Mineral and whole-rock geochemistry of high-Al podiform chromitites in the Fizh Massif of the Cretaceous Oman ophiolite: origin of hydrous N-MORB melts in a nascent forearc setting

The Oman ophiolite is one of the largest and best exposed ophiolites in the world and has >450 chromitite deposits. We report here a newly identified chromitite deposit in the Wadi Rajmi, Oman. This deposit occurs within a dunitic envelope, which is surrounded by harzburgite, and consists of both massive and disseminated chromitite types. The Rajmi peridotites represent depleted upper mantle rocks that underwent >20% partial melting and experienced metasomatism by melts and fluids derived from a subducting slab. They demonstrate geochemical affinities similar to those of the Izu-Bonin-Mariana forearc peridotites, supporting their formation in a forearc environment. The Rajmi chromitites have low Cr# values and are classified as high-Al chromitites. They have geochemical compositions comparable with those of chromitites crystallized from mid-ocean ridge basalt (MORB)-type melts. However, the chromites in these high-Al chromitites contain various silicate inclusions (e.g. amphiboles and micas), indicating the hydrous and atypical MORB nature of their parental magmas. Combined with the mineralogical and geochemical characteristics of the country rocks, we posit that the parental melts of the Rajmi high-Al chromitites have a MORB-like affinity derived from partial melting of a nascent forearc mantle.