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.
Highlights
Zircon is typically associated with crystallization from evolved igneous melts and the lack of zircon in mafic and ultramafic igneous rocks is commonly noted (e.g. Hoskin and Schaltegger 2003)
The relationship of zircon to the ultramafic host is often uncertain. We investigate this in situ mineral assemblage at Loch an Daimh Mor and present new field, petrographic, microtextural, geochemical and isotopic evidence on these exceptionally large and abundant zircon crystals and describe their petrological association with the Lewisian ultramafic host rocks. We suggest that these rocks record a common process involving metasomatic interaction between crustal melts and ultramafic host rocks with potentially important implications for the geochemical behaviour of high field strength elements (HFSE) in the mantle
The intense fracturing may create a blocky style of zoning in the cores of some large megacrysts, with broad fracture-related zones of dark luminescent zircon superimposed on areas with oscillatory zoning (Fig. 4a)
Summary
Zircon is typically associated with crystallization from evolved igneous melts and the lack of zircon in mafic and ultramafic igneous rocks is commonly noted (e.g. Hoskin and Schaltegger 2003). Zircon is typically associated with crystallization from evolved igneous melts and the lack of zircon in mafic and ultramafic igneous rocks is commonly noted From supposedly mantle wedge environments, as inherited grains from slab components (Liu et al 2009; Li et al 2016). Such zircon often lacks the petrological context in which their generation in mantle peridotite might be assessed as they are either separated from their host rocks by natural processes or by sample preparation techniques. The Lewisian Gneiss Complex provides an excellent example of where the petrological context between zircon grains and ultramafic host rocks is preserved. The link between large zircon and ultramafic rocks is striking both here and at other localities
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