Post-crystallisation modification of the igneous layering in the Nunarssuit and West Kûngnât syenites, South Greenland

Abstract

AbstractThe chemistry and textures of the minerals in the layered syenites of Nunarssuit and West Kûngnât record a complex post-crystallisation history. The initial cumulus phases were alkali feldspar, pyroxene and olivine, apatite and opaque oxides. Grain size distributions of the pyroxenes and olivines have been modified since crystallisation, possibly during transport of these grains in a density current, prior to deposition in layers. Olivines and pyroxenes re-equilibrated with intercumulus melt. The lower ratio of mafic phases to intercumulus fluid in leucocratic layers has resulted in pyroxene and olivine in these layers undergoing more re-equilibration that in melanocratic layers, consequently pyroxenes and olivines in leucocratic layers are more ferroan. Pyroxenes reacted with either intercumulus or deuteric fluid to form amphibole. In West Kûngnât pyroxene and amphibole re-equilibrated via a later fluid. Olivines reacted with deuteric fluid to produce biotites, probably at temperatures < 550°C. After crystallisation alkali feldspars exsolved, the exsolution textures coarsened in the presence of deuteric fluids with a magmatic origin, possibly at temperatures as low as 450°C. Zoning patterns of apatites and zircons were modified during interactions with either, or both intercumulus or deuteric fluids. Leucocratic layers underwent more modification than melanocratic ones, probably due to the higher concentration of intercumulus melt in the former leading to a higher concentration of deuteric fluids as water exsolved from the magma during cooling. The syenites underwent little compaction during solidification.