Zircon Morphology and Geochemical Diversity During Closed-System Crystallization of the Skaergaard Intrusion

Abstract The textures and chemistry of zircon in the Eocene Skaergaard intrusion, related to the East Greenland flood basalts and opening of the North Atlantic Ocean, are used to unravel a wide range of competing physicochemical processes in a shallow magma reservoir that cooled and crystallized as a closed system. This study involved detailed microscopy, SEM-cathodoluminescence imaging and LA-ICP-MS trace element analysis of zircon from mineral separates and directly in thin sections. Samples represent all major components of the Skaergaard intrusion, a suite of late granophyres and granophyric sills (Tinden, Sydtoppen), and hosting Precambrian gneiss. Zircon occurs primarily within interstitial crystalline pockets characterized by two distinct mineral assemblages that are related to crystallization from late-stage conjugate immiscible Si- and Fe-rich melts. Marked variations in zircon morphology occur throughout the intrusion. Large skeletal crystals, acicular needles, euhedral zircon with stubby or prismatic terminations, and wafer grains with feathery internal textures are typical of the Upper Border Series and Sandwich Horizon. In contrast, anhedral zircon with sector zoning is found throughout the Layered Series. Apatite, rutile, and thorite inclusions are abundant in Skaergaard zircon. Titanium-in-zircon temperatures for Skaergaard cumulates (total range = 579–861°C; Q1–Q3 = 711–777°C) and MELTS-modelled zircon saturation temperatures (790–845°C) for variable initial Zr concentrations indicate crystallization from highly fractionated near-solidus melts. The extremely variable abundance, morphology, and trace element chemistry (e.g. Th/U, Nb/Yb, Eu/Eu*, Ce/Nd, Yb/Dy) of Skaergaard zircon result from the combined effects of numerous processes. These include (1) crystallization of primocryst phases prior to zircon saturation, (2) extensive fractionation of interstitial melt, (3) late-stage liquid immiscibility in the consolidating cumulate pile, (4) disequilibrium crystallization triggered by late vapour saturation and volatile loss, (5) co-crystallization of accessory phases, and (6) secondary zircon growth as a result of the intrusion of the 660-m-thick Basistoppen sill above the just-solidified Sandwich Horizon. The remarkable morphological and geochemical diversity of zircon in the Skaergaard intrusion, unprecedented in the plutonic environment, demonstrates the critical role of distinct crystallization environments between the floor, walls, roof, and centre of the magma body during closed-system solidification of this sub-volcanic magma reservoir.