Mineralogy and Geochemistry of Peridotites and Chromitites in the Aladag Ophiolite (S. Turkey): Melt Evolution of the Cretaceous Neotethyan mantle

The Aladag ophiolite (AO, also known as Pozanti-Karsanti ophiolite) is one of the largest Neotethyan ophiolites in Turkey and represents a Cretaceous oceanic lithosphere derived from the Inner Tauride seaway of Neotethys. Its peridotites consist of spinel-harzburgites, dunites and chromitites. Harzburgites display depleted mineral and whole-rock geochemistry, indicating high degrees of partial melting. They contain Pd- and Pt-rich interstitial sulphides that crystallized from sulphur-saturated melts during melt-rock interactions, and their spoon-shaped REE patterns indicate metasomatism by light rare earth element (LREE) enriched fluids derived from a subducting slab. Chromitites in both the mantle and mantle-crust transition zone (MCTZ) rock units exhibit similar geochemical compositions, typical of high-Cr chromitites. They also show similar chondrite-normalized PGE patterns, characterized by Os, Ir, Ru and Rh enrichments relative to Pt and Pd. Parental melts of the AO chromitites were sulphur-undersaturated and boninitic in composition. The presence of microdiamond, moissanite, zircons, and needle-shaped clinopyroxene exsolutions in chromite grains suggests that their formation might have originally involved high-T and high-P conditions within the mantle transition zone (MTZ) prior to their participation in further melting and oceanic crust formation at shallow depths in a forearc tectonic setting during intra-oceanic subduction initiation within the Inner Tauride seaway.