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

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 similar to 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 similar to 21-19 Ma, and a preliminary report on zircons from pelitic gneiss in the upper sequence gives a U-Pb age of similar to 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 Pb-206/U-238 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 similar to 40-36 Ma, and that the sedimentary protolith was deposited after similar to 53-47 Ma. These metamorphic ages are consistent with the reported ages of similar to 37-36 Ma plutonic rocks in the upper sequence, but contrast with the similar to 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 similar to 19 Ma as a result of dextral reverse tectonic movement.