CENOZOIC MAGMATIC ACTIVITIES AND STAGES IN THE BAIYUN SAG, NORTHERN MARGIN OF SOUTH CHINA SEA

The Baiyun Sag is a second-order depression of the Pearl River Mouth Basin located on the northern margin of the South China Sea. Since Cenozoic, it has suffered strong magmatic activities characterized by both intensive eruption and intrusion. By integration of geological and seismic data, this paper summarized the seismic features of different types of magmatic bodies and some Cenozoic magmatic bodies were distinguished from the others. Spatial distribution pattern of the magmatic bodies was mapped. It is found that the Cenozoic magmatic bodies are mainly distributed on uplifts or higher parts around the Baiyun sag, especially on the south side. Magmatic bodies are few within the sag. Using the traditional classification of magmatic stages based upon contact relationship, thickness of sediment cover and stratigraphic correlation, the Cenozoic magmatic activities in the Baiyun sag might be grouped into four periods: 32~23.8 Ma, 23.8~10.5 Ma, 10.5~5.5 Ma, and 5.5 Ma up to present. Spatial distribution maps are compiled for each period. The first period corresponds to the transforming stage from faulting to depression. The next three, however, are all within the stage of depression. The magmatic activities weakened with time.