As an important component of the Central Asian Orogenic Belt (CAOB), the Alxa Block has been a key area for studying the tectonic evolution history of the PaleoAsian Ocean (PAO). This study carried out LA-ICPMS zircon U-Pb dating for two types of gabbros from the Beidashan region in the Alxa Block, yielding two rockforming ages of 293±2 Ma and 267±2 Ma (early and middle Permian). The early Permian gabbro (293±2 Ma) shows cumulate texture, subalkaline, tholeiitic, high Mg# value (64), positive Eu anomaly, low rare earth elements (REE), enrichment of large ion lithophile elements (LILE; Ba, K, Sr), depletion of high field strength elements (HFSE; Th, Nb, Ta, Hf, Zr), and low positive average εHf(t) value (0.91). All of these characteristics indicate that the earlyPermian gabbro derived from a lithospheric mantle metasomatized by slabderived fluids or melt in an oceanic subduction tectonic setting. The middle Permian gabbro (267±2 Ma) is alkaline, enriched in LILE (Ba, K) and REE, but indistinct Nb and Ta depletion, and high positive average εHf(t) value (3.5), indicating that the middlePermian gabbro derived from the depleted mantle with crustal contamination in a continental intraplate extensional setting. Combined with previous research, this study can conclude that one late Paleozoic (328~262 Ma) ultramaficmafic rock belt extending from Longshoushan in the west to the Langshan in the east occurs in the northern margin of Alxa block and formed in the subduction and closure of the PAO. The northern margin of Alxa Block was an oceanic subduction environment during late Carboniferous to late earlyPermian, and transferred into a continental intraplate extensional environment during late earlyPermian to middlePermian, which restricts that timing of the final closure of the PAO in the northern margin of Alxa Block was between late earlyPermian and middlePermian.