LA-ICP-MS zircon U-Pb age and geochemistry of the Early Permian Halinudeng granite in northern Alxa area, western Inner Mongolia

The Halinudeng pluton is located in the northern margin of Alxa area. LA-ICP-MS U-Pb dating of zircons from the pluton yields an age of 284.0±1.8 Ma , indicating that the formation of this pluton is Permian instead of Silurian as previous considered. The main rock types of the pluton include biotite-plagioclase granites and biotite monzogranites, with high SiO2(67%-72%), CaO (2.2%-4.4%) and Al2O3 (14.5%-17.2%). Geochemical characteristics indicate that they are metaluminous-weakly peraluminous (A/CNK=0.90-1.07) and calc-alkaline to high-K calc-alkaline rocks (Na2O=3.7%-4.3%，K2O=2.1%-2.9%), assuming characteristics of I-type granite. The rocks have lower REE contents(35.2×10-6-130.6×10-6) with relative enrichment of LREE and positive Eu anomaly (δEu=1.04-1.90). The primitive mantle normalized trace element patterns are characterized by the enrichment of the LILE(K, Rb, Ba, Sr) and depletion of the HFSE (Ta, Nb, P, Yb, Y) , showing the features of arc granites. It is considered that the northern margin of Alxa might still have had some arc-related magmatic activities during the Early Permian period.