The Cathaysia Block and the Jiangnan Orogenic Belt in southeastern South China Block are important areas for the distribution of Precambrian basements and Phanerozoic Nb- Ta and other rare- metal deposits. Understanding the relationship between rare- metal mineralization and Precambrian basement composition, and how rare- metal occurrence in basement metamorphic rocks influences Nb- Ta contents and Nb/Ta ratios in magma generated by partial melting are important fundamental scientific problems. This study analyzes the Nb and Ta contents of different minerals in metamorphic rocks from the Cathaysia Block and Jiangnan Orogenic Belt. Combining whole- rock geochemistry, mineral compositions, mineral proportions, metamorphic temperature, and partial melting modeling, we identify the key factors controlling the Nb- Ta contents and variations in biotite, muscovite, and other metamorphic minerals. We also discuss the influence of melted minerals on Nb- Ta contents in melts under different conditions. Our results show that biotite is the most Nb- Ta- enriched rock- forming mineral in the metamorphic rocks of South China, with mean Nb and Ta contents of 64. 1×10-6 and 4. 93×10-6, respectively, exhibiting similar compatibility between Nb and Ta. Muscovite is another Nb- Ta- rich metamorphic mineral, albeit with slightly lower concentrations than biotite. Under subsolidus conditions, Nb is more compatible than Ta in muscovite, resulting in a high Nb/Ta ratio of 16. 9, which makes it a potential reservoir of high Nb/Ta. Pyroxene, amphibole, garnet, and feldspar have low Nb and Ta contents and make little effect on the Nb- Ta enrichment and differentiation in the system. The Nb- Ta contents and Nb/Ta ratios of biotite and muscovite, as well as their partition coefficients with bulk rocks, are mainly controlled by host- rock composition, mineral assemblage, mineral composition, and metamorphic temperature. Nb- Ta contents and partition coefficients of biotite and muscovite show positive correlations with metamorphic temperature and inverse correlations with mineral proportions, reflecting the modal abundance effect. During partial melting and melt extraction, the Nb- Ta contents in mica sharply decrease due to low Nb- Ta contents in the rocks and changes in mineral compositions and partition coefficients. In subsolidus conditions, these changes are not controlled by those factors but rather by the degree of melting, i. e. , the “melting effect.” Based on partial melting modeling of a two- mica schist from the eastern Nanling region, we conclude that Nb- Ta enrichment and differentiation in melts mainly depend on source compositions, melted mineral assemblage, oxygen fugacity, and melting degree. High oxygen fugacity and pressure effectively promote Nb- Ta enrichment in melts. Modeling results suggest that at pressure of 0. 6 GPa and FMQ+2 oxygen fugacity, partial melting may produce melts with Nb- Ta contents up to 45. 1×10-6 and 3. 44×10-6, respectively, 2. 65 times higher than the source. Fractional crystallization modeling indicates that normal crystallization differentiation cannot lead to significant Nb- Ta enrichment in residual melts. Extreme differentiation, even reaching the fluid- rich stage, must be needed to promote significant Nb- Ta enrichment.