The magmatic rocks formed in the forearc setting are considered to possibly record key evidences for subduction initiation. In this paper, the niobium-rich gabbro and plagiogranite are newly reported in the Corridor Nan-shan ophiolitic melange zone of the North Qilian Orogen. Based on field observation, combined with comprehensive studies of petrology, geochemistry, isotope geochemistry and chronology, the time of initial subduction of proto-Tethys Ocean is constrained and its evolution model and tectonic implication are discussed. Zircon U-Pb dating results reveal a weighted average ²⁰⁶Pb/²³⁸U zircon age at 512±4 Ma for a niobium-enriched gabbro sample, and 522±3 Ma and 519±1 Ma for two plagiogranite samples. The niobium-rich gabbro samples have Nb contents of 7.49×10^-6~10.80×10^-6, TiO₂ of 1.50%~2.08%, Nb/U values of 11.9~13.4 and (Nb/La)_N > 0.5, which are obviously higher than those of island-arc basalts. εNd(t) values of Nb-enriched grabbro are between +4.38 and +5.78. Plagiogranite samples have low K₂O content (0.31%~1.66%) and low ratios of K₂O/Na₂O (0.05~0.43). The chondrite-normalized rare earth element pattern is relatively flat, with weakly positive Eu or negative anomalies. The plagioclase samples are relatively enriched in large ion lithophile elements such as Ba, Sr and U, and depleted in high field strength elements such as Nb and Ta. Combing their positive εNd(t) value (+3.35) and highly positive εHf(t) ranging (mostly between 6.2 to 12.9), we suggest an oceanic plagiogranite. The new data, along with the regional geology, indicate the Baijingsi plagiogranites are resulted from partial melting of the young and hot subducting oceanic slab at shallow depths in relatively high temperature during subduction initiation stage. Subsequently, the partial melting of subducting slab led to product of adakitic melts, which metasomatized or interacted with the upper mantle wedge peridotites, then the partial melting of metasomatized mantle peridotites gave rise to Nb-enriched basaltic magma. These data indicate that initial stage of subduction of the Prototethyan ocean (Paleo-Qilian ocean) occurred in the Early Cambrian.