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
Namık Aysal, Mehmet Keskin, Irena Peytcheva, Olgun Duru, 2018. "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", Petroleum Geology of the Black Sea, M. D. Simmons, G. C. Tari, A. I. Okay
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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.
Supplementary material: Tables of representative analyses are available at https://doi.org/10.6084/m9.figshare.c.3841276
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The Black Sea remains one of the largest underexplored rift basins in the world. Future success is dependent on a better understanding of a number of geological uncertainties. These include reservoir and source rock presence and quality, and the timing of migration of hydrocarbons relative to trap formation. An appreciation of the geological history of the Black Sea basins and the surrounding orogens is therefore key. The timing of basin formation, uplift of the margins, and of facies distribution remain issues for robust debate. This Special Publication presents the results of 15 studies that relate to the tectono-stratigraphy and petroleum geology of the Black Sea. The methodologies of these studies encompass crustal structure, geodynamic evolution, stratigraphy and its regional correlation, petroleum systems, source to sink, hydrocarbon habitat and play concepts, and reviews of past exploration. They provide insight into the many ongoing controversies concerning Black Sea regional geology and provide a better understanding of the geological risks that must be considered for future hydrocarbon exploration.