Petrology and geochemistry of the Neo-Tethyan volcanism as revealed in the Ankara melange, Turkey

The Ankara melange in north-central Turkey represents a typical convergent margin melange which was developed within the Izmir–Ankara–Erzincan suture zone as the northern branch of the Neo-Tethys closed between the Rhodope–Pontide continental fragment in the north and the Kirsehir metamorphic massif in the south. From north to south, the Ankara melange includes three mappable units, a metamorphic block melange, limestone block melange, and ophiolitic melange, that are distinguished based on the nature of their constituent block types. The ophiolitic melange occurs in a structurally lowermost tectonic position as part of a south-directed imbricate thrust system and contains two separate groups of magmatic rocks characterizing different modes of volcanism in the Neo-Tethys. One group contains several incomplete fragments of the Neo-Tethyan oceanic lithosphere that are considered as ophiolite massifs and mafic and ultramafic rock clasts in a serpentinized matrix. The ophiolitic massifs (Edige and Kalecik) consist of lower crustal and upper mantle units of the Neo-Tethyan oceanic lithosphere that are intruded at all structural levels by doleritic to microgabbroic dike swarms. The other group of rocks includes mainly basaltic massive to pillow-lava flows locally intercalated with turbiditic limestone, red argillite, and radiolarite breccias and occurs as clasts and individual megablocks in a tuffaceous–shaly matrix. Dike intrusions in the ophiolitic massifs fall into two separate geochemical groups on the basis of their rock chemistry. One group of dikes, combined with basaltic lava fragments from the melange, has subalkaline characteristics and shows incompatible element abundances characteristic of N-MORB chemistry. The second group of dikes displays typical island-arc tholeiite signatures with large-ion lithophile (LIL) enrichment and high-field strength element (HFS) depletion compared to N-MORB. Mafic extrusive rocks of the second group of magmatic rocks in the ophiolitic melange have REE and incompatible element concentrations characteristic of oceanic island type (OI) alkaline basalts. Combined with their stratigraphic relations with the deep marine sedimentary rocks, these alkaline mafic volcanic rocks are interpreted to represent volcanic build-up and/or seamount related volcanism on the Neo-Tethyan ocean floor. ree distinct types of volcanism recorded in the ophiolitic melange and the regional geological constraints suggest the following evolutionary history for the Neo-Tethyan ocean floor: the oceanic lithosphere as preserved in the ophiolitic massifs of this study was developed at a mid-ocean ridge system within the northern branch of the Neo-Tethys during the Cretaceous. Point-source volcanism associated with mantle plume activities formed volcanic build-ups and/or seamounts on the newly formed Neo-Tethyan ocean floor. This Neo-Tethyan oceanic lithosphere was subsequently situated in a suprasubduction zone environment, where the subduction related magmatic plumbing system produced dike swarms with island arc tholeiite chemistry. This evolutionary history of the magmatic rocks is consistent with the general history of the eastern Mediterranean region and indicates multiple and distinct episodes of volcanism associated with ocean floor generation in the Neo-Tethys.