Geochemistry of basalts of late period of Early Cretaceous from Jiangshan-Guangfeng, SE China and its tectonic significance
YU Xinqi1, SHU Liangshu1, YAN Tiezeng2, YU Yunwen2, ZU Fuping1, WANG Bin11. Department of Earth Sciences, Nanjing University, Nanjing 210093, China; 2. Zhejiang Institute of Geological Survey, Hangzhou 512026, China
The Xinjiang basin and JinhuaQuzhou basin, originating from late period of Early Cretaceous, are all the components of the NEEtrend Ganhang Fault Basin Zone of the Southeast China, and the JiangshanGuangfeng area is located in east end of the Xinjiang basin. Erupted basalts in this area have 98～105 Ma KAr isotopic age, and their geochemical characteristics show that these basalts are divided into two types: tholeiite and alkaline olivine basalt. The NdSr isotopic features show that all of these basalts have the trend of forming from enriched mantle source, and that the mantle nonequality mixing with the crustal materials has caused the difference. The tholeiites from Chentangwu area, Jiangshan City, have medium 87Sr/86Sr ratio (0.705 4～0.707 0) and lower εNd(t) (1.69～0.70), indicating that the magma of these basalts came from an unchanged mantle, they spreaded large area of the basin by extending on the continental margin. Those alkaline olivine basalts from Lixian Village and Guangfeng Town have relative high 87Sr/86Sr ratio (0.707 6～0.708 0) and relatively lower value of εNd(t) (-6.43～-5.18), indicating that these magma came from an enriched mantle by mixing the subducted crustal material, and they erupted and covered middle part of the basin. The results show that the formation and the evolution of the basalts of the Ganhang Fault Basin are intimately related with the subduction of the Pacific Plate during late period of Early Cretaceous to Late Cretaceous. The subduction angle of Pacific plate increased from a very low angle to a median angle and the subductive boundary migrated oceanward to the southeast, resulting in the extension of the SE China continental margin and all the phenomenons above.