Metasomatism and the crystallization of zircon megacrysts in Archaean peridotites from the Lewisian complex, NW Scotland

Abstract

Zircon megacrysts are locally abundant in 1–40 cm-thick orthopyroxenite veins within peridotite host rocks in the Archaean Lewisian gneiss complex from NW Scotland. The veins formed by metasomatic interaction between the ultramafic host and Si-rich melts are derived from partial melting of the adjacent granulite-facies orthogneisses. The interaction produced abundant orthopyroxene and, within the thicker veins, phlogopite, pargasite and feldspathic bearing assemblages. Two generations of zircon are present with up to 1 cm megacrystic zircon and a later smaller equant population located around the megacryst margins. Patterns of zoning, rare earth element abundance and oxygen isotopic compositions indicate that the megacrysts crystallized from crustal melts, whereas the equant zircon represents new neocryst growth and partial replacement of the megacryst zircon within the ultramafic host. Both zircon types have U–Pb ages of ca. 2464 Ma, broadly contemporaneous with granulite-facies events in the adjacent gneisses. Zircon megacrysts locally form > 10% of the assemblage and may be associated to zones of localized nucleation or physically concentrated during movement of the siliceous melts. Their unusual size is linked to the suppression of zircon nucleation and increased Zr solubility in the Si-undersaturated melts. The metasomatism between crustal melts and peridotite may represent an analog for processes in the mantle wedge above subducting slabs. As such, the crystallization of abundant zircon in ultramafic host rocks has implications for geochemistry of melts generated in the mantle and the widely reported depletion of high field strength elements in arc magmas.

Zircon Solubility in Solute‐Rich Supercritical Fluids and Zr Transfer From Slab to Wedge in the Deep Subduction Process

“…While supercritical fluids may be initial agents for leaching elements from the slab, they are likely to separate into immiscible hydrous melts and aqueous fluids at the slab‐mantle interface in response to changes in pressure, temperature, and especially bulk rock compositions (Chen et al., 2018; Manning, 2004). As the process continues, hydrous melts will react with mantle peridotites to form metasomatic domains containing zircon (e.g., Faithfull et al., 2018; Kalfoun et al., 2002; Li et al., 2016; Malaspina et al., 2006), whereas aqueous fluids may ascend to the arc magma source regions (Malaspina et al., 2006). In this case, zirconium mobilization is controlled not only by zircon solubility in supercritical fluids but also by partitioning of Zr between hydrous melts and aqueous fluids.…”

“…Zirconium could be efficiently extracted from subducted crust rocks by solute‐rich supercritical fluids, as evidenced by newly formed zircon in orogenic peridotites at slab‐mantle interface (Faithfull et al., 2018; Li et al., 2016). However, the question arises as to whether or not zirconium can be transferred to the source of arc magma or to the volcanic arc.…”

“…In this case, zirconium mobilization is controlled not only by zircon solubility in supercritical fluids but also by partitioning of Zr between hydrous melts and aqueous fluids. HFSEs are hence transportable from subducting slab to mantle wedge, but the characteristic HFSEs depletion recorded in arc magmas may partly result from fluid-rock interactions at the slab-mantle interface, during which most HFSEs were scavenged by newly formed zircon and rutile (Faithfull et al, 2018;Kalfoun et al, 2002;Li et al, 2016). Such a depletion may indicate a lower solubility of HFSEs relative to LILEs but does not necessarily mean immobility of these elements in subduction-zone fluids.…”

“…More recent experiments gave extremely low solubility of zircon and rutile in dilute aqueous fluids (Audétat & Keppler, 2005; Antignano & Manning, 2008; Bernini et al., 2013; Tropper & Manning, 2005), which is consistent with the “conservative” nature of these elements in shallow geological processes. However, the ubiquitous presence of zircon and rutile in orogenic peridotites (Faithfull et al., 2018; Kalfoun et al., 2002; Li et al., 2016; Malaspina et al., 2006) and as daughter minerals of fluid inclusions and vein minerals in high‐pressure and ultrahigh‐pressure metamorphic rocks (Ferrando et al., 2005; Gao et al., 2007; Rubatto & Hermann, 2003; Zhang et al., 2008) provides compelling evidence that HFSEs are soluble and transportable in fluids, making the paradigm “HFSE immobility in subduction zones” no longer tenable.…”

Origin(s) and geodynamic significance of Archean ultramafic–mafic bodies in the mainland Lewisian Gneiss Complex, North Atlantic Craton

“…Metaperidotite is variably veined by separate carbonate-and orthopyroxene-rich veins, with the latter more numerous than the former (Faithfull et al, 2018). The orthopyroxene-dominated veins, which display sharp contacts with the surrounding ultramafic rocks, are 1-40 cm thick and contain centimetre-scale zircon crystals interpreted as associated with Inverian metasomatism (Faithfull et al, 2018). Adjacent metagabbrodominated mafic rocks, which occur in association with intermediate gneiss, are poorly-exposed and locally contain coarse garnet.…”