Abstract: LA-ICP-MS zircon U-Pb geochronology and geochemistry of the monzonite in Dongfu pluton of Tuquan area was studied to explore the tectonic background of its formation.The results show that the monzonite was formed in Late Jurassic(161.8±1.1 Ma).The monzonite displays slightly negative Eu anomalies, enrichment of LILEs and depletion of HFSEs, suggesting that its primitive magma was derived from partial melting of the crust.High content of Sr shows the existence of a small amount of residual plagioclase in the source region.The enrichment of LREEs and depletion of HREEs suggest that the minerals such as pomegranate were enriched in HREEs in the source region.The residues of plagioclase and pomegranate reveal that the monzonites were formed in the environment with medium-high pressure, and were products of the partial melting of the thickened crust which delaminated or collapsed to a certain extent.Combined with regional geology, the authors hold that the formation of the monzonites was related to the post-collisional tectonic environment of the evolution in Mongolia-Okhotsk suture zone, and the southwestern part of the Mongolia-Okhotsk Ocean was closed in Late Jurassic.