Akmaz, Recep Melih; Uysal, Ibrahim; Su, Ben-Xun; Sen, Ahmet D.; Hu, Wen-Jun; Seitz, Hans-Michael; Mueller, Dirk; Eslami, Alireza

This study presents a complete alteration sequence in chromitites from the Kizildag ophiolite (southern T & uuml;rkiye), documenting three distinct chromite types that record processes related to Neotethyan subduction. Chromitites exhibit predominantly lenticular forms and occasionally banded textures, embedded within mantle peridotites and the mantle-crust transition zone, specifically harzburgite and dunite. Our analysis categorizes chromite grains into three groups: fresh, altered, and porous. Fresh chromites are characterized by high contents of Cr2O3 ranging from 42.69 to 57.08 wt% and Al2O3 from 12.46 to 25.34 wt%. In contrast, altered chromites, typically located along fractures or at the edges of fresh grains, show significant variability in chemical composition, with Cr2O3 content between 43.34 and 63.94 wt% and Al2O3 between 2.91 and 14.84 wt%. Porous chromites, distinguished by their porous structure, present Cr2O3 contents from 44.58 to 52.66 wt% and Al2O3 from 3.00 to 8.38 wt%. Notably, altered and porous chromites demonstrate substantial enrichments in Fe and trace elements such as Mn, Ni, Zn, Co, and Ga, indicating a shift due to metamorphic processes. The observed high Cr# (0.68-0.93) and low Mg# (0.25-0.65) values in rims of the altered and porous chromite further illustrate the metamorphic influence on chromite, with greenschist-facies conditions indicated by the Cr-Al-Fe3+ diagram at temperatures below 450 degrees C. The alteration dynamics reflect a progression that initiates with the olivine matrix and advances through reactions involving fresh chromites. This progression results in mass loss, the formation of chlorite, and the porous structures. Our findings contribute to the understanding of metamorphic alteration processes in chromitites and provide a detailed account of the geochemical trends (such as Fe/Mg exchange and trace element enrichment) that reflect greenschist-facies metamorphism. The evidence of chlorite formation and the prevalence of porous textures highlight the influence of metamorphic fluids over serpentinization, offering a model for chromite alteration in supra-subduction zones. This research elucidates the metamorphic history of chromitites in the Kizildag ophiolite.

Uysal, Ibrahim; Sen, Ahmet D.; Su, Ben-Xun; Ersoy, Yalcin; Demir, Yilmaz; Pan, Qi-Qi; Hu, Wen-Jun; Bagci, Utku; Godard, Marguerite

The processes governing ophiolite formation and evolution remain incompletely understood. The K & imath;z & imath;lda & gbreve; ophiolite, a well-preserved remnant of the Neo-Tethyan oceanic lithosphere in the Anatolian region, southern T & uuml;rkiye, 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: (1) those with very low modal clinopyroxene, interpreted as highly depleted harzburgites and preserving primary mineral assemblages, and (2) 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 17-25% 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.