Ultrapotassic Volcanism from the Waning Stage of the Neotethyan Subduction: a Key Study from the Izmir-Ankara-Erzincan Suture Belt, Central Northern Turkey

Upper Cretaceous volcano-sedimentary successions in the Central Pontides of Turkey, related to the closure of the Tethys Ocean, include a variety of alkaline ultrapotassic igneous rocks that have been classified as leucititic, lamprophyric and trachytic based on their mineral paragenesis. Although the ultrapotassic rocks display a range of K2O contents (0.9-8.4 wt %) that may partly reflect alteration processes, they display subduction-related trace element signatures characterized by significant enrichment of large ion lithophile elements and light rare earth elements relative to high field strength elements and heavy rare earth elements and depletion of Nb and Ta. However, their initial Nd-Sr isotope compositions plot within the mantle array. The nature of the mantle source of their parental magmas is inferred to be highly complex, involving contributions from several different components based on contrasting geochemical and isotopic features: (1) a depleted mantle source, which is indicated by unradiogenic Sr-87/(86)Sri (0.70449-0.70609) and radiogenic Nd-143/Nd-144(i) (0.51252-0.51269); (2) an obvious requirement of mantle phlogopite to explain the high potassium contents; (3) slab-derived fluids, which are indicated by ultra-low delta O-18(cpx) ratios regardless of the ultrapotassic rock type (2.4-5 parts per thousand), with high Ba/La and Nb/Ta, low Th/La and the most radiogenic Nd-143/Nd-144(i); (4) a contribution from subducted sediments giving rise to low Ce/Pb ratios and high Th contents; (5) the introduction of convective mantle into the source region with an asthenospheric Pb isotope signature. Whereas the differentiation of silica-undersaturated leucititic and lamprophyric magmas was driven by heteromorphic reactions, owing to the absence of major and trace element variations between the resultant rock types, the formation of silica-saturated trachytic rocks was the result of assimilation-fractional crystallization processes. We propose that a complex sequence of subduction events, starting from at least the Middle Triassic, caused metasomatism of the depleted mantle source and the generation of the Late Cretaceous ultrapotassic parental magmas, facilitated by slab roll-back followed by slab tearing.