The southern Great Xing’an Range is the most critical Sn-polymetallic metallogenic belt in northeast China. However, the tectonic setting of the Early Cretaceous magmatic-metallogenic “flare-up” event remains uncertain. This paper presents an integrated study on the occurrence, petrology, zircon U-Pb ages, whole-rock geochemistry, and in situ zircon Hf isotopes for Wenduerchagan granites of Xi Ujimqin Banner, central-eastern Inner Mongolia. These granites consist primarily of granite porphyry (with ages of 137 ± 1 Ma and 138 ± 1 Ma) and (porphyritic) alkali feldspar granite (with an age of 141 ± 2 Ma), corresponding to the early Early Cretaceous. They are A-type granites characterized by high silicon, alkali, and TFeO/MgO contents while being depleted of Ba, Nb, Ta, Sr, P, and Ti. They show right-dipping trend rare-earth element distribution characteristics with negative Eu anomalies (Eu/Eu* = 0.01–0.20) and weak heavy rare-earth element fractionation ((Gd/Yb)N = 0.77–2.30). They demonstrate homogeneous zircon Hf isotopic compositions (positive εHf(t) values from +5.3 to +7.1 and young two-stage Hf model ages of 851–742 Ma) and high zircon saturation temperatures (av. 810°C). These geochemical characteristics indicate that Wenduerchagan granites originated from the partial melting of juvenile crust under high-temperature and low-pressure conditions. Wenduerchagan granites most likely formed in a post-collisional compression-extension transition regime caused by the closure of the Mongol–Okhotsk Ocean, when combined with regional geology. Such a transition regime can probably be attributed to the upwelling of the asthenospheric mantle caused by the break-off of a subducted Mongol–Okhotsk oceanic slab. Upwelling asthenospheric mantle provided sufficient energy and favorable tectonic conditions for magmatism and mineralization of the Early Cretaceous.