The Ordovician Majiagou Formation in the Ordos Basin has become the most popular carbonate formation in the Ordos Basin, because of its potential for oil and gas exploration, but its age attribution is still controversial. The Upper Member of the Ordovician Majiagou Formation (referred to as the “Ma- 5” Member) is characterized by the presence of two layers of tuffs, which are commonly used as regional marker beds for stratigraphic correlation. Investigation of the zircon ages of these tuffs can help establish the chronostratigraphic framework of the Majiagou Formation and provide insights into its depositional period and tectonic settings. Methods: We performed zircon SHRIMP U- Pb dating and zircon microzonation in situ Hf isotope characterization of the Ma- 5 Member tuffs in the Well S148 and the Well T56 in the Ordos Basin.Results: The zircon SHRIMP U- Pb dating data of tuff shows that the 206Pb/238U age of tuff samples from the Well T56 ranges from 442.3 Ma to 474.3 Ma, with a weighted average age of 460.9±2.9 Ma, and that of tuff samples from the Well S148 ranges from 442.0 to 471.6 Ma, with a weighted average age of 458.4±2.4 Ma. The measured n(176Hf)/n(177Hf) ratios of in- situ zircon Hf isotopes range from 0.282687 to 0.280820, and εHf(t) values vary from 6.67 to 11.32, with two- stage model ages ranging from 720 to 1024 Ma.Conclusions: Based on the zircon dating results of the tuffs of the Majiagou Formation, the deposition of the Majiagou Formation is constrained to the Middle Ordovician Darriwilian to Upper Ordovician Sandbian stages. Comparison of the zircon U- Pb ages and Hf isotope characteristics of the tuffs surrounding the basin suggests that the volcanic tuffs of the Majiagou Formation may have originated from the North Qinling Mountains and/or North Qilian Orogenic Belts in the southwestern part of the basin. Furthermore, it is inferred that prior to the emplacement of the tuffs, the North Qinling and/or North Qilian Ocean initiated a northward subduction and entered a stage of ocean—land transition, indicating the transformation of the western and southern margins of the basin into an active continental margin during the late Middle Ordovician.