17:15 〜 19:15
[MIS17-P07] 根田茂帯綱取ユニット砕屑性ジルコン複合化学分析に基づいた付加体成長史と前期古生代テクトニクス進化史の解明
キーワード：砕屑性ジルコン、U–Pb年代、微量元素、Hf同位体比
The Pacific-type orogeny, which began at ca. 600 Ma due to subduction of an oceanic plate along the northeastern Gondwana margin, contributed to the formation of the basements constituting the present Japanese Islands. These basements include accretionary complexes, high P/T metamorphic belts, and granitic batholiths, which serve as crucial geological evidence for elucidating the tectonics and igneous processes in the subduction zones at the time. However, the geological units that formed before the Permian in the Japanese Islands are limited in their distribution, leaving the pre-Permian tectonic history in the eastern margin of the Gondwana continent unresolved.
The Tsunatori Unit of the Nedamo Belt, distributed in the Kitakami Massif of NE Japan, has been interpreted as an Early Carboniferous accretionary complex. However, the biostratigraphic constraints on its depositional age remain ambiguous. This is because the age ranges of the index fossils such as Trilonche cf. vetusta (Middle Devonian to Early Carboniferous) are broad, and its occurrence is limited in the Tsunatori Unit. In this study, we performed U–Pb dating, trace-element analysis, and Hf isotopic analysis on detrital zircons from 10 sandstone samples of the unit, aiming to constrain the accretionary age of the unit and elucidate the magmatic and tectonic history of the northeastern Gondwana margin.
The U–Pb dating revealed two distinct groups of sandstone samples. One group exhibited a Youngest Cluster Age (2σ) of ca. 400 Ma, while the other showed a Youngest Cluster Age ranging from 430 to 480 Ma. The age histograms of the former group showed a single peak from 400 to 500 Ma, whereas the latter one exhibited multiple peaks at 400-500 Ma, 500-700 Ma, 800-1200 Ma, and 1400-1800Ma. The sandstone close to the strata where the Devonian to Early Carboniferous index fossils were previously reported exhibited age histograms characteristic of the former group. Trace-element analysis of the zircons with U–Pb ages between 600 and 400 Ma revealed a systematic decrease in U/Yb ratios from 1.0 to 0.3 after 430 Ma.
Given that the sandstones of the Tsunatori Unit contain abundant volcanic lithic fragments, including volcanic glass, the detrital zircons forming the Youngest Cluster are likely syn-sedimentary zircons derived from contemporaneous magmatism. Therefore, the Youngest Cluster Age of each sample represents its depositional age, indicating the Tsunatori Unit was formed by multiple accretion events at ca. 400 Ma and 430-480 Ma. The ca. 400 Ma depositional age is also consistent with the age range of the index fossils.
Changes in the zircon U-Pb age histogram corresponding to the depositional ages of the sandstones, along with the temporal variation in U/Yb, suggest that, before 430 Ma, the Tsunatori Unit was formed in the vicinity of igneous rocks formed in a relatively mature crust and the northeastern Gondwana continental crust. Between 430 and 400 Ma, the development of an immature oceanic island arc restricted the supply of above-mentioned mature crusts, leading to the dominant input of island-arc crustal components into the trench.
The Tsunatori Unit of the Nedamo Belt, distributed in the Kitakami Massif of NE Japan, has been interpreted as an Early Carboniferous accretionary complex. However, the biostratigraphic constraints on its depositional age remain ambiguous. This is because the age ranges of the index fossils such as Trilonche cf. vetusta (Middle Devonian to Early Carboniferous) are broad, and its occurrence is limited in the Tsunatori Unit. In this study, we performed U–Pb dating, trace-element analysis, and Hf isotopic analysis on detrital zircons from 10 sandstone samples of the unit, aiming to constrain the accretionary age of the unit and elucidate the magmatic and tectonic history of the northeastern Gondwana margin.
The U–Pb dating revealed two distinct groups of sandstone samples. One group exhibited a Youngest Cluster Age (2σ) of ca. 400 Ma, while the other showed a Youngest Cluster Age ranging from 430 to 480 Ma. The age histograms of the former group showed a single peak from 400 to 500 Ma, whereas the latter one exhibited multiple peaks at 400-500 Ma, 500-700 Ma, 800-1200 Ma, and 1400-1800Ma. The sandstone close to the strata where the Devonian to Early Carboniferous index fossils were previously reported exhibited age histograms characteristic of the former group. Trace-element analysis of the zircons with U–Pb ages between 600 and 400 Ma revealed a systematic decrease in U/Yb ratios from 1.0 to 0.3 after 430 Ma.
Given that the sandstones of the Tsunatori Unit contain abundant volcanic lithic fragments, including volcanic glass, the detrital zircons forming the Youngest Cluster are likely syn-sedimentary zircons derived from contemporaneous magmatism. Therefore, the Youngest Cluster Age of each sample represents its depositional age, indicating the Tsunatori Unit was formed by multiple accretion events at ca. 400 Ma and 430-480 Ma. The ca. 400 Ma depositional age is also consistent with the age range of the index fossils.
Changes in the zircon U-Pb age histogram corresponding to the depositional ages of the sandstones, along with the temporal variation in U/Yb, suggest that, before 430 Ma, the Tsunatori Unit was formed in the vicinity of igneous rocks formed in a relatively mature crust and the northeastern Gondwana continental crust. Between 430 and 400 Ma, the development of an immature oceanic island arc restricted the supply of above-mentioned mature crusts, leading to the dominant input of island-arc crustal components into the trench.