This paper presents an investigation of the petrology and mineral assemblage of altered basalts from the lower part of Hole U1502B of the International Ocean Discovery Program (IODP), which was drilled from the Continent-Ocean Transition Zone in the northern margin of the South China Sea. According to the altered minerals and their proportions, the studied samples were subdivided into the slightly (<50% altered minerals), moderately (50%–80% altered minerals), and heavily (>80% altered minerals) altered basalts. There was a significant decrease in the CaO content of the altered basalts as the degree of alteration increased. The altered minerals in the slightly and moderately altered basalts were mainly Fe-rich chlorite, albite, and quartz with minor epidote, carbonate, and actinolite, which should have experienced high-temperature hydrothermal interactions mainly at 280–340 ℃. The altered minerals of the heavily altered basalts were mainly albite, quartz, kaolinite, and Fe-Mg oxide hydroxide with minor Fe-rich chlorite, clay minerals, and pyrite. Thus, the heavily altered basalts should have experienced high- to medium-temperature hydrothermal interactions, which were subsequently impacted by a low-temperature alteration. High-temperature hydrothermal interaction should occur during the formation of the mid-ocean ridge under the high-temperature and low water/rock environment due to the influence of heat extracted from magma lenses beneath the ridge axes. However, the low-temperature alteration in a high water/rock environment can be attributed to later strong tectonic activities in the local area. The basalts from Holes U1502B and U1500B, located in a narrow transition zone between the continent and ocean in the northern margin of the South China Sea (SCS), show remarkably different structures with distinct hydrothermal reactions and alterations. This indicates a rapid tectonic transition from the initial rupture of the continental margin to the subsequent spreading of the mid-ocean ridge in the South China Sea.