Genesis and high-pressure evolution of the Koycegiz ophiolite (SW Turkey): Mineralogical and geochemical characteristics of podiform chromitites

Textural and compositional characteristics of mineral inclusions in podiform chromitites flourish thoughtful information about the genesis of the Koycegiz ophiolite (SW Turkey). In the chromitite bodies, olivine, orthopyroxene, clinopyroxene, and sulfide inclusions are mostly sub-rounded or globular. Less abundant needleshaped silicate inclusions (10–50 μm-long) are aligned along crystallographic orientations in the host magnesiochromite grains. Raman spectroscopy and micro analytical data indicate that these lamellar inclusions have consistently a diopside composition. Uncommon, discrete micro-diamonds and moissanite (SiC) particles, observed in some magnesiochromite grains, may suggest formation under ultra-high pressure (UHP) superreduced (SuR) conditions. The Koycegiz chromitites are characterized by variable platinum-group element (PGE) abundances (80–441 ppb), with unfailing IPGE > PPGE contents. Fractionated chondrite-normalized PGE patterns of chromitites and associated peridotites and data scatter in the Pd/Ir versus Pt/Pt* space indicate chromitite formation by partial melting of the associated mantle rocks. The variable 187Os/188Os(i) (0.1278–0.1380) and γOs ( 0.38 to +7.54) values demonstrate extensive contamination of crustal components, likely in a supra-subduction zone (SSZ) setting. Combined with the occurrence of unusual ‘exotic’ mineral inclusions, geochemical and isotopic characteristics of the Koycegiz chromitites replicate superimposed deep mantle processes, recycling and subduction related phase transformations.