Mylonitic potassic granitoids from the Yagan metamorphic core complex on Sino-Mongolian border: a mark of transition from contractile to extensional tectonic regime
中蒙边界亚干变质核杂岩糜棱状钾质花岗岩—早中生代收缩与伸展构造体制的转换标志

Occurrences of large-scale thrusts and extensional tectonics in Middle and Late-Mesozoic are a striking feature of the tectonics in the North China Block. This paper, however, further recognizes Early Mesozoic mylonitic granites with extensional tectonic setting from the Yagan metamorphic core complex (MCC). a typical representative of extensional tectonics. They have features of potassic or shoshonitic granite types and their tectonic discriminations show extensional setting. The zircon U-Pb dating of the Shuiquangou mylonitic pluton yields 228 + 7Ma. These granitoids were strongly deformed by syn-tectonic magmatic flow and high-temperature solid flow. All these characteristics suggest that adjustment, thinning and extensional deformations at the middle-low crustal levels might have started from the early Mesozoic, i.e. much more earlier than the final formation (Mesozoic) of the large-scale extensional tectonics, i.e. the MCC. Integration of all these characteristics displays a peculiar tectonic dynamic mechanism and setting.