Nb and Ta are geochemical twins, yet their ratio varies considerably in igneous rocks. It is generally supposed that felsic melts coexisting with residual rutile will have elevated Nb/Ta ratios relative to their basalt precursors, which is pivotal to our understanding of deep crustal melting and continent formation throughout Earth’s history. This study reports petrological, bulk-rock and mineral geochemical, and Sr-Nd-Mo-Hf isotopic data on Eocene adakitic rocks in central Tibet. These rocks can be petrologically and geochemically divided into an amphibole-bearing high-Nb/Ta and an orthopyroxene-bearing low-Nb/Ta group, which were formed by partially molten eclogitized lower crust based on the geochemistry of their early-crystallized minerals. The two groups have similar Sr–Nd–Hf isotope compositions and abundances of strong incompatible elements, suggesting that their source protoliths share identical compositions. Besides, the values of TiO₂, Ba, Zr/Sm, and La/Yb are comparable between the two groups of rocks, eliminating the possibility of variations in Nb/Ta-sensitive minerals (e.g., rutile, amphibole, biotite, and titanite) in the source as the cause of their Nb/Ta variation. The contrasting mineral assemblages of the two group rocks, along with the higher Ce/Mo ratios and lower δ⁹⁸/⁹⁵Mo values in the low-Nb/Ta group, indicate the source of the low-Nb/Ta contains less water than that of the high-Nb/Ta group. These findings, combined with experimental results and trace element partial melting modeling, lead us to propose that the D_{rutile Nb/Ta} responsible formation of the low-Nb/Ta group is higher than that of the high-Nb/Ta group, due to the decreased water content in the source. Thus, the central Tibetan adakitic rocks provide the first natural example demonstrating that the Nb/Ta ratios in felsic rocks derived from rutile-bearing eclogite depend on the physical conditions of source melting, which are not always higher than the protolith. Our study also highlights the potential of employing Mo isotopes in adakitic rocks to test this hypothesis for those that are not petrologically distinguishable.

Article link: https://doi.org/10.1016/j.gca.2024.08.003