May 2014 LIP of the Month

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The Scourie Dyke Swarm, Lewisian portion of the North Atlantic Craton

Elemental and isotopic insights into the origin of an unusual dyke swarm

Hannah S.R. Hughes1‡ and Joshua H.F.L. Davies2,3*

1School of Earth and Ocean Sciences, Cardiff University, UK

2Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada

3Isotope Geochemistry, Geochronology and Thermochronology Group, Department of Earth Sciences, University of Geneva, Switzerland


Extracted from and full details provided in:

Davies, J.H.F.L. and Heaman, L.M. (2014) “New U-Pb baddeleyite and zircon ages for the Scourie dyke swarm: A long-lived large igneous province with implications for the Paleoproterozoic evolution of NW Scotland”. Precambrian Research (in press).

Hughes, H.S.R., McDonald, I., Goodenough, K.M., Ciborowski, T.J., Kerr, A.C., Davies J.H.F.L. and Selby D. (2014) “Enriched lithospheric mantle keel below the Scottish margin of the North Atlantic Craton: evidence from the Palaeoproterozoic Scourie Dyke Swarm and mantle xenoliths”. Precambrian Research (in press).


The Lewisian Gneiss Complex (LGC) crops out in NW Scotland and the Outer Hebridean Islands. It comprises mid-late Archaean tonalite-trondhjemite-granodiorite (TTG) gneisses, with minor mafic-ultramafic and metasedimentary components, that have been reworked by several Late Archaean and Palaeoproterozoic tectonic events. The Complex contains an extensive swarm of early Palaeoproterozoic mafic to ultramafic dykes (the Scourie Dykes; Fig. 1). The Scourie dykes are geologically interesting for a number of reasons.  They intrude the LGC in between periods of high-grade metamorphism, the earlier Badcallian at ~2.7 Ga and Inverian at ~ 2.5 Ga events and the later Laxfordian which starts at ~1.9 Ga.   The dykes were initially used as time markers to distinguish between the older and younger metamorphic events (Peach et al. 1907; Sutton and Waton, 1950).

The dykes are all in the same orientation (NW-SE) with no obvious cross cutting relationships.   Parallel orientation is often used as evidence of synchronicity in dyke swarms (e.g. Ernst and Buchan, 1997), however, high precision U-Pb dating from two of the dykes in the Scourie swarm indicated that they intruded ~400 Ma apart (Heaman and Tarney, 1989).  The extremely long time span between two dykes of the same orientation, in apparently the same swarm has not been previously identified.  Some of the Scourie dykes, around the hamlet of Scourie (after which the dykes are named) have extremely low δ18O values of ~2 ‰ (Cartwright and Valley, 1991).  Mantle derived basaltic rocks almost invariably have δ18O values of ~5.5 ‰ if they are unaltered (Eiler, 2001), and typical hydrothermal alteration or crustal contamination results in heavier isotopic compositions (see compilation of Valley et al. 2005).  Cartwight and Valley, (1991) suggested that the oxygen isotopic composition was the result of incorporating subducted oceanic crust that had a low δ18O as a result of interaction with 0 ‰ seawater into the source of the dykes.  If this explanation was correct, it would strongly support the presence of plate tectonics in the late Archean. 

The elemental geochemistry of the dykes indicates that they are not the products of melting from typical depleted mantle, or primitive mantle sources (Weaver and Tarney, 1981).  They are enriched in trace elements but many of the dykes are also ultramafic requiring high degrees of melting to produce their major element geochemistry.  The mixture of high degrees of partial melting of the mantle combined with the enriched trace elements led Weaver and Tarney, (1981) to suggest that the mantle source for the dykes was anomalously enriched in trace elements.  Crustal contamination of the dyke magma has been discounted due to extremely depleted Th and U contents in the gneisses of the Assynt terrane, which is not reflected in the dykes. 

