The North Qilian orogenic belt in northwestern China is an elongate tectonic unit situated between the North China plate in the north and the Qaidam plate in the south, and is resulted from collision of two plates in Caledonian stage. Shihuigou in Yongdeng county locates in the eastern sector of Northern Qilian Mountains, crossing the island--arc zones of the Ordovician age. Zongpu Group is distributed over the Shihuigou area, and consists of medium--basic volcanic rocks, volcanic clastic rocks interbedded with cherts, limestone, slate and metamorphic sandstone. The geochemistry of the cherts reflects that the cherts coexisting with island-arc volcanic rocks all formed the background of continental margin basin. The rare earth elements reflect that these cherts formed in the deep-water basin far from continent where terrestrial material had no obvious effect to them. It is inferred that northern Qilian located in a tectonic background of archipelagic ocean where existed a wide ocean but multi--island in the Ordovician.