U–Pb ages of zircons from metamorphic rocks in the upper sequence of the Hidaka Metamorphic Belt, Hokkaido, Japan: Identification of two metamorphic events and implications for regional tectonics

Abstract

The Hidaka Metamorphic Belt is a well-known example of island-arc crustal section, in which metamorphic grade increases westwards from unmetamorphosed sediment up to granulite facies. It is divided into lower (granulite to amphibolite facies) and upper (amphibolite to greenschist facies) metamorphic sequences. The metamorphic age of the belt was considered to be 55 Ma, based on Rb-Sr whole-rock isochron ages for granulites and related S-type tonalities. However, zircons from the granulites in the lower sequence yield U-Pb ages of 21-19 Ma, and a preliminary report on zircons from pelitic gneiss in the upper sequence gives a U-Pb age of 40 Ma. In this paper we provide new zircon U-Pb ages from two pelitic gneisses in the upper sequence to assess the metamorphic age and also the maximum depositional age of the sedimentary protolith. The weighted mean 206 Pb/ 238 U ages from a biotite gneiss in the central area of the belt yield 39.6 ± 0.9 Ma for newly grown metamorphic rims and 53.1 ± 0.9 Ma for the youngest detrital cores. The ages of zircons from a cordierite-biotite gneiss in the southern area are 35.9 ± 0.7 Ma for metamorphic rims and 46.5 ± 2.8 Ma for the youngest detrital cores. These results indicate that metamorphism of the upper sequence took place at 40-36 Ma, and that the sedimentary protolith was deposited after 53-47 Ma. These metamorphic ages are consistent with the reported ages of 37-36 Ma plutonic rocks in the upper sequence, but contrast with the 21-19 Ma ages of metamorphic and plutonic rocks in the lower sequence. Therefore, we conclude that the upper and lower metamorphic sequences developed independently but coupled with each other before 19 Ma as a result of dextral reverse tectonic movement.

The emplacement ofin situgreenstones in the northern Hidaka belt: The tectonic relationship between subduction of the Izanagi–Pacific ridge and Hidaka magmatic activity

“…The Nakanogawa Group was metamorphosed to form the upper metamorphic sequence around 40-37 Ma (Kemp et al, 2007; Y. Takahashi et al, 2021). We speculate that another heat source must have existed at that time, but no obvious candidate has yet been identified.…”

Repeated metamorphism in the pelitic granulites of the Hidaka metamorphic belt, Hokkaido, Japan: Implications for the formation of the present‐day trench‐arc‐basin system in NE Asia

“…Protoliths of the metasediments are thought to have originated from the Nakanogawa Group in the east (Figure 1c), which has maximum depositional age of 66–48 Ma, and are suggested to have been deposited in a near‐trench setting close to the Eurasian continental margin during the Cretaceous–Eocene (Kimura, 1994; Nanayama et al, 2019). Metamorphic zircon ages reported for the granulites in the lower sequence include 39.6 ± 0.9 Ma for Crd‐Bt‐gneiss (Takahashi et al, 2021), 35.9 ± 0.9 Ma for Bt‐gneiss (Takahashi et al, 2021), 20.6 ± 1.0 Ma for Grt‐Opx‐Crd‐gneiss (Usuki et al, 2006) and 19.3 ± 0.3 Ma for Grt‐Opx‐gneiss (Kemp et al, 2007).…”

“…Maeda and Kagami (1996) suggested that the granulites were formed from accretionary wedge materials in a forearc setting in response to a thermal pulse triggered by subduction of the Izanagi–Pacific ridge during the late Palaeocene (c. 55 Ma). However, in more recent studies, zircon U–Pb dating from the HMB has yielded younger ages such as 39.6–32.5 Ma in pelitic granulites (Kemp et al, 2007; Ono, 2002; Takahashi et al, 2021) and 20.6–18.5 Ma in mafic granulites (Kemp et al, 2007; Kimura, 1996; Takahashi et al, 2021; Usuki et al, 2006). The difference in the age has led to confusion as to whether the HMB is a continuous sequence or a mosaic of different lithologic units that have undergone distinct metamorphic processes (Takahashi et al, 2021).…”

“…However, in more recent studies, zircon U–Pb dating from the HMB has yielded younger ages such as 39.6–32.5 Ma in pelitic granulites (Kemp et al, 2007; Ono, 2002; Takahashi et al, 2021) and 20.6–18.5 Ma in mafic granulites (Kemp et al, 2007; Kimura, 1996; Takahashi et al, 2021; Usuki et al, 2006). The difference in the age has led to confusion as to whether the HMB is a continuous sequence or a mosaic of different lithologic units that have undergone distinct metamorphic processes (Takahashi et al, 2021). The HMB contains two important pieces of evidence in terms of reconstructing its geological history: (1) staurolite occurs as an early mineral in the pelitic granulites (Osanai & Owada, 1990), indicating that an early phase of metamorphism has been overprinted by later granulite facies metamorphism; and (2) the emplacement of tonalites and granites during the late Eocene (37.5 Ma) and again during the early Miocene (c. 19 Ma) indicates two stages of magmatism (Takahashi et al, 2021).…”

“…The difference in the age has led to confusion as to whether the HMB is a continuous sequence or a mosaic of different lithologic units that have undergone distinct metamorphic processes (Takahashi et al, 2021). The HMB contains two important pieces of evidence in terms of reconstructing its geological history: (1) staurolite occurs as an early mineral in the pelitic granulites (Osanai & Owada, 1990), indicating that an early phase of metamorphism has been overprinted by later granulite facies metamorphism; and (2) the emplacement of tonalites and granites during the late Eocene (37.5 Ma) and again during the early Miocene (c. 19 Ma) indicates two stages of magmatism (Takahashi et al, 2021). Our ability to reconstruct the metamorphic and magmatic evolution depends on establishing the relative timing of, and the relationships between, the granulite facies metamorphism, magmatism, and the tectonic transition of the NE Asian continental margin, which are still poorly constrained.…”