Subduction initiation of the Neotethyan Ocean recorded in chromite deposits: A case study of the Kızıldağ ophiolite, southern Türkiye

Abstract

The chromitites of the Kızıldağ ophiolite (KO) are primarily characterized by massive and disseminated textures within refractory mantle peridotites, predominantly harzburgite and dunite, with occasional occurrences of banded and nodular chromitites. Based on chromite composition, these chromitites can be classified into two groups exhibiting a broad compositional range: intermediate (0.55 ≤ Cr# < 0.70, TiO₂ = 0.08–0.51 wt%) and high-Cr (0.70 ≤ Cr# ≤ 0.77, TiO₂ = 0.05–0.28 wt%). The compositions of minor and trace elements align consistently with variations in Cr#. Total PGE (Os, Ir, Ru, Rh, Pt, Pd) contents in these chromitites are generally less than 500 ppb, with many samples showing enrichment of IPGE (Os, Ir, Ru) relative to PPGE (Rh, Pt, Pd). However, certain samples from both groups display elevated PGE concentrations (ƩPGE_intermediate = 718 ppb, ƩPGE_High-Cr = 866 ppb) compared to other samples. The dominant platinum group mineral inclusions in chromite are Ru-rich laurite [(Ru, Os)S₂], with a single osmiridium (Os-Ir alloy) grain also observed suggesting formation under high-temperature and low ƒS₂ conditions. Primary base metal mineral inclusions include millerite and pentlandite. Additionally, some chromite crystals contain single or multi-phase silicate inclusions, such as clinopyroxene, orthopyroxene, olivine, and amphibole. Parental melt compositions, calculated from Al₂O₃ and TiO₂ contents of chromites, indicate that most high-Cr chromitites originated from boninitic melts, whereas intermediate chromitites and some high-Cr samples derived from melts with intermediate compositions. Trace element compositions of amphibole and clinopyroxene inclusions in the chromite crystals from the intermediate group further support crystallization from hydrous intermediate melts. It is proposed that the intermediate chromitite group formed from partial melts influenced by fluids from the subducted oceanic crust during the early stages of subduction initiation, while high-Cr chromitites originated from hydrous melts derived from a more depleted mantle wedge in later stages.