Objective The Early Paleozoic magmatisms in the southern Qinghai−Tibet Plateau are important for researching the nature of the continental margin of the northern margin of the Gondwana continent and the subduction process of the Proto−Tethys Ocean. Based on the reaserch of the Late Cambrian−Early Ordovician rhyolite found in the Zhakang area in the northern part of Xainza County, Lhasa block, this paper discusses the petrogenesis and geological significance of the rhyolite, so as to further constrain the time limit of the Zhakang unconformity in the Xainza area, and provide a basis for understanding the geological evolution process of the northern margin of the Gondwana continent in the Early Paleozoic.

Methods In this study, we report the zircon U−Pb age, rock geochemistry and zircon Hf isotope analysis of rhyolites in Zhakang area, northern Xainza County, Lhasa Block, Qinghai−Tibet Plateau.

Results The results show that the weighted average age of zircon ²⁰⁶Pb/²³⁸U of rhyolite is 485±5 Ma. The rock geochemistry shows high silicon, rich alkali, rich aluminum, low phosphorus and low magnesium. The content of SiO₂ is 75.10%~77.39%, the content of Al₂O₃ is 10.74%~12.90%, the content of K₂O+Na₂O is 6.65%~7.99%, the content of P₂O₅ is 0.03%~0.11%, the content of MgO is 0.27%~0.35% and the A/CNK is 1.20~1.61, which is greater than 1.1. It belongs to a set of strongly peraluminous shoshonite series. The character in rare earth elements of the rhyolite is enrichment of light rare earth elements and relatively flat right−inclined curve of heavy rare earth elements. The fractionation of light and heavy rare earth elements is obvious, accompanied by obvious negative Eu anomalies (δEu=0.44~0.48). The Zhakang rhyolite has high Rb (368.43×10⁻⁶~489.42×10⁻⁶) content, low Zr/Hf (31.97~37.35) and Nb/Ta (12.17~15.32) values, indicating that strong crystallization differentiation occurred during its formation. In the SiO₂−Zr diagram and ACF diagram, the samples fell into the S−type granite area, showing the characteristics of highly differentiated S−type granite. The ε_Hf (t) value of zircon varies from −2.0 to −5.5, with an average of −3.7, which is negative, indicating the rhyolite might be the product of partial melting of the crust. The two−stage model age of Hf isotope is 1581~1752 Ma, indicating that the source area may be Mesoproterozoic crustal material.

Conclusions This study believes that the Late Cambrian−Early Ordovician rhyolite in the Zhakang area may be formed under the tectonic background of the subduction of the proto−Tethys oceanic crust to the northern margin of the Gondwana continent.