The Bosphorus Volcano: remnants of an ancient volcano on an ancient city

In many ancient and active volcanic provinces, dyke systems are commonly disposed radially and concentrically around the volcanic chimney, represented by a central intrusive body in many eroded examples. In Istanbul, NW Turkey, late Cretaceous dykes, which are emplaced into the pre-Cretaceous basement consisting of sedimentary rocks of Palaeozoic and Triassic ages, have both patterns. In and around Istanbul, late Cretaceous magmatism is now represented by three types of igneous structures distributed into three separate outcrop areas, (1) The Çavuşbaşı pluton, (2) volcano-sedimentary units and (3) dykes, although the volcanics contain some dykes that intrude them. The zircon U–Pb crystallization age determined in this study from the Çavuşbaşı pluton is 67.24 ± 0.32 Ma, consistent with the ages of 67.91 ± 0.63 and 67.59 ± 0.5 Ma given in the literature. Geobarometry results we obtained from the crystal chemistry of amphiboles indicate that the Çavuşbaşı pluton was emplaced at a shallow depth between 3.2 and 4.7 km making it an epizonal pluton. It is surrounded by an indistinct contact aureole of about 500 m in width. Volcano sedimentary units, preserved mostly to the north of Istanbul along the Black Sea coast were deposited on the slopes of a volcano characterised by agglomerates and flows with some sills and dykes. Three types of dykes have been identified in the region: (1) lamprophyre dykes, (2) diabase dykes and (3) intermediate to felsic dykes (72.49 ± 0.79 to 65.44 ± 0.93 Ma). The presence of these dykes and the pluton with different petrology and crystallisation depths indicates the presence of a magma plumbing system beneath the study area consisting of a set of magma chambers, some of which are interconnected to each other, emplaced at variable crustal depths (i.e., polybaric crystallization). Upper Cretaceous dykes are concentrated around the Çavuşbaşı pluton and extend almost as far as 30 km away from the pluton. The intrusion of the plutonic body of the Çavuşbaşı pluton created a domal structure in the basement rocks. The formation of this dome structure may have controlled the stress field and the orientation of the dyke system. Similar patterns are observed in the British Tertiary igneous province, Galapagos volcanoes, Boa Vista (Cape Verde), Summer Coon volcano and Spanish Peak Mountain, Dike Mountain (Colorado), Vesuvius, Etna, Stromboli and in connection with many other volcanoes around the world. We suggest that the Upper Cretaceous volcanic edifice, composed of the Çavuşbaşı Granite pluton (chimney), dykes (the radial and concentric dykes around the chimney) and the lavas and agglomerates (slopes of the volcanic mountain) in the Istanbul region is related to an arc volcano similar to the andesitic volcanoes in Sumatra and Java in Indonesia, in Kamchatka, and those in Japan and the Philippines; we name it the Bosphorus Volcano.