LA-ICP-MS zircon U-Pb age of A-type granite from the Shibanjing area of middle Beishan orogenic belt, Inner Mongolia, and its constraint on closure time of Beishan Ocean

The Shibanjing A-type granite intrusion is located in the middle section of the Beishan orogenic belt of Inner Mongolia.Its lithologic composition is dominated by syenogranites, followed by monzogranite.The age of the granite obtained by LA-ICP-MS zircon U-Pb dating is 395.6±4.9 Ma(MSWD=3.6, n=15), suggesting Early Devonian.The rocks belong to the meta-aluminium-weak peraluminous high potassium calc-alkaline series and are characterized by high silicon(SiO₂=72.71%~76.43%), rich alkali(K₂O+Na₂O=7.80%~9.23%), low aluminum(Al₂O₃=12.09%~13.73%), poor magnesium(MgO=0.06%~0.51%), calcium(CaO=0.44%~1.69%)and K₂O > Na₂O.The chondrite-normalized REE patterns of the granite belong to the "seagull" pattern of the right-type, with significant negative Eu anomalies(δ Eu=0.02~0.35, averaging 0.16).The granitic rocks are enriched in high field strength elements(e.g., Zr, Hf, U and Th)and large ion lithophile elements(e.g., K and Rb)but depleted in P, Ti, Ba, Sr.All these characteristics resemble features of A-type granites which originated from the partial melting of lower crust under high temperature conditions and the subsequent fractional crystallization of feldspar, apatite, titanite and some other rocks.The tectonic discriminant diagram indicates that it has the characteristics of A₂ granite, which was formed in the post-collision extension tectonic environment, indicating that the closure time of the Beishan Ocean represented by the Niuquanzi-Xichangjing ophiolite belt was prior to the Early Devonian, and that the structure of the area in the Early Devonian had changed from a compressional system to an extensional system.