Petrogenesis and tectonic setting of late Permian–Early Triassic granitic rocks in the western Qaidam Basin, Qinghai–Tibetan Plateau

Abstract

Occurring after the early Palaeozoic collisional orogeny and continental deep subduction, late Palaeozoic tectonic magmatism in the northern Qaidam Tectonic Belt (NQTB) related to the Palaeo-Tethys Ocean has become a critical issue. We performed a detailed geochronological and geochemical study of Lenghu (LH) granitic rocks in the western NQTB to obtain better constraints on the subduction processes of the Palaeo-Tethys Ocean in the northern Qinghai–Tibetan Plateau. LA–ICP–MS U–Pb dating of zircons from LH granitic rocks in the western NQTB yields emplacement ages of 253 ± 4 Ma and 264 ± 4 Ma, indicating a two-stage emplacement process. Geochemical analyses show that both the late Permian and Early Triassic granitic rocks have low A/CNK and A/NK values and are enriched in LILEs (Rb, Th, Ba, Zr, and Hf) and depleted in HFSEs (Sr, P, Ti, Nd, and Ta). The SREE contents range from 36 to 189 ppm, and the samples are enriched in LREEs (LREE/HREE =1.5–7.3). Values of (La/Yb)N range between 0.9 and 6.5, and all samples show negative Eu anomalies (δEu =0.2–0.6), with typical characteristics of island arc magmatism. The two-stage granitic rocks have εHf(t) values of -3.7 to +8.0 and yield a two-stage depleted mantle Hf model age of 1.51-0.78 Ga. The data suggest that the late Permian–Early Triassic granitic rocks were derived from partial melting of Proterozoic crust-related mafic to intermediate rocks with significant crust–mantle interactions. The results, combined with regional magmatic information, indicate that the South Kunlun Oceanic plate might have experienced plate rollback in the late Permian and plate retraction in the Triassic under the Kunlun–Qaidam Block.