Plagioclase Fractionation in Troctolitic Magma

Abstract

Troctolitic intrusive rocks poor in augite are common in certain Proterozoic anorthosite complexes and related rocks.The Lower Zone of the Kiglapait intrusion, Labrador, consists of 1570 km troctolite today and possibly 2900 km before erosion. In this augite-free Lower Zone, plagioclase fractionation is as low as 1 6%An km of cumulate thickness and averages 3%An km.When augite crystallizes after 84% of the magma has crystallized, the fractionation becomes as much as 17%An km of cumulate.Why such a difference? It is clear from first principles of phase equilibria that fractionation accelerates with saturation in augite, but not so clear that the difference should be so great.The answer is to be found in the silica-poor nature of troctolitic magma that is critically undersaturated in silica. This low-silica effect reduces the activity of the NaSi albite component relative to the CaAl anorthite component in the plagioclase, thereby favoring the An component of the liquid and crystals and weakening the fractionation process. As the normative augite component in the magma rises from the base of the Lower Zone to the base of the Upper Zone, the activity of silica also rises slightly and its consequent effect on plagioclase composition tends to diminish. Liquid fractionation paths derived from observed crystal paths, when plotted in the system Diopside^Anorthite^Albite, rise across the liquidus fractionation lines toward diopside and reach augite saturation near the 1atm cotectic. They produce plagioclase compositions 10 mol % higher in An than pure liquidus fractionation lines predict. The key criterion for the troctolitic fractionation of plagioclase composition is the absence of Ca-poor pyroxene from the rocks. Noritic magmas, by contrast, have higher activities of silica and more effective fractionation of plagioclase. A parallel fractionation of olivine is also retarded in the Lower Zone by the accumulation of ferric iron in the liquid until augite and titanomagnetite crystallize in the Upper Zone. KEY WORD: plagioclase fractionation; troctolite melts; silica activity; layered intrusions; Kiglapait intrusion