Abstract
In the southern part of the Hidaka belt, andalusite-sillimanite type metamorphic rocks, tonalites and gabbros are widely exposed. To clarify tectono-thermal evolution of the belt, K-Ar ages of 43 biotite and 5 hornblende separated from metapelites, amphibolites, tonalites and gabbro were determined. These ages are divided into two groups according to their distribution area, the group of 35-29 Ma (biotite age) is in the northeastern part and that of 19-17 Ma (biotite age) is in the southwestern part. Cooling rates of the older and younger groups are estimated using biotite and hornblende K-Ar ages to have been about 120 °C/Ma and about 23 °C/Ma, respectively. The rocks of the younger group had been strongly mylonitized under ductile deformation during their uplifting. It is estimated that narrow transitional zone or fault, which separates the K-Ar ages into two groups, would exist along the mountain ridge of NW-SE direction in the studied area.
On the basis of these K-Ar ages with geological and petrological studies which have been already published, the tectono-thermal evolution of the study area can be divided into four stages: (1) stage of the regional metamorphism and anatexis (60-41 Ma), (2) stage of emplacement of tonalite body formed by anatexis and subsequent contact metamorphism by the tonalite intrusions (ca. 40 Ma), (3) stage of upthrusting and mylonization of lower crustal rocks (36-17 Ma), and (4) stage of uplifting and cooling of these rocks.
