Fe isotope systematics and their role in constraining front- and rear-arc processes in Iran Cenozoic magmatism: Beyond compositional and radiogenic isotopes evidence

Abstract

The Cenozoic arc of Iran preserves archetypal exposures of the continental arc developed during the north-eastward subduction of the Neo-Tethys oceanic slab and records the history of flare-up magmatism within the Zagros Orogen. Here we present a new dataset of high-precision Fe–Sr–Nd isotopic compositions of Eocene–Oligocene frontal- and rear-arc (i.e., pre-collisional) magmatic products associated with subduction of the Neo-Tethys Ocean beneath Iran to provide new constraints on the fractionation of Fe isotopes and cycling of Fe within these tectonic settings. The δ⁵⁶Fe values range from +0.07 ‰ to +0.15 ‰, falling within the range of arc-related rocks worldwide. The rear arc samples display higher δ⁵⁶Fe (+0.135 ± 0.010 (2σ)) than samples from the coeval frontal arc (+0.090 ± 0.006 (2σ)). The low Fe³⁺/ƩFe values of all investigated samples in the range from 0.26 to 0.36 and the lack of negative correlation between δ⁵⁶Fe and Fe³⁺/ƩFe argue against incorporation of any significant sulfate- and/or carbonate-bearing fluids. Based on geochemical and isotopic data, we postulate that the high-flux magmatic events (i.e., flare-ups) are related to partial melting of a garnet-free lherzolite mantle source and limited crustal assimilation. Our results also demonstrate that the Fe isotope fractionation in the Zagros orogen is primarily governed by fractional crystallization of Fe²⁺-rich phases (e.g., olivine, clinopyroxene), particularly in rear-arc domains where deeper crystallization promotes heavier δ⁵⁶Fe signatures. Although changes in oxidation state can affect Fe isotope fractionation when iron is added or lost from the system, the relatively narrow range in Fe³⁺/ΣFe ratios and the absence of a systematic covariation with δ⁵⁶Fe suggest that redox-driven effects were likely secondary with respect to crystal–melt partitioning in this setting. This stands in marked contrast to global arc systems, particularly oceanic arcs, where slab-derived oxidizing fluids typically dominate over magmatic differentiation, underscoring the Zagros as a case where source composition and fractional crystallization outweigh subduction-related redox processes.