Evolution of Middle-Late Triassic granitic intrusions from the Badaguan Cu-Mo deposit, Inner Mongolia: Constraints from zircon U-Pb dating, geochemistry and Hf isotopes

Abstract

The Great Xing’an Range in the eastern part of the Central Orogenic Belt is one of the major polymetallic metallogenic belts in China. The Badaguan Cu-Mo deposit, in the Erguna massif forms part of this belt and is a typical porphyry deposit associated with multiple granitic intrusions, which are temporally and genetically related to the formation of this deposit. In this paper, we present new zircon U–Pb ages, Hf isotopic data, and whole-rock major and trace element geochemistry on the Badaguan granitoids. Zircon U–Pb data indicate that the intrusions formed during two major phases: ca. 244–237 Ma and ca. 232–229 Ma. The Middle Triassic (ca. 244–237 Ma) phase is represented by a suite of medium-K calc-alkaline granite, K-feldspar granite, biotite granite, and porphyritic granite with I-type affinities. In general, they are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs). Their zircon εHf (t) values and two-stage model ages (TDM2) range from −1.5 to +8.5 (with a mean value of +4.9) and from 732 to 1366 Ma (with a mean value of 959 Ma), respectively. In contrast, the Late Triassic (ca. 232–229 Ma) intrusions dominantly comprise a suite of high-K calc-alkaline I-type granite porphyry and granodiorite porphyry. These rocks are enriched in LILEs and LREEs but depleted in HREEs and HFSEs, such as Nb, Ta and Ti. Their εHf (t) and TDM2 values range from +3.4 to +6.2 (with a mean value of +5.0) and from 865 to 1047 Ma (with a mean value of 942 Ma), respectively. These results suggest that the primary magmas of the Middle and Late Triassic intrusive rocks were co-magmatic and were mainly derived from the partial melting of juvenile thickened lower crust. In addition, the primary magma forming the Middle Triassic complex also assimilated a small amount of ancient crustal material and experienced a remarkable degree of fractional crystallization prior to its emplacement. In contrast, the source magma of the Late Triassic complex probably underwent interaction between of minor mantle-derived melt and crustal components prior to emplacement. The Cu in Badaguan deposit was likely sourced from the mantle-derived melt, whereas the Mo was contributed by the host rocks with high Mo contents.