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[SGL22-P06] Provenance of the Early–Middle Jurassic sandstone from the accretionary complex of the northern part of the Mino Belt, Southwest Japan
Keywords:Mino Belt, zircon, U–Pb age, Hf isotopic composition, accretionary complex
Introduction
The comparison of (1) the detrital-zircon-age spectrum and (2) zircon U-Pb age distribution of igneous rocks in the world is a powerful tool for analyzing the provenance of the detrital zircons. However, since the age distributions of igneous rocks can be similar in two or more areas, different provenance areas can be proposed from the same age spectrum. A similar argument has been made for the Mino Belt in SW Japan (e.g., Fujisaki et al., 2014; Tokiwa et al., 2019). Hence, this study aims to make a provenance analysis of the accretionary complex (AC) of the northern part of the Mino Belt, using two indices from detrital zircons: U–Pb ages and Lu–Hf isotopic compositions.
Samples and geological setting
We took four sandstone samples: one (Ks1) from the Kasugano Unit and three from the Imajo (Im1), Onishi (Im2), and Samondake (Im3) formations of the underlying Imajo Unit. The Kasugano Unit is a Sinemurian–Aalenian mélange consisting of blocks of chert, siliceous shale, basalt, and sandstone included in a black-shale matrix (Hattori and Yoshimura, 1982). The Bajocian–Bathonian Imajo Unit consists of coherent siliciclastic rocks and chert (Hattori and Yoshimura, 1982; Wakita, 1982; Kobayashi, 1998).
Method
We performed the U-Pb dating by using the laser ablation-Inductively coupled plasma mass spectrometer (LA-ICPMS) equipped in the Graduate School of Environmental Studies, Nagoya University. We measured Lu–Hf isotopic compositions with the LA-MC-ICPMS equipped in the Tono Geoscience Center, Japan Atomic Energy Agency. Then we calculated the εHf (t) values [176Hf / 177Hf ratios at the time of crystallization relative to the CHUR (Chondritic Uniform Reservoir), assuming that they evolved from the depleted mantle] and tried to find the provenance.
Result and discussion
The attached figure shows the U-Pb age spectra, the 206Pb/238U age of youngest single grain (YSG), and the εHf (t)-age plots of the detrital zircons.
Previous studies showed that 950–720 Ma and 500–100 Ma igneous rocks occur in the eastern part of the Central Asian Orogenic Belt (CAOB), and 2800–2400 Ma, 2200–1600 Ma, and 400–100 Ma igneous rocks occur in the Yanshan Fold Belt (YFB), North China Craton (Yang et al., 2006; Xu et al., 2019). The εHf (t) values of these igneous zircons are as follows: -6–+18 for 500–100 Ma zircons of the CAOB, and -2–+10, -10–+4, and -24–-2 for the 2800–2400 Ma, 2200–1600 Ma and 400–100 Ma zircons of the YFB, respectively (see attached εHf (t)-age plots).
We have reached the following conclusions from the εHf (t)-age plots. The provenance of the Kasugano Unit (Ks1) was most likely the eastern part of the CAOB. On the other hand, the 257–191 Ma and 185–180 Ma zircons of sample Im1 (Imajo formation) were likely provided from the CAOB and the YFB, respectively; the provenance must have covered the boundary between the two entities. Finally, the provenance of the Onishi (Im2) and Samondake (Im3) formations were most likely the YFB. Hence, the subduction zone that formed the AC of the Mino Belt was on the eastern margin of the CAOB in the Sinemurian–Aalenian. However, it moved relatively southward to the North China Craton by the Bajocian–Bathonian along the dextral shear zones subparallel to the East Asian continental margin (Otoh et al., 1989).
References
Fujisaki et al., 2014: J. Asian Earth Sci., 88, 62–73 / Hattori and Yoshimura, 1982: News Osaka Micropaleontol. Spec. Vol., 5, 103–116 / Kobayashi, 1998: J. Fac. Sci. Shinshu Univ., 33, 27–63 / Otoh et al., 1989: J. Tectonic Res. Group Japan, 34, 75–84 / Tokiwa et al., 2019: J. Asian Earth Sci., 184, 103970 / Wakita, 1982: News Osaka Micropaleontol. Spec. Vol., 5, 153–171 / Xu et al., 2019: Earth Sci., 44, 1620–1646 (in Chinese) / Yang et al., 2006: Earth Planet. Sci. Lett., 246, 336–352.
