Abstract:The Cathaysia Block and the Jiangnan Orogenic Belt in the southeastern part of the South China Block are important areas for the distribution of Precambrian basements and Phanerozoic Nb-Ta and other rare-metal deposits. Whether the rare-metal mineralization is related to the Precambrian basement composition, and how the occurrence of rare metals in the basement metamorphic rocks affects the Nb-Ta contents and Nb/Ta ratios in the magma generated by partial melting are important fundamental scientific problems. In this study, the Nb and Ta contents of different minerals of the metamorphic rocks in the Cathaysia Block and Jiangnan Orogenic Belt are analyzed in detail. Combining whole-rock geochemistry, mineral compositions, mineral proportions, metamorphic temperature, and partial melting modelling, this study reveals the critical factors controlling the Nb-Ta contents and variations in biotite, muscovite and other metamorphic minerals, and discusses the influence of melted minerals on the Nb-Ta contents in melts in different conditions. Analytical results show that biotite is the most enriched Nb-Ta rock-forming mineral in the metamorphic rocks of South China, have mean Nb content of 64.1×10-6 and mean Ta content of 4.93×10-6, with similar compatibility between Nb and Ta. Muscovite is another metamorphic mineral containing high Nb-Ta contents, slightly lower than biotite. Niobium is more compatible in muscovite than Ta in subsolidus conditions, with an high Nb/Ta ratio of 16.9, being a potential high Nb/Ta reservoir. Pyroxene, amphibole, garnet and feldspar have low Nb and Ta contents, and make little effect on the enrichment and differentiation of Nb-Ta in the system. The Nb-Ta contents and Nb/Ta of biotite and muscovite, and partition coefficients with bulk rocks are mainly controlled by the host-rock composition, mineral assemblage, mineral composition, and metamorphic temperature. The Nb-Ta contents and partition coefficients of biotite and muscovite show positive correlation with metamorphic temperature, and inverse correlation with mineral proportions, reflecting the modal abundance effect. When partial melting occurs and melt is extracted, the Nb-Ta contents in mica sharply decrease owing to low Nb-Ta contents in rocks and changes in mineral compositions and partition coefficient, which is not controlled by those factors in subsolidus conditions, but by the degree of melting, i.e. "melting effect". Based on the partial melting modelling of a two-mica schist from the eastern Nanling region, it is concluded that the Nb-Ta enrichment and differentiation of melts mainly depend on source compositions, melted mineral assemblage, oxygen fugacity and melting degree. High oxygen fugacity and pressure can effectively promote the Nb-Ta enrichment in melts. The modelling results suggest that at pressure of 0.6 GPa and oxygen fugacity of FMQ+2, partial melting may produce the melt 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 modelling indicates that normal crystallization differentiation can’t lead to significant Nb-Ta enrichment in residual melt. Extreme differentiation, even reaching the fluid-rich stage, must be needed to promote significant Nb-Ta enrichment.