In this contribution, we present an updated and combined data set of whole-rock major and trace elements, platinum group elements (PGE), Re-Os isotopes (Hughes et al. 2014) and U-Pb geochronology (Davies & Heaman, 2014) for the Scourie Dykes based on new sampling campaigns carried out across the Lewisian.  We also present new mantle xenolith whole-rock geochemical data for the subcontinental lithospheric mantle (SCLM) beneath the Lewisian at Loch Roag, Isle of Lewis (Hughes et al., 2014).  These new data are then used to suggest potential magma sources and provide insights into the geochemistry of Archaean (SCLM) and melting regimes. We also redefine the ‘Scourie’-aged dyking event and demonstrate the protracted and pulsed intrusion phases that formed the Scourie Dyke Swarm.

Regional Geology

The LGC is a small fragment of the much larger North Atlantic Craton (NAC; Fig. 1c) that includes Greenland, Labrador and parts of eastern Canada.  The NAC consists of Archaean protoliths with zones of Palaeoproterozoic reworking. Tectonic events within the LGC fragment were as follows: (1) 3.0-2.8 Ga – magmatic protolith formation, (2) 2.8-2.5 Ga high-grade Badcallian and Inverian metamorphism and deformation including initiation of shear zones, (3) ~2.4 Ga – intrusion of the mafic and ultramafic  Scourie dyke swarm followed by subsequent intrusion of mafic dykes at ~2.0 Ga, (4) 2.0-1.8 Ga – Laxfordian metamorphic event which comprises the formation and accretion of arc terranes, marked by calc-alkaline granitoid emplacement and related volcanics, and formation of localised high-grade shear zones, (5) 1.8-1.5 Ga – continuation of the Laxfordian involving calc-alkaline igneous activity, amphibolite-facies metamorphism and deformation, and crustal anatexis (Park, 1994; Park, 1995; Kinny et al., 2005; Goodenough et al., 2013; Vernon et al., 2014).

The mainland LGC was traditionally sub-divided into three districts (Peach et al., 1907; Sutton & Watson, 1951): the Central granulite-facies district, which was only partially affected by Laxfordian- reworking, and the North and South amphibolite-facies districts that were extensively reworked during the Laxfordian (Fig. 1a). Recent geochronological data have shown that these crustal blocks also have different protolith ages, and they are now considered to represent separate terranes divided by major shear zones (Friend & Kinny, 2001; Love et al., 2003; Kinny et al., 2005; Park et al., 2005; Goodenough et al., 2010; Love et al., 2010). The number of terranes or ‘blocks’, and the details of this model are still contentious, so that the timing and composition of the Scourie Dykes are important to constrain this terrane model further (Goodenough et al., 2010). The terrane nomenclature of Kinny et al. (2005) has been adopted here (Fig. 1) with the Rhiconich, Assynt and Gruinard Terranes being broadly comparable to the North, Central, and South districts respectively.

The Assynt and Rhiconich Terranes are separated by the Laxford Shear Zone, which is perhaps the best example of a terrane boundary in the LGC between two crustal blocks (Goodenough et al., 2010). The Assynt Terrane consists of banded TTG gneisses with mafic-ultramafic pods and lenses on scales from 1 cm to 10 m. The granulite-facies gneisses are coarse-grained rocks containing clinopyroxene, relic orthopyroxene, hornblende, plagioclase and occasionally retrograde biotite (Johnstone & Mykura, 1989). However, primary granulite-facies lithologies are relatively rare.  The majority of the Assynt gneisses display some degree of amphibolite-facies reworking, typically in discrete Laxfordian shear zones (Kinny et al., 2005). The Assynt gneisses are also characterised by a marked depletion in U, Th, and variability in the abundance of large ion lithophile elements (Sheraton et al., 1973).

Figure 1. (a) Simplified map of NW Scotland, showing main geological units and the Scourie Dykes and location of Loch Roag mantle xenolith site (b). MTZ is Moine Thrust Zone, delineating the Caledonian Foreland. Numbered localities indicate approximate dyke sampling locations. Terrane nomenclature based on (Kinny et al., 2005). (c) Larger scale simplified map showing the early Palaeoproterozoic relationship between the Lewisian of Scotland and the North Altantic Craton, as part of Laurentia and Baltica, based on Buchan et al. (2000). Figure from Hughes et al. (2014).

