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

Christophe Ballouard (1) , Marc Poujol (1) , Philippe Boulvais (1) , Yannick Branquet (2, 1, 3) , Romain Tartese (4, 5) , Jean Louis Vigneresse (6)

1 GR - Géosciences Rennes
2 ISTO - Institut des Sciences de la Terre d'Orléans - UMR7327
3 Métallogénie - UMR7327
4 IMPMC - Institut de minéralogie, de physique des matériaux et de cosmochimie
5 OU - The Open University [Milton Keynes]
6 GeoRessources

In their late stages of evolution, peraluminous granitic melts exsolve large amounts of fluidswhich can modify the chemical composition of granitic whole-rock samples. The niobium/tantalum (Nb/Ta) ratio is expected to decrease during the magmatic differentiation of graniticmelts, but the behavior of both elements at the magmatic-hydrothermal transition remainsunclear. Using a compilation of whole-rock geochemical data available in the literature, wedemonstrate that fractional crystallization alone is not sufficient to explain the distribution ofNb-Ta in most peraluminous granites. However, we notice that most of the granitic samplesdisplaying evidence of interactions with fluids have Nb/Ta < 5. We propose that the decreaseof the Nb/Ta ratio in evolved melts is the consequence of both fractional crystallization andsub-solidus hydrothermal alteration. We suggest that the Nb/Ta value of ~5 fingerprints themagmatic-hydrothermal transition in peraluminous granites. Furthermore, a Nb/Ta ratio of ~5appears to be a good marker to discriminate mineralized from barren peraluminous granites.

Domaines: Géochimie

Christophe Ballouard, Marc Poujol, Philippe Boulvais, Yannick Branquet, Romain Tartese, et al.. Nb-Ta fractionation in peraluminous granites: A marker of the magmatic-hydrothermal transition. Geology, 2016, 44 (3), pp.231-234. ⟨10.1130/G37475.1⟩. ⟨insu-01270578⟩