Within the Late Cretaceous melange complex of the Izmir-Ankara suture zone in central Sakarya area, north-western Turkey, a megablock with radiolarian cherts associated with basaltic pillow lavas has been dated by radiolarians. The studied radiolarian assemblage and conodonts yielded an early Late Carnian age. This age is the oldest obtained from the chert blocks all along the suture belt and has important implications for the rifting/opening age of the Izmir-Ankara branch of the Tethys ocean in north-western Turkey. Based on this new data, it is concluded that during the Late Triassic the Izmir-Ankara seaway was connected to the other Tethyan oceanic branches and deep enough to provide chert sedimentation and exchange radiolarians with the main open oceans. It is further suggested that the long-lasting misinterpretation of a Liassic rifting/opening of the Izmir-Ankara ocean should be revised. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.