Geology of the Scourie Dyke Swarm

The approximately NW-SE trend of the Scourie Dyke Swarm is thought to have been controlled by a pre-existing crustal fabric that was exploited during crustal extension (Tarney, 1973). Many of the Scourie Dykes have been deformed and metamorphosed by Laxfordian shearing, but their igneous mineralogy and textures are still preserved at some localities in the Assynt Terrane. Similar dykes (the ‘Older Basics’) have also been described from the Outer Hebrides (Fettes & Mendum, 1987) but were not sampled during our studies.

In the Assynt Terrane, two generations of Scourie dykes are apparent: a mafic suite trending NW-SE, and an ultramafic E-W trending suite (Weaver & Tarney, 1981b; Tarney & Weaver, 1987). Both suites are steep-sided, ranging in width from 30 to 100m, and some dykes can be traced along strike for up to 15km, often with tapering offshoots (Weaver & Tarney, 1981b). They display sharp contacts with the host gneisses, and obliquely cross-cut pre-existing fabrics in the surrounding gnessies.

Petrological and geochemical features of the Scourie Dyke Swarm in the Assynt Terrane were reported by Tarney (1963; 1973), Weaver & Tarney (1981b), and Tarney & Weaver (1987) who grouped the dykes according to their original igneous mineralogy and composition. Four suites of dyke were identified: a main ‘quartz-dolerite’ or ‘dolerite’ suite (comprising 90-95% of all dykes in the Assynt Terrane), and comparatively minor ‘bronzite-picrites’ (here referred to as ‘picrites’), ‘olivine-gabbro’, and rare ‘norite’ suites (Fig. 2 shows the typical petrology of the dolerite and norite dykes).

Laxfordian amphibolite-facies metamorphism of the dykes is common as shown by variable amounts of amphibolisation to tremolite and actinolite (Tarney, 1963). Garnet occurs in some Assynt Terrane dolerite dykes, sometimes as coarse stringers (e.g., ‘the Graveyard dyke’, Scourie Bay). Garnet is also prevalent in dykes around Gairloch (Gruinard Terrane) as patches or lenses. These dykes have compositions distinct from other members of the Scourie Dyke Swarm (Park 2002) and this may have facilitated the growth of garnet during metamorphism. Although primary igneous mineralogy is retained to some degree at localities in the Assynt Terrane, an amphibolite-facies overprint is often present (see Fig. 2). In the Rhiconich Terrane, the dykes have been pervasively deformed and amphibolitised during the Laxfordian, with the development of a strong and pervasive mineral foliation defined by aligned hornblendes.

Figure 2. Thin section evidence for igneous and metamorphic textures in the dykes.  A) Dolerite dyke shown (in xpl) with exsolution lamellae in pyroxene and growth of actinolite and hornblende pseudomorphing pyroxene.  B) Dolerite dyke showing high degrees of alteration of plagioclase. C) Norite dyke showing elongate zircon together with with biotite, ilmenite and orthopyroxene.  D) Dolerite dyke showing slightly rounded zircon within a quartz crystal.  E) Dolerite dyke showing large zircon crystal within late stage silica saturated melt.  F) Dolerite dyke showing baddeleyite within plagioclase, apatite and magnetite, the large centre baddeleyite has a thin zircon rim along its lower edge.  Mineral abbreviations are from Siivola et al. (2007) except baddeleyite, which is abbreviated to bad.  Mineral abbreviations are; act – actinolite, ap – apatite, bt – biotite, cpx – clinopyroxene, czo – clinozoisite, ep – epidote, ilm – ilmenite, Mg-hbl magnesiohornblende, opx – orthopyroxene, pl – plagioclase, qtz – quartz, ser – sericite, and zrn – zircon. Figure from Davies & Heaman (2014).

