Petrology of Evolved Peralkaline Rocks and the Unusual Cases for Volcanic Eudialyte and Metamorphic Catapleiite

In the frame of the Mineralogical Association of Canada Short Course on Peralkaline Rocks and Associated Mineral deposits (May 24–May 25, 2026), petrological aspects on the formation and evolution of evolved peralkaline phonolites are summarized. The parental magmas for such rocks may be alkali basaltic to basanitic or foiditic (melilititic/nephelinitic), and mineral assemblages indicate variable crystallization conditions during their formation, especially with respect to water activity (aH2O) and redox conditions (fO2). Under special circumstances, volatiles (especially halogens) are sufficiently enriched to form otherwise rare halogen-bearing phases, such as sodalite, and various alkali-rich HFSE- and REE-bearing minerals that can be Cl-rich (e.g., eudialyte) or F-rich (e.g., wöhlerite). This is typically reached in shallow-level plutonic examples, and the formation of such agpaitic mineral assemblages in foid syenites received much research during the last years, as some of them represent major resources for critical metals. Recent work on peralkaline lavas from Tanzania and elsewhere shows that the necessary conditions for the formation of agpaitic mineral assemblages can be reached even during syn-effusive stages. It is proposed that sodalite formation during magmatic stages may be a key prerequisite, as it buffers Cl activity in the residual magmas. This can promote precipitation of Cl-bearing eudialyte in volcanic rocks as long as sufficient HFSE enrichment is achieved. Relatively few deformed and metamorphosed peralkaline meta-nepheline syenites are known, but in contrast to the magmatic occurrences, these are typically sodalite-free and contain jadeitic aegirine. Whether this reflects primary magmatic features and preserved deuteric alteration processes or results from metamorphic recrystallization is discussed. For the meta-nepheline syenites from Norra Kärr (Sweden), the potential case for metamorphic catapleiite formation is discussed.