Petrogenesis of Middle Triassic Mafic Enclaves and Host Granodiorite in the Eastern Kunlun Orogenic Belt, NW China: Implications for Continental Crust Growth in Syn-Collisional Setting

The mechanism of the Triassic continental crust growth in the East Kunlun Orogenic Belt (EKOB) is highly controversial. In this contribution, we present comprehensive data on the Yuegelu granodiorite and its mafic microgranular enclaves (MMEs) from the eastern segment of the EKOB, including zircon U–Pb geochronology and Hf isotope, mineral chemistry, whole-rock geochemistry and Sr–Nd isotopes, and in situ plagioclase Sr isotope, to constrain the genesis of these rocks and shed new insights on the continental crust growth. The MMEs are coeval with the host granodiorite at ∼240 Ma and display abundant quenching textures such as acicular apatites and quartz phenocrysts rimmed by amphibole. They typically exhibit low SiO₂ but high TiO₂, Fe₂O₃^T, MnO, and MgO concentrations and have similar Sr–Nd–Hf isotopic compositions to the host rock. We suggest that the Yuegelu MMEs are late cognate cumulates derived from the same parental magma with the host rock as a result of pressure quenching rather than the hybrids of crustal and mantle magmas. The granodiorite is medium- to high-K calc-alkaline, metaluminous I-type granite with relatively high SiO₂, but low Al₂O₃, CaO, and Fe₂O₃^T contents. The granodiorite is enriched in Rb, K, and Pb but depleted in Nb, Ta, Sr, P, and Ti, resembling a bulk continental crust. However, Yuegelu granodiorite has much more depleted Sr–Nd–Hf isotopic compositions than those of the mature crustal materials, indicating a significant mantle contribution. Based on Sr–Nd isotopic modeling, the Yuegelu granodiorite could be generated by partial melting of the Paleo-Tethys Oceanic crust (∼80%) with the overlying terrigenous sediments (∼20%). The partial melting of oceanic crust fragments in the syn-collisional setting in the Middle Triassic contributed substantially to the continental crust growth in the EKOB.