Migration of the Upper Cretaceous subduction-related volcanism towards the back-arc basin of the eastern Pontide magmatic arc (NE Turkey)

Abstract

The Eastern Pontide active continental margin extends along the eastern part of the Black Sea with an E–W trend and comprises the parallel oriented northern, southern and axial zones. Each zone is fault bound and distinguished by distinctive magmatism, lithofacies and stratigraphy. In the northern zone, from the coast of the Black Sea to Torul, the cessation of Liassic bimodal volcanism and accumulation of anoxic carbonates of the Valanginian–Barremian was followed by bimodal volcanism which evolved from a tholeiitic (TH)–calc-alkaline (CA) to high K–CA composition. These volcanics erupted sporadically in asymmetric deep basins until the end of the Senonian. In the southern zone, around Torul and Gümüşhane, Liassic bimodal volcanism and the accumulation of the Malm–Cenomanian platform carbonate was succeeded by Campanian volcanism, i.e. much later than volcanism in the northern zone. A second epoch of CA–high K–CA andesitic volcanism occurred in the southern zone and is intercalated with a Campanian red pelagic limestone which is also widespread in the northern zone, and known as the marker lithologic unit in the eastern Pontides. South of the southern zone, in the Bayburt–Maden area, the pelagic limestone of the Early Cretaceous and overlying ophiolitic olistostromal mélange (which contains Cenomanian MOR, WP and IA basaltic pillow lavas) are overlain by high–K calc-alkaline Campanian andesitic volcanic rocks, implying that the ophiolitic mélange was formed in a back-arc environment. The major and trace element geochemistry and REE patterns of the Upper Cretaceous volcanic rocks suggest a variation from hydrous IA to anhydrous back-arc melting conditions on the subduction zone and the migration of arc magmatism laterally from the northern to the southern zones during the Senonian. This change to dehydration melting and the migration of the subduction-related Late Cretaceous volcanism towards the south in the eastern Pontide Magmatic arc, requires a south-dipping subduction polarity during the Senonian. Copyright © 1999 John Wiley & Sons, Ltd.