An overview of the mantle xenoliths from Loch Roag, Northwest Scotland (the margin of the North Atlantic Craton)

Hughes, H., Upton, B.G.J., Faithfull, J. , Macdonald, I., Downes, H. and Loocke, M. (2015) An overview of the mantle xenoliths from Loch Roag, Northwest Scotland (the margin of the North Atlantic Craton). 2nd European Mantle Workshop, Wroclaw, Poland, August 25th-28th, 2015. (Unpublished)

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At Loch Roag, on the western edge of the Outer Hebridean island of Lewis, a vertical ENE-WSW trending monchiquite dyke (50-150cm wide) cross-cuts Archaean-Palaeoproterozoic Lewisian gneisses. This fine-grained and aphanitic dyke has been dated to 45.2 ± 0.2 Ma (Faithfull et al., 2012) and is thus the youngest known magmatic intrusion in the British Isles, intruded some ~10 Ma later than the initial opening of the North Atlantic. It was discovered in the late 1970s during quarrying to widen a farm track. Aside from its recent age, the dyke has other features that mark it as unique in the British Isles, if not in western Europe. Axial portions of the dyke carry an assemblage of mantle and lower crustal xenoliths (including shonkinites (syenites), anorthoclasites, anorthosites, glimmerites, mafic granulites, pyroxenites, wehrlites and spinel lherzolites) which are distinct from those known from any other UK xenolith site, and megacrysts including gem quality sapphires (Upton et al. 1983; Menzies et al. 1987). More than 300 specimens were collected in the 1980s (exhausting the currently exposed portions of the dyke) and are now curated. For the purposes of this presentation, we will primarily focus on the spinellherzolite mantle xenoliths. The spinel lherzolite xenoliths display a range of textures, from sub-equigranular and granoblastic, to porphyroblastic. Whole-rock analyses of xenolith samples were originally presented by Hunter and Upton (1987), and further bulk geochemical analysis has beenconducted by Hughes et al. (2015), including platinum-group elements (PGE) and Au. Bulk PGE abundances are equivalent to estimates of ‘primitive upper mantle’ and are notably Ptrich. Cu abundances are elevated above cratonic or Archaean-Proterozoic mantle peridotite compositions worldwide and currently available in the published literature. The xenoliths are also light REE enriched and depleted in Nb, Ta and Ti. Thus the Loch Roag xenoliths are enriched relative to cratonic lithospheric mantle, probably because of their position on the margin of the North Atlantic Craton. Crystal sizes are typically 1-2mm but orthopyroxene porphyroblasts may be up to 10mm and olivine compositions range Fo92 to Fo88 (Upton et al. 2011). ‘Primary’ clinopyroxene compositions indicate a cryptic metasomatism event (Hughes et al. 2015). Sulphides are abundant and may be categorized into distinct ‘groups’ according to their petrographic setting within the xenoliths and trace element composition. For example, lherzolite xenoliths demonstrate modal metasomatism in the form of corroded grains of spinel mantled by anorthoclase-pyroxene symplectites. In these, the pyroxenes adjacent to the spinel-anorthoclase pairs are enriched in Na. Feldspars range from potassic-oligoclase to anorthoclase, and spinel has a wide range of Cr2O3 contents, from 6.2–31.4 wt.%. Crucially, rounded ‘droplets’ of base metal sulphides, in PGE and bearing PtS (cooperite) are found strictly within these symplectites, indicating the same metasomatic event was responsible for PGE enrichment (particularly Pt) of this portion of the SCLM. Similar PGE-rich sulphide droplets have rounded inclusions of CaCO3, possibly indicating a carbonatitic metasomatic agent, as supported by the xenolith’s light REE enrichment andother isotopic evidence. A model-age of ~2.5 Ga was obtained from the metasomatisedrocks by Long et al. (1991), suggesting that the Loch Roag-type lherzolite protolith is Archaean-Palaeoproterozoic, unlike other mantle xenoliths from western Europe south of Scandinavia where the SCLM has experienced different Phanerozoic orogenic events.

Item Type:Conference or Workshop Item
Glasgow Author(s) Enlighten ID:Faithfull, Dr John
Authors: Hughes, H., Upton, B.G.J., Faithfull, J., Macdonald, I., Downes, H., and Loocke, M.
Subjects:Q Science > QE Geology
College/School:University Services > Library and Collection Services > Museum and Art Gallery
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