EOCENE SLAB BREAKOFF EVENTS IN THE TETHYAN SYSTEM: CHEMICAL GEODYNAMICS OF POST-COLLISIONAL MELT EVOLUTION AND MAGMATISM

The early Eocene tectonics of the Tethyan realm, stretching from the Balkan-Anatolian peninsulas in the west through the Caucasus-Iran to Tibet in the east, is characterized by continental collision- induced slab breakoff magmatism and basin evolution. We show here that a series of coeval but separate collisional events within different seaways of the Tethyan system were responsible for slab detachment and asthenospheric heat input beneath the Eocene orogenic belts. Partial melting of the subcontinental lithospheric mantle and assimilation/FC processes produced evolved magmas that developed the post-collisional magmatic units in discrete, E-W-trending belts, straddling the early Eocene suture zones in Anatolia, Iran, Azerbaijan and Tibet. Geochemical characteristics of these slab breakoff magmatic assemblages display major differences, however, indicating that the mantle melt sources and the evolutionary paths of their magmas might have differed significantly along-strike of the suture zones. The lower Eocene volcanic units from western, central and eastern Anatolia are mainly calc-alkaline, whereas the coeval volcanic rocks from Azerbaijan, northern Iran and southern Tibet (Linzizong volcanics) consist of high-K, calc-alkaline to shoshonitic affinities. The melt source of the former group was asthenospheric mantle that was modified by subduction-related components, whereas that of the latter group was a relatively deeper mantle source whose melt products were strongly affected by contributions from continental crust during magma ascent. We present a regional geodynamic model, which explains the magmatic, tectonic and landscape evolution of the Eocene collisional belts via slab breach in an otherwise continuous, Cenozoic subduction zone tectonics within the entire Tethyan system.