A chronological study of seamount rocks in the South China Sea basin provides a great opportunity to understand the expansion and evolution history of the sea basin.In this paper,we analyzed the40Ar/39Ar age of trachytic samples collected from the Shuangfeng seamounts in the northwestern sub-basin of the South China Sea.The two samples yielded plateau ages of 23.80 0.18 and 23.29 0.22 Ma,respectively,which indicate magmatic activity in late Oligocene which helpful constraints the expansion time of the northwest sub-basin.Previous studies suggested that the northwestern sub-basin and southwestern sub-basin have experienced a relatively consistent expansion in the NWeSE direction followed by a late expansion of the eastern sub-basin.We concluded that the expansion of the northwestern sub-basin began prior to ca.24 Ma,which also implicated magmatic events of a late or stop expansion of the northwestern sub-basin combined with our results of40Ar/39Ar age data and previous geophysical data.
