Mineralogy and petrology of the late cretaceous rinkite-bearing Bas,o<spacing diaeresis>ren nepheline-sodalite syenite, Central Türkiye

Late Cretaceous nepheline-sodalite syenites from the Bas,oren area in central-eastern T & uuml;rkiye include complex Na-K-Ca-Fe-bearing silicates such as rinkite-(Ce) and represents the first occurrence of agpaitic rocks among the extensive Late Cretaceous magmatic units in the region. The Bas,oren nepheline syenites include three distinct rock types: (1) rinkite-bearing early-, (2) rinkite-free late-magmatic rocks, and (3) pyrochlore-bearing, cumulate-like rocks akin to lujavrite found in other layered alkaline igneous complexes such as the Il & iacute;maussaq, South Greenland and the Lovozero, Kola Peninsula, Russia. The Bas,oren rocks exhibit a sodic affinity and demonstrate silica-undersaturated compositions at a minimum pressure of 1 kbar, consistent with their highly fractionated characteristics. The alkalinity indexes [AI = molar [(Na + K) / Al)] and Fe# values [= molar Fe / (Fe + Mg)] of the early-and late-magmatic rocks vary in the ranges of 0.88-1.09, and 0.89-1.00, respectively. The clinopyroxene in these rocks are essentially aegirine augite in composition. However, the cumulate-like lujavritic rocks display higher AI values of 1.16-1.70 and Fe# values of 0.99-1.00 and contain aegirine. Both the early-and late-magmatic samples exhibit several common characteristics, including pronounced depletion in Ba, Sr, Ti, and P, a limited presence or complete absence of high field strength elements relative to the large ion lithophile elements, and considerable variability in the abundances of rare earth elements (REE). The early-magmatic rocks show steep patterns from light REE to medium REE (La to Eu), and convex-upward patterns from medium to heavy REE (Gd to Lu). The late-magmatic samples exhibit similar patterns to the early-magmatic ones but have lower REE abundances. However, the lujavritic rocks display significantly lower REE abundances with convex-downward patterns. Overall, the nepheline-sodalite syenites in the Bas,oren area represent undersaturated melts that were formed by extreme fractionation of silica-undersaturated K-alkaline basaltic magmas generated in a back-arc setting.