Abstract The Western Pontide Magmatic Belt consists of two different magmatic series corresponding to two distinct periods of intense volcanism, separated by a pelagic limestone marker horizon resting on a regional unconformity. The first stage of magmatism and associated extensional tectonic regime prevailed in the region between the Middle Turonian and Early Santonian. During the first stage, magmas were derived from a depleted mantle source containing a clear subduction signature. The extrusives intercalated with marine clastic sediments and pelagic carbonates associated with thick debris-flow horizons and olistoliths. Based on geochemistry and depositional features, the first stage is interpreted as an extensional ensialic arc setting developed in response to northwards subduction of the Tethys Ocean beneath the southern margin of Laurasia. During the Late Santonian, the volcanism stopped and the whole region suddenly subsided with the deposition of a thin, but laterally continuous, pelagic limestone horizon. This subsidence may imply the break-up of the Laurasian continental lithosphere and the beginning of oceanic spreading in the Western Black Sea Basin. The intensified extension is interpreted to be linked to the southwards rollback of the subducting slab. During the second stage in the Campanian, magmas were derived from two contrasting mantle sources: (1) a depleted lithospheric mantle enriched by a subduction component; and (2) an enriched asthenospheric mantle which is similar to that of the ocean island basalts (OIB). The depleted lithospheric source may be linked to the subcontinental lithospheric mantle of Laurasia, which was metasomatized by the previous Tethyan subduction event rather than by an active arc magmatism. Lavas derived from the depleted source are abundant throughout the stratigraphic column, whereas those from the enriched source dominate the end of the second stage. The presence of the alkaline lavas may indicate thinning of the lithosphere and upwelling of the asthenospheric mantle in the matured stages of rifting. We argue that the main cause of both rifting and temporal change in magma generation was the steepening and rollback of the northwards subducting slab of the Tethys Ocean. The aforementioned rollback also caused the Istanbul Zone to be moved to the south, and colliding with the Sakarya Zone in the south during the Maastrichtian. Based on geochemical, stratigraphic, palaeontological and sedimentary data, we suggest that the oceanic Western Black Sea Basin opened as an intra-arc basin during Turonian–Santonian time. Supplementary material : The full geochemical dataset in MS Excel workbook format is available at https://doi.org/10.6084/m9.figshare.c.3841255

Abstract We report new U–Pb zircon ages, major and trace element data, mineral chemistry, and Sr–Nd isotopic analyses of the mafic–intermediate dykes and intrusions in the İstanbul Zone. Mafic dykes are represented by calc-alkaline to alkaline lamprophyre and diabase. Intermediate dykes and subvolcanics are andesitic to dacitic in composition and calc-alkaline in character, while intrusive rocks (stocks and small plutons) are granodioritic and dioritic in composition. New zircon U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) dating yielded ages from 72.49 ± 0.79 (Upper Cretaceous–Campanian) to 65.44 ± 0.93 Ma (Lower Paleocene–Danian) for the intermediate dykes, and 58.9 ± 1.8 Ma (Upper Paleocene–Thanetian) for a small granodiorite stock. 87 Sr/ 86 Sr (i) values of the mafic and intermediate dykes and small stocks span a range from 0.703508 to 0.706311, while their 143 Nd/ 144 Nd (i) values vary from 0.512614 to 0.512812 and eNd (i) values from 5.09 to 1.24. Nd TDM model ages range between 0.46 and 0.77 Ga. Dykes are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) relative to high field strength elements (HFSEs). Normal-type mid-ocean ridge basalt (N-MORB)-normalized multi-element spidergrams of the majority of the mafic and intermediate dykes display a clear subduction signature, except a subset, which cut the Palaeozoic of İstanbul and the upper part of the Upper Cretaceous volcanics in the north of İstanbul (i.e. feeder dykes of the Kısırkaya Formation) and show a clear ocean island basalt (OIB) signature indicating that the melts feeding the dyke system during the Upper Cretaceous–Paleocene period were derived from two contrasting mantle sources: (1) initially a lithospheric mantle modified by subducted slab-derived melts which sourced the magmas with a clear subduction signature; and (2) followed by an asthenospheric mantle from which basic magmas with OIB signature. Petrological models indicate the interaction of these two discrete magma series via magma-mixing processes. Geothermometric calculations based on the composition of amphiboles are in the range of 769–953 and 938–994°C. Geobarometric calculations indicate crystallization depths ranging over an interval between 3.0 and 20.2 km, implying a polybaric crystallization. The oxygen fugacity (logƒO 2 ) values vary between −10.10 and −13.07 bar in the dykes cutting the Upper Cretaceous volcanics, and from −8.71 to −10.33 bar in intermediate dykes cutting the İstanbul Palaeozoic unit. H 2 O melt contents change between 4.91–6.89 and 4.82–7.51%, respectively implying that the dykes were emplaced at mid to shallow crustal levels. Dyke complexes of the İstanbul zone are interpreted to have been emplaced in a rifted volcanic arc margin related to the opening of the Black Sea during the Late Cretaceous–Paleocene period. Supplementary material: Tables of representative analyses are available at https://doi.org/10.6084/m9.figshare.c.3841276

