Multiple mantle melting and metasomatism during the initial stages of subduction: Multi-geochemical and Os-isotope evidence from well-preserved Kızıldağ ophiolite, S Türkiye

Abstract The processes governing ophiolite formation and evolution remain incompletely understood. The Kızıldağ ophiolite, a well-preserved remnant of the Neo-Tethyan oceanic lithosphere in the Anatolian region, offers critical insights into subduction-related processes and geodynamics. This study presents new element data for mineral and whole-rock samples, combined with Re-Os isotopic compositions of mantle peridotites and associated crustal rocks and extrusives. Petrographic observations identify two distinct groups of peridotites: (i) those with very low modal clinopyroxene, interpreted as highly depleted harzburgitesand preserving primary mineral assemblages, and (ii) depleted harzburgites containing secondary clinopyroxene with elevated trace element contents, inferred to have crystallized from percolating melts and thus reflecting metasomatic modification. Spinel and orthopyroxene compositions indicate 17 to 25% degrees of partial melting, with evidence for melting in both garnet and spinel stability fields. However, mineral and whole-rock compositions also record subsequent cryptic and modal metasomatism. Crustal rocks and extrusives exhibit geochemical signatures transitional between fore-arc basalts and boninites, consistent with subduction initiation settings. Variations in Os isotope ratios (187Os/188Os) in crustal rocks further reveal melt-rock interactions and the incorporation of radiogenic Os from subducted oceanic crust. The integration of petrographic, geochemical, and isotopic data supports a four-stage tectonic model for subduction initiation and mantle wedge evolution, consistent with global models proposed for supra-subduction zone (SSZ) ophiolites, particularly those of the Tethyan domain. These findings provide new constraints on mantle melting, metasomatism, and subduction initiation processes and contribute to a broader understanding of SSZ mantle evolution in both regional and global contexts.