The dual origin of granites from fluid-absent and fluid-fluxed melting of the juvenile lower crust: New constraints from Mo isotopes and phase equilibrium modeling

Abstract

Water plays a pivotal role in the differentiation of continental crust and the formation of granitic magmas. However, it is still hotly debated how to identify fluid regimes during crustal anatexis. Molybdenum (Mo) isotopes have shown the potential to trace fluid activity during magmatic and metamorphic processes and thus are a valuable tool to identify the presence of free fluid during crustal anatexis. This is exemplified by our previous study on the Early Paleozoic sodic adakitic rocks from the Dabie orogen, eastern China, which were explained to originate from fluid-fluxed melting of juvenile crust based on their Mo isotope characteristics. However, the Early Paleozoic potassic granites (K2O/Na2O = 0.81–1.13) in the Dabie orogen—spatially and temporally associated with sodic granites—exhibit distinct geochemical signatures (e.g., higher K2O/Na2O ratios and lower Sr/Y, La/Yb ratios). Sr–Nd isotope results indicate that the potassic granites originate from the juvenile lower crust, consistent with the sodic granites. To evaluate the controlling factors behind the geochemical differences between the two groups of granites, we conducted whole-rock Mo isotope analyses of the potassic granites and compared them with the sodic granites and coeval mafic rocks. The results reveal that the potassic granites exhibit δ98Mo values (–0.41 to 0.23 ‰, median = 0.02 ‰) similar to adjacent contemporaneous mafic rocks (–0.17 to 0.21 ‰, median = 0.02 ‰), whereas the sodic granites display systematically higher δ98Mo values (–0.13 to 0.74 ‰, median = 0.22 ‰). Simulation calculations indicate that neither pure fractional crystallization (FC) nor AFC (assimilation-FC) process can account for the observed differences in K2O/Na2O ratios and Mo isotope compositions between the potassic and sodic granites. Instead, such differences can be well explained by partial melting of the same mafic source rocks at different fluid regimes. Further phase equilibrium modeling indicates that the addition of fluids can promote preferential plagioclase breakdown and amphibole retention in the source rocks, thereby generating or amplifying high Sr/Y and La/Yb ratios in melts. In contrast, pressure variations alone cannot account for the observed K2O/Na2O ratio differences. These results indicate that distinct fluid regimes in the source region exert the primary control on the geochemical disparities between the two groups of granites. The fluid-fluxed melting could be caused by fluids exsolved from crystallization of the underplated hydrous mafic arc magmas or released by the breakdown of hydrous minerals in the juvenile lower crust. As a comparison, the fluid-absent melting of the mafic crust is primarily driven by the breakdown of hydrous minerals, including biotite and amphibole, generating potassic melts with Mo isotope compositions similar to the source rocks. This study provides compelling evidence for the control of fluids on the Mo isotope composition of granitic magmas and thus offers a new means to distinguish between the fluid-absent and fluid-fluxed melting regimes.