1324 ± 25 Ma, consistent with a concordant analysis of 1312 ± 14 Ma (Th/U = 2.1). Zircon grains in the amphibolitized granulite sample (1028_3) exhibited concentric zoning without core-rim age variation, yielding a weighted mean age of 219.4 ± 2.1 Ma.
These results suggest that the mafic granulite originated as a mafic dike intruded at ~1.3 Ga. Consequently, the mafic lithologies experienced only Triassic crustal metamorphism. The Triassic low-pressure granulite metamorphism can be subdivided into a prograde stage (~232 Ma) and a later stage of zircon growth in the presence of anatectic melt (~212 Ma). Notably, Mesoproterozoic (~1.3 Ga) mafic dike swarms associated with the breakup of the Nuna (Columbia) supercontinent have been reported in the North China Craton, such as the Yanliao Large Igneous Province (Zhan et al., 2017). This strongly supports a tectonic correlation between the Oki metamorphic rocks and the North China Craton.
References:
Cho et al. (2021) Lithos, 396-397, 106217.
Kawabata et al. (2022) J. Metam. Geol., 40, 257-286.
Takahashi et al. (2023) Island Arc, 32(1), e12497.
Zhan et al. (2017) EPSL, 465, 112-125.
In this study, we present results from SHRIMP zircon U-Pb dating of two mafic samples from Oki-Dogo Island. In the mafic granulite sample (1206B), zircon grains were classified as either optically clear or dusty. Cathodoluminescence (CL) imaging revealed concentric zoning in the clear grains, with darker cores (Th/U < 0.05) and brighter rims (Th/U > 0.2), while dusty grains displayed dark, irregular-shaped cores (very high Th/U). The low-Th/U cores and high-Th/U rims yielded weighted mean ages of 232.1 ± 1.9 Ma and 212.1 ± 2.2 Ma, respectively. Although U-Pb data from the dark irregular-shaped cores were largely discordant, a discordia line yielded an upper intercept age of 1324 ± 25 Ma, consistent with a concordant analysis of 1312 ± 14 Ma (Th/U = 2.1). Zircon grains in the amphibolitized granulite sample (1028_3) exhibited concentric zoning without core-rim age variation, yielding a weighted mean age of 219.4 ± 2.1 Ma.
These results suggest that the mafic granulite originated as a mafic dike intruded at ~1.3 Ga. Consequently, the mafic lithologies experienced only Triassic crustal metamorphism. The Triassic low-pressure granulite metamorphism can be subdivided into a prograde stage (~232 Ma) and a later stage of zircon growth in the presence of anatectic melt (~212 Ma). Notably, Mesoproterozoic (~1.3 Ga) mafic dike swarms associated with the breakup of the Nuna (Columbia) supercontinent have been reported in the North China Craton, such as the Yanliao Large Igneous Province (Zhan et al., 2017). This strongly supports a tectonic correlation between the Oki metamorphic rocks and the North China Craton.
References:
Cho et al. (2021) Lithos, 396-397, 106217.
Kawabata et al. (2022) J. Metam. Geol., 40, 257-286.
Takahashi et al. (2023) Island Arc, 32(1), e12497.
Zhan et al. (2017) EPSL, 465, 112-125.