Evidence of Liquid Immiscibility in Alkaline Ultrabasic Dikes at Callander Bay, Ontario

Monchiquitic and camptonitic dikes radiating from an alkaline carbonatite intrusion display sharply bounded, ellipsoidal, coarse-grained segregations of predominantly leucocratic minerals, identical in both composition and zoning to minerals in the matrix. These oeelli are not found in chilled margins, but elsewhere occur in layers or segregations within the intrusives. Similar phenomena are seen in rocks of similar composition, both plutonic and volcantic, the world over. Field relations, detailed mineralogy, and macro- and micro-chemical studies strongly suggest that they result from liquid immiscibility in ultra basic alkaline magma. Hydrothermal melting experiments show that olivine-bearing dikes with carbonate-rich ocelli melt to a homogeneous silicate liquid, and an immiscible CO2-rich fluid phase. Kaersutite-bearing lamprophyre with felsic ocelli yields a homogeneous silicate liquid on first melting, but on further heating this liquid dissociates into two immiscsible portions. The data strongly suggest that an initial carbonated nepheliniti magma could gives rise by successive immiscibility to carbonatite, melteigite, and nepheline syenite liquids.