10:00 〜 10:15
[MIS23-05] ジルコンU–Pb年代およびマンガンノジュール中の放散虫化石による北部北上帯の付加年代制約
キーワード：ジュラ紀付加体、凝灰岩、砕屑性ジルコン、5万分の1地質図幅「門」
Studies on the formation of accretionary complexes provide information of subducted deep ocean floors and the subduction process itself. The accretionary age of accretionary complexes is estimated based on the age of clastic sediments at the trench. Radiolarians are probably the most widely used age indicator and detrital zircon U–Pb dating has been used by a number of studies in recent years. These two have weaknesses: radiolarians become mostly unextractable by contact metamorphism and detrital zircons do not always indicate sedimentary age. The Jurassic accretionary complex of the North Kitakami Belt has exactly such issues: Metamorphism by Cretaceous plutons renders radiolarians mostly unavailable, and investigations of detrital zircons showed the scarcity of grains that were formed close to the depositional age. In the face of these problems, we attempted a different approach to estimate accretionary ages by targeting tuffs for zircon U–Pb dating and manganese nodules (Mn-nodules) for radiolarian extraction. Both Mn-nodules and zircons in tuffs have been investigated by previous studies for accretionary ages of accretionary complexes, but not extensively, due to their relatively rare occurrence.
The current study area is the Kado Discrict of the 1: 50,000 Quadrangle Series, situated in the central area of the North Kitakami Belt. Tectonostratigraphic division of Jurassic accretionary complexes in the northeast part of the study area has been established based on detailed lithostratigraphic research and sporadic reports on radiolarians and detrital zircons. Previous data indicate the presence of Middle Jurassic to Late Jurassic accretionary units. The southwest part has been targeted for lithostratigraphy, but this was before recognition of accretionary complexes, and no age data have been obtained from clastic rocks. We analyzed nine tuff samples from across the Kado District and three Mn-nodules from the southwest part of the district. In addition, detrital zircons in three sandstone samples were investigated for comparison. Zircons were separated and analyzed by laser-ablation inductively coupled mass spectrometry by Kyoto Fission Track Co. Ltd., and Mn-nodules were treated with 36% hydrochloric acid to obtain radiolarians.
Our results, in concert with data from previous studies, indicate the presence of latest Triassic to Late Jurassic accretionary units that are distributed so that younger and structurally lower units lie generally towards the northeast (oceanward). For each unit, the age of tuffs and Mn-nodules are almost completely consistent with the stratigraphic order expected from oceanic plate stratigraphy. On the other hand, most sandstone samples yielded youngest clusters of zircons that are either considerably older than the expected age or have very large uncertainties of age. Results from samples in the northeast part is consistent with previous studies. Samples from the southwest part with no previous accretionary age data showed the wide distribution of Middle Jurassic accretionary complexes and the first evidence of latest Triassic accretionary complexes in the study area. The general younging trend of accretionary age in the North Kitakami Belt is interrupted by kilometre-scale folds and faults with vertical displacements. The former has been recognized by previous studies, but significance of the latter is highlighted for the first time. Thus, zircon U–Pb dating of tuffs and microfossils from Mn-nodules successfully enabled us to obtain information of accretionary history and post-accretionary structural development in the North Kitakami Mountains.
The current study area is the Kado Discrict of the 1: 50,000 Quadrangle Series, situated in the central area of the North Kitakami Belt. Tectonostratigraphic division of Jurassic accretionary complexes in the northeast part of the study area has been established based on detailed lithostratigraphic research and sporadic reports on radiolarians and detrital zircons. Previous data indicate the presence of Middle Jurassic to Late Jurassic accretionary units. The southwest part has been targeted for lithostratigraphy, but this was before recognition of accretionary complexes, and no age data have been obtained from clastic rocks. We analyzed nine tuff samples from across the Kado District and three Mn-nodules from the southwest part of the district. In addition, detrital zircons in three sandstone samples were investigated for comparison. Zircons were separated and analyzed by laser-ablation inductively coupled mass spectrometry by Kyoto Fission Track Co. Ltd., and Mn-nodules were treated with 36% hydrochloric acid to obtain radiolarians.
Our results, in concert with data from previous studies, indicate the presence of latest Triassic to Late Jurassic accretionary units that are distributed so that younger and structurally lower units lie generally towards the northeast (oceanward). For each unit, the age of tuffs and Mn-nodules are almost completely consistent with the stratigraphic order expected from oceanic plate stratigraphy. On the other hand, most sandstone samples yielded youngest clusters of zircons that are either considerably older than the expected age or have very large uncertainties of age. Results from samples in the northeast part is consistent with previous studies. Samples from the southwest part with no previous accretionary age data showed the wide distribution of Middle Jurassic accretionary complexes and the first evidence of latest Triassic accretionary complexes in the study area. The general younging trend of accretionary age in the North Kitakami Belt is interrupted by kilometre-scale folds and faults with vertical displacements. The former has been recognized by previous studies, but significance of the latter is highlighted for the first time. Thus, zircon U–Pb dating of tuffs and microfossils from Mn-nodules successfully enabled us to obtain information of accretionary history and post-accretionary structural development in the North Kitakami Mountains.