Abstract:

The Zhongtiaoshan area in which Precambrian rocks widely occurred is an important part of the Trans-North China Orogen (TNCO). The Neoarchean TTG is mainly distributed in the Main Range of Zhongtiaoshan Mountain extending in NE direction and the Tongshan “window” of Wangwushan Mountain in EW direction. There occurs mainly the Huping complex composed of biotite plagioclase gneiss, tonalite and Songjiashan gourp in the Tongshan “window”. LA-MC-ICPMS Zircon U-Pb dating shows that the age of the biotite plagioclase gneiss is (2 530±13) Ma and its ε_Hf（2 530 Ma）values range from 3.89 to 7.12.The weighted average ²⁰⁷Pb/²⁰⁶Pb age of the Tonalitic gneiss is(2 551.4±2.7) Ma，and its ε_Hf（t） values from 5.49 to 9.67, which are higher than that of depleted mantle. The approximate ε_Hf（2.55 Ga） values of the Neoarchean depleted mantle in the TNCO are from 8.2 to 9.5, which is also proved by isotopic composition of the TNCO researched recently and the ε_Hf（t）of the tonalitic gneiss. It can be concluded that the mantle of the TNCO was largely depleted before 2.55 Ga. The ε_Hf（t） values of the tonalitic gneiss in Huping complex suggest that the age of the ocean ridge subduction or interocean subduction of young ocean crust is 2.55 Ga, and the subduction played an important role in formation of the tonalitic gneiss in the Huping complex.

Key words: Trans-North China Orogen, Zhongtiaoshan Mountains, Huping complex, zircon U-Pb age, isotope