Zircon U-Pb age, Hf isotope, and geochemistry of Late Permian to Triassic igneous rocks from the Jiapigou gold ore belt, NE China: Petrogenesis and tectonic implications

The Jiapigou gold ore belt is located in the northeastern corner of the wedge-shaped North China Craton and is hosted by Neoarchean basement gneiss and amphibolite, along with late Palaeozoic and Mesozoic granitoids. In this paper, whole-rock major and trace element geochemistry together with zircon U-Pb ages and Lu-Hf isotope compositions are reported for Late Permian to Triassic igneous rocks from the gold belt. The dominant population of zircons in all these rocks shows oscillatory growth zoning in CL images, and relatively high Th/U ratios (0.10-5.23) that are indicative of a magmatic origin. Zircon U-Pb data indicate that the igneous rocks formed during two major phases: ca. 252 Ma and ca. 230-213 Ma. Geochemically, the Late Permian (ca. 252 Ma) diorites are high-K calc-alkaline in composition and low total Fe2O3, MgO, and CaO contents, and represented by enrichment in LILEs relative to HFSEs and HREEs, all of which are indicative of a postcollision high-K calc-alkaline granite affinity. In contrast, the Late Triassic (ca. 230-213 Ma) igneous rocks are mainly quartz syenite porphyries, felsites, and granodiorites, and are variable medium- to high-K calc-alkaline series and exhibit a negative Eu anomaly with A-type affinities, indicating an extensional environment. In addition, in situ Hf isotopic analyses of zircons from four dated samples reveal that they have epsilon(Hf)(t) values of -3.0 to +2.2, with two-stage model (T-DM2) ages range from 964 to 1,210 Ma, indicating that they probably originated from the partial melting of a dominantly Mesoproterozoic to Neoproterozoic crustal source. On the basis of the geochemical data and regional geological investigations, we further propose that the Jiapigou gold ore belt in the Late Permian was formed in a postcollisional tectonic setting, whereas an extensional tectonic setting in the Late Triassic that caused lithospheric thinning during a postorogenic event.