Transitional High-Al to Intermediate-Cr Podiform Chromitites from Wadi Zikt (Khor Fakkan Block, United Arab Emirates): Evidence for Reactive Melt Percolation during Nascent Forearc Evolution

Abstract

Abstract Podiform chromitites at Wadi Zikt in the Khor Fakkan Block of the United Arab Emirates constitute a previously undescribed occurrence within the mantle section of the Semail ophiolite. We combine field relations, electron-probe microanalysis, in situ LA–ICP–MS mineral analyzes and whole-rock Fe isotopes to evaluate their origin. Chromian-spinels show wide compositional variability (40.98–51.64 wt.% Cr2O3 and 17.14–26.13 wt.% Al2O3, with Cr# = 0.51–0.67 and Mg# = 0.53–0.72), classifying them as transitional high-Al to intermediate-Cr. Olivine is highly forsteritic (Fo91–95), whereas clinopyroxene exhibits extreme magnesian numbers (Mg# up to 0.96); these highest Mg values may reflect subsolidus Mg–Fe exchange with the abundant coexisting spinel during post-magmatic cooling. Empirical modeling of spinel chemistry yields parental melt compositions (Al2O3 ~13–15 wt%) consistent with mid-ocean ridge basalt (MORB)-like forearc basalts. Trace element systematics indicate that interstitial clinopyroxene did not crystallize from the primary parental melt but rather formed as a product of late-stage reactive porous melt–rock interaction and mantle refertilization. Whole-rock Fe isotope compositions (δ56Fe +0.056 to +0.064‰) are compatible with interaction between basaltic melt and peridotitic mantle. The chemistry of chromian-spinel and associated silicates, along with the occurrence of chromitite pods within dunite envelopes suggest that Wadi Zikt chromitites crystallized in a nascent forearc setting during the onset of Neo-Tethyan subduction, driven by the reactive percolation of asthenospheric forearc basalt melts that were progressively modified by incipient slab-derived aqueous fluids.