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 εNd(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. (author)