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