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 ²⁰⁶Pb/²³⁸U 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.

中文翻译：

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日本北海道日高变质带上层变质岩中锆石的U–Pb年龄：两个变质事件的识别及其对区域构造的意义

日高变质带是岛弧地壳断面的一个著名例子，其中变质坡度从未变质沉积物向西直至粒状相向西增加。它分为下部（粒状至闪石相）和上部（两性方至绿片岩相）变质序列。根据花岗石的Rb – Sr全岩石等时年龄和相关的S型声调，该带的变质年龄被认为是〜55 Ma。然而，从较低的收益率序列的锆石粒ü -〜21 Pb年龄- 19马，并从上面的序列泥质片麻岩锆石的初步报告给出了U形-〜40马的Pb年龄。在本文中，我们提供了新的锆石U –铅在上层序列中来自两个胶质片麻岩的年龄，以评估变质年龄以及沉积原生质岩的最大沉积年龄。来自带中部黑云母片麻岩的加权平均²⁰⁶ Pb / ²³⁸ U年龄，新生长的变质轮缘为39.6±0.9 Ma，最年轻的碎屑岩芯为53.1±0.9 Ma。南部地区堇青石-黑云母片麻岩的锆石年龄为变质缘，年龄为36.5±0.7 Ma，最年轻的碎屑岩心年龄为46.5±2.8 Ma。这些结果表明，上层序列的变质作用发生在〜40 – 36 Ma，而沉积的原生质岩是在〜53 –之后沉积的。47马。这些变质年龄与上层序列中〜37–36 Ma的深成岩年龄一致，但与下层序列中〜21–19 Ma的变质和深成岩年龄相一致。因此，我们得出的结论是，由于右旋逆构造运动，上，下变质层序列独立发育，但在〜19 Ma之前彼此耦合。