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Detrital zircon geochronology of sandstones from Jurassic and Cretaceous accretionary complexes in the Kanto Mountains, Japan: implications for arc provenance

Kazumasa Aoki・Yukio Isozaki・Shuhei Sakata・Tomohiko Sato・Shinji Yamamoto・Takafumi Hirata

In order to obtain information about provenance of terrigenous clastics that make up most　of the Jurassic-Cretaceous accretionary complex (AC) of SW Japan, we have conducted detrital zircon　U-Pb dating (laser-ablation inductively coupled plasma-mass spectrometry – LA-ICP-MS) of two Jurassic　sandstones and four Cretaceous sandstones from the Mitsumine area, Kanto Mountains near Tokyo.
Except for one sample, the youngest U-Pb age data obtained from each sample are 159.6 ± 4.9, 122.7 ±　3.8, 97.5 ± 2.3, 96.7 ± 4.3 and 87.7 ± 1.9 Ma that are consistent with microfossil ages previously　reported　from shales in the same units, confirming the polarity of tectonic-downward growth of the ACs across　the Butsuzo Tectonic Line, previously estimated from biostratigraphic dating. The new zircon U-Pb age　data also reveal that the Jurassic and Cretaceous sandstones contain many ca. 200-160 Ma and ca. 120-90　Ma detrital zircons, respectively. These results indicate that granitic arc-batholiths corresponding to each　age were widely exposed in the fore-arc region when the sandstones were deposited. Detrital zircon data　including this study have revealed that the Late Jurassic and Cretaceous sandstones are significantly low　in population of Proterozoic ages than the Middle Jurassic sandstone, suggesting that a major provenance　shift occurred between middle and late Jurassic. Arc batholith formation derived from oceanic-plate　subduction typically causes high topographic relief in a fore-arc. Thus, it is likely that the formation of　the granitic batholiths played an important role in the provenance shift, which had an important effect on　the supply of terrigenous clastics in the Jurassic-Cretaceous fore-arcs in Japan. The large amount of Late　Jurassic and Cretaceous batholiths might　serve as barriers to restrict the supply route of the terrigenous　clastics from the back-arc side.