09:15 〜 09:30
[SGL23-02] 巨摩山地桃の木亜層群のジルコンU-Pb年代とその意義
キーワード：ジルコン、U-Pb、伊豆衝突帯
Momonoki Subgroup distributed in the western part of Yamanashi Prefecture has been considered to be trough-fill sediments formed in a trough located between Honshu arc and Izu-Bonin-Mariana arc before their arc-arc type collision has started. Previous studies have classified Momonoki Subgroup into three formations; the Lower Formation (or Yunokawa Fm.) is composed of mudstone, conglomerates, and alternation of sandstone and mudstone, the Middle Formation (or Maruyama Fm.) is composed of mudstone and alternation of sandstone and mudstone, and the Upper Formation (or Senshirosawa Fm.) is mainly composed of conglomerates (Tamura et al., 1984; Aoike, 1999; Kuroda, 2000, B.S. thesis). Depositional age and provenance of Momonoki Subgroup will provide important constraint on the timing of the collision. Previous researches have indicated that it was formed 16-15 Ma (Koyama, 1993) or 15-13.5 Ma (Aoike, 1999) or 12 Ma (Amano, 1991) based on biostratigraphy and estimated cooling age of the Kaikomagatake pluton, and that its provenance was mainly Honshu arc (Okuzawa and Hisada, 2008). In this study, U-Pb dating of zircon were applied to reconsider the depositional age and provenance of Momonoki Subgroup.
U-Pb ages of zircon grains obtained from tuff and sandstones of Momonoki Subgroup were analyzed. The zircon crystals for analysis were handpicked from heavy fraction of constituent particles separated from samples by high voltage pulsed power selective fragmentation device (SELFRAG-Lab) and stamp mill. The measurement was carried out using LA-ICP-MS installed at the National Museum of Nature and Science. Tuff sandwiched in the Lower Formation shows concordant ages concentrated at 23.5 Ma. Sandstones in the Lower Formation show concordant ages ranging from 21 to 2364 Ma. Arkose sandstone in the Middle Formation show the concordant ages ranging from 13 to 2184 Ma and concentrated around 14.6 Ma. The U-Pb age spectrums of sandstones between 100 and ~250 Ma show similarity with those of samples from Shimanto Belt (Tokiwa et al., 2018).
The depositional age of the Momonoki Subgroup has been previously estimated as 16-15 Ma (Koyama, 1993) or 15-13.5 Ma (Aoike, 1999) or 12 Ma (Amano, 1991), but our new zircon age revealed that the Lower Formation may be as old as 23.5 Ma since the tuff in the Lower Formation is considered to be air-fall volcanic ash because of the concentration of zircon U-Pb ages and the absence of other ages. Though the arkose sandstone in the Middle Formation was deposited at 14.6 Ma or later, it is not clear whether the zircons that give concentrated U-Pb age around 14.6 Ma were derived from air-fall volcanic ash or from granite in the hinterland. The 9 My difference in depositional age between tuff in the Lower Formation and the arkose sandstone in the Middle Formation suggests that there may be structural discontinuities in the Momonoki Subgroup, or that the geological setting of the Momonoki Subgroup may have been different from trough-fill sediments.
The similarity of the U-Pb age spectrums of detrital zircons between the Momonoki Subgroup and Shimanto Belt suggests that the provenance of the Momonoki Subgroup is considered to have been mainly the Shimanto Belt with some influence from the Jurassic accretionary complex, with minimum contribution from the Izu-Bonin arc. In addition to the results of zircon U-Pb dating, the sandstones and conglomerates of the studied Momonoki Subgroup contain very few igneous rock fragments and conglomerates, which further provide evidence that sediment supply from the Izu-Bonin arc was volumetrically small. These results suggest that the collision between the Izu-Bonin arc and the Honshu arc may have started after 14.6 Ma, but further research is required for estimating the precise timing of the collision.
U-Pb ages of zircon grains obtained from tuff and sandstones of Momonoki Subgroup were analyzed. The zircon crystals for analysis were handpicked from heavy fraction of constituent particles separated from samples by high voltage pulsed power selective fragmentation device (SELFRAG-Lab) and stamp mill. The measurement was carried out using LA-ICP-MS installed at the National Museum of Nature and Science. Tuff sandwiched in the Lower Formation shows concordant ages concentrated at 23.5 Ma. Sandstones in the Lower Formation show concordant ages ranging from 21 to 2364 Ma. Arkose sandstone in the Middle Formation show the concordant ages ranging from 13 to 2184 Ma and concentrated around 14.6 Ma. The U-Pb age spectrums of sandstones between 100 and ~250 Ma show similarity with those of samples from Shimanto Belt (Tokiwa et al., 2018).
The depositional age of the Momonoki Subgroup has been previously estimated as 16-15 Ma (Koyama, 1993) or 15-13.5 Ma (Aoike, 1999) or 12 Ma (Amano, 1991), but our new zircon age revealed that the Lower Formation may be as old as 23.5 Ma since the tuff in the Lower Formation is considered to be air-fall volcanic ash because of the concentration of zircon U-Pb ages and the absence of other ages. Though the arkose sandstone in the Middle Formation was deposited at 14.6 Ma or later, it is not clear whether the zircons that give concentrated U-Pb age around 14.6 Ma were derived from air-fall volcanic ash or from granite in the hinterland. The 9 My difference in depositional age between tuff in the Lower Formation and the arkose sandstone in the Middle Formation suggests that there may be structural discontinuities in the Momonoki Subgroup, or that the geological setting of the Momonoki Subgroup may have been different from trough-fill sediments.
The similarity of the U-Pb age spectrums of detrital zircons between the Momonoki Subgroup and Shimanto Belt suggests that the provenance of the Momonoki Subgroup is considered to have been mainly the Shimanto Belt with some influence from the Jurassic accretionary complex, with minimum contribution from the Izu-Bonin arc. In addition to the results of zircon U-Pb dating, the sandstones and conglomerates of the studied Momonoki Subgroup contain very few igneous rock fragments and conglomerates, which further provide evidence that sediment supply from the Izu-Bonin arc was volumetrically small. These results suggest that the collision between the Izu-Bonin arc and the Honshu arc may have started after 14.6 Ma, but further research is required for estimating the precise timing of the collision.