Abstract:Thinlayered and laminated tuff was developed in the Wulalike Formation in the northwestern Ordos Basin.This tuff bed was produced by volcanic ashfall deposits which can reclassify the Ordovician stratigraphic framework of Ordos Basin and provide importantconstrains on the plate tectonic setting of the northwestern Ordos Basin as well as major volcanism—magma activities. In this contribution,wecarried SHRIMP UPb dating and Hf isotope on tuff sample collected from Zhuozishan area, located in Wuhai City,Inner Mongolia.LAICPMS UPb dating on zircons has yielded a weighted average of 4549±34Ma(MSWD=002). SHRIMP UPb dating on zircons from the tuff sample has yielded a set of concordant ages of 4332±33~4642±35Ma with the mean age 4564±42Ma(MSWD=19). Accordingly,the main depositional period of Wulalike Formation,which is still controversial,is redefined to Darriwilian—Sandbian,Middle—Late Ordovician. In situ zircon isotope analysis yieldedεHf(t) = +402 ~ +1098,n(176Hf)/n(177Hf)ratios of 0282 619~0282 806 and twostage model age of 729~1178Ma. According to their regional distribution, it is thought that these tuffs derive from volcanic eruption in the North Qinling or/ and North Qilian region. The zircon age and magmas source are in accordance with the tuffs from Wuhai city to the tuffs(or Kbentonites) from the southwestern Ordos Basin, which means all the western and southern Ordos Basin affected by the North Qilian or/ and North Qinling Early Paleozoic orogenic belt during Middle—Late Ordovician.