Petrological evolution and tectonic implications of the late cretaceous leucite-bearing basalts and lamprophyres of the Ankara-Erzincan suture belt

Transition from a long-lasting period of subduction to continental collision represents one of the most complex environments for magma generation. Transformation of an arc-system into a collisional orogeny will be usually accompanied with volcanism that geochemically mimics the older arc lavas, but with important ingredients derived from the continental lithosphere. Moreover, this syn- collisional episode will evolved into a post-collisional phase during which volcanism may also be extremely variable. Post-collisional volcanism is usually ascribed to combinations of several processes, including slab detachment, delamination, slab roll back and extension, and each one of these processes may involve different sources and melting regimes. In the simplest case, the magma source will be conductively- heated, sub-continental mantle lithosphere usually heavily metasomatised. Additionally complicating factors may be the involvement of the slab tear or ridge subduction, that substantially change the thermal structure of the mantle, as well as metasomatic processes resulting in the generation of more silica undersaturated magmas. The Cretaceous period in the Alpine-Himalayan Mountain Belt is best characterized by the convergence between continents that terminated the northern branch of NeoTethys Ocean. This subduction was followed by diachronous collision of the megacontinents Laurasia and Gondwana with numerous continental slivers during Tertiary, which resulted in formation of a suture belt. The middle segment of the suture belt is named the Izmir-Ankara-Erzincan Suture Zone (IAESZ). It corresponds to the Vardar Suture Zone in Eastern Europe and the Sevan-Akera Zone in the Lesser Caucaus. The collision started first during the early Paleocene in western Anatolia, and slightly later further east. Although the exact timing of this event is still under discussion, further evidence for subduction termination is provided by postcollisional magmatism, as its volcanic products abundantly overlie heterogeneous basement units juxtaposed before Eocene. Upper Cretaceous time represents part of a global subduction period along Eurasian margin which caused the formation of the Pontide magmatic arc in Anatolia, the Apuseni-Banat-Timok-Srednogorie belt situated in south-eastern Europe and the Somkheto–Karabakh arc in Lesser Caucaus. The subduction initiation is dated as Middle Jurassic to Upper Cretaceous based on the volcanoclastic series around the Lesser Caucasus, whereas in the Pontides the initiation of subduction is proposed to be in the Middle Jurassic period, although this age is a matter of ongoing discussion. It is generally accepted that the European margin is represented by a huge southward growing, subduction-accretionary complex that is at least 100 km wide, starting from Permian or Triassic in the Central Pontides. Along Central Pontides, in Upper Cretaceous, subduction generated a back arc basin represented by Black Sea, and simultaneously formed a fore-arc accretionary prism. The arc volcanism is dominated by Upper Cretaceous-Paleocene calcalkaline magmatic rocks. However, slighly younger volcanism occurred in the fore-arc sedimentary melange, including a number of dykes of ultrapotassic, High-K and adakitic lavas, which are atypical for an active convergent margin In the Eastern Pontides, the leucite-bearing volcanism was associated with the final phase of subduction and was coeval with the initiation of collision.Therefore active arc volcanism worldwide, dominantly shows calkaline affinity, and there are only a few examples where ultrapotassic rocks occur, such as in Indonesia, Mexico and Japan etc. The earliest age reported for Pontide arc volcanism is Turonian. Younger, well defined forearc basins in which Cenomanian and Maastrichtian aged turbidites, pelagic limestones intercalated with lava flows and related clastics were deposited, also host the potassic products of Upper Cretaceous volcanism. However, in the case of the Central Pontides, it is not possible to simply draw the lithotectonic belts, representing continental arc units, fore arc units and melange units. Therefore slab rollback simultaneous with the growth of an accretionary prism may have caused the arc to migrate southward which is evident by stratigraphical features and paleomagnetic data which suggests approximately 450 km shift of arc magmatism from Turonian to Campanian. The most reliable time constraints come from U-Pb zircon and Re-Os ages which documented arc hinge migration from north to south during a 14 Myr time period in Srednogorie Zone. Upper Cretaceous volcono-sedimentary succession related to the consumption of the Tethys Ocean includes a variety of alkaline ultrapotassic rocks that were identified as dykes, stocks and lava flows in the Central Pontides of Turkey. This PhD study focuses on these rocks that are classified as leucititic, lamprophyric and trachytic varieties based on their mineral paragenesis by evaluting their major and trace element geochemistry, Sr, Nd and Pb isotopes and mineral chemistry. Although the detailed stratigraphical work reveals that the ultrapotassic rocks are related to suture belt rocks and forearc units in the Upper Cretaceous Pontide Arc, 40 Ar/ 39 Ar radiometric age analysis are also presented to constrain timing and establish their geodynamic setting. Although these ultrapotassic rocks display a variation of K 2 O contents (0.89-8.39 wt.%) that were induced by alteration processes (mainly analcimization), they are characterised by significant enrichment of LILE and LREE relative to HFSE and HREE with Nb and Ta depletion which are very common features for subduction- related magmas. Initial strontium and neodymium isotope compositions of ultrapotassic rocks are restricted to the mantle array field (87Sr/86Sr (i) :0.70449-0.70609, 143Nd/144Nd (i) : 0.51252-0.51268) and indicate that a depleted mantle source was an important component of the source for these ultrapotassic magmas. The nature of the source region is highly complex and involved contributions of several components.