Mineralogy and geochemistry of peridotites and chromitites in the Aladag Ophiolite (southern Turkey): melt evolution of the Cretaceous Neotethyan mantle

Abstract

The Aladag Ophiolite (also known as the Pozanti-Karsanti Ophiolite) is one of the largest Neotethyan ophiolites in Turkey and represents Cretaceous oceanic lithosphere derived from the Inner Tauride seaway of Neotethys. Its peridotites consist of spinel harzburgites, dunites and chromitites. The harzburgites display a depleted mineral and whole-rock geochemistry, indicating high degrees of partial melting. They contain Pd- and Pt-rich interstitial sulphides, which crystallized from S-saturated melts during melt–rock interactions. Their spoon-shaped rare earth element patterns indicate metasomatism by fluids, derived from a subducting slab, which were enriched in light rare earth elements. The chromitites in both the mantle and mantle–crust transition zone rock units show similar geochemical compositions, typical of high-Cr chromitites. They also show similar chondrite-normalized platinum group element patterns characterized by Os, Ir, Ru and Rh enrichments relative to Pt and Pd. The parental melts of the Aladag Ophiolite chromitites were S-undersaturated and boninitic in composition. The presence of microdiamond, moissanite, zircon and needle-shaped clinopyroxene exsolutions in chromite (chr) grains suggests that their formation might have originally involved high-temperature and high-pressure conditions within the mantle transition zone before their participation in further melting and the formation of oceanic crust at shallow depths in a forearc tectonic setting during the initiation of intra-oceanic subduction within the Inner Tauride seaway. Supplementary material: Sampling and analytical methods, representative microprobe analyses and whole-rock major and trace element compositions are available at https://doi.org/10.6084/m9.figshare.c.4153631 Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways