Mineralogy and petrology of the late cretaceous rinkite-bearing Başören nepheline-sodalite syenite, Central Türkiye

Abstract

Late Cretaceous nepheline-sodalite syenites from the Başören area in central-eastern Türkiye 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 Başören 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 Ilímaussaq, South Greenland and the Lovozero, Kola Peninsula, Russia. The Başören 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 Başören area represent undersaturated melts that were formed by extreme fractionation of silica-undersaturated K-alkaline basaltic magmas generated in a back-arc setting.