Faruk BERBER, Kaan SAYİT, Mehmet Cemal GÖNCÜOĞLU

Geochemistry and U-Pb ages from the Kösdağ Metavolcanics in the southern Central Pontides (Turkey): Complementary data for early Late Cretaceous island arc development in the Northern Neotethys

The Kösdağ Metavolcanics (KMs) in the southern Central Pontides are exposed between the İzmir-Ankara-Erzincan Suture Belt in the south and the Sakarya Composite Terrane in the north. They comprise an approximately 40-km-long tectonic unit, bounded by the splays of the North Anatolian Transform Fault in the north and the Kızılca Thrust in the south. The basement of the unit mainly consists of metabasalts, metaandesites, and metarhyolites, with well-developed blastomylonitic textures, which are interlayered by recrystallized pelagic limestone and chert. Late Cretaceous pelagic limestones of the Dikmen Formation disconformably overlie the basement. Geochemically, the KMs exhibit enrichment in Th and La relative to Nb (and Ti), indicating subduction-related magmatic signatures. The KMs are subdivided into two main types, as Type 1 and Type 2, based on their relative Zr-Hf enrichment/depletion features. All of the members of the KMs have a subalkaline nature (Nb/Y = 0.08–0.19 for Type 1; Nb/Y = 0.05–0.13 for Type 2) and display a calc-alkaline affinity. The high Zr/Nb (38.1–52.9 for Type 1, 21.8–41.2 for Type 2), low Zr/Y (4.07–5.25 for Type 1, 1.58–2.44 for Type 2), and Nb/Y (0.08–0.14 for Type 1, 0.05–0.10 for Type 2) signatures of the KMs indicate that they have derived from a depleted source, which has been modified by a subduction component. The laser ablation inductively-coupled plasma mass spectrometry (LAICP-MS) U-Pb zircon ages of the metarhyolite samples ranged between 94.64 ± 0.77 Ma and 113.2 ± 2.3 Ma, suggesting the presence of an intraoceanic subduction zone during the early Late Cretaceous within the İzmir-Ankara-Erzincan branch of the Neotethys Ocean in the Central Pontides.

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