Abstract: Two basalt samples from the Gajinxueshan Group were dated by using single zircon U Pb isotope dilution method. The two lower intercept points yield age of 362±8 Ma and 296±7 Ma respectively, whereas the two upper intercepts yield 2 525±12 Ma and 1387±59 Ma respectively. The former are interpreted to date the formation age of the basalt, suggesting that the Jinshajiang ocean opened in the early Carboniferous. The latter represent the age of the underlying basement, implying a Proterozoic Yangtze type continental crust at depth. All these indicate that the Jinshajiang ocean was formed by rifting on the margin of the Yangtze continent.