Is the Nb-Ta Fractionation a Marker of an Interaction with Fluids in Peraluminous Granites?

In their last stage of evolution, highly evolved peraluminous melts can exsolve a significant amount of fluids which can change their chemical composition and lead to the deposition of mineralization. The Nb/Ta ratio is expected to decrease naturally during differentiation but the behavior of both elements at the magmatichydrothermal transition is still unclear. Here, we present a compilation of some of the whole rock geochemical data available in the literature for crustal peraluminous granites. We noticed that for low Nb/Ta values, most peraluminous granites display evidences of fluid interaction such as: a large increase of the highly incompatible element contents (Cs, Sn, F, Li, Rb, W), a significant increase of the Tetrad effect and a decrease of the K/Rb ratio to values as low as 150. We propose that the Nb/Ta values around 5 mark the magmatichydrothermal transition and that the decrease of the Nb/Ta ratio in highly evolved melts is the consequence of both fractional crystallization and sub- to post-solidus magmatic-hydrothermal alteration. We also propose that low Nb/Ta values can be used as markers to discriminate barren granites from mineralization-bearing granites.