Petrogenesis of early Permian granitic dykes in the Wulanhuduge area, central Inner Mongolia, North China: constraints from geochronology, geochemistry, and Sr–Nd–Pb isotopes

Early Permian granitic dykes are well developed in the Wulanhuduge area, central Inner Mongolia, North China. In this study, we investigated the petrography, geochronology, and whole-rock geochemistry of the granite porphyry dykes in the Wulanhuduge area. Laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating yielded ²⁰⁶Pb/²³⁸U ages of 289–288 Ma for these granite porphyry dykes, indicating they were emplaced in the early Permian. These granitic dykes are high in silica and alkali contents, and low in total Fe₂O₃, MgO, CaO, and P₂O₅ contents. They show enrichment in large-ion lithophile elements such as Rb, Ba, Th, U and K, and depletion in high field strength elements such as Nb, Ta, and Ti, typical of arc-like magma. Their Sr–Nd–Pb isotopic compositions indicate low initial ⁸⁷Sr/⁸⁶Sr ratios (0.70306–0.70564), positive ε_Nd(t) values (+3.3 to +3.9), and radiogenic Pb isotopes with (²⁰⁶Pb/²⁰⁴Pb)_i of 18.080–18.616, (²⁰⁷Pb/²⁰⁴Pb)_i of 15.497–15.555, and (²⁰⁸Pb/²⁰⁴Pb)_i of 37.713–38.175. These geochemical data, along with petrological characteristics, suggest that they belong to high K calc-alkaline I-type granites and were generated by the partial melting of the mafic rocks from the pre-existing juvenile arc crust in a post-subduction extensional setting caused by slab breakoff. Therefore, the emplacement of these granite porphyry dykes in the Wulanhuduge area may represent the end stage of the subduction–accretion process in central Inner Mongolia.