Chromitite–Mafic Dike Interfaces as Archives of Syn-kinematic Fluid–Rock Interaction, Chromium Mobility and Ca-metasomatism, Nain Ophiolite, Central Iran

Abstract The effects of rodingitisation on chromitite and the controls on chromium mobility during fluid–rock interaction remain poorly understood, especially along structurally controlled reaction zones where deformation enhances fluid infiltration. This study examines the combined effects of calcium metasomatism and syn-kinematic deformation at chromitite–mafic dike interfaces in the Nain ophiolitic massif, Central Iran, using electron backscatter diffraction, electron microprobe analysis, whole-rock geochemistry, and Raman spectroscopy to characterise microstructural evolution, mineral chemistry, and mineral phases across a chromitite—slightly rodingitised dike interface. The chromitite body (~15 cm thick, ~3 m long) is deformed, hosted within sheared, serpentinised harzburgite, and spatially associated with boudinaged, slightly rodingitised mafic dikes. Two chromite generations are identified: an early disseminated generation with high Cr# and Mg# reflecting supra-subduction zone crystallisation, and a second generation at chromitite margins containing polymineralic silicate inclusions (clinopyroxene, orthopyroxene, olivine, pargasite, phlogopite, aspidolite, talc, chlorite, garnet, apatite, sulfides) with lower Cr# (50–59). Their restricted occurrence, hydrous mineralogy, subsolidus temperatures (<700 °C), and textural affinity with the host matrix point to modification of pre-existing chromite by hydrothermal fluids, neoformation by direct precipitation from those fluids, or both. The chromitite–dike contact hosts a brecciated reaction zone (2 mm–1 cm) recording intense rodingitisation, comprising zoned grossular rimmed by Cr-rich andradite, diopside, xonotlite, clinochlore, and calcite. Thermodynamic modelling constrains this assemblage to < 450 °C and oxygen fugacities of FMQ − 4 to FMQ + 3, under CH₄-dominated fluid conditions. Zoned grossular shows oscillatory Cr# variation, with spinel relicts confined to high-Cr# zones, indicating chromite-derived Al, Cr, and Fe 2+. Fish-hook-shaped chromite grains and etched, garnet-sealed fractures suggest coupled dissolution–precipitation during fluid interaction. These observations support a three-stage evolution of the Nain chromitite–rodingite assemblage, involving: (1) magmatic chromitite formation in a supra-subduction setting; (2) syn-kinematic dike emplacement, accompanied by modification of pre-existing marginal chromite by hydrothermal fluids, neoformation through direct precipitation from high-temperature hydrothermal fluids, or both—a process that generated the hydrous, alkali-rich multiphase silicate inclusions rather than through primary melt entrapment; and (3) subsequent Ca-metasomatism below ~450 °C, which produced garnet + chlorite + clinopyroxene + xonotlite assemblages and generated oscillatory Cr-, Al-, and Fe-zoning in garnet through nonlinear, reaction-driven, self-organizing crystallisation.