U–Pb zircon ages of the Nakanogawa Group in the Hidaka Belt, northern Japan: Implications for its provenance and the protolith of the Hidaka metamorphic rocks

Abstract

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 58–57 Ma, zircons from sandstone from the upper part of the unit dated to 56–54 Ma, and zircons from acidic tuff from the uppermost part dated to 60–56 Ma and 69–63 Ma. Both of the tuff U–Pb ages are significantly older than the youngest radiolarian fossil age (66–48 Ma). Therefore, the maximum depositional age of the turbidite facies in the northern unit is 58 Ma and the younger age limit, estimated from the fossil age, is 48 Ma. In the southern unit, homogeneous zircons from turbidite sandstone dated to 58–57 Ma. Thus the depositional age of this turbidite facies was interpreted to be 66–56 Ma from the fossil age, probably close to 57 Ma. Most of the zircon U–Pb ages from the Nakanogawa Group are younger than 80 Ma, with a major peak at 60 Ma. This result implies that around Hokkaido volcanic activity occurred mainly after 80 Ma. Older zircon ages (120–80 Ma, 180–140 Ma, 340–220 Ma, 1.9 Ga, 2.2 Ga, and 2.7 Ga) 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 (66–48 Ma). The depositional ages of the lower sequence of the Hidaka Metamorphic Zone and the Nakanogawa Group are probably the same.

Origin of the early Cenozoic belt boundary thrust and Izanagi–Pacific ridge subduction in the western Pacific margin

“…Cretaceous accretionary complexes in Hokkaido are dominated by greenstone of various origins, such as seamounts, normal oceanic crust, oceanic plateau material and island arcs (Ueda & Miyashita, ). In contrast, the Paleocene–Eocene complexes are dominated by terrigenous sediments (Kiminami & Kontani, ; Nanayama, ; Nanayama et al, ). The in situ eruption of mid‐oceanic ridge type basaltic magma (MORB) into the Paleogene accretionary complex has been reported (Miyashita & Yoshida, ) and it was suggested that the Kula‐Pacific ridge–trench encounter occurred during the Paleocene.…”

Organic matters and acid‐sulfate alteration in Itomuka mercury mine, Hokkaido, Japan: Implications for the transportation and deposition mechanisms of Hg

“…Recently, the age of Hidaka Group was revised from pre‐Tertiary to Paleogene by Nanayama et al . (). Figure is the geological map of Itomuka area based on these previous studies.…”

“…Geological map of the Itomuka mercury mine (modified after Sugimoto et al ., , the age of Hidaka Group is taken from Nanayama et al ., ). Line A‐A′ shows the section line in Figure …”

“…The geology of Itomuka area was reported by Saitoh et al (1960), Hasegawa et al (1961), and Sugimoto et al (1972). Recently, the age of Hidaka Group was revised from pre-Tertiary to Paleogene by Nanayama et al (2017). Figure 1 is the geological map of Itomuka area based on these previous studies.…”

“…Kamishiyubetsu Formation is in fault contact with the upper Miocece volcanic and pyroclastic rocks (Itomuka Formation). This formation is widely distributed from the F I G U R E 1 Geological map of the Itomuka mercury mine (modified after Sugimoto et al, 1972, the age of Hidaka Group is taken from Nanayama et al, 2017). Line A-A 0 shows the section line in Figure 2 Yamato deposit to the Tobu deposit in Itomuka area and consists of dacitic volcanic breccia, dacite and andesitic lava.…”