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

Faithfull, J. W. , Dempster, T. J. , MacDonald, J. M. , Reilly, M. and EIMF, (2018) Metasomatism and the crystallization of zircon megacrysts in Archaean peridotites from the Lewisian complex, NW Scotland. Contributions to Mineralogy and Petrology, 173, 99. (doi: 10.1007/s00410-018-1527-5) (PMID:30930466) (PMCID:PMC6404683)

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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.

Item Type:Articles
Additional Information:These analyses were supported by NERC facilities grants (IMF610/1016).
Glasgow Author(s) Enlighten ID:Dempster, Dr Tim and MacDonald, Dr John and Faithfull, Dr John
Authors: Faithfull, J. W., Dempster, T. J., MacDonald, J. M., Reilly, M., and EIMF,
College/School:College of Science and Engineering > School of Geographical and Earth Sciences > Earth Sciences
University Services > Library and Collection Services > Museum and Art Gallery
Journal Name:Contributions to Mineralogy and Petrology
ISSN (Online):1432-0967
Published Online:17 November 2018
Copyright Holders:Copyright © 2018 The Authors
First Published:First published in Contributions to Mineralogy and Petrology 173:99
Publisher Policy:Reproduced under a Creative Commons License

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