Eastern Anatolia is one of the best examples of an active continental collision zone in the world. It comprises one of the high plateaus of the Alpine-Himalaya mountain belt, with an average elevation of ∼2 km above sea level. Almost two-thirds of this plateau is covered by young volcanic units related to collision. They range in age from 11 Ma to Recent and have a thickness of up to 1 km in places. The collision-related volcanic province is not confined to Eastern Anatolia, but extends across much of the Caucasus in the east, including Eastern Turkey, Armenia, Azerbaijan, Georgia, and Southern Russia, spanning a distance of some 1000 km. The region covered by the collision-related volcanic sequences comprises a regional domal shape (∼1000 km in diameter), and this unique morphology is comparable to that of the Ethiopian high plateau except for its north-south shortened asymmetrical shape. Recent geophysical data reveal that the lithospheric mantle is exceptionally thin or absent beneath this regional dome, indicating that the dome is currently supported by the asthenospheric mantle. Because of these features, the Eastern Anatolia–Iranian plateau and the Lesser Caucasus region as a whole can be regarded as the site of a “melting anomaly” or “hotspot” closely resembling the setting proposed for mantle plumes. However, geologic and geochemical data provide evidence against a plume origin. Instead, the results of recent geophysical studies, coupled with geologic, geochemical, and experimental findings, support the view that both domal uplift and extensive magma generation can be linked to the mechanical removal of a portion or the whole thickness of the mantle lithosphere, accompanied by passive upwelling of normal-temperature asthenospheric mantle to a depth as shallow as 40–50 km. Mechanical removal of the mantle lithosphere might be controlled by delamination in the north beneath the Erzurum-Kars plateau, while it might be linked to slab steepening and break-off in the south. Therefore, magma generation beneath Eastern Anatolia may have been controlled by adiabatic decompression of the asthenosphere. The Eastern Anatolian example is important in showing that not only plumes but also shallow plate tectonic processes have the potential to generate regional domal structures in the Earth's lithosphere as well as large volumes of magma in continental intraplate settings.

In Northeastern Anatolia, the Erzurum-Kars plateau comprises the northernmost part of a volcanic province related to the collision between the Eurasian and Arabian continents. It contains an almost complete record of the volcanism from 11 Ma to 1.5 Ma. Volcanic units on the plateau are calc-alkaline in character and contain a distinct subduction signature. There was a systematic temporal variation in volcanic activity all over the plateau that can be seen in terms of three stages: it initiated with bimodal volcanic products between 11 and 6 Ma (the early stage), turned abruptly into a unimodal intermediate volcanism dominated by andesitic lavas between 6 and 5 Ma (the middle stage), and finally reverted to bimodal activity between 5 and 1.5 Ma (the late stage). These three stages were diachronous as the volcanic succession got progressively younger from the west to the east. The temporal variations were strongly dependent upon the depth of the magma chambers from which the volcanic products were derived. Lavas of the early and late stages were derived from relatively shallow chambers (<10–13 km) that fractionated anhydrous phases and assimilated a minor amount of crustal material or none. In contrast, those of the middle stage were sourced by large, deeper (>13 km), compositionally zoned chambers where amphibole was a fractionating phase and assimilation and fractional crystallization was an important process. The isotopic compositions of the volcanic units do not exhibit a systematic temporal variation on the Erzurum-Kars plateau; instead they exhibit spatial changes. Lavas from the western part of the plateau are much more unradiogenic interms of their Pb isotopic ratios than those from the eastern part. These variations are possibly related to the composition and the amount of crustal material assimilated by the magmas, and hence indicate the existence of two different and isotopically distinct crustal domains beneath the plateau: (1) the Rhodope-Pontide fragment in the west and (2) the Northwest Iranian fragment in the east.