Plenty of granitic rocks are exposed in the western Junggar region which has been regarded as one of the most typical areas in the world where the post-collisional vertical continental growth has been recorded. Previous studies were concentrated on the representative intrusions exposed in the eastern part of this region, such as Miaoergou granite pluton, Akebasitao granite pluton, Kelamayi granite pluton, Hongshan granite pluton, Hatu granite pluton and Baogutu adakitic pluton. However, rather insufficient studies were devoted to the granite pluton exposed in Tangbale-Mayile area in the western part of this region. In this paper, the study was concentrated on the geology, geochemistry and U-Pb chronology of the Laba intrusion in the Mayileshan Mountain in order to provide some new data for the discussion on the post-collisional magmatism along the southwestern margin of the western Junggar accretionary orogenic belt. The Laba pluton intruded into the Lower Carboniferous Baogutu Formation. This intrusion occupies about 23.95 km2 in the plan view, with the long axis extending about 8 km in NS direction and an EW width of about 5 km. The pluton is composed of granodiorites, quartz-diorites, tonalites and granites, predominantly of granites. Among them, the granodiorite and the granite belong to calc-alkaline to high-K calc-alkaline series and metaluminous to peraluminous rocks. The σ value of granitic rocks varies from 1.09 to 1.91, averagely 1.48；and the A/CNK value varies from 0.87 to 1.34，averagely 1.07, indicating that these rocks are metaluminous to peraluminous. The A/CNK values of two samples are 1.25 and 1.34 respectively, suggesting that they are peraluminous. The chondrite-normalized patterns of the samples assume LREE-enriched and HREE flat with a (La/Yb)N value varying from 1.04 to 1.39, 1.19 on average. Relative to the primitive mantle, the trace element spider diagram of the samples indicates that these rocks are enriched in large ion lithophile elements (K, Rb and Ba) and depleted in high strength field elements (Nb, Ta, Ti and P). The Laser-ICP-MS mean 206Pb/238U ages of 287 ± 5 Ma and 295.1 ± 2.3 Ma were obtained for two representative samples from this pluton, indicating that the age of this pluton is Early Permian. This age is similar to the age of post-collisional granites in the western Junggar region with a peak of 310～295 Ma. On the basis of an integration of geological and geochemical characteristics of this pluton and regional geology, the Laba pluton is held to be a magmatic product of the post-collisional extension in the western Junggar region. The pluton has similar trace element geochemical characteristics to island-arc granites, implying that its magma source had been metasomatized by previous subduction fluids. The closure of paleo-oceans in western Junggar and the adjacent northern Tianshan region happened earlier than 316 Ma. In other words, these regions were under a post-collisional extension episode during the period of Late Carboniferous to Early Permian. Both the oceanic ridge subduction and the subducted oceanic crust melting models cannot be employed to interpret the formation of the Laba granitic pluton.