A record of metasomatism and crustal contamination of the Mediterranean lithosphere in chromitites of the Orhaneli Ophiolite Complex (NW Türkiye)

Abstract

• Orhaneli chromitites include Middle Devonian to Mesoproterozoic zircon and rutile . • Chromite trace elements points a metasomatic signature in relics within chromitites. • More data should be sought for ultrahigh-pressure history of Orhaneli chromitites. This study provides the first-ever report of U-Pb isotopic ages of zircon and rutile from high-Cr chromitites of the Orhaneli Ophiolite Complex (OOC). The chromites within the OOC chromitites yield Cr-numbers between 0.77 and 0.86, Mg-numbers between 0.46 and 0.71 and TiO 2 < 0.26 wt%. They exhibit V, Ga and Ni depletion and Sc, Ti, Zn, Co and Mn enrichment relative to MORB spinels, with significant Sc and Ti positive anomalies. This, and the Al 2 O 3 and TiO 2 compositions of the parental melts, suggest that the studied chromitites formed from island arc melts of boninitic affinity, in a supra-subduction environment. In addition to zircons, chromites host other solid mineral inclusions, including Os- and Ir-dominant platinum-group minerals (PGM), base-metal sulfides (BMS), alloys, ubiquitous silicate (some as micron-sized oriented lamellae), and two poorly defined Fe-Cr-O and Ti-Si-C grains. The zircons show Th/U ratios typical for magmatic zircons from felsic melts, and present Middle Devonian to Mesoproterozoic U-Pb isotopic ages. Although the reworked/resorbed forms of the zircons suggest a xenocrystic origin, coupling of U-Pb data from some zircons and rutile with the Ti positive anomaly in the chromites raises the possibility for an earlier Neoproterozoic metasomatic event affecting the relics now included within the chromitite bands. The current state of knowledge about the silicate micro-lamellae within the chromites has prompted reassessment of their early interpretation as direct evidence of deep mantle recycling. We currently suggest a more cautious approach on this issue by seeking additional evidences for an ultrahigh-pressure (UHP) history.