Transition From Low-K to High-K Calc-Alkaline Magmatism at Approximately 84Ma in the Eastern Pontides (NE Turkey): Magmatic Response to Slab Rollback of the Black Sea

Compositional changes of magma in space and time can be used to infer mantle dynamics and crust-mantle interaction that are responsible for the generation of magma and evolution. This study reveals, for the first time, the presence of magma compositional transition at approximately 84Ma in the eastern Pontides arc (NE Turkey), providing important insights into the mantle dynamics responsible for the generation of the extensive Late Cretaceous arc magmatism. The Late Cretaceous granitoids can be divided geochemically into two groups. Group I samples were emplaced at 91-86Ma and are characterized by low K2O/Na2O, whereas Group II samples were emplaced at 83-78Ma and display high K2O/Na2O. Group I samples have positive zircon epsilon(Hf) (t), whole-rock (Nd) (t), and mantle-like zircon O-18, which significantly differs from the Group II samples with negative to positive epsilon(Hf) (t), negative whole-rock (Nd) (t), and enhanced zircon O-18. The Nd-Hf-O isotopic compositions of the Group I and II samples can be quantitatively interpreted as being derived from partial melting of the low-K amphibolite source within the juvenile lower arc crust that incorporated 5-30% and 50-60% enriched components from the basement rocks into the magmatic systems, respectively. Considering the occurrences of regional thermal subsidence and extensional structure during the Late Cretaceous, the compositional transition from Group I to II and the coeval magmatic flare-up represented by Group II samples can be linked with the slab rollback of the southward subducting Black Sea (Paleotethys) oceanic lithosphere.