The Hebukesaier ophiolite was intruded by Early Silurian rhyolite porphyry in the Saier Mountain region of northern West Junggar. The rhyolite porphyries exhibit obviously high SiO₂ (70.88%~73.36%), Al₂O₃ (14.30%~15.31%) and extremely low MgO (0.36%~1.09%) values, and display relatively high Sr and low Y values with high Sr/Y (22.26~78.42) and (La/Yb)_N (20.27~24.24) ratios. Moreover, both rhyolite porphyries in the Saier Mountain and Early Silurian-Early Devonian adakitic rocks in the north of Xiemisitai Mountain show high positive εHf(t) values and young Hf model ages, and have low K₂O values, Mg^# values as well as compatible elements (Cr, Co, Ni). All these features prove that in Early Silurian-Early Devonian period these rocks in Saier Mountain and the north of Xiemisitai Mountain were formed by partial melting of lower juvenile crust rather than by subducted oceanic crust. From Early Ordovician to Early Devonian, the north of West Junggar underwent intra-oceanic subduction, arc-arc collision, the closure of the ocean and post-collisional extension processes. The ocean in the north of West Junggar might have been closed before Early Silurian, followed subsequently by post-collisional extension. In the post-collisional extension stage, the mantle-derived materials migrated upward and were mixed with juvenile crust-derived materials, forming a mixed magma chamber. Lots of 435~400 Ma A₂-type granites, I-type granites and a small amount of adakitic rocks were derived from this mixed magma chamber, and then the massive vertical crustal accretion was completed in this period.