During a 1:50000 regional geological survey in Chaoyangtun, a ductile shear zone as the contact between the marble of the Neoproterozoic Haigou lithogroup and an Archean metamorphic monzonitic granite was discovered. It is confirmed that this is an outcrop of the boundary between the northern margin of the North China block and its active continental margin,i. e. , the boundary between a geosyncline and a platform. The discovery provides the firsthand material to study the tectonic framework and has an important significance to reconstruct the erogenic history of the area.