In the Mesozoic basaltic rocks from northern Guangxisouthern Hunan, there are abundant deep-source enclaves, which can be divided into peridotite, gabbro and intermediate-acid gneiss. Based on petrology and geochemistry of these rocks, the present paper indicates that Mesozoic basaltic rocks are products of partial melting of peridotite instead of being related to gabbro and intermediate-acid gneiss which are merely accidental enclaves. It is noteworthy that gabbro and gneiss in these deep-source enclaves show genetic connection of fractional crystallization. They are evolutionary products of underplated basaltic magma, which was intruded between the crust and the mantle in Proterozoic. The gabbro is of the cumulus phase of underplated magma, whereas the gneiss is the product of residual magma after the fractional crystallization of underplated magma.