According to the principles of thermodynamics, partial melting is a from of phase transition. The protolith will melt when it moves across the solidus along the p-T path. The incongruent partial melting, which is a usual behavior for the natural rocks, can leave a residuum with different bulk composition and mineral assemblage (lithology) from the protolith. Therefore, the residuum can be different from the protolith in "metamorphic facies" and lithology. The "C-type adakites" contain hydrous minerals such as hornblende and biotite, and they cannot be generated by dry melting. The experimental petrology studies show that the mafic (SiO₂ < 50%) system is much easier than intermediate or acidic systems in generating the "eclogitic" residuum during the dehydration melting under the condition p=1.0～2.0 GPa. Meanwhile, the major oxides other than silica in the protolith also control the portion of minerals in residuum and, subsequently, influence the content of Sr, Y and HREE in the melt. Accordingly, the geochemical signal of high Sr and low HREE for the "C-type adakite" is determined by multiple factors such as the p-T condition of melting, the major oxide composition and the trace element content of the protolith. The melts produced from mafic or intermediate-mafic protolith with high potash content (K₂O≈1.0%) contain too much aluminum or sodium, and differ significantly from the "C-type adakite" in composition. However, the mafic protolith with moderate potash content can generate high-silica "C-type adakite" with low degree of melting, but fails to produce the intermediate "C-type adakite".