This paper mainly focused on the mafic rocks in southwest Xitieshan area, which are composed of basalt, diabase and gabbro derived from the same primitive magma. Detailed geochemical studies show that the basalt in Xitieshan area have high concentrations of Fe, Ti, P and K, and are also characterized by OIB-type trace element patterns, displaying significant enrichment of LREE and LILE as well as HFSE. These data indicate that the mafic rocks were probably formed in an intraplate environment. Zircon U-Pb dating of the gabbro yielded a ²⁰⁶Pb/²³⁸U weighted average age of (520.8±7.0) Ma. Combined with the chronology and geochemical data of the region, the authors infer that the North Qaidam was located in a tectonic background of archipelagic ocean where there existed a multi-island oceanic basin in Early Paleozoic period.