Nb-Ta fractionation in peraluminous granites: a marker of the magmatic-hydrothermal transition

On the basis of geochemical and mineralogical features, Ballouard et al. (2016) argue that magmatic fractionation alone cannot explain the formation of leucogranites with low Nb/Ta ratios. Instead, they propose that low ratios are better explained by post-magmatic interaction with F-bearing fluids. Although fluids may certainly play an important role in the formation of rare metal leucogranites, the model proposed by Ballouard et al. lacks key details. We contend that the principal features they attribute to magmatic-hydrothermal processes are better explained by the magmatic fractionation model.