Geochemical Characteristics and Tectonic Implications of Nayixiong Formation Basalts in Eastern Qiangtang Basin, Tibet

Li Xueren; Wang Jian; Wan Youli; Fu Xiugen

Abstract：The Nayixiong Formation basalts in the eastern Qiangtang basin were the eruption products of the last stage of rift evolution,and their diagenetic age and geochemical characteristics provided important constraints for the closure time of the rift and the tectonic evolution of the Permian.Based on the profile geological survey,LA-ICP-MS zircon U-Pb dating and whole rock analysis of the Nayixiong Formation basalts were carried out in this study.Nayixiong Formation basalt samples yield the concordant age with a weighted mean 206 pb/238U age of 257.2±2.9 Ma.The geochemical characteristics of the basalts show a tholeiitic basalt affinity,with slight enrichment of Ta and slight negative Nb anomalies,as well as exhibiting no Eu-anomalies.The Nayixiong Formation basalts were likely resulted from the interaction between the upwelling asthenosphere and the lithospheric mantle that formed by the predated underplating.we propose that the basalts formed in an extensional setting after the closure of the Permian rift.All of the Permian basalts geochemical data show a transitional trend that the Early Permian-Late Permian basalts gradually evolved from OIB continental basalt to volcanic arc basalt,demonstating that the Permian rift undergoing the opening,rifting and closing from Early Permian to Late Permian in eastern Qiangtang terrane.The Nayixiong Formation basalts formed in an extensional setting after the amalgamation of intra-plate rift,confirming that the Qiangtang terrane was a tectonic transitional phase experiencing extension and closure of intra-plate rift (limited ocean) in the Late Permian.