Detailed petrographic and geochemical studies show that the diabase dyke swarms in the western segment of the southern Altun tectonic belt belong to tholeiite series. The major elements of the diabase dykes are characterized by intermediate TiO2(1.19%～1.59%)，high MgO(5.51%～7.88%)，poor K2O(0.04%-84%)and P2O5(0.10%～0.20%)and Na2O>K2O；Characteristics of high field strength elements contents show that the diabase dyke swarms are of E-type MORB or transitional types from continental (intraplate) rift basalt to mid-ocean ridge basalt. The diabase dyke swarms have high contents of ∑REE and unremarkable fractionation between LREE and HREE ［(La/Yb)N=1.93～3.61，LREE/HREE=3.0～4.10］. Chondrite_normalized REE diagrams show a slightly enriched LREE patterns. Combined with the basalt tectonic discrimination diagrams，it is held that the diabase dyke swarms in the western segment of the southern Altun tectonic beltmight have been formed in a transitional tectonic environment from rift to MORB, i.e., the tectonic environment of an initial small oceanic basin.