FEATURES OF THE TERTIARY VOLCANISM AROUND SEA OF MARMARA

In the region around the sea of Marmara, limited by the boundaries of the 1:500 000 scale Istanbul Quadrangle, the volcanism starting in Upper Cretaceous and intermittently continuing through the end of Upper Miocene has been differentiated into five different stages, namely Upper Cretaceous, Eocene, Oligocene, Lower-Middle Miocene and Upper Miocene, and the volcanic outcrops situated in the region have been dated. Together with the detailed petrographic studies, nine samples from different areas and stages have been dated by K/Ar method, resulting in that the oldest and the youngest lava is of 74.3 ± 1.0 million years old (Upper Cretaceous) and 8.9±0.2 years old (Upper Miocene), respectively. Of these, belonging to the first four stages are mostly calcalkaline (some of the Eocene aged samples are tholeiitic) and are of basalt, basaltic andesite, trachyandesite, andesite, dacite, rhyolite type, whereas that of belonging to the fifth stage are alkaline and of basanite, basalt and trachybasalt types. The pyroclastics of various size and the tuffs of the first four volcanism stages crop out in a wide area. The Upper Cretaceous volcanics have completely formed beneath the sea. On the other hand, some of Eocene volcanics have formed beneath the sea which are seen intercalated with sediments while the others have formed on land. The lavas of Oligocene, Lower-Middle Miocene and Upper Miocene age have formed on land and are observed to be intercalated with lacustrine sediments, in places. Of the lavas stranded along the Black sea coast, the Upper Cretaceous aged ones have formed in a group of island arc volcanics and have been produced in a subduction zone and the Eocene, Oligocene and Lower-Middle Miocene aged ones have formed in an environment of compression during and after the collision and have been produced from a material of crust and mantle mixture. It is proposed that the Upper Miocene aged alcaline basaltic volcanics have formed in an environment of extension by the uplift of mantle after the change of tectonic regime in Middle Miocene.