Geochronology, geochemistry and isotope systematics of a mafic–intermediate dyke complex in the İstanbul Zone. New constraints on the evolution of the Black Sea in NW Turkey

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. 87Sr/86Sr(i) values of the mafic and intermediate dykes and small stocks span a range from 0.703508 to 0.706311, while their 143Nd/144Nd(i) values vary from 0.512614 to 0.512812 and eNd(i) values from 5.09 to 1.24. NdTDM 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 (logƒO2) 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. H2Omelt 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.

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

“…We note that a slab rollback model associated with the northward subduction of the northern branch of the Neotethys lithosphere has also been proposed to explain the generation of the Late Cretaceous magmatic rocks both in the EP (e.g., Çinku et al, ; Karsli et al, ; Karsli, Caran, et al, ) and the western Pontides (e.g., Aysal et al, ; Keskin & Tüysüz, ) on the basis of an assumed southward migration of magmatism that was inferred from the paleomagnetic data (Çinku et al, ). However, as discussed earlier, existing data indicate that the Late Cretaceous magmatism did not show a simple southward migration but covers a broader region not only in the northern zone (Ordu‐Güdül) but also in the southern zone (Gümüşhane‐Bayburt) of the EP (Figure b).…”

The phanerozoic palaeotectonics of Turkey. Part I: an inventory

“…The associated magmatic arc crops out discontinuously north of and within the city of İstanbul and its andesites and andesitic basalts have yielded ages ranging from 75 Ma to 65 Ma (Yavuz and Yılmaz 2009;Fig. 9 A simplified rendering of the detailed seismic profile DG-1256 of Özaydın and Erol (1981) that clearly shows the southerly imbrication of an ophiolite and the ophiolitic mélange along the southern rim of the Strandja Mountains in Thrace Aysal et al 2017). One microdiorite from the southeast of the city gave an age of 58 ± 1.8 Ma indicating the survival of the arc magmatism here into the late Palaeocene.…”

Major Late Cretaceous Mass Flows in Central Turkey Recording the Disruption of the Mesozoic Continental Margin

“…There are two lines of evidence for such an orientation: (a) the east‐west trending Istanbul Zone and the eastern Sakarya Zone were amalgamated before the Late Jurassic in the central Pontides (Okay et al, ), whereas an oceanic embayment, the Intra‐Pontide Ocean, existed between the Istanbul Zone and the western Sakarya Zone until the latest Cretaceous; this necessitates a bend between the western and eastern Sakarya zones. (b) The arc magmatism in the western Sakarya Zone starts in the Campanian (approximately 78 Ma; e.g., Aysal et al, ). In contrast, there is ample evidence for earlier arc magmatism in the eastern Sakarya Zone (Eastern Pontides, 91–72 Ma; e.g., Aydınçakır, ; Eyuboglu, ; Kaygusuz et al, ; Liu et al, ; Özdamar, ).…”