Geochronology, Geochemistry, and Hf Isotope of Yanchangbeishan Adamellite of Lenghu Area in Qinghai

The adamellite develops in Yanchangbeishan of Lenghu area in Qinghai, which is located in the west segment of the north margin of Qaidam. The samples have higher SiO₂ (74.98%-76.92%) and K₂O (4.44%-5.93%) contents, but lower MgO (0.04%-0.07%) and CaO (0.43%-0.69%) contents. The Rittmann indices of the rock range in 2.43-2.79, which shows that it belongs to high potassium calc-alkaline series. The alumina saturation average index is 1.01, which indicates that the adamellite belongs to peraluminous granites. Moreover, the samples are intensively depleted in HFSEs (Nb, Ta), with enriched LILEs (K, Rb, Ba, Pb), reflecting the characteristics of island-arc magma. LA-ICP-MS zircon U-Pb dating shows that the formation age of the rock is 252±3 Ma. The zircon Hf isotope analysis reveals that ¹⁷⁶Hf/¹⁷⁷Hf ratios, varying in 0.282 958 to 0.283 032, have high positive ε_Hf(t) (8.75-14.50) and two-stage Hf model ages (averaged at 385 Ma). In the ¹⁷⁶Hf/¹⁷⁷Hf -ε_Hf(t) discrimation diagrams, these zircons are plotted nearly on the depleted mantle evolution line. It can be concluded that the rock chiefly derived from the new basaltic lower crust which is originated from depleted mantle. Based on the regional geological background and the geochemical characteristics of the masses, we consider that Yanchangbeishan adamellite bears similar characteristics of its primary magma with mafic-ultramafic rocks of Devonian, which were supposed to be originated from partial melting of the young basaltic lower crust related to subduction of Zongwulong small oceanic basin from south. Thereby, it reveals that the north margin of Qaidam was in volcanic arc or active continental margin tectonic environment during Permian epoch.