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

Two pairs of high-field-strength elements (Nb-Ta and Zr-Hf) display contras ting behaviour during the evolution of a peraluminous granitic pluton that is composed of leucomonzogranites and subordinate greisen-hosted topaz-bear ing leucogranites. Zr and Hf progressively decrease with differentiation an d their ratio remains the same, close to the chondritic value (similar to 3 6). The crystallization of zircon which contains the bulk of these two elem ents in the rocks, does not modify the Zr/Hf ratio. With increasing differe ntiation, 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 gre isens. Tantalum correlates positively with Rb, Li and F and is strongly enr iched in the evolved rocks, implying that an addition of internally derived mineralizing fluids and fluid fractionation played a key role in the varia tion 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 mant le and chondrites (similar to 17.5) suggesting that fluid fractionation mig ht have been an important process during the evolution of the continental c rust. (C) 2000 Elsevier Science B.V. All rights reserved.