Nb-Ta fractionation by amphibole and biotite during magmatic evolution: Implications for the low Nb/Ta ratios of continental crust

Abstract

How did the low niobium/tantalum (Nb/Ta) ratios of continental crust form is a hotly debated issue? Here, we report the Nb Ta features of a well-characterized magmatic rock series, mainly composed of amphibole and biotite cumulates, from the Northern Dabie orogenic belt in China. The Nb/Ta ratios of whole-rock (19.5–26.2), amphibole (16.8–41.1), and biotite (21.1–49.6) are negatively correlated with their Mg # values, indicating that the separation of amphibole and biotite could not only cause Nb Ta fractionation but also reduce the Nb/Ta ratios of residual magmas during magmatic evolution. Furthermore, we compile the Nb/Ta ratios of global arc magmas ( n = 15,696) to examine the role of amphibole and biotite in the differentiation of arc magmas. The Nb/Ta ratios of global arc magmas display a negative correlation with SiO 2 contents and positive correlations with Dy/Yb and Ba/Rb ratios. Additionally, the compositions of the upper continental crust (UCC) exactly fall on the evolution line of Nb/Ta vs. SiO 2 and Nb/Ta vs. Ba/Rb, yet significantly falls below the evolution line of Nb/Ta vs. Dy/Yb. These observations show that amphibole and biotite are two important magmatic phases necessary for processes of arc magma differentiation and that they play a role together in the formation of low Nb/Ta ratios of the continental crust. As a whole, we suggest that amphibole- and biotite-rich rocks stored in the deep crust are one of the potential hidden reservoirs with higher Nb/Ta ratios, which may contribute to the depleted Nb during the formation process of continental crust. • Nb Ta fractionation of arc magmas could be induced by amphibole and biotite. • Amphibole- and biotite-rich rocks in deep crust are potential high Nb/Ta reservoirs.