Highly siderophile elements and mineral chemistry of chromitites from the Kırdağ ophiolite (NE Turkey): Constraints on genesis and subduction initiation

Abstract

This study presents the first petrological and geochemical characterisation of chromitites from the mantle section of the Kırdağ ophiolite (NE Turkey), focusing on concentrations of highly siderophile elements (HSE; PGE and Au) and the mineralogical features of chromian spinel and olivine. Field and petrographic observations found that the chromitites occur as small bodies, predominantly with disseminated texture and less commonly as semi-massive structures, hosted within dunite lenses enclosed by harzburgitic mantle rocks. Chromian spinels exhibit Cr# values of 0.66–0.68 and Mg# values of 0.65–0.72, classifying the Kırdağ chromitites as intermediate-Cr type ophiolitic chromitites. The low TiO₂ content (0.18–0.32 wt%) suggest derivation from melts transitional between boninite and island arc tholeiitic composition. Olivine occurs both as inclusions in spinel and within the interstitial matrix, with forsterite (Fo) contents higher in inclusions (96.21–97.18) compared to the matrix (94.25–94.68). The low Ca concentrations in olivine support formation from a highly magnesian melt interacting with a depleted mantle source. The chromitites display low total PGE concentration (73–153 ppb), relative enrichment in Ir-group (IPGE) over Pd-group (PPGE), and low Au content (<5–8 ppb), consistent with typical ophiolitic chromitite signatures. Pd/Ir ratios (0.77–2.33) and Pt anomalies (Pt/Pt∗ = 0.24–0.76) indicate that partial melting was the primary control on PGE distribution, although increasing Pd/Ir ratios suggest a minor role for magmatic fractionation. The estimated parental magma composition, characterised by low TiO₂ (0.29–0.44 wt%), moderate Al₂O₃ (13.52–13.80 wt%), and FeO/MgO ratios (0.82–1.02), points to arc-related affinity. These results suggest that the Kırdağ chromitites formed during the early stages of subduction initiation, in a supra-subduction zone forearc setting, from a progressively evolving mantle source.