U-Pb zircon geochronology and geochemistry of mafic and intermediate dykes in the Istanbul-Zonguldak Tectonic Unit (NW Turkey): evidence for Devonian and Carboniferous magmatism and the north-subducting Rheic Ocean in Far East Avalonia

Abstract

The Istanbul-Zonguldak Tectonic Unit, which is devoid of Devonian-Carboniferous magmatism, is a continental fragment of Far East Avalonia. It was deformed by thrusting due to the collision with the Sakarya terrane with Minoa origin during the Carboniferous, and forms the Variscan foreland. The tectono-magmatic evolution of Devonian-Carboniferous, thus consumption evolution of the Rheic Ocean in Far East Avalonia, is poorly known. Diabase and basaltic andesite dykes show Late Devonian to Late Carboniferous igneous crystallization ages in the Istanbul-Zonguldak Tectonic Unit (NW Turkey). They have aphanitic to vitrophyric textures. U-Pb dating on igneous zircons from diabase and basaltic andesite dykes yielded Late Devonian age of ca. 381.3 +/- 1.8 Ma (2 sigma) and Late Carboniferous ages of ca. 306.1 +/- 3.3 to 301.5 +/- 1.1 Ma (2 sigma), respectively. Geochemically, Late Devonian magmatism shows tholeiitic affinities and contains subduction and with-in plate components, but Late Carboniferous magmatism displays calc-alkaline signatures and includes subduction components. The Late Devonian dykes intruded into the continental block during the rifting of the Pripyat-Dnieper-Donets Basin in Baltica as a result of the north-subducting Rheic Ocean. While it proceeded to subduct under Far East Avalonia along the magmatic arc that was similar to the Hanseatic arc in East Avalonia, Late Carboniferous dykes emplaced into it in an arc-related setting. Also, Late Carboniferous magmatism that is related to magmatic arc displays that the collision of the Istanbul-Zonguldak Tectonic Unit and Sakarya terrane, described as the Variscan orogeny in the Pontides did not form during the Carboniferous.