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.

Uysal, Ibrahim; Akmaz, Recep Melih; Su, Ben-Xun; Demir, Yilmaz; Sen, Ahmet Dundar

The chromitites of the K & imath;z & imath;lda & gbreve; ophiolite (KO) are primarily characterized by massive and disseminated textures within refractory mantle peridotites, predominantly harzburgite and dunite, with occasional occurrences of banded and nodular chromitites. Based on chromite composition, these chromitites can be classified into two groups exhibiting a broad compositional range: intermediate (0.55 <= Cr# < 0.70, TiO2 = 0.08-0.51 wt%) and high-Cr (0.70 <= Cr# <= 0.77, TiO2 = 0.05-0.28 wt%). The compositions of minor and trace elements align consistently with variations in Cr#. Total PGE (Os, Ir, Ru, Rh, Pt, Pd) contents in these chromitites are generally less than 500 ppb, with many samples showing enrichment of IPGE (Os, Ir, Ru) relative to PPGE (Rh, Pt, Pd). However, certain samples from both groups display elevated PGE concentrations (Sigma PGE(intermediate) = 718 ppb, Sigma PGE(High-Cr) = 866 ppb) compared to other samples. The dominant platinum group mineral inclusions in chromite are Ru-rich laurite [(Ru, Os)S-2], with a single osmiridium (Os-Ir alloy) grain also observed suggesting formation under high-temperature and low & fnof;S-2 conditions. Primary base metal mineral inclusions include millerite and pentlandite. Additionally, some chromite crystals contain single or multi-phase silicate inclusions, such as clinopyroxene, orthopyroxene, olivine, and amphibole. Parental melt compositions, calculated from Al2O3 and TiO2 contents of chromites, indicate that most high-Cr chromitites originated from boninitic melts, whereas intermediate chromitites and some high-Cr samples derived from melts with intermediate compositions. Trace element compositions of amphibole and clinopyroxene inclusions in the chromite crystals from the intermediate group further support crystallization from hydrous intermediate melts. It is proposed that the intermediate chromitite group formed from partial melts influenced by fluids from the subducted oceanic crust during the early stages of subduction initiation, while high-Cr chromitites originated from hydrous melts derived from a more depleted mantle wedge in later stages.

Sipahi, Ferkan; Sen, Cueneyt; Sadiklar, M. Burhan; Demir, Yilmaz

Sipahi, Ferkan; Sadiklar, M. Burhan; Sen, Cueneyt; Demir, Yilmaz

Demir, Yilmaz; Uysal, Ibrahim; Sadiklar, M. Burhan; Sipahi, Ferkan

Vein type Köstere ores were deposited along cracks formed as a result of NE-SW fault system in the late Cretaceous felsic volcanic rocks in the northeastern Black Sea Region of Turkey. Total silver content of the Köstere ore is controlled by total silver content of the hydrothermal solution, homogenization temperature of the ore minerals and modal abundances of the tetrahedrite-tennantite serie minerals. The Zn/Cd ratios of sphalerites from the Köstere deposit are close to those of granite-related classical vein-type hydrothermal deposits. Furthermore Co/Ni ratio in pyrite separates, higher than unit, suggests an acidic source for the ore forming fluids. Homogenization temperatures of 200-370 °C and salinity of 0-9.85 eq. wt% NaCl were obtained on the basis of fluid inclusions studies measured in quartz minerals. These data indicate that ore formation conditions for the Köstere ore are similar with the vein-type epithermal-mesothermal deposits, related to the magmatic fluids. The overall low salinities of fluids (on average: 5.4 eq. wt% NaCl) suggest that meteoric waters were involved in mineralization. The possibility of interactions between ascending hydrothermal fluids and dilute meteoric waters is supported by geological features and mineralogical data which indicate a shallow depth of mineralization. Torul Pluton, which is the younger acidic unit (72 my) in the area, is suggested to be related to the formation of Köstere deposit.