Geochemistry of mantle peridotites and chromitites from Upper Cretaceous ophiolites in NW Türkiye: Insights into abyssal to forearc mantle settings

Abstract

This paper presents new data on whole-rock and mineral chemistry, trace elements of clinopyroxene (Cpx) and chromite, along with detailed petrographic observations of mantle peridotites and chromitites from the NW Anatolian (Harmancık-Orhaneli, Bursa) ophiolites to better understand contrasting geochemical signatures and different stages of lithospheric evolution along the İzmir-Ankara-Erzincan Ocean (İAEO) in northern Türkiye. The mantle peridotites, characterized by abundant high-grade podiform chromitites in Harmancık and banded chromitites in Orhaneli, are particularly notable. The mantle rocks include depleted- lherzolites, Cpx-rich harzburgites, harzburgites, Cpx-poor harzburgites, and dunites. Compositional variations in the mantle peridotites from both Harmancık and Orhaneli reflect continuous evolution driven by melt depletion and metasomatic enrichment occurring in both mid-ocean ridge (MOR) and supra-subduction zone (SSZ) environments. Our modeling suggests that the depleted lherzolites and Cpx-rich harzburgites in the Harmancık area are residues of anhydrous MOR-type melting of a fertile mid-ocean ridge mantle (FMM), with partial melting degrees of approximately 12–17% and 15–22%, respectively. In contrast, the harzburgites, Cpx-poor harzburgites, and dunites in the Harmancık area represent residues after 20–25% partial melting of a depleted MORB-type mantle, which itself was a residue following around 22% partial melting of an FMM during subduction initiation in the Late Cretaceous. The mantle peridotites in the Orhaneli area are comparatively more depleted, comprising Cpx-rich harzburgites, harzburgites, and dunites. This is supported by highly depleted rare earth element (REE) contents, similar to SSZ-type peridotites, indicating residues after 20–35% partial melting of a depleted MORB-type mantle (DMM). Furthermore, the compositions of parental magmas, as well as the trace element contents of chromites from the high-Cr chromitites and associated dunites in both Harmancık and Orhaneli, suggest that they were generated from boninitic melts in a supra-subduction zone (SSZ) environment. All the data suggests that the peridotites with high-grade podiform chromitites in the Harmancık region form the deeper mantle levels, whereas the banded chromitites within extensive dunite bodies in the Orhaneli region suggests proximity to the Moho transition zone, derived from the İAE Ocean in Late Cretaceous.