Abstract：In Southern Tianshan Mountains of the Tonghuashan ophiolitic tectonic belt, the plagiogranite is exposed at the top of ultrabasic rocks, showing fault contact relationship. The concordant age of (406.5±5.0)Ma was obtained for the plagiogranite by the zircon micro-domain LA-ICP-MS rare earth elements analysis and U-Pb age dating，indicating that the intrusive epoch of plagiogranite was Early Devonian, and that this age represents the lower limit age of the Tonghuashan ophiolitic formation.