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

J M Macdonald,C Magee,K M Goodenough

doi:10.1144/sjg2017-004

Abstract

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. Supplementary material: Electron microprobe analyses and analytical spot locations are available at: https://doi.org/10.6084/m9.figshare.c.3809545

Highlights

Unravelling the geological history of polymetamorphic basement gneiss complexes is often difficult because mineral fabrics and metamorphic assemblages formed in older tectonothermal events are commonly overprinted by those formed during younger metamorphism and deformation (e.g. Holdsworth et al 2001)
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
Because of the relatively large proportion of quartz, and plagioclase to a lesser extent, in the TTG gneiss xenolith compared to the Scourie Dyke, we suggest that the contemporaneous retrogression of both rock types progressed at a faster rate within the dyke; i.e. there was a greater amount of pyroxene available that could retrogress to hornblende

Summary

Introduction

Unravelling the geological history of polymetamorphic basement gneiss complexes is often difficult because mineral fabrics and metamorphic assemblages formed in older tectonothermal events are commonly overprinted by those formed during younger metamorphism and deformation (e.g. Holdsworth et al 2001). Hoskin and Schaltegger 2003). These processes may obscure our understanding of early tectonothermal events. Phenomena such as reaction softening and strain localisation can result in spatially heterogeneous tectonothermal overprinting Oliot et al 2010; White 2004). This is because structures generated by reaction softening and strain localisation (e.g. shear zones) may channel fluid flow, which is generally required to promote retrograde metamorphic reactions This is because structures generated by reaction softening and strain localisation (e.g. shear zones) may channel fluid flow, which is generally required to promote retrograde metamorphic reactions (e.g. White and Knipe 1978)

Results

Discussion

Conclusion