Contrasting behaviour of Nb/Ta and Zr/Hf ratios in a peraluminous granitic pluton (Nova Scotia, Canada)

Two pairs of high-field-strength elements (Nb–Ta and Zr–Hf) display contrasting behaviour during the evolution of a peraluminous granitic pluton that is composed of leucomonzogranites and subordinate greisen-hosted topaz-bearing leucogranites. Zr and Hf progressively decrease with differentiation and their ratio remains the same, close to the chondritic value (∼36). The crystallization of zircon which contains the bulk of these two elements in the rocks, does not modify the Zr/Hf ratio. With increasing differentiation, Nb and Ta show an increase accompanied by a distinct decrease of the Nb/Ta ratio from 14 in leucomonzogranites to 2 in leucogranites and greisens. Tantalum correlates positively with Rb, Li and F and is strongly enriched in the evolved rocks, implying that an addition of internally derived mineralizing fluids and fluid fractionation played a key role in the variation of the Nb/Ta ratio in rocks with K/Rb ratios < 150. The values of the Nb/Ta ratio in the continental crust are typically lower than those of mantle and chondrites (∼ 17.5) suggesting that fluid fractionation might have been an important process during the evolution of the continental crust.