As a key step of the Wilson cycle, subduction initiation is regarded as a start hallmark of the ocean-continent transformation. However, the crust-mantle interaction and crustal growth processes related to subduction initiation remain enigmatic. Here we report a newly recognized complete magmatic sequence containing three rock assemblages in the Hongguleleng ophiolitic complex (HOC) in West Junggar, NW China. Group I contains typical Mid-Oceanic Ridge (MOR)-derived rocks including abyssal peridotite, gabbro, plagiogranite, and basalt. Group II consists of forearc basalt (FAB) and boninitic gabbro and has a high continentality index (CI > 130), similar to the rock assemblages in the Mariana proto-arc. Group III composed of calc-alkaline andesite and arc tholeiite basaltic andesite exhibits middle CI values (50 < CI < 100), suggesting that they form in a transitional oceanic arc. Zircon U-Pb ages show that the N-MORB gabbro and plagiogranite samples are emplaced at 483–481 Ma, and the boninitic rocks, calc-alkaline andesites, and arc tholeiite basaltic andesites have the crystalline ages of 472–467 Ma, 456 Ma, and 452 Ma, respectively. An increasing trend of Ba/Th and V/Ti ratios from Group I to Group II suggests the addition of aqueous fluid and elevated oxidation in the mantle source during subduction initiation. However, it exhibits an uptrend in Th/Nb and (La/Sm)cn ratios but a downtrend in Ba/Th ratios from Group II to Group III, indicating a significant contribution of slab-derived sediments to the mantle during oceanic arc evolution. Combined with regional data, we consider that there developed an E-W–trending sub-ocean basin of the Paleo-Asian Ocean spanning northern West Junggar and East Kazakhstan during the Middle Cambrian–Ordovician. The MOR-type and forearc ophiolites in northern West Junggar as well as our results suggest that the ocean happened mid-ocean ridge spreading at ca. 512–478 Ma and subduction initiation at ca. 476–467 Ma, and developed a transitional oceanic arc at ca. 456–452 Ma.