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摘要
Zircon U-Pb ages of two acidic tuff and two turbidite sandstone samples from the Nakanogawa Group, Hidaka Belt, were measured to estimate its depositional age and the development of the Hokkaido Central Belt, northeast Japan. In the northern unit, homogeneous zircons from pelagic acidic tuff from a basal horizon dated to 58-57Ma, zircons from sandstone from the upper part of the unit dated to 56-54Ma, and zircons from acidic tuff from the uppermost part dated to 60-56Ma and 69-63Ma. Both of the tuff U-Pb ages are significantly older than the youngest radiolarian fossil age (66-48Ma). Therefore, the maximum depositional age of the turbidite facies in the northern unit is 58Ma and the younger age limit, estimated from the fossil age, is 48Ma. In the southern unit, homogeneous zircons from turbidite sandstone dated to 58-57Ma. Thus the depositional age of this turbidite facies was interpreted to be 66-56Ma from the fossil age, probably close to 57Ma. Most of the zircon U-Pb ages from the Nakanogawa Group are younger than 80Ma, with a major peak at 60Ma. This result implies that around Hokkaido volcanic activity occurred mainly after 80Ma. Older zircon ages (120-80Ma, 180-140Ma, 340-220Ma, 1.9Ga, 2.2Ga, and 2.7Ga) give information about the provenance of other rocks in the Hidaka Belt. It is inferred that the Nakanogawa Group comprises protoliths of the upper sequence of the Hidaka Metamorphic Zone, which therefore has the same depositional age as the Nakanogawa Group (66-48Ma). The depositional ages of the lower sequence of the Hidaka Metamorphic Zone and the Nakanogawa Group are probably the same.
- 出版日期2018-3
