Zircon U-Pb Ages, Sr-Nd Isotopes and Geological Significance of Early Jurassic Volcanic Rocks from Southwest Fujian

An Early Mesozoic volcanic belt with nearly EW distribution develops in South Hunan, Southeast Hunan, South Jiangxi and Southwest Fujian of the South China continental margin, which is represented by the volcanic rocks of Fankeng Formation from Tangpu and Wuhu villages of Yongding County in the Southwest Fujian. The zircon U-Pb dating results indicate that the volcanic rocks of Fankeng Formation formed in 189±6 Ma-195±2 Ma and 184±2 Ma, i.e., the Early Jurassic. In order to reveal the magma source and tectonic setting of the Early Jurassic volcanic rocks in the Southwest Fujian, the lithology identification, major and trace elements, and Sr-Nd isotope data are presented in this paper. The Daly gap of the basic end-member and the acid end-member of volcanic rocks of Fankeng Formation reaches to about 20, and they therefore constitute a typical bimodal association of volcanic rocks. The basic end-member basalt (tephrite) has high TiO2 and TFe2O3 (all Fe) values, and high ratios of (Rb/Sr)N, (La/Nb)N, (Ba/Nb)N and (Rb/Yb)N ratios, which are all higher than the original mantle values. They are also characterized by the higher ∑REE, enrichment in light rare earth elements, and negative anomalies of Eu. Their Isr values are between 0.704 4 and 0.707 5 and the εNd(t) are between -1.19 and 4.30, respectively, indicating that the magma source was derived from the deeper asthenosphere mantle and some components were derived from the lithosphere mantle. The (Rb/Sr)N and (Rb/Yb)N ratios of acid end-member rhyolite (dacite) are all higher than the initial mantle values. However, the (Ba/Nb)N ratios are all less than the initial mantle values and the (La/Nb)N ratios are both high and low. They are characterized by the high ∑REE, enrichment in light rare earth elements, and obvious anomalies of Eu. Their Isr values are between 0.701 2 and 0.705 3 and the εNd(t) are between -5.40 and -1.95, respectively, indicating that the magma source is the crust-mantle misciblemagma, which is mainly composed of the crust-derived materials with involvement of the mantle-derived components. The Early Jurassic bimodal volcanic rocks in Southwest Fujian were formed in the intraplate with a special tectonic position near the continental margin. They were influenced by the effect of low angle subduction of paleo-Pacific plate-back arc extension-basaltic magma underplating. © 2019, Editorial Department of Earth Science. All right reserved.