The southern part of a lamprophyre dyke swarm from West Otago, New Zealand comprises monchiquites and camptonites. Dykes are compositionally similar to volatile-rich nephelinite or nepheline basanite, and contain leucocratic ocelli and irregular vein segregations of nepheline benmoreite composition. Ocelli are commonly zoned with an upper globule or centrally disposed core of analcime-carbonate and an alkali feldspar-kaersutite-rich lining.Virtually identical zoning is developed in minerals common to both lamprophyre host and ocellus. Kaersutite is progressively enriched in Fe, Mn, Alvi, and Na + K, and depleted in Mg, and Ti towards the rim. Titanaugite is concentrically zoned to aegirine-augite, and is sector zoned with prism sectors enriched in Ti and Al (and to a lesser extent in Fe and Na) compared to basal sectors. Brown micas are zoned from titanphlogopite cores to titanbiotite and titaniferous lepidomelane rims.In monchiquites glass occurs as an intersertal phase and as an ocellus infilling. The glass is phonolitic and contrasts sharply with the composition of the ultrabasic host, indicating that production of extreme differentiates without intermediate compositions is possible by fractional crystallization. Identical glass compositions in both lamprophyre groundmass and ocellus preclude liquid immiscibility. Instead, it is suggested that ocelli are segregations of residual liquids drawn into gas vesicles as a consequence of reduction of the vapour phase, volume during cooling. The analcime-carbonate globule within the ocellus is interpreted as the condensed vapour phase.At Haast River, in the northern part of the swarm the dyke association of peridotite, camptonite, trachyte, tinguaite, and carbonatite is readily explained in terms of this model. Fractional crystallization of camptonite results in phonolitic late stage liquids which, if separated from the lamprophyre host crystallize as sodalite tinguaites, leaving a more ultrabasic residuum whose composition is equivalent to damkjernitic peridotite. Vesiculation of the camptonite results in separation of a volatile phase (fluid immiscibility) which is carbonate and alkali-rich, the source of both carbonatites and the agent responsible for fenitization. © 1979 Oxford University Press.
