The Longwanggou Formation of Caotangou Group is widely exposed in the western part of northern Qinling Orogenic Belt, which is of great importance for understanding the early Paleozoic tectonic evolution of the Qinling Orogenic Belt. The acidic tuff in the Longwanggou Formation yields an LA- ICP- MS zircon age of 457. 4±3. 8 Ma. The acidic tuff samples in the Longwanggou Formation have SiO 2 (72. 06%~74. 49%) and Na 2O+K 2O (5. 35%~8. 11%), and low Al 2O 3(13. 91%~ 16. 07%) contents. The rocks are geochemically featured by LILE enrichment and HFSE depletion, low Mg# values and Nb/Ta ratios and obvious negative anomalies (δEu=0. 34~0. 86). The acidic tuff in the Longwanggou Formation has the characteristics of crustal source. The acidic tuff in the Longwanggou Formation and Late Ordovician- Early Silurian magmatic rocks have gecchemical characteristics of arc magmatic rocks, indicating that they were formed in a subduction environment. Combined with the early Paleozoic ultrahigh pressure metamorphic belt, the distribution of magmatic rocks and the sedimentary characteristics of Caotanggou Group, we suggest that the subduction of the Shangdan Ocean was still ongoing in the Early Ordovician.