THE CONSECUTIVE CLOSURE PHASES OF THE IAPETUS, RHEIC AND PALAEO-TETHYS IN THE ISTANBUL ZONE: U-PB AGES FROM CROSS-CUTTING DYKES

The İstanbul Zone is considered to have undergone multiple deformational phases. This tectonic unit with its base exposed in the Sünnice Massif consists of migmatic quartzo-feldspathic gneisses (Demirci Paragneisses) and low-grade meta-volcanics (Yellice meta-volcanics; 1782±130 Ma), which are intruded by the Dirgine Granite (576-565 Ma). The basement is tectonically overlain by a conformable sedimentary sequence comprising the fluvial clastics (Kurtköy Formation) that overlie the lacustrine deposits (Kocatöngel Formation). The terrestrial rocks are covered by the beach deposits (Aydos Formation), the shelf deposits (Fındıklı Formation-Sandbian-Lochkovian- corresponding to the Gözdağ-Dolayoba-Istinye Formations in İstanbul) and the offshore deposits (Ferizli-Yılanlı Formations-Lochkovian-Visean, equivalents of the Kartal-Tuzla Formations in İstanbul).

We offer field and U-Pb age data linked to variously oriented dykes cross-cutting some of these units. The Demirci paragneiss is intruded by N80⁰E trending dacite dykes (485.7±3.6 Ma) and N15⁰W trending dacite dykes (443.0±5.4 Ma) with a collisional tectonic setting; N6⁰E trending basaltic andesite dykes (301±1.6 Ma), N75⁰E trending granite dykes (262.9±3.4 Ma), N80⁰W trending andesite dykes (261.4±1.7 Ma) with subduction signatures. The Yellice meta-volcanics are intruded by N40⁰-50⁰E trending basalt dykes (268.2±2.4 Ma) with a subduction signature. N18⁰E trending basaltic andesite dykes (318±16 Ma) with a subduction signature crosscut the Yılanlı Formation.

The Cambro-Ordovician and Ordovician-Silurian ages suggest events linked to the Caledonian Orogeny, first and second closure phases of the Tersseyre-Tornquist Sea- a branch of the Iapetus. The Ordovician-Silurian age is also in agreement with interbedded volcanoclastics rocks of andesitic and trachytic compositions in the sandstone-siltstone of the Gözdağ Formation having a lower age of Darriwilian and an upper age of Telychian. The Upper Carboniferous ages are related to the Variscan Orogeny-late closure phase of the Rheic, and the Permian ages are associated with the Hercynian Orogeny-initial closure phase of Palaeo-Tethys.