LA-ICP-MS zircon U-Pb dating and tectonic setting of the monzogranites in the Fengleishan area of Beishan orogenic belt, Inner Mongolia

Monzogranites in Fengleishan are located in northern Beishan and situated between two ophiolitic mélange belts of Hongshishan-Baiheshan-Pengboshan and Jijitaizi-Xiaohuangshan, having a special tectonic affinity. Zircons from the monzogranites yielded a weighted mean LA-ICP-MS U-Pb age of (320±1) Ma, representing the intrusion age. The samples are characterized by high content of SiO₂ and Al₂O₃ with Rittmann indexes of 1.58-2.74 (< 3.3) and alkaline ratios of 2.35-4.05, belonging to calc-alkaline granitoids. Their A/NCK values range from 0.92 to 1.15 and display metaaluminous to peraalmuinous affinities. They have relatively high content of total REE and are characterized by LREE-enriched patterns ((La/Yb)_N=4.60-8.83)and negative Eu anomalies (δEu=0.56-0.88, 0.72 in average). They also show strong enrichment of incompatible elements (e.g., Th and U) and large ion lithophile elements (e.g., Rb and K) but depletion of high field strength elements (e.g., Nb, Ta, Ti and P). Their normalized patterns are similar to those of upper continental crust (UPC), indicating participation of materials from the UPC in their magma source. High values of Th (8.58-21.62 μg/g) and low ratios of Nb/Ta (8.75-15.72) point to a crust-dominated magma source. On the tectonic discriminant diagrams of Rb-(Nb+Y) and R₁-R₂, most of the samples are plotted in the pre-collisional volcanic arc granite area. Based on associated lithological characteristics, it is proposed that monzogranites in the Fengleishan were formed by pre-collisional volcanic arc magmatism, which was related to oceanic crust subduction in the Hongshishan-Baiheshan-Pengboshan belt. Combined with regional geological background, it is inferred that the closure of the small ocean in the Hongshishan-Baiheshan-Pengboshan belt should postdate the early period of late Carboniferous.