An Early Garnet Redox-Filter as an Additive Oxidizer in Lower Continental Arc Crust Traced Through Fe Isotopes

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.