A study was made of the geochemical characteristics of the greenschist unit discovered first by us in the Saiwusu, in western Langshan Mountain, North China craton. It was found that the greenschist is NEE trending, its outcrop being about 20 km, and with a thickness of 20～274 m. The geochemistrical measurement showed that its SiO₂ content is 48.03%～53.93%, and that it is rich in light REE and incompatible elements. The protolith recovery showed that the greenschist originated from the metamorphism of arc-island basalt, suggesting that the western Langshan was occupied by an active continental margin at that time, with a special distribution and thickness variation of the greenschist, indicating that the arc-island took place in a short period and in a limited area. This discovery is important for deciphering the formation and attributes of the north China craton.oft-com:office:office" />