Spheroidal structures in arfvedsonite lujavrite, Ilı́maussaq alkaline complex, South Greenland—an example of macro-scale liquid immiscibility

Abstract

Lujavrites are meso- to melanocratic agpaitic nepheline syenites and are the most highly evolved rocks of the Ilı́maussaq complex, South Greenland. Spheroidal bodies measuring up to 20 cm in diameter occur locally in arfvedsonite lujavrite. They consist of a core rich in analcime and arfvedsonite and a rim rich in analcime, aegirine and a low temperature potassium feldspar. The host lujavrite is dominated by albite and arfvedsonite. The igneous lamination of the host continues undisturbed through rim and core of spheroids and the arfvedsonite crystals in host, rim and core have identical chemical compositions. There are sharp contacts between host and rim and rim and core. Host, rim and core contain the same accessory minerals and have almost identical chemical compositions, the main differences are high H 2O and Na 2O but low SiO 2 in the core and high Fe 2O 3 in the rim. The spheroids are proposed to have been formed by separation of immiscible interstitial H 2O-rich globules of magma from the host lujavrite magma at a late stage in the crystallization of the lujavritic melt.