Abstract: Mineralogical (microscopy, microprobe, SEM, XRD, DTA-TG) and geochemical studies of Paleogene(Eocene?) volcanic rocks in the Polatlı region, exposed as domes and dykes, revealed the formation mechanism ofanalcime minerals within these rocks. The analcimes in these basic rocks were observed with clinopyroxenes and feldspars. The lack of aqueous minerals, aside from analcime, and the detection of residual leucite though XRD,DTA-TG, microprobe analyses, and microscopy, suggest that analcime formed through an ion-exchange process from leucite during post-magmatic evolution. SEM images further support this hypothesis, revealing rough, irregular,cracked, and microporous surfaces of analcime, which are characteristic of this ion-exchange transformation. Additionally, geochemical analyses show that the volcanic rocks were ultrapotassic at the time of their formation,with high K2O content not saturated with silica; thus, providing conditions conducive to leucite crystallization. These results suggest that volcanic activity related to continental rifting and tectonic stress during the collision and postcollisional period after the closure of the northern branch of Neotethys produced the volcanic rocks in the Polatlı region. The ultrapotassic characteristics of the Polatlı volcanic rocks, which are confined to a relatively limited area,suggest that they formed from a heterogeneous potassium-enriched mantle source. This source underwent partial melting, incorporating high-K continental components added to the mantle during subduction; thus, enabling the formation of leucite and subsequently analcime in the later stages of magmatic evolution. In this study, the analcime observed in these volcanic rocks subsequently formed from leucite through ion exchange during post-magmatic processes.