The Zircon LA-ICP-MS U-Pb dating and geochemistry of the Yagan intermediate-acid volcanics in the northern margin of the Alxa block: Constraints on regional tectonic evolution

The northern Alxa block is the key area for studying the closing of the Paleo-Asian Ocean, Owing to the Permian Volcanics have been rarely investigated in the Yagan area, Leading to the uncertainty tectonic evolution of Yagan Fault in Paleozoic, and importartly restriction for the time of the closing of the Paleo⁃Asian Ocean in northern Alxa block. In this paper, we carried out a systematic study of the petrography, geochemistry, zircon U-Pb dating for the intermediate-acid volcanic, which are located in the south of the Yagan Fault. The intermediate-acid volcanics in the Yagan region have mainly composed of andesite, rhyolite, dacite, rhyolitic crystal tuff and dacitic crystal tuff. The zircon LA-MC-ICP-MS U-Pb dating results show that the rhyolite and dacite are formed during the Early Permian in 278±2.2Ma, 279 ±2.4 Ma, 283±1.4Ma.The Yagan intermediate-acid volcanics are characterized by high K calc-alkaline or calc-alkaline. are relatively enriched in large in lithophile elements, such as Rb, U, Th, and depletions in high field strength elements, such as Nb, Ta, P and Ti. They show right-leaning chondrite-normalized patterns, which means enrichment in LREE and depletions in HREE, and exhibit relatively moderate-strong negative Eu anomalies (average value of δEu=0.42~0.67). Based on the formation age and geochemical characteristics, it is concluded that the The Yagan intermediate-acid volcanics are likely formed in a continental margin arc setting, and products of the subduction of the paleo-Asian ocean during Early Permian. Combined with regional geological background, it is inferred that the closure of the paleo-Asian ocean in the northern margin of the Alxa block should postdate the late period of Early Permian.