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[SGL23-P02] Forearc volcanism in NE Japan during opening of the Sea of Japan: petrological characteristics and magmatic evolution for the Miocene mafic lavas in Motegi district.
Keywords:Miocene volcanism, Fore-arc Northeast Japan, Bulk rock chemistry, Opening of the Sea of Japan, Basaltic andesite, Andesite
The lower Miocene Nakagawa Group (Ichiba, Motokozawa, Yamanouchi, and Motegi formations) unconformably overlies the Jurassic Yamizo Group, in the Motegi area in southeastern Tochigi Prefecture. These strata were deposited during the opening of the Japan Sea. Previous studies have conducted geologic mapping, paleomagnetic analysis, structural analysis, radiometric dating, and bulk rock chemical analysis. However, volcanic activity and magmatic evolution remain unclear. The basaltic andesite to andesite lavas (hereafter mafic lavas) in the Yamanouchi and Motegi formations have been considered as same genesis [1, 2], however, no petrological evidence have been obtained. We conducted field surveys, petrological analysis, bulk rock chemical analysis to elucidate their volcanism.
The Yamanouchi Formation unconformably overlies the Motokozawa Formation and consists mainly of basaltic to andesite volcaniclastic rocks with interbedded clinker-textured lavas [2]. The Motegi Formation, which conformably overlies the Yamanouchi Formation [2, 3], consists mainly of felsic pyroclastic rocks, including tuff, tuff breccia, and pyroclastic breccia with pumice. Dacite intrusive rock is found in these formations [2].
The mafic lavas and volcanic clasts in volcaniclastic rocks in the Yamanouchi Formation are same. They have porphyritic textures and different from the mafic intrusions with aphyric texture in the Yamanouchi Formation. The mafic lavas interbedded in felsic pyroclastic rocks of the Motegi Formation are also similar to that of the Yamanouch Formation.
The Yamanouchi and lower Motegi formations have similar strike and dip, indicating no age gaps between them. Their bulk rock chemistry show a linear trend on Harker diagrams, with SiO2 from 52 to 62 wt.%, classifying them as tholeiitic series. Mafic lavas in both formations are same bulk rock chemistry, that are similar to Quaternary tholeiitic volcanic rocks in the Northeast Japan arc. Both lavas are cpx-opx basaltic andesite to andesite with hyalo-pilitic and hyalo-ophitic groundmass textures.
We finds that the mafic lavas in the Yamanouchi and Motegi formations have similar lithofacies and petrological characteristics, suggesting these lavas are same volcanic activity. The tectonic setting of this volcanism is likely to the Quaternary in the Northeast Japan arc. During the strata of the Motegi Formation was formed, both mafic and felsic volcanoes were acted. In the Yamanouchi Formation, SiO2 content gradually increases from lower to upper parts. These mafic lavas are chemically different from other volcanic rocks of aphyric basalts in the Motokozawa Formation [4] and the dacite intrusive rocks in the Motegi Formation [5] in Harker diagrams. This suggests that the Yamanouchi and Motegi mafic lavas are same. In contrast, the Motokozawa aphyric basalts and Motegi dacite intrusive rocks have different chemical composition to the volcanic rocks in the Quaternary Northeast Japan arc [4, 5, 6, 7]. The fact that the mafic lavas in the Yamanouchi and Motegi formations which are like Quaternary volcanic rocks, are sandwiched between periods of unusual volcanic activity as described above, suggests magma diversity during the opening of the Sea of Japan.
References
[1]Izaki et al. (1985) Sci. Rep. Fac. Edu., Fukushima Univ. [2]Hoshi and Takahashi (1996a) J. Geol. Soc. Japan. [3]Hoshi and Takahashi (1996b) J. Geol. Soc. Japan. [4]Shuto et al. (1985b) J. Japan. Assoc. Mineral., Petrol. Econ. Geol. [5]Shiramizu et al. (1983) J. Japan. Assoc. Mineral., Petrol. Econ. Geol. [6]Takahashi et al. (1995) Mem. Geol. Soc. Japan. [7]Yamamoto and Yamasaki (2023) J. Geol. Soc. Japan.
The Yamanouchi Formation unconformably overlies the Motokozawa Formation and consists mainly of basaltic to andesite volcaniclastic rocks with interbedded clinker-textured lavas [2]. The Motegi Formation, which conformably overlies the Yamanouchi Formation [2, 3], consists mainly of felsic pyroclastic rocks, including tuff, tuff breccia, and pyroclastic breccia with pumice. Dacite intrusive rock is found in these formations [2].
The mafic lavas and volcanic clasts in volcaniclastic rocks in the Yamanouchi Formation are same. They have porphyritic textures and different from the mafic intrusions with aphyric texture in the Yamanouchi Formation. The mafic lavas interbedded in felsic pyroclastic rocks of the Motegi Formation are also similar to that of the Yamanouch Formation.
The Yamanouchi and lower Motegi formations have similar strike and dip, indicating no age gaps between them. Their bulk rock chemistry show a linear trend on Harker diagrams, with SiO2 from 52 to 62 wt.%, classifying them as tholeiitic series. Mafic lavas in both formations are same bulk rock chemistry, that are similar to Quaternary tholeiitic volcanic rocks in the Northeast Japan arc. Both lavas are cpx-opx basaltic andesite to andesite with hyalo-pilitic and hyalo-ophitic groundmass textures.
We finds that the mafic lavas in the Yamanouchi and Motegi formations have similar lithofacies and petrological characteristics, suggesting these lavas are same volcanic activity. The tectonic setting of this volcanism is likely to the Quaternary in the Northeast Japan arc. During the strata of the Motegi Formation was formed, both mafic and felsic volcanoes were acted. In the Yamanouchi Formation, SiO2 content gradually increases from lower to upper parts. These mafic lavas are chemically different from other volcanic rocks of aphyric basalts in the Motokozawa Formation [4] and the dacite intrusive rocks in the Motegi Formation [5] in Harker diagrams. This suggests that the Yamanouchi and Motegi mafic lavas are same. In contrast, the Motokozawa aphyric basalts and Motegi dacite intrusive rocks have different chemical composition to the volcanic rocks in the Quaternary Northeast Japan arc [4, 5, 6, 7]. The fact that the mafic lavas in the Yamanouchi and Motegi formations which are like Quaternary volcanic rocks, are sandwiched between periods of unusual volcanic activity as described above, suggests magma diversity during the opening of the Sea of Japan.
References
[1]Izaki et al. (1985) Sci. Rep. Fac. Edu., Fukushima Univ. [2]Hoshi and Takahashi (1996a) J. Geol. Soc. Japan. [3]Hoshi and Takahashi (1996b) J. Geol. Soc. Japan. [4]Shuto et al. (1985b) J. Japan. Assoc. Mineral., Petrol. Econ. Geol. [5]Shiramizu et al. (1983) J. Japan. Assoc. Mineral., Petrol. Econ. Geol. [6]Takahashi et al. (1995) Mem. Geol. Soc. Japan. [7]Yamamoto and Yamasaki (2023) J. Geol. Soc. Japan.