40Ar-39Ar isotope chronological evidence for the orogeny in the eastern segment of the Central Asian-Mongolian orogenic belt

Abstract：A kyanite schist belt containing phengite is found in metamorphic rocks of the Hulan Group in the Hongqiling area of south-central Jilin, tectonically in the eastern segment of the Central Asia-Mongolia orogenic belt. Plateau ages of 223.57±0.8 and 224.41±4.0 Ma and isochron ages of 221.9±2.79 Ma and 223.27±4.53 Ma of micas in biotite gneiss of the Lower Member and phengite in kyanite schist of the Upper Member of the Hulan Group are obtained by the continuous stepwise 40Ar-39Ar heating method. The same method also gives discordant plateau ages of 302±6.3 and 246.00±5.70 Ma for hornblende peridotites of island-arc tholeiite type intruded into the Hulan Group. These ages thus confine the time limits of the closing of the Paleo-Asian-Mongolian ocean and Mesozoic collisional orogeny to at least 302-225 Ma and suggest that the area probably underwent a continuous process from continent-continent collisional orogeny to overlapping.