Liquid immiscibility and melt segregation in alkaline lamprophyres from Labrador

Sannaites (alkaline lamprophyres with Ti-rich mica and/or amphibole and K-feldspar) forming part of the Aillik Bay alkaline dyke swarm contain two varieties of leucocratic globular structures. Type I are interpreted to represent immiscible silicate liquids, whereas type II are caused by segregation of late-stage melt. Type I globules are characterised by enrichment in Al₂O₃, SiO₂, Na₂O and K₂O, and depletion in CaO, MgO, FeO (total Fe), TiO₂ and P₂O₅ relative to the sannaite groundmass. Mafic minerals inside and outside the type I globules have cores of similar composition, but rims which follow different trends. Type II globular structures are commonly zoned; they have a carbonate-analcite inner zone interpreted as the site of a former gas bubble, and an outer zone representing late-stage liquid. Minerals in the outer zones show extreme zonation along chemical trends similar to those in the groundmass, but contrasting with those in type I globules. Boundary nucleation is common in type II but rare in type I. The Aillik Bay sannaites are believed to lie at the alkali + alumina-rich extremity of the miscibility gap. Rocks with type II structures frequently have a two-stage groundmass which may be caused by metastable immiscibility compositionally adjacent to the stable miscibility gap.