Based on 175 petrochemical data, this paper has defined chemical series and rock assemblages of Carboniferous volcanic rocks in the northern half of west Tianshan Mountains and dealt with their geochemical characteristics. On such a basis, the trench--arc basin system of this area is investigated.The upper unit of Middle Carboniferous in North Tianshan Mountains represents the active continental margin of Junggar plate, whereas the lower unit belongs to sediments of a forearc basin environment. Rocks of the ophiolite suite in north Tianshan Mountains are remnants of a slowly spreading ancient ooeanic crust.Lower Carboniferous volcanic rooks and Middle Hercynian granitoids along the northern margin of Central Tianshan Mountains represent the main part of the extremely thick continental island arc, while Middle and Lower Carboniferous rocks along the Awulale belt are of volcanic assemblages in an internal island arc environment. The Yishijilike belt is a back-arc basin with limited extension.