Geological and geochemical characteristics of Late Ordovician volcanic levels of the Gözdağ Formation in the İstanbul-Zonguldak Tectonic Unit (NW Turkey): Implications for global events during the Ordovician

The İstanbul-Zonguldak Tectonic Unit is regarded as the easternmost fragment of Avalonia-Carolina and stands for Far East Avalonia. Its stratigraphy also is characterized by discontinuous sedimentation from Late Ediacaran to Late Carboniferous. In the western part of the block, known as the İstanbul Terrane, the Gözdağ Formation is represented by lagoonal sedimentary rocks consisting of shale-sandstone with limestone of Middle Ordovician-Lower Silurian age. Here, I report on stratigraphic positions and petrographical and geochemical data of fine- and coarse-grained tuffs and lavas in the Late Ordovician strata of the Gözdağ Formation. The fine- and coarse-grained tuffs have pyroclastic and the lavas have porphyritic, vitrophyric and aphanitic textures. The fine-and coarse-grained tuffs are Sandbian and Katian ages, and the lavas are Hirnantian ages, according to the stratigraphic positions of the Late Ordovician volcanic rocks. Geochemically, the-fine grained tuffs have High-K calc-alkaline character and the coarse-grained tuffs and lavas have calc-alkaline affinity. They are devoid of noticeable with-in plate components, as deduced by the presence of obvious negative Nb anomalies, and they have subduction signatures. In conjunction with the data from literature, the Sandbian fine-grained tuffs were deposited in a lagoonal depocenter in the İstanbul-Zonguldak Tectonic Unit during the Taconic orogeny formed in the earliest late Ordovician between the Piedmont Terrane and Laurentia. The Katian coarse-grained tuffs were deposited in a shallow marine in it during the last phase of the consumption of the Teisseyre-Tornquist Ocean from volcanoes occurred in volcanic arc-related settings. Besides, I the Hirnantian lavas flowed in a lagoonal depocenter in it during the soft-docking of Avalonia and Baltica, known as the pre-Caledonian orogeny.