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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

By
Namık Aysal
Namık Aysal
1
Department of Geological Engineering, Faculty of Engineering, İstanbul University, 34320 Avcılar, İstanbul, Turkey
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Mehmet Keskin
Mehmet Keskin
1
Department of Geological Engineering, Faculty of Engineering, İstanbul University, 34320 Avcılar, İstanbul, Turkey
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Irena Peytcheva
Irena Peytcheva
2
Geological Institute, Bulgarian Academy of Sciences, Sofia, Bulgaria
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Olgun Duru
Olgun Duru
3
Graduate School of Science and Engineering, İstanbul University, 34126 Vezneciler, İstanbul, Turkey
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Published:
January 01, 2018

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.

Supplementary material: Tables of representative analyses are available at https://doi.org/10.6084/m9.figshare.c.3841276

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Contents

Geological Society, London, Special Publications

Petroleum Geology of the Black Sea

M. D. Simmons
M. D. Simmons
Halliburton, UK
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G. C. Tari
G. C. Tari
OMV, Austria
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A. I. Okay
A. I. Okay
Eurasia Institute of Earth Sciences, Turkey
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Geological Society of London
Volume
464
ISBN electronic:
9781786203724
Publication date:
January 01, 2018

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