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

Abstract : 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.
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Submitted on : Monday, February 8, 2016 - 11:13:39 AM
Last modification on : Tuesday, July 9, 2019 - 2:26:04 PM

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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, Geological Society of America, 2016, 44 (3), pp.231-234. ⟨10.1130/G37475.1⟩. ⟨insu-01270578⟩

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