Geochemical and Nd-Sr-Pb-O isotopic constrains on Permo-Triassic magmatism in eastern Qaidam Basin, northern Qinghai-Tibetan plateau: Implications for the evolution of the Paleo-Tethys

Eastern Qaidam Basin of the northern Qinghai-Tibetan plateau is located in a transitional zone between the Permo-Triassic Paleo-Tethyan orogenic belt in the south and the early Paleozoic Qilian orogenic belt in the north. Here we present geochemical and Sr-Nd-Pb-O isotopic data for the Permo-Triassic plutons in eastern Qaidam Basin. Bulk-rock geochemical data and regional geological studies indicate that these plutons consist mainly of subduction-related high-K calc-alkaline metaluminous, I-type granitoids, which occurred during the northward subduction of the Paleo-Tethyan oceanic lithosphere below the southern continental margin of the Kunlun-Qaidam terrane. The εNd(t) values of these Permo-Triassic granitoids are between -9.4 and -3.0, and εSr(t) values are from -20.33 to +168.20. Nd isotopic compositions indicate that the granitoids can come from a pre-existed materials formerly originated from an enriched mantle (EM II) source. The T_DM2 model ages of 1.28-1.78Ga implies that the arc-induced Triassic granitoids were derived melts of Meso-Proterozoic basement rocks of the Kunlun-Qaidam terrane that is bounded by the early Paleozoic Qilian suture zone to the north and the Triassic Kunlun suture zone in the south. The Permo-Triassic granitoids yield initial ratios of ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb values from 18.295 to 19.096, 15.617 to 15.692, and 37.960 to 38.531, respectively. The Pb isotope composition of the granitoids is very similar to that of the Mesozoic granitoids from the western segment of the east of the study area. Geochemical analyses of the plutons, integrated with previous LA ICP-MS U-Pb zircon dating, reveal two series of Permo-Triassic arc magmatisms in eastern Qaidam Basin. Both the series of magmatism display reversed trends with the classic Bowen's reaction series. The new geochemical evidence suggest that the arc magmatism in eastern Qaidam Basin was induced by fluid-fluxing melting of an enriched lithospheric mantle and rock composition is best explained by mixing of mantle-derived mafic magma mixed with felsic rocks of the Qaidam basement.