Phase Equilibrium Constraints on Intensive Crystallization Parameters of the Ilimaussaq Complex, South Greenland

Abstract

The 1·13 Ga Ilímaussaq intrusive complex, South Greenland, is composed of various types of alkali granite and silica-undersaturated alkaline to agpaitic nepheline syenites related to three subsequently intruded magma batches. Mineral chemistry indicates continuous fractionation trends within each rock type, but with distinct differences among them. The last, peralkaline magma batch is the most fractionated in terms of XFemafic mineral, feldspar composition and mineral assemblage. This indicates that an evolving magma chamber at depth discontinuously released more highly fractionated alkaline melts. Fluid inclusions in some sodalites record a pressure drop from 3·5 to 1 kbar indicating that crystallization started during magma ascent and continued in the high-level magma chamber. On the basis of phase equilibria and preliminary fluid inclusion data, crystallization temperature drops from >1000°C (augite syenite liquidus) to <500°C (lujavrite solidus) and silica activity decreases from ∼0·8 to <0·3. An almost pure methane fluid phase at high temperatures and an almost pure aqueous fluid phase in the last crystallization stages of the agpaitic rocks indicate a strong increase in water activity. NaCl activity drops from 0·4 during magmatic sodalite crystallization to <0·01 (3 wt % NaCl equiv) in the late magmatic aqueous fluids. Relative oxygen fugacity [ΔFMQ, where FMQ is fayalite–magnetite–quartz)] depends on silica and water activity via two solid–solid buffer reactions. It decreases during fractionation in the augite syenite from about FMQ – 1 to below FMQ – 4, but increases in the peralkaline stage. The extreme peralkaline fractionation trend appears to be governed by low water activity and low SiO2 activity in the parental melt. Only then is methane a stable fluid phase during most of the crystallization history, which prevents early unmixing of an aqueous NaCl-bearing fluid phase.