ZIRCON U-Pb AGES AND GEOCHEMICAL CHARACTERISTICS OF THE HAMAERWULA PLUTON IN INNER MONGOLIA

The Hamaerwula granite-porphyry, outcropping in the eastern slope of the middle section of Daxinganling Mountains, was formed in Early Cretaceous. The U -Pb isochron dating of zircon shows that the weighted average of apparent age is 124.9±2.5 Ma, representing the intrusion era of Hamaerwula pluton. The chemical composition of the rock reveals high SiO₂ (71.262%-77.61%, averagely 74.48%), rich alkali (6.32%-8.77% of K₂O+Na₂O, averagely 7.46%), high K₂O/Na₂O ratio (1.1-2.2), rich Al and scarce Ca, Fe and Mg, belonging to high-K calc-alkaline series. The trace elements show enrichment in Sr, Rb, Th and Ce and relative deficiency of Zr, Hf, Sm, Y and Yb. The rare earth elements distribution curves tend right. The light rare earth elements are concentrated with well fractionation and obvious Eu negative anomaly. The heavy rare earth elements are relatively deficient with poor fractionation. Combining with structure formation background, it is considered that the Hamaerwula granite-porphyry is granitoid pluton of late orogenic epoch.