Abstract: The İstanbul-Zonguldak Tectonic Unit is regarded as the eastern most fragment of Avalonia-Carolinaand designated as Far East Avalonia. Its stratigraphy is characterized by discontinuous sedimentation from Late Ediacaran to Late Carboniferous. In the western part of the block, known as İstanbul Terrane, the Gözdağ Formationis 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- andcoarse-grained tuffs and lavas in the Late Ordovician strata of the Gözdağ Formation. The fine- and coarse-grainedtuffs have pyroclastic and the lavas have porphyritic, vitrophyric and aphanitic textures. The fine-and coarse-grained tuffs are of Sandbian and Katian ages, and the lavas have Hirnantian ages, according to the stratigraphic positionsof the Late Ordovician volcanic rocks. The fine-grained tuffs have high potassium calc-alkaline, and the coarsegrained tuffs and lavas have a calc-alkaline character. 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 data from the literature, the Sandbian fine-grained tuffs were deposited in a lagoonal depocenter in the İstanbul Zonguldak Tectonic Unit in the earliest Late Ordovician due to multiple Plinian-type eruptions during the last phaseof the Taconic orogeny, which formed between the Piedmont Terrane and Laurentia. The Katian coarse-grained tuffs were products of volcanic activities formed in the arc settings during the last stage of depletion of the TeisseyreTornquist Ocean, lying between Avalonia and Baltica. The Hirnantian lavas were formed by flowing in a lagoonaldepocenter of the İstanbul-Zonguldak Tectonic Unit during the soft-docking of Avalonia and Baltica, known as thepre-Caledonian orogeny