The Limu granites in Guangxi Province, rich in tin—tungsten deposit and tantalum—niobium deposit, are typical rareelement granites in South China. There are some puzzling occurrences of silicate rocks that are intimately coexisted and show contrasting textures in the granites. On the basis of petrography study, we apply thermodynamic analysis to these abnormal phenomena to understand the evolution of magma. The results constitute the chain of evidence of magmatic evolution, verify with each other and correspond to the results of tests from thermodynamic equation. The study result illustrated that the liquid—gas fractionation is the most important mechanism in the formation of silicate rocks and deposit. As waterrich magma moves up to emplace, large scale liquid—gas fractionation takes place, the liquid separates into a new liquid phase and bubbles. Rapid reduction of P(H2O) in the liquid system raises the solidus and liquidus temperatures abruptly, and causes the new liquid phase crystallizing quickly to form porphyritic texture rock. Some of the bubbles (vesiculation) rise to the top and upper parts of the magma chamber, and the others escape from the magma into the overlying rock, then they burst and separate into residual vapor and residual liquid phases. Later, the residual vapor can cause autometasomatism of granite, or crystallize to form pegmatite. The residual vapor is the main carrier transporting oreforming metals too. The mineralization of altered granitetype tin—niobium—tantalum deposit is related to the autometasomatism of granite led by residual vapor, and the mineralization of veinstype tin—tungsten deposit is related to the crystallization of residual vapor in voids