Geological Evolution of the Ankara Mélange (Central Anatolia, Turkey) in İzmir-Ankara-Erzincan Suture Zone (IAESZ)

The Pontides (Laurasian affinity) in the north and the Anatolides-Taurides (Gondwana affinity) in the south constitute two of the main tectonic units of Turkey. They were once separated by a Mesozoic Neo-tethyan ocean. The İzmir-Ankara-Erzincan Suture Zone (IAESZ) represents the boundary between them along which the Neo-tethyan Ocean was subducted. The Ankara Mélange is located approximately in the centre of the of the IAESZ, and is one of the first mélanges described. A part of the Ankara Mélange, the Eldivan region was studied to reveal its structure, origin and age. There are three main units: an ophiolite slice, an ophiolitc mélange and flyschoidal sedimentary sequence. These N-S striking units were imbricated along with thrust faults verging towards the west. Peridotites, pyroxenites, rare layered gabbros, isotropic gabbros, diabases and plagiogranites (trondhjemites) are found within the ophiolite slice. Although a pseudo-stratigraphic contact between mantle and crustal rocks cannot be observed, the ophiolite slice has a partial internal structure observed from bottom to top with peridotites, gabbros and diabases. Three plagiogranite samples yielded Early Jurassic U- Pb zircon ages of 177.4 ± 1.0 Ma, 176.2 ± 3.1 Ma and 177.1 ± 2.1 Ma. The ophiolitic mélange is composed of basalts, radiolarian cherts, pelagic limestones, mudstones and shallow marine (neritic) limestone blocks. The age of shallow marine limestones is determined as Late Jurassic – Early Cretaceous based on Crescentiella sp. and Verneuilinoides sp. Previous geochemical studies revealed that basalts within the ophiolitic mélange show OIB characteristics, while gabbros, diabases and plagiogranites in the ophiolite slice show SSZ characteristics. The presence of ocean island basalts and shallow marine limestones within the ophiolitic mélange indicates an oceanic seamount environment during the Early Jurassic-Early Cretaceous. The flyschoidal sedimentary rocks probably represents accreted Late Jurassic-Late Cretaceous (mostly Cenomanian) fore-arc deposits. Preliminary results show that the Ankara Mélange located in IAESZ in the Eldivan region is composed of tectonic units, which were formed during Early Jurassic to Early Cretaceous and accreted during different periods to the southern margin of Laurasia.