The comparison of (1) the detrital-zircon-age spectrum and (2) zircon U-Pb age distribution of igneous rocks in the world is a powerful tool for analyzing the provenance of the detrital zircons. However, since the age distributions of igneous rocks can be similar in two or more areas, different provenance areas can be proposed from the same age spectrum. A similar argument has been made for the Mino Belt in SW Japan (e.g., Fujisaki et al., 2014; Tokiwa et al., 2019). Hence, this study aims to make a provenance analysis of the accretionary complex (AC) of the northern part of the Mino Belt, using two indices from detrital zircons: U–Pb ages and Lu–Hf isotopic compositions.
Samples and geological setting
We took four sandstone samples: one (Ks1) from the Kasugano Unit and three from the Imajo (Im1), Onishi (Im2), and Samondake (Im3) formations of the underlying Imajo Unit. The Kasugano Unit is a Sinemurian–Aalenian mélange consisting of blocks of chert, siliceous shale, basalt, and sandstone included in a black-shale matrix (Hattori and Yoshimura, 1982). The Bajocian–Bathonian Imajo Unit consists of coherent siliciclastic rocks and chert (Hattori and Yoshimura, 1982; Wakita, 1982; Kobayashi, 1998).
Method
We performed the U-Pb dating by using the laser ablation-Inductively coupled plasma mass spectrometer (LA-ICPMS) equipped in the Graduate School of Environmental Studies, Nagoya University. We measured Lu–Hf isotopic compositions with the LA-MC-ICPMS equipped in the Tono Geoscience Center, Japan Atomic Energy Agency. Then we calculated the εHf (t) values [176Hf / 177Hf ratios at the time of crystallization relative to the CHUR (Chondritic Uniform Reservoir), assuming that they evolved from the depleted mantle] and tried to find the provenance.
Result and discussion
The attached figure shows the U-Pb age spectra, the 206Pb/238U age of youngest single grain (YSG), and the εHf (t)-age plots of the detrital zircons.
Previous studies showed that 950–720 Ma and 500–100 Ma igneous rocks occur in the eastern part of the Central Asian Orogenic Belt (CAOB), and 2800–2400 Ma, 2200–1600 Ma, and 400–100 Ma igneous rocks occur in the Yanshan Fold Belt (YFB), North China Craton (Yang et al., 2006; Xu et al., 2019). The εHf (t) values of these igneous zircons are as follows: -6–+18 for 500–100 Ma zircons of the CAOB, and -2–+10, -10–+4, and -24–-2 for the 2800–2400 Ma, 2200–1600 Ma and 400–100 Ma zircons of the YFB, respectively (see attached εHf (t)-age plots).
We have reached the following conclusions from the εHf (t)-age plots. The provenance of the Kasugano Unit (Ks1) was most likely the eastern part of the CAOB. On the other hand, the 257–191 Ma and 185–180 Ma zircons of sample Im1 (Imajo formation) were likely provided from the CAOB and the YFB, respectively; the provenance must have covered the boundary between the two entities. Finally, the provenance of the Onishi (Im2) and Samondake (Im3) formations were most likely the YFB. Hence, the subduction zone that formed the AC of the Mino Belt was on the eastern margin of the CAOB in the Sinemurian–Aalenian. However, it moved relatively southward to the North China Craton by the Bajocian–Bathonian along the dextral shear zones subparallel to the East Asian continental margin (Otoh et al., 1989).
References
Fujisaki et al., 2014: J. Asian Earth Sci., 88, 62–73 / Hattori and Yoshimura, 1982: News Osaka Micropaleontol. Spec. Vol., 5, 103–116 / Kobayashi, 1998: J. Fac. Sci. Shinshu Univ., 33, 27–63 / Otoh et al., 1989: J. Tectonic Res. Group Japan, 34, 75–84 / Tokiwa et al., 2019: J. Asian Earth Sci., 184, 103970 / Wakita, 1982: News Osaka Micropaleontol. Spec. Vol., 5, 153–171 / Xu et al., 2019: Earth Sci., 44, 1620–1646 (in Chinese) / Yang et al., 2006: Earth Planet. Sci. Lett., 246, 336–352.