Abstract: The Mazhashan syenites in the Bachu area were formed in the Late Permian with the zircon SHRIMP age ranging from 286 to 277Ma, and these ages are consistent with the ages (300-260 Ma) of the post-collisional granites in the southern Tianshan orogen. These syenites are high in silica and alkaline with SiO₂=62.98%-67.29 % and K₂O+Na₂O=10.08%-11.22% , and often show Na₂O>K₂O. Most of the samples have positive Eu anomalies and slightly negative Sr and significantly negative P and Ti anomalies. They formed in an intraplate rifting setting as they have geochemical affinity with A1type granites with Ga/Al =（2.84-3.28）×10^－4 and Zr+Nb+Ce+Y =（405.11-606.49）×10^－６. The syenites have more depleted Sr-Nd isotopic compositions (ε_Nd(t) =+3.4-+4.3, I_Sr=0.703 03-0.703 44) and younger Nd model ages (T_DM=0.6-0.7 Ga) than the coeval mafic dikes. Geochemical data indicate that the syenites were originated from the remelting of the underplated mafic lower crust which composition had been changed by the asthenospheric upwelling. Combining with regional geological data, the authors found that the magmatic activity differences between the Tarim Basin and the South Tianshan Orogen in the Permian stage were probably related to their different tectonic localities. The Permian magmatism in the Tarim Basin was related to the active rifting triggered by the asthenospheric upwelling, this is different from the magmatism during the postcollisional extension in the South Tianshan Orogen, which was controlled by the preexistent mobile zone (suture) or by the extensionresulted detachment zones.

Key words: syenites, petrogenesis, tectonic implication, SHRIMP zircon dating, Bachu County