The magmatic differentiation of cooling arc lavas on their way to the surface is dominated by fractional crystallization, which predominantly occurs at lower crustal arc levels (25–40 km) at continental margins. The magmatic storage in the deep crust is complex and remote, and rarely studied compared to shallow magma fractionation, but carries key information for the formation and evolution of the lower continental crust. At convergent continental margins, mantle‐derived melts must pass through a complex system of rocks in the lower arc crust. In particular, melts inevitably transition through deep "hot" (melt‐rich) or "cold" (i.e., water‐rich) zones, collectively termed here lower crustal arc zones (LoCAZ) to account for both scenarios. Their role in driving melt modification, including formation and oxidation of calc‐alkaline suites, remains unclear. Here we report stable Fe isotope compositions of 40 well‐characterized Neoproterozoic mafic dikes from the northwestern margin of the Yangtze Block, representing three stages of continental arc evolution. Rocks from the Tongde dikes display strong correlations between δ57Fe values and trace element indices of garnet fractionation (e.g., high La/Yb and depleted heavy REE patterns). An increase in heavy Fe isotopes with garnet fractionation indicates a redox‐filtering of melts through LoCAZ. We thus propose that retention of Fe2+ in deep crustal garnet is one driver for elevated fO2 and the calc‐alkaline trend in continental margin rocks. With continental arc rocks being on average more oxidized than intra‐oceanic arc lavas, garnet redox‐filtering may be an additive oxidizer in thicker arc crust. The Tongde dike swarms may represent a rare, volumetrically small, direct example of melts ejected out of the lower arc crust. The complex mixing and mingling of melts reported for most arcs and their plutons may obscure this redox process contributing to continental crust growth. Plain Language Summary: Calc‐alkaline rocks are widely distributed at convergent continental margins, where oceanic lithosphere is subducted beneath continental lithosphere (e.g., Andes). They generally display low Fe concentrations and high oxygen fugacity, and have important information of formation and evolution of the continental crust. However, generation and oxidation of calc‐alkaline rocks are still unclear. Here, we report stable Fe isotopes for a series of Neoproterozoic basaltic to andesitic dikes in South China. Some of them consist of calc‐alkaline rocks that experienced fractional crystallization of garnet‐bearing assemblages, suggesting that they were formed in the deep crust and reached garnet saturation under high pressures. Considering that garnet contains large proportions of ferrous iron and has much light Fe isotopes, we thus propose that garnet fractionation in deep crust is one dominant factor controlling formation of the oxidized calc‐alkaline rocks at convergent continental margins. Garnet acts as an additive oxidizer in thick arc crust and results in elevated oxygen fugacity in continental arc rocks over intra‐oceanic arc rocks. Key Points: The Tongde dikes formed by fractional crystallization of garnet‐bearing assemblages in lower continental crustal arc zoneGarnet acts as a redox filter in lower continental arc crust but only during the early stages of melt evolutionEarly Fe depletion (forging a calc‐alkaline trend) forms a possible additive process for creating oxidized arc melts at convergent continental margins [ABSTRACT FROM AUTHOR]