Abstract: The Xishadegai intrusion is located in middle segment of the northern margin of the North China Craton, intruding into the Paleoproterzoic Wulashan Group. It is composed of porphyritic syenogranites, which serve as the host rock for the Xishadegai porphyry molybdenum deposit. Zircon LA-MC-ICP-MS U-Pb dating yielded a weighted average ²⁰⁶Pb/²³⁸U age of 220.20±1.80 Ma, indicating that the intrusion formed during the Late Triassic. The syenogranites are high silica (SiO₂ content ranging from 75.09 to 77.06 wt.%), alkali-rich (K₂O+Na₂O content ranging from 7.76 to 8.34 wt.%), peraluminous (A/CNK>1), low in Ti, Fe, Mn, Mg, Ca, and P, enriched in Rb, Th, and U, and depleted in Ba, Sr, and Eu, suggesting a highly fractionated, high-potassium calc-alkaline I-type granite, rather than A-type granite as previously thought. The syenogranites exhibit low (⁸⁷Sr^/86Sr)_i ratios (0.703954-0.705263), negative εNd(t) values (-14.91 to -13.40) and zircon εHf(t) values (-16.42 to -13.30), along with ancient two-stage Nd and Hf model ages (2087-2207 Ma and 2103-2292 Ma, respectively), suggesting that magma derived from the ancient lower crust. Combined with the geochemical features and regional geological data, it is inferred that the Xishadegai intrusion formed in the post-collisional extensional setting related to the collision between the North China Craton and the Siberian plate. The magma suffered a significant mineral fractionation crystallization and a certain degree of crustal assimilation and contamination . The high degree of crystalline differentiation and high oxygen fugacity of the magma are key to the formation of the Xishadegai molybdenum deposit.