SHRIMP U-Pb zircon dating and petrogeochemistral characteristics of the intermediate-acid intrusive rocks in the Aoergai copper deposit of Inner Mongolia

Abstract：The Aoergai copper deposit is located in Xingfuzhilusumu area of Bairin Right Banner in Inner Mongolia, and its tectonic setting is Late Paleozoic accretion orogenic belt of southern Da Hinggan Mountains. The samples collected in this study were intermediate-acid intrusive rocks. Zircon SHRIMP U-Pb dating results show that the granodiorites were formed at 245.4±1.8 Ma （MSWD=1.4）, being the product of magmatism in early Indosinian period. The granodiorites in the Aoergai copper deposit have similar petrogeochemistral characteristics to adakite. The granodiorites yielded the following data: SiO2≥56 wt.%（65.03~70.11 wt.%），Al2O3≥15 wt.%（14.45~15.49 wt.%），MgO＜3 wt.%（1.18~1.61 wt.%），Y＜18μg/g（5.85~6.83μg/g），Yb≤1.9 μg/g（0.416~0.637μg/g），Sr＞400μg/g（380~642 ug/g）, LREE/HREE=11.60~12.24，and δEu=1.00~1.18, indicating that they were formed at a great depth and under strong stress. The granodiorites have high εNd (t) values（+3.9~+4.4）, low (87Sr/86Sr)i values（0.70380~0.70412）, and young Nd model ages (578~657Ma)，suggesting that the magma might have been derived from a depleted mantle source in Neoproterozoic. Based on regional tectonic settings, the authors consider that the granodiorites in the Aoergai copper deposit were most likely derived from partial melting of mafic materials in the depleted mantle in a compression tectonic setting after the disappearance of the Paleo-Asian Ocean. Based on regional analysis, the authors suggest that the tectonic setting of the plutons was an environment of conversion from compression to extension in Indosinian period.