Abstract: The intrusions and volcanic rocks formed in Late Paleozoic are widespread in Sawuer area, the north of West Junggar, but the forming environment has been controversial. The Tasite pluton is an important multiple granitic intrusion which consists of gabbro, granodiorite, monzonite granite, K-feldspar granite and some other rocks. According to U-Pb age analysis, the formation age of the gabbro from the Tasite pluton is 345.3±2.3 Ma, suggesting the middle stage of Early Carboniferous. The geochemical studies indicate that the average of δ is 3.04, A/CNK ratios(0.76~0.85) is lower than 1.1, which belongs to papa-aluminum and calc-alkali rocks. This pluton is enriched in LILE (Rb, Sr, Ba, U, K), depleted in HFSE (Nb, Ta, T, Zr, Hf) and HREE (Yb, Y), with no Eu anomalies, which is similar to features of post-collisional magmatic rocks in West Junggar. It can be deduced that the gabbro might have formed in the extrusion to extension stage of the post-collisional period. The zircon U-Pb age of the Tasite pluton in this paper is the oldest age of post-collisional magmatic rocks in Junggar reported so far, thus providing a new evidence for confining the beginning time of post-collisional period in West Junggar.