14:00 〜 14:15
[HCG28-02] Detrital Zircon and Provenance Analysis of Sedimentary Rock in the Nemuro and Urahoro Groups, Eastern Hokkaido, Northern Japan
キーワード：Nemuro Group, Sandstone, Urahoro Group, U-Pb dating, Zircon
This study aims to examine the development processes of the Cretaceous to Paleogene sedimentary basins in the collisional zone between North East Japan and Kuril Arc systems on the basis of the U-Pb age distribution of detrital zircon grains in sandstones. The study area is located in the eastern part of Hokkaido Island, a part of the Nemuro Belt, which has been interpreted as a complex of deposits of forearc and foreland basins. Recently, the paleomagnetic analysis suggested that the Nemuro Belt complex records collisional process associated with block rotation that was driven by tectonic activity. This tectonic event is likely to cause uplifting of colliding arc crusts, and thus sediment provenance analysis in the depositional sequences of the Nemuro Belt will provide information about the exact timing and mode of the collision of two arc systems. For this purpose, this study investigated the Nemuro and Urahoro Groups that are well exposed in the Nemuro Belt complex area.
We report the first record of the U-Pb age distributions of detrital zircon grains in the Nemuro Belt. Fifty-five samples of sandstones were collected from outcrops. Detrital zircons were separated using standard mineral separation techniques at the Kyoto University. Approximately 200 grains were mounted for each sample, and their U-Pb age was measured by LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) to estimate the temporal variation of sediment provenances. In total, we evaluated 1053 LA-ICP-MS U-Pb detrital zircon ages, and these U-Pb sandstones analyses resulted four stratigraphic intervals with age peaks: (1) Early Jurassic (200-175 Ma), (2) Late Cretaceous (100-69 Ma), (3) Paleocene (66-56 Ma), and (4) Eocene (55 and 41 Ma). Dating results also define two detrital zircon U-Pb type signatures based on a similarity pattern of age populations. Type 1 signatures are found in the sandstones of the Shiomi, Beppo, Harutori, Tenneru, Yuubetsu, and Shitakara Formations and are characterized by peaks in the Paleogene (66-41 Ma), and type 2 signatures are found in the Hamanaka and Akkeshi Formations and are distinguished by the persistence of Mesozoic grains in the Early Jurassic (200-175 Ma). This result implies that the significant transition of the sediment provenance areas occurred at the uppermost horizon of the Nemuro Group (~60 Ma), which can be associated with the progression of the collision event between two island arcs.
We report the first record of the U-Pb age distributions of detrital zircon grains in the Nemuro Belt. Fifty-five samples of sandstones were collected from outcrops. Detrital zircons were separated using standard mineral separation techniques at the Kyoto University. Approximately 200 grains were mounted for each sample, and their U-Pb age was measured by LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) to estimate the temporal variation of sediment provenances. In total, we evaluated 1053 LA-ICP-MS U-Pb detrital zircon ages, and these U-Pb sandstones analyses resulted four stratigraphic intervals with age peaks: (1) Early Jurassic (200-175 Ma), (2) Late Cretaceous (100-69 Ma), (3) Paleocene (66-56 Ma), and (4) Eocene (55 and 41 Ma). Dating results also define two detrital zircon U-Pb type signatures based on a similarity pattern of age populations. Type 1 signatures are found in the sandstones of the Shiomi, Beppo, Harutori, Tenneru, Yuubetsu, and Shitakara Formations and are characterized by peaks in the Paleogene (66-41 Ma), and type 2 signatures are found in the Hamanaka and Akkeshi Formations and are distinguished by the persistence of Mesozoic grains in the Early Jurassic (200-175 Ma). This result implies that the significant transition of the sediment provenance areas occurred at the uppermost horizon of the Nemuro Group (~60 Ma), which can be associated with the progression of the collision event between two island arcs.