Palaeogene K‐Ar ages from the Kamuikotan metamorphic rocks, southern area of the Kamuikotan Gorge, central Hokkaido, northern Japan

Abstract

Mica separates from 44 metamorphic rocks (mostly pelitic) from the southern area of Kamuikotan Gorge, central block of the Kamuikotan zone were dated using the K-Ar method. The ages of white micas, separated from pelitic schists, range from 51 to 74 Ma. The Palaeogene K-Ar ages are supported by 40Ar-39Ar dating. Judging from mineral paragenesis and X-ray diffraction studies (RM values) of white micas, most of the pelitic schists belong to the intermediate pressure type. In the Orowen River district, the northern part of the study area, biotite from metavolcanic rocks of a lower pressure assemblage yields the youngest K-Ar ages (45-50 Ma). Although the Kamuikotan zone was considered to have been a subduction-related metamorphic belt along a Mesozoic consuming margin, the time span of the metamorphism and the lowering of metamorphic pressure in younger rocks suggest that: (1) exhumation of the younger Kamuikotan rocks accompanied the erosion of forearc basin sediments (the Yezo Supergroup); (2) the increase in the thermal gradient was probably related to the approach of the Kula-Pacific ridge to the subduction zone; and (3) the main stage of exhumation of the Kamuikotan rocks ceased before westward movement of the Pacific plate, which induced the collision of the Hidaka-Nemuro belts (palaeo-Kurile arc-trench system) with the Kamuikotan zone.

Fission track and U–Pb zircon ages of psammitic rocks from the Harushinai unit, Kamuikotan metamorphic rocks, central Hokkaido, Japan: constraints on metamorphic histories

“…(), recalculated using the decay constant of Steiger and Jäger () as cited in Nakagawa and Nakano (), and from Imaizumi and Ueda (), Nakagawa and Nakano (), Shibakusa and Itaya (), Ota et al (), Watanabe et al . (), Iwasaki et al (), and Ota (). The 40 Ar– 39 Ar age data of Takigami () are almost consistent with the dates obtained for same samples by the K–Ar method (Imaizumi & Ueda ) and as such are not shown.…”

“…Little research has been undertaken on the deformation recorded in the Kamuikotan metamorphic rocks. Ota (1999) defined four deformation phases in these Kamuikotan metamorphic Bikerman et al (1971), recalculated using the decay constant of Steiger and Jäger (1977) as cited in Nakagawa and Nakano (1987), and from Imaizumi and Ueda (1981), Nakagawa and Nakano (1987), Shibakusa and Itaya (1992), Ota et al (1993), Watanabe et al (1994), Iwasaki et al (1995), and Ota (1999. The 40 Ar-39 Ar age data of Takigami (1987) are almost consistent with the dates obtained for same samples by the K-Ar method (Imaizumi & Ueda 1981) and as such are not shown.…”

Tectonic accretion of a subducted intraoceanic remnant arc in Cretaceous Hokkaido, Japan, and implications for evolution of the Pacific northwest

“…13). From the Early Cretaceous to the Late Cretaceous, subduction zone metamorphism in the Kamuikotan zone transformed from a typical blueschist facies series to a high‐pressure–intermediate facies series (Sakakibara & Ota 1994; Iwasaki et al . 1995; Ota 1999).…”

“…13). From the Early Cretaceous to the Late Cretaceous, subduction zone metamorphism in the Kamuikotan zone transformed from a typical blueschist facies series to a high-pressure-intermediate facies series (Sakakibara & Ota 1994;Iwasaki et al 1995;Ota 1999). Trench tectonics converted from accretion of seamount volcanics with lesser turbidites, accompanied by tectonic erosion, to accretion of voluminous trench-fill turbidites (Ueda et al 2000(Ueda et al , 2001.…”

Revised stratigraphy and zircon U-Pb age data of the Hitachi metamorphic formations in the southern Abukuma Mountains, and reconstruction of the basement of the Northeast Japan Arc before the opening of the Japan Sea

“…U-Pb , Ishiwatari and Tsujimori, 2003;Santosh , 2011; , , 1998 -, , 1966 , Iwasaki et al, 1995, Isozaki, 1996Fig. 13a 20102010Otofuji et al, 1986, , 20101990Fig.…”