17:15 〜 18:30
[SGL23-P02] Detrital zircon geochemistry in the Shimanto Accretionary Complex: Tectonic constraints on formations of the Jurassic to Cretaceous batholiths in the eastern Asian margin
キーワード：ジルコン、U-Pb年代測定、Hf同位体比
Most of the Phanerozoic granite batholiths in the North and South China blocks have a tectonical-petrogenetical relation to the Pacific-type orogeny. In connection with this aspect, the major orogenic components including the batholiths of the Jurassic and Cretaceous are relatively well preserved especially in SW Japan. However, it is difficult to estimate the paleo-geologic structures even in the Jurassic and Cretaceous components, because they suffered from later disturbances such as tectonic and surface erosions. In this study, we compared the detrital zircon U–Pb ages and Hf isotopic ratios obtained from fifteen terrigenous clastic rocks in the Cretaceous Shimanto Accretionary Complex of the Shikoku, SW Japan. This comparison is a key to understand the Jurassic-to-Cretaceous spatial-temporal distributions of the batholiths connected with the inner structures of the subduction zone.
The zircon age spectra from the formations before ca. 100 Ma show two age clusters of Permian to middle Jurassic (from ca. 280 to 160 Ma) and early-to-middle Cretaceous (ca. 130 to 100 Ma). On the other hand, those from the formations after ca. 100 Ma show a single age peak of middle-to-late Cretaceous (< ca. 130 Ma). The major change in the age distributions suggests an oceanward migration of the magmatic front in the late Cretaceous (ca. 100 Ma), and the large batholiths formed after 100 Ma played a role in controlling terrigenous supply routes between coeval back-arc and fore-arc domains.
The Hf isotopic ratios (εHf(t)) obtained from the analyzed zircons show characteristics values in corresponding to their crystallization ages; moderate juvenile to intermediate values (+6 to -4) for ca. 220 to 200 Ma, moderate juvenile to highly evolved values (+10 to -26) for ca. 200 to 180 Ma, moderate to highly evolved values (-4 to -30) for ca. 180 to 130 Ma, moderate juvenile to intermediate values (+10 to -4) for ca. 130 to 100 Ma, and intermediate to moderate evolved values (+4 to -10) for < ca. 100 Ma.
Integration of these results with the U–Pb age spectra leads to suggestion that the late Jurassic to early Cretaceous granites were generated from the older materials at the inland areas of the China blocks; on the other hand, the late Cretaceous granites were originated from the younger materials at the coastal areas. Moreover, subducting oceanic crusts and/or asthenospheric materials are also suggested to be involved in the early Cretaceous granites. Probably, the major change of source materials in relation to each granite was close to the transition in the position of magmatic front at the time in the North and South China blocks.
The zircon age spectra from the formations before ca. 100 Ma show two age clusters of Permian to middle Jurassic (from ca. 280 to 160 Ma) and early-to-middle Cretaceous (ca. 130 to 100 Ma). On the other hand, those from the formations after ca. 100 Ma show a single age peak of middle-to-late Cretaceous (< ca. 130 Ma). The major change in the age distributions suggests an oceanward migration of the magmatic front in the late Cretaceous (ca. 100 Ma), and the large batholiths formed after 100 Ma played a role in controlling terrigenous supply routes between coeval back-arc and fore-arc domains.
The Hf isotopic ratios (εHf(t)) obtained from the analyzed zircons show characteristics values in corresponding to their crystallization ages; moderate juvenile to intermediate values (+6 to -4) for ca. 220 to 200 Ma, moderate juvenile to highly evolved values (+10 to -26) for ca. 200 to 180 Ma, moderate to highly evolved values (-4 to -30) for ca. 180 to 130 Ma, moderate juvenile to intermediate values (+10 to -4) for ca. 130 to 100 Ma, and intermediate to moderate evolved values (+4 to -10) for < ca. 100 Ma.
Integration of these results with the U–Pb age spectra leads to suggestion that the late Jurassic to early Cretaceous granites were generated from the older materials at the inland areas of the China blocks; on the other hand, the late Cretaceous granites were originated from the younger materials at the coastal areas. Moreover, subducting oceanic crusts and/or asthenospheric materials are also suggested to be involved in the early Cretaceous granites. Probably, the major change of source materials in relation to each granite was close to the transition in the position of magmatic front at the time in the North and South China blocks.