4th International Turkish Geology Symposium in Adana, Turkey, Turkey, 1 - 04 September 2011, pp.119
Volcano-sedimentary units of the Late Cretaceous age are exposed as an approximately W-E extending belt throughout the whole extent of southern cost of the Black Sea from Bulgaria to Georgia. These successions are of great importance in understanding the Mesozoic evolution of the Alpine-Himalayan belt, since they contain a well-preserved record of coeval sedimentation and volcanism.
Geochemical studies on these volcanic units to date have revealed two important findings: (1) the lavas have a distinct arc-signature and, (2) volcanism starts with calc-alkaline lavas and changes gradually or abruptly into more alkalic (e.g. shoshonitic) series. These findings led most researchers to agree with a view of an ensialic arc tectonic setting since such an evolutionary trend is quite a common phenomenon for matured arcs all over the world. The Late Cretaceous also marks the period during which the Black Sea basin was opened.
To better understand the genesis, tectonic setting and temporal and spatial variations of the Late Cretaceous volcanism, we studied an area located between Ereğli (Zonguldak) and İnebolu (Kastamonu) in Western Pontides and sampled two formations containing volcanic intercalations: (1) Dereköy (Middle Turonian-Santonian) and (2) Cambu (Campanian). Our geochemical database indicates that lavas of the Dereköy formation are calc-alkaline in character (CA) and contain anhydrous fractionation phases. In contrast, lavas of the Cambu formation contain polybaric crystallisation assemblages: (1) unhydrous (POAM) and (2) hydrous. Hydrous lavas of the Cambu formation are all CA in character with a distinct arc signature, while those containing POAM phases are either alkaline (with a within-plate signature) or mildly alkaline.
Our geochemical data show that lavas displaying variable alkalinity may be explained by a model involving mixing of magmas derived from two contrasting sources: (1) metasomatised sub-continental lithosphere with a distinct subduction signature and (2) an asthenospheric source with a within plate signature. We propose that the magma generation was associated with lithospheric thinning which resulted in the derivation of magma from progressively deeper zones in the mantle during the opening of Black Sea as a back-arc basin.