Abstract: Geochronology and geochemistry of the Aolansandui pluton in Horqin Right Wing Middle Banner of Inner Mongolia were studied. Zircon U-Pb dating demonstrates that the weighted mean ²⁰⁶Pb/²³⁸U age for the zircons from the syenogranite is 220±1Ma, suggesting Late Triassic. The syenogranite belongs to low-Sr and high-Yb type granitoids. The syenogranite is characterized by high SiO₂, high alkali, high TFeO/MgO ratios, and low CaO, low MgO and lower TiO₂ values. The syenogranite is characterized by "sea-gull" REE patterns and significant negative Eu anomaly as well as depletion of Sr and Ba and enrichment of Yb, Zr and Y. The high Rb/Sr and Rb/Nb ratios of the rock indicate a crustal origin. Comprehensive analysis shows that the syenitegranite was formed by low pressure felsic crust partial melting. In (Y+Nb)-Rb, (Yb+Ta)-Rb, Nb-Y-Ce diagrams combined with regional tec-tonic evolution, the authors hold that the syenogranite was formed in a post-orogenic setting. In the Middle Late Triassic period, the regional tectonic regime in Horqin Right Wing Middle Banner of Inner Mongolia underwent significant changes, and experienced as-thenosphere upwelling after squeezing into the orogenic lithosphere and thinning.