Geochronology and geochemistry of the Aolansandui pluton in Horqin Right Wing Middle Banner of Inner Mongolia

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.