Geochemical, mineralogical and Re-Os isotopic constraints on the origin of Tethyan oceanic mantle and crustal rocks from the Central Pontides, northern Turkey

Chromite, ultramafic and mafic rocks from Eldivan, Yapraklı, Ayli Dağ, Küre, Elekdağ and Kızılırmak in northern Turkey have been studied to determine their mineral and whole-rock geochemical, and Re-Os isotope geochemical characteristics. Most of the studied peridotites display depleted but commonly V-shaped chondrite-normalized rare-earth element (REE) patterns while some peridotites as well as pyroxenites from all areas exhibit light REE depleted patterns. Olivine (forsterite 82 to 92 mol%) and spinel (chromium number 13 to 63) in the studied peridotites exhibit a wide range of compositions. Compositions of spinels suggest that peridotites from Eldivan, Ayli Dağ and Küre experienced relatively large degrees of partial melting (~ 15 and 19 wt%), whereas those of the Kızılırmak area most likely reflect lower melting degrees (~ 4–6 wt%). Whole-rock and mineral chemical data indicate that the ultramafic rocks are similar to abyssal and supra-subduction zone peridotites. The ultramafic rocks of the investigated areas exhibit a wide range of ¹⁸⁷Re/¹⁸⁸Os (0.12 to 6.6) and measured ¹⁸⁷Os/¹⁸⁸Os (0.122–1.14), while the basaltic rocks from Küre, Eldivan and Kızılırmak areas have high ¹⁸⁷Re/¹⁸⁸Os (128–562) and measured ¹⁸⁷Os/¹⁸⁸Os (0.724–1.943). On the other hand, chromite from Eldivan, Elekdağ and Kızılırmak show high Os contents (21.81–44.04 ppb) and low ¹⁸⁷Re/¹⁸⁸Os (0.015–0.818) and ¹⁸⁷Os/¹⁸⁸Os (0.122–0.133). Re-Os model ages (T_Chur) for all analyzed samples yielded scattered ages ranging from Jurassic to Proterozoic. Overall, geochemical data are interpreted to reflect different degrees of partial melting, melt – rock interactions and metasomatic effects that produced a heterogeneous mantle in a supra-subduction setting.