The Sanjiang area in Northeast China hosts the sole exposed Mesozoic siliceous rock series in northern China, which holds significant geochemical characteristics for unraveling its genesis and tectonic setting, particularly to trace the temporal and spatial initiation of paleo-Pacific subduction. In the Tongjiang area north of Sanjiang, the "Heitongdi 1 Well" reveals a silicic rock system from the late Triassic to Early Jurassic Dajiashan Formation, spanning approximately 400 meters. Through field geological investigation, microscopic observation, and petrogeochemical analysis of core samples from this well, we have conducted a comprehensive study on the origin and tectonic environment of these siliceous rocks. Radiolarian fossils are observed in the siliceous rocks of the Tongjiang area. The mean Al/(Al+Fe+Mn) value in the siliceous rock samples was 0.74, with no discernible Ce anomaly and Ce/Ce* values ranging from 1.02 to 1.54, (La/Ce)_SN≈1, no significant difference in Y/Ho values, a mean Y/Ho value of 30.02, and a mean Eu/Eu* value of 1.17, indicating that these siliceous rocks were formed in a continental margin island arc environment. The present study also provides compelling evidence for the initiation of subduction of the Paleo-Pacific plate beneath the Eurasian plate during the late Triassic period, leading to the establishment of an island arc environment along the eastern margin of Jiamusi Block. The inferred location of the subduction suture between these two plates is situated westward from Tongjiang drilling hole.