Extreme Mo isotope variations recorded in high-SiO2 granites: Insights into magmatic differentiation and melt-fluid interaction

The Mo stable isotope system has been used to trace material recycling during subduction-related processes, but the behav-ior of Mo isotopes during magmatic evolution (e.g., crystal-melt fractionation and melt-fluid interaction) remains con-tentious, especially in high-SiO2 granites. This study addresses the issue of Mo isotope variation in high-SiO2 granites by measuring bulk-rock and mineral Mo isotopes of biotite granites (BGs) and garnet-bearing two-mica granites (GBGs) from the well-characterized Zhengga granite pluton (southern Tibet, China). The GBGs have similar Sr-Nd-O isotope composi-tions to those of the BGs but show higher SiO2 and lower TiO2, MgO, total Fe2O3, and CaO contents, and represent the prod-ucts of advanced fractionation of the BG magmas. The BGs have lower Mo contents (0.02-0.07 ppm) and higher delta 98/95Mo values (-0.54% to 0.22%) compared with the GBGs (0.029-2.121 ppm and-0.97% to-0.41%, respectively). Analysis of major silicate minerals suggests that substantial segregation of biotite and feldspar with high delta 98/95Mo values of 0.00% to 0.38% and-1.06% to 0.57% (most within-0.58% to 0.13%) could have driven the GBGs and the late-stage crystalline phase of garnet (-1.22% to-0.98%) towards very low delta 98/95Mo values. However, the trend of decreasing delta 98/95Mo with indices of magma differentiation is not linear: one group of GBGs show increasing Mo contents and decreasing delta 98/95Mo val-ues with decreasing Y, Ho, and Dy contents; while the other group display increasing Mo contents and slightly decreasing delta 98/95Mo values with respect to the increasing contents of Y, Ho, and Dy. These two contrasting behaviors can be ascribed to further crystal fractionation and melt-fluid interaction in a closed magmatic-hydrothermal system. This is also evidenced by the formation of two types of garnets with different contents of Mo and rare earth elements in these two groups of GBGs. Closed-system fluid saturation is inferred to have driven the silicate melt to be enriched in 98Mo, which limited the decrease in melt delta 98/95Mo caused by crystal fractionation. These observations are supported by quantitative geochemical modeling. We conclude that both fractional crystallization and melt-fluid interaction control Mo isotope fractionation in high-SiO2 granites and that Mo isotopes are useful for tracing the evolution of high-SiO2 igneous rocks. (c) 2022 Elsevier Ltd. All rights reserved.