Petrogenesis of two stages of mafic dykes and associated granites in the Kunlunhe area, East Kunlun orogen, China: implications for Permian-Triassic tectonic evolution

The East Kunlun Orogen (EKO), a core segment of the Central Orogenic Belt in the northeastern Tibet Plateau, experienced pivotal Palaeozoic-Mesozoic tectonic evolution closely linked to the closure of the Palaeotethys Ocean and subsequent continental collision. The Kunlunhe area within the EKO preserves well two stages of mafic dykes and associated I-type granites, making it an ideal natural laboratory to trace regional geodynamic transitions. This study presents systematic zircon U-Pb geochronology, whole-rock geochemistry, and Lu-Hf/Sr-Nd isotope data for these magmatic rocks. Zircon U-Pb dating yields three groups of coeval ages: Permian mafic dykes (255.7 ± 1.7 Ma, 257.2 ± 1.3 Ma) and their associated granites (257.1 ± 1.9 Ma, 259.0 ± 1.3 Ma); and Late Triassic mafic dykes (224.8 ± 2.6 Ma) with coeval granites (225.0 ± 1.9 Ma). Geochemically, the Permian mafic dykes exhibit subduction-related signatures, including positive Eu anomalies (Eu/Eu*=1.12–1.20), high initial 87Sr/86Sr ratios (0.71042–0.71176), and low εNd(t) values (−7.34 to −3.03), indicating derivation from a mantle wedge modified by subduction fluids and subsequent crustal contamination. The coeval Permian granites are high-K calc-alkaline I-type rocks, formed by crustal melting with contributions from the enriched mantle. In contrast, the Triassic mafic dykes show primitive mantle-like geochemical features and relatively higher εHf(t) values, while their associated granites display mantle-like isotopic compositions. Correlations between Nb/Ta-Zr/Hf ratios and spatial variations in εHf(t) further confirm a slab breakoff event. Combined with regional tectonic context, these results demonstrate that the Permian magmatism (ca. 255–259 Ma) was related to the subduction of the A’nyemaqen Ocean, whereas the Triassic magmatism (ca. 225 Ma) resulted from asthenospheric upwelling triggered by Middle Triassic (ca. 240 Ma) slab breakoff. Collectively, the two-stage magmatic rocks recorded the tectonic transition of the EKO from oceanic subduction to post-collisional extension, providing critical insights into the evolution of the Palaeotethys Ocean and continental orogenic processes.