The Solonker zone (Solonker-Sonidzuoqi-Xilinhot) of northern Inner Mongolia is thought by most authors to represent the suture between the southern margin of the Siberian plate and the northern margin of the North China plate. The subduction- and collision-related granitoids that occur along the suture zone in southern Sonidzuoqi are selected for isotopic dating. Two conclusions are achieved: (1) Two phases of arc magmatism happened in the area, at about 490 Ma and 310 Ma, revealed by zircon U-Pb SHRIMP method; the collisional granites were emplaced at the time interval of 230 - 250 Ma (Rb-Sr whole-rock and zircon U-Pb); (2) Final collision and suturing between the two plates is supposed to happen during the period of 230~310 Ma based on our new data. This does not support the "late Devonian collision" model advocated by most local geologists, rather, is consistent with a "late Permian collision" model proposed by Sengor et al. (1993).