Abstract: The Laojunmiao metamorphic complex, exposed on the northeastern margin of the Junggar basin, consists mainly of quartz schists of greenschist facies, with some marbles bearing pre Devonian tabulates and crinoids resting on it. 40 Ar/ 39 Ar dating of muscovite from the complex yields a plateau age of 461.5±0.2 Ma and an isochron age of 462.0±4.1 Ma, indicating that its metamorphism occurred in the late Middle Ordovician. In combination with regional geology, it is supposed that the basement of the basin is composed of the Paleozoic continental crust.