@misc{oai:niigata-u.repo.nii.ac.jp:00004437,
 author = {Ota, Tsutomu},
 month = {Mar},
 note = {The Kamuikotan metamorphic rocks in central Hokkaido, Japan, are typical example produced by subduction-related high pressure and low temperature (high-P/T) metamorphism in the world. They have preserved several lines of evidence showing that they had been formed in the subduction zone with a non-steady state thermal structure. Such evidence is based on investigations in the field, structural analyses, metamorphic petrology and radiometric dating of the Kamuikotan metamorphic rocks. They are summaried as follows. The Kamuikotan metamorphic rocks in the Kamuikotan Gorge area are divided into major two geologic sequences: melange with serpentinite matrix containing a large amount of metamorphic blocks (Tokiwayama melange unit) and coherent metamorphic sequence. The latter is subdivided into three units: Pankehoronai, Harushinai and Biei units, based on the geology, metamorphism and radiometric age. Major four stages of deformation, D1, D2, D3 and D4 stages, are discriminated based on available textural relation among meso- and microscopic deformation structures in the coherent metamorphic sequence. The D1 and D2 stages correspond to subduction-related structures to produce melange complex at trench and underplating of the complex at deep level of subduction zone, respectively. The D3 stage is an exhumation process of the metamorphic complex accompanied with ductile deformation. Subsequently, the metamorphic complex has been gently bent in the D4 stage. Judging from the mineral paragenesis in the basic rocks, the Tokiwayama melange, Biei and Harushinai units are metamorphosed at higher-P/T conditions with geothermal gradient less than 10 ℃/km than the Pankehoronai unit (geothermal gradient; about 15 ℃/km). However, remnants of minerals of the Pankehoronai unit, which were produced in the earlier stage of deformation (D2 stage), shows that the Pankehoronai unit had been metamorphosed at similar conditions to the Biei and Harushinai units. Radiometric ages of phengite dated by K-Ar method are considarably scattered and range from 133 Ma to 62 Ma. The ages of units with higher P/T conditions are older than those of units with lower P/T conditions. Detailed investigations of dated specimens in term of deformation and recrystallization degree indicate that each apparent age of specimen with complicated history never denotes a specific time, and that the intermediate ages within the above range are mixture ages of different events due to incomplete recrystallization during the later metamorphic event. The specimens showing older ages are nearly free from the later metamorphic event (lower P/T conditions). Thus, the Kamuikotan metamorphic rocks are clearly divided into two types, i.e., typical high-P/T metamorphic rocks formed in Early Cretaceous time and rocks belonging to high-P/T intermediate group metamorphosed in Early Eocene time. Such change of the Kamuikotan metamorphic P/T gradient is related that the ages and the relative motions of subducted oceanic plates had been changed in early Cretaceous to early Paleogene time., 学位の種類: 博士（理学）. 報告番号: 甲第1169号. 学位記番号: 新大院博（理）甲第60号. 学位授与年月日: 平成8年3月21日, 新大院博（理）甲第60号},
 title = {Evolution of the Kamuikotan high-P/T metamorphic belt in central Hokkaido,Japan : with special reference to the relation between white mica K-Ar ages and deformation},
 year = {1996}
}