Dykes as physical buffers to metamorphic overprinting: an example from the Archaean–Palaeoproterozoic Lewisian Gneiss Complex of NW Scotland

MacDonald, J.M. , Magee, C. and Goodenough, K.M. (2017) Dykes as physical buffers to metamorphic overprinting: an example from the Archaean–Palaeoproterozoic Lewisian Gneiss Complex of NW Scotland. Scottish Journal of Geology, 53(2), pp. 41-52. (doi: 10.1144/sjg2017-004)

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The early history of polymetamorphic basement gneiss complexes is often difficult to decipher due to overprinting by later deformation and metamorphic events. In this paper we integrate field, petrographic and mineral chemistry data from an Archaean tonalitic gneiss xenolith, hosted within a Palaeoproterozoic mafic dyke in the Lewisian Gneiss Complex of NW Scotland to show how xenoliths in dykes may preserve signatures of early tectonothermal events. The Archaean tonalite– trondhjemite–granodiorite (TTG) gneisses of the Lewisian Gneiss Complex are cut by a suite of Palaeoproterozoic (c. 2400 Ma) mafic dykes, the Scourie Dyke Swarm, and both are deformed by later shear zones developed during the upper greenschist- to lower amphibolite-facies Laxfordian event (1740 – 1670 Ma). Detailed field mapping, petrographic analysis and mineral chemistry reveal that a xenolith of TTG gneiss entrained within a Scourie dyke has been protected from amphibolite-facies recrystallization in a Laxfordian shear zone. Whereas the surrounding TTG gneiss displays pervasive amphibolite-facies retrogression, the xenolith retains a pre-Scourie dyke, clinopyroxene-bearing metamorphic assemblage and gneissic layering.We suggest that retrogressive reaction softening and pre-existing planes of weakness, such as the c. 2490 Ma Inverian fabric and gneiss–dyke contacts, localized strain around but not within the xenolith. Such strain localization could generate preferential flow pathways for fluids, principally along the shear zone, bypassing the xenolith and protecting it from amphibolite-facies retrogression. In basement gneiss complexes where early metamorphic assemblages and fabrics have been fully overprinted by tectonothermal events, our results suggest that country rock xenoliths in mafic dykes could preserve windows into the early evolution of these complex polymetamorphic areas.

Item Type:Articles
Additional Information:The fieldwork was carried out under UK Natural Environment Research Council DTG NE/G523855/1 and British Geological Survey CASE Studentship 2K08E010 to JMM.
Glasgow Author(s) Enlighten ID:MacDonald, Dr John
Authors: MacDonald, J.M., Magee, C., and Goodenough, K.M.
Subjects:Q Science > QE Geology
College/School:College of Science and Engineering > School of Geographical and Earth Sciences > Earth Sciences
Research Group:Solid Earth
Journal Name:Scottish Journal of Geology
Publisher:Geological Society Publishing House
ISSN (Online):2041-4951
Published Online:29 June 2017
Copyright Holders:Copyright © 2017 The Authors
First Published:First published in Scottish Journal of Geology 53(2): 41-52
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher
Data DOI:10.6084/m9.figshare.c.3809545.v1

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