Petrogenesis of the late Cretaceous Turnagoel intrusion in the eastern Pontides： Implications for magma genesis in the arc setting

A series of Cretaceous plutons is present in the eastern Pontides of northeastern Turkey.The Turnagl intrusion is the least studied and,thus,the least understood plutons in the orogen.This intrusion consists of hornblende-biotite granodiorites emplaced at 78 Ma based on LA-ICP-MS U-Pb zircon dating.It is of sub- alkaline affinity,belongs to the medium- to high-K calc-alkaline series,and displays features typical of Itype granites.The rocks of the intrusion are enriched in large-ion lithophile elements and light rare earthelements with negative Eu anomalies（Eu/Eu^*= 0.69-0.82）,but are deficient in high-field-strength elements.They have a small range of (⁸⁷Sr/⁸⁶Sr)_i（0.7060-0.7063）,ε_Ndi（-2.6 to -3.1）,and δ¹⁸O（+8.1 to +9.1） values.Their Pb isotopic ratios are ²⁰⁶Pb/²⁰⁴Pb = 18.63-18.65,²⁰⁷Pb/²⁰⁴Pb = 15.62-15.63,and ²⁰⁸Pb/²⁰⁴Pb = 38.53-38.55.The fractionation of plagioclase,hornblende,and Fe-Ti oxides had key functions in the evolution of the Tumagl intrusion.The crystallization temperatures of the melts ranged from 758 to 885℃ as determined by zircon and apatite saturation thermometry.All these characteristics,combined with the low values of K₂O/Na₂O and（Na₂O + K₂O）/（FeO^t+MgO + TiO₂）,as well as the high values of （CaO + FeO^t + MgO + TiO₂）.suggest an origin by dehydration melting from a metabasaltic lowercrustal source.