5:45 PM - 6:00 PM
[R8-10] Re-proposal of the metamorphic zone division in the Horokanai area of the Kamuikotan Belt based on the mineral paragenesis and phengite K-Ar Age.
Keywords:Phengite K-Ar age, Triple Point bluecshit, Horokanai area, Kamuikotan Belt
According to the K-Ar age of phengite (Phn) and the deformation style of the rock, Sakakibara et al. (2007) proposed a zone division in the Horokanai area of the Kamuikotan belt, such as Horokanai unit (HKU) characterized by the occurrence of blueschist (BS) with Phn K-Ar ages of 135-120 Ma, and Biei unit (BIU) by the missing of BS and Phn K-Ar ages of 115-100 Ma. However, we confirmed the occurrence of BS from Mt. Shirakke through Jyari River to Mt. Numaushi, where Sakakibara et al. (2007) classified as BIU, in addition, the Phn K-Ar dating in the area has not been reported.To clarify these inconsistencies, we performed Phn K-Ar dating collected from the Jyari River and Mt. Numashi and their surrounding area. The results are as follows:HKU: EP256: Etanbetsu Pass, 115.1Ma, KD12/KD13: Numaushi River East, 123.3/106.6Ma, 126.9/101.8Ma.BIU: KHK121: Mt. NUmaushi,120.2Ma, KHK331/KHK333: Jyari-River, 109.2Ma/105.6Ma. Since Phns in the studied BS are generally fine-grained, the particle size of the dated fraction was adjusted to 0.2-2 micron and coarse-grained (50-120 micron) Phn fractions in KD12/KD13 were also dated. The coarse-grained fraction of KD12/KD13 showed 123.3/126.9 Ma, and their fine-grained fractions were 106.6/101.8 Ma, and the difference reached 12-25 Ma. The phenomenon of younger fine-grained fractions has also been confirmed in a few reports (Kurosegawa, Sato et al., 2014; Kanto Mountains, Lu et al., 2022), and there are various theories as to why. In order to eliminate the influence of particle size, the fine-grained fraction age is considered, i.e., 115.1~106.6 Ma for HKU and 120.2~105.6 for BIU. Phn K-Ar ages (107-102 Ma) reported from BS missing area of BIU also overlap with abovementioned BS ages (120-101Ma). Therefore, the blueschist occurrence region, which Sakakibara et al. (2007) regarded as the Biei unit, has lost the reason to distinguish it from the Horokanai unit in terms of mineral combination and metamorphic age.