Petrology of Nepheline Syenite Pegmatites in the Oslo Rift, Norway: Zirconium Silicate Mineral Assemblages as Indicators of Alkalinity and Volatile Fugacity in Mildly Agpaitic Magma

Agpaitic nepheline syenites contain complex zirconium silicate minerals as primary magmatic phases; agpaitic liquidus mineral assemblages are controlled by compositional parameters such as alkalinity and concentrations of water and halogens in the magma. The Larvik Plutonic Complex in the late Palaeozoic Oslo Rift, Norway consists of hypersolvus monzonite (larvikite) and different varieties of nepheline syenite. The nepheline syenite members have miaskitic mineralogy (i.e. zircon stable), except for a suite of late, mildly agpaitic pegmatites, which contain zirconium silicate minerals of the wöhlerite, rosenbuschite and eudialyte groups, in some cases together with zircon. These minerals form part of the liquidus mineral assemblage of the nepheline syenite magma together with fluorite and the rock-forming minerals alkali feldspar, aegirine(–augite), alkali amphibole, biotite, nepheline and sodalite. Low-variance mineral assemblages with two or three zirconium silicate minerals, fluorite and the major rock-forming minerals can be modelled in terms of phase equilibria in a multi-component system involving the sodium disilicate component of the melt and the volatile components water, fluorine and chlorine. A semiquantitative isothermal and isobaric petrogenetic grid in log activity space constructed from the observed mineral assemblages and the compositions of the coexisting minerals indicate that there are at least three trends of evolution leading from a miaskitic to an agpaitic crystallization regime in peralkaline nepheline syenites: (1) the increasing alkalis trend, typically leading to eudialyte-dominated liquidus assemblages (and possibly further to hyperagpaitic residual liquids); (2) the increasing water trend, leading to catapleiite crystallization; (3) the increasing fluorine trend, giving transitionally agpaitic liquidus assemblages with wöhlerite, låvenite and hiortdahlite, with or without zircon, but eventually with magmatic fluorite; this type of evolution is applicable to weakly agpaitic systems such as the Oslo Rift pegmatites. The evolution of such magmas will most probably terminate at liquidus boundaries also involving eudialyte and/or rosenbuschite, and they are unlikely to evolve to more strongly agpaitic compositions.