The latest tectonic magmatism in the Buqingshan–A’nyemaqen tectonic mélange belt: evidence from zircon U–Pb geochronology of intermediate–basic dikes, northern Tibetan Plateau, China

This study focuses on the zircon U–Pb geochronology and geochemistry of intermediate–basic dikes from the Buqingshan–A’nyemaqen tectonic mélange belt (BTMB) along the southern margin of the East Kunlun orogenic belt (EKOB). Zircons from a diorite dike show oscillatory zoning and relatively high Th/U ratios (0.47–2.43), indicating that they are of magmatic origin. Using LA–ICP–MS, zircons of the diorite dike yield a U–Pb age of 205 ± 1 Ma (MSWD = 0.88), implying that the diorite dikes were formed in the Late Triassic (Rhaetian) and also represented the latest tectonic magmatism in the BTMB. Geochemical analyses show that the rocks have low SiO2 (51.96–59.33 wt%), low Al2O3 (10.49–13.95 wt%), and low alkaline (4.00–5.29 wt.%), and thus belong to the subalkaline magma series. The contents of rare earth elements (REEs) are 80.23–189.19 ppm, with weakly negative to weakly positive Eu anomalies (δEu = 0.50–1.10). The trace element geochemistry is characterized by negative anomalies of Nb, Hf, P, Ti, and Sr and by positive anomalies of Th, La, Nd, Sm, Zr, and Eu. The diorite dikes, the product of a mafic magma formed at high temperature (~ 777 °C), were derived by partial melting of the mantle with possible admixture of crustal material. The intermediate–basic dikes in the BTMB are the products of mantle enriched upward and emplaced along tensional faults in the crustal–relaxation stage after the subduction–collision of the Bayan Har and East Kunlun Blocks during the Late Hercynian–Early Triassic.