Liquid immiscibility and melt segregation in alkaline lamprophyres from Labrador

Stephen F. Foley*

*Corresponding author for this work

Research output: Contribution to journalArticle

52 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)127-137
Number of pages11
JournalLithos
Volume17
Issue numberC
DOIs
Publication statusPublished - 1984
Externally publishedYes

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