Based on a study of Cenozoic basalts in southwest Japan where mantle upwelling is considered to have occurred, the degree of melting for primary magmas and weight fractions of residuals phases are quantitatively estimated using mass balances for major and compatible components among magma, residual phases and source. Assemblage and composition of the residual phases are obtained from melting experiments of primary magmas, and various source compositions were assumed. The calculation shows an increase of degree of melting toward the upwelling center from approximately 4 to 20%. This is consistent with a change in the residue from a spinel lherzolite to a harzburgite, which was determined by melting experiments. We also obtain constraints on the source composition. Peridotite with more than 3.0 wt % Al2O3 and 2.0 wt % CaO and a high Al2O3/CaO ratio of approximately 1.5 is suitable as a source. Incompatible element concentrations of the source for Cenozoic basalts in southwest Japan are calculated assuming a batch melting model. At the center of upwelling, the source is anomalously enriched in incompatible elements (up to 200 times compared with primitive mantle model) excepting Nb, while that of upwelling margin has primitive or slightly enriched concentrations. These variations can be explained by addition of fluid (flux melting) and subsequent upwelling of enriched material from the deep mantle (e.g., lower-upper mantle boundary or core-mantle boundary). The flux melting model also explains the low potential temperature of approximately 1300 ¿C of the upwelling mantle. Cenozoic volcanism in southwest Japan may have been caused with a plume with abundant volatile and incompatible components rather than a plume of anomalously high temperature (e.g., Hawaii). ¿ American Geophysical Union 1992