Abstract: The Irtysh complex is located between Altai and East Junggar terranes in Northern Xinjiang and underwent strong metamorphism and deformation, which recorded the tectonic amalgamation of the two terrains. In order to constrain the amalgamation time of the Altai and East Junggar terrains, we used detrital zircon U−Pb ages to analyze and discuss the provenance. The youngest detrital zircon age obtained from the metaclastic rock sample is 354 Ma. Based on a comprehensive analysis of new and published detrital zircon U−Pb age data from the Irtysh complex and a comparison with regional tectonic and magmatic U−Pb age data, this paper proposes that the Irtysh complex may contain older detritus from the passive continental margin of the Altai terrane, in addition to the Late Carboniferous syn−collisional deposits from both of the Altai and East Junggar terranes and only the East Junggar terrane, suggesting that the Altai and East Junggar terranes had been amalgamated when the Irtysh complex was formed.