Petrology and geochemistry of post-collisional Middle Eocene volcanic units in North-Central Turkey: Evidence for magma generation by slab breakoff following the closure of the Northern Neotethys Ocean

The Eocene volcano-sedimentary units of Northern Anatolia are confined into a narrow zone trending parallel to the Intra Pontide and İzmir–Ankara–Erzincan sutures, along which the northern branch of the Neotethys Ocean was closed during a period between Late Maastrichtian and Paleocene. The Middle Eocene formations overlie both the imbricated and highly deformed units of the suture zone, which are Paleocene or older in age, as well as the formations of adjacent continental blocks with a regional disconformity. Therefore, they can be regarded to be post-collisional. These units are composed of subaerial to shallow marine sedimentary beds (i.e. the Örencik formation) at the base and a subaerial volcanic unit (i.e. the Hamamözü formation) in the middle and at the top. This sudden facies change from marine to subaerial environment in the Middle Eocene is a common phenomenon across northern Turkey, implying that a regional uplift event occurred possibly across the suture zone before the initiation of the volcanism during Lutetian. The Middle Eocene lavas span the whole compositional range from basalts to rhyolites and display a calc-alkaline character except for alkaline to mildly-alkaline lavas from the top of the sequence. All lavas display a distinct subduction signature. Our geochemical data indicate that calc-alkaline lavas were derived from a subduction-modified source, whereas alkaline to mildly-alkaline lavas of the late stage were possibly sourced by an enriched mantle domain. Magmas evolved in magma chambers emplaced possibly at two different crustal levels. Magmas in deeper (> 13 km) and possibly larger chambers fractionated hydrous mafic minerals (e.g. amphibole and biotite), two pyroxenes and plagioclase and assimilated a significant amount of crustal material. Intermediate to acid calc-alkaline lavas and pyroclastics were derived from these chambers. Magmas in the shallower chambers, on the other hand (~ < 12 km), crystallized anhydrous mineral assemblages, assimilated little or no crustal material and fed basic to intermediate lavas in the region. Both deep and shallow chambers were periodically replenished by mafic magmas. We argue that a slab breakoff model explains better than any alternative model (i) why the volcanism during the Middle Eocene was confined into a rather narrow belt along the suture zone, (ii) why it initiated almost contemporaneous with a regional uplift after the continental collision event, (iii) why it postdated arc volcanism along the Pontides in the north by 15–20 My, (iv) why it assimilated significant amount of crustal material, and (v) why alkalinity of lavas increased in time.