Abstract：There outcrop many fresh greenschist tectonic blocks in the western segment of the southern Altyn Tagh marginal tectonic belt. Intensive petrological and geochemical studies show that: the protoliths of the greenschist are basic volcanic rocks (tholeiites), and in addition three types of basalt are also recognized, namely, oceanic island (within plate) basalt (OIB), mid-oceanic ridge basalt (E-MORB) and island arc basalt (IAT). The E-MORB have similar geochemical characteristics, with low Ti (TiO2=0.93%-1.75%, average 1.34%) and P (P2O5=0.09%-0.15%, average 0.13%) and the high field strength elements Zr=(79-146)×10-6, Hf=(2.6-3.5)×10-6 and Nb=(6.6-15.3)×10-6, being LREE-depleted or slightly enriched. IAT have lower Ti (TiO2=0.4%) and P (P2O5=0.08%) than those of MORB and the high field strength elements Zr=56×10-6, Hf=1.6×10-6 and Nb=5.9×10-6, being LREE-slightly enriched. OIB have high Ti (TiO2 = 1.96%-3.08%, average 2.49%) and P (P2O5=0.23%-0.34%, average 0.26%), with the high field strength elements Zr=(204-237)×10-6, Hf=(4.2-7.2)×10-6 and Nb=(14.6-22.1)×10-6, being LREE-enriched. The presence of the OIB, E-MORB and IAT assemblage suggests the existence of an ocean basin in the southern Altyn Tagh marginal tectonic belt during the Early Paleozoic.