Abstract: The paper comprehensively studies the petrography, geochemistry, and LA-ICP-MS zircon U-Pb dating of the syenogranite in the middle section of Yilehuli Mountain. The results show that the syenogranite is characterized by high Si and rich alkali contents, poor Mg and Ca contents, and low FeO^T/MgO ratio, belonging to quasialuminous-weakly peraluminous and high-K calc-alkaline series. The rock, with enriched LREEs, depleted HREEs and negative Eu anomaly, as well as enriched LILEs (Rb, Th, and K) and depleted HFSEs (Nb, Ta, Ti, and P) and LILEs (Ba, Sr, and La), is identified as a typical low Ba-Sr, highly fractionated I-type granite, formed by partial melting of crustal materials. The LA-ICP-MS zircon U-Pb dating yields a weighted mean age of 143.90 ± 1.20 Ma, indicating the early stage of Early Cretaceous. Combined with regional geological data, the pluton is the product of partial melting of crustal materials under the post-orogenic extensional setting within the Mongolia-Okhotsk Ocean tectonic domain, where the crustal delamination led to lithospheric thinning and upwelling of basic magma to contact with the crust.