Early Permian deformational ages of Late Carboniferous basalt dykes in the İstanbul-Zonguldak Tectonic Unit: Implications for the Variscan orogeny in the Pontides

The İstanbul-Zonguldak Tectonic Unit (NW Turkey) is regarded as the unmetamorphosed continental fragment of Far East Avalonia in the Pontides. It is to the east of the Rhodope-Strandja Massif, which is a part of the metamorphosed section of Far East Avalonia, and is to the north and west of the Sakarya terrane. The Variscan orogeny in the Pontides defines as the collision of the Sakarya terrane with the İstanbul-Zonguldak Tectonic Unit during the early Carboniferous. But, the Late Carboniferous arc magmatism (c. 306-301 Ma) in the eastern sector of the İstanbul-Zonguldak Tectonic Unit rejects this view. Here, I present analytical data of basalt dykes in the western sector of the İstanbul-Zonguldak Tectonic Unit. Basalt dykes have porphyritic and holohyaline textures. Geochemically, they display calc-alkaline affinities and show depletion in Nb relative to Ce. They contain subduction components and are associated with the arc-related geodynamic setting. U–Pb dating on igneous zircons from two basalt dykes yielded Late Carboniferous ages of ca. 321.6 ± 1.6 and 311.4 ± 0.75 Ma (2σ), and their Pb-loss ages from the white spot in zircons calculated Early Permian ages of ca. 295.1 ± 1.1 to 285.0 ± 1.3 Ma and 295.5 ± 1.2 to 284.0 ± 1.4 Ma, respectively. In conjunction with the data from the literature, the Late Carboniferous arc magmatism (c. 321-311 Ma) in the western side of the İstanbul Zonguldak Tectonic Unit corresponds to the Late Carboniferous arc magmatism (c. 306-301 Ma) in the eastern side of the İstanbul Zonguldak Tectonic Unit, thus indicating that the Rheic Ocean continued to subduct under Far East Avalonia during the Carboniferous. As for the Pb-loss ages obtained from the Late Carboniferous arc basalt dykes, the earliest-latest Cisuralian ages (c. 295-285 Ma) correspond to regional deformation events at the Carboniferous-Permian boundary in the Rhodope-Strandja Massif (c. 298-296 Ma) and at the ending of the early Permian in the Sakarya terrane (c. 282-275 Ma), respectively. All in all, I suggest that the docking of Far East Avalonia, including the İstanbul-Zonguldak Tectonic Unit and Rhodope-Strandja Massif, with the Sakarya terrane formed during the early Permian instead of the early Carboniferous.