Zircon U-Pb dating of the Shijiangshan Late Jurassic-Early Cretaceous A-type granite in Xi Ujimqin Banner of Inner Mongolia and its tectonic setting

Lying along the Hegenshan collisional orogenic suture zone in Xi Ujimqin Banner of Inner Mongolia, the Shijiangshan Atype granite intruded into Early Carboniferous Diyanmiao-Baiyinbulage ophiolite as well as Early Permian Shoushangou Formation and Dashizhai Formation, and consists mainly of monzogranites. The granite is geochemically characterized by high SiO₂ (74.18%~77.16%), K₂O (4.31%~5.07%), high absolute alkali values (Na₂O+K₂O=8.44%~9.16%), low Al₂O₃, CaO, MgO, TiO₂, P₂O₅, Sr, Ba, Eu, Ti, P values, and higher Ga/Al (3.98~6.09), (Na₂O+K₂O)/CaO, K₂O/MgO, TFeO/MgO, Rb/Nb, Y/Nb, Sc/Nb ratios. It is characterized by typical flat or slight right-inclined gull-wing shaped REE distribution patterns with obvious negative Eu anomalies (δEu=0.01~0.19). The Shijiangshan monzogranite exhibits typical geochemical characteristics of A-type granite, being significantly different from I, S and M type granites in geochemistry. According to the chemical subdivision diagrams of the A-type granitoids, the Shijiangshan A-type granite belongs to aluminous A₂-type granitoid, formed and emplaced in a post-orogenic extension setting. LAICP-MS zircon U-Pb dating shows that the ages of the granite are 159.8±1.3Ma, 143.1±1.3Ma and 136.20±0.69Ma, suggesting that it was produced during Late Jurassic to Early Cretaceous and that the Hegenshan suture zone was at the post orogenic extension stage in the Late Jurassic-Early Cretaceous period.