Geochronology of the Scourie Dykes

A total of ten ‘Scourie’ dykes have been dated by TIMS U-Pb geochronology on zircon/baddeleyite and nine have emplacement dates within the range ~2410-2375 Ma, a span of ~40 m.y. We propose that this is the main period of Scourie dyke emplacement.  Within this main period of dyke emplacement, three discrete events can be recognized: an older suite 2418-2408 Ma consisting of the ultramafic dykes, the main suite at ~2395-2390 Ma consisting of doleritic and noritic dykes, and a late stage event at ~2375-2385 Ma consisting again of doleritic dykes (Fig. 3). Only one dyke so far has been dated at ~2000 Ma, this dyke was identified by Heaman and Tarney (1989), and is likely a minor component of magmatism within the swarm. We therefore propose that only dykes formed during the main intrusion event at ~2.4 Ga (identified here) be referred to as Scourie dykes sensu stricto, thereby avoiding confusion with the different Paleoproterozoic dykes in the region that span an emplacement history of more than 400 m.y. 

The new dyke ages allow an initial chronology of events to be constructed indicating how the dyke compositions vary over time (Fig. 3), which is an important step in understanding the origin of the dykes.  Petrologically the Scourie dykes can be subdivided into 4 categories: bronzite picrite, norite, olivine gabbro and dolerite (Tarney, 1973).  Weaver and Tarney (1981b) noted that these 4 petrologic classes of dykes could be separated into two geochemical groups, likely originating from different mantle sources.  The ultra-mafic dykes form one group with enriched trace element contents relative to primitive mantle.  The mafic dykes form the second group, which cannot be related to the first through crystal fractionation (Weaver and Tarney, 1981b).  The second group also contains enriched trace element compositions (relative to primitive mantle) but less fractionated rare earth element ratios (see geochemistry section below).  Oxygen isotopic analysis of the dolerite and olivine gabbro dykes indicate that they are not contaminated by the surrounding Lewisian gneiss (at least in the area around Scourie, Cartwright and Valley, 1991) and that the geochemistry of these samples is representative of their source.  If the different geochemical groups do sample separate sources, the U-Pb data indicate that the ultramafic dykes were emplaced before the mafic dykes (see Fig. 3). 

In addition to the emplacement ages of the dykes, the U-Pb data also provide information on the timing of Pb-loss.  A somewhat surprising result is that most of the dykes analyzed here have Caledonian lower intercept ages rather than a disturbance related to the Laxfordian orogeny.  The lack of Laxfordian Pb-loss for the majority of the dykes could indicate that the dykes were still at mid-crustal levels and at high temperature (>250°C) during this time.   Alpha tracks in zircon don’t accumulate at temperatures >250°C (Murakami et al., 1991; Meldrum et al., 1999), and without damage to the zircon crystal lattice, Pb-loss is unlikely (Nasdala et al., 1998).  MacDonald et al. (2013) discovered that zircons from the LGC only record Laxfordian ages when they have been physically deformed by shearing during the Laxfordian orogeny.  In order to study these zircons, MacDonald et al. (2013) sampled highly deformed gneiss, and discovered only ~5% of the zircons were affected by Laxfordian Pb mobility.  We intentionally sampled the least deformed areas of the dykes, in an effort to avoid, or at least minimize, deformation-associated Pb-loss.  The lack of Laxfordian Pb-loss could therefore reflect the sampling strategy utilized in this study, combined with the high temperatures associated with their storage at mid- crustal levels throughout most of the Precambrian.

Uplift and erosion of the LGC occurred during the Caledonian orogeny and the development of the Moine Thrust (Holdsworth et al., 2007; Goodenough et al., 2011). During this tectonic activity, the Scourie dykes were transported to higher crustal levels, exposed to Caledonian fluids, and lower temperatures where radiation damage in zircon could accumulate.   It is interesting that most of the zircons from the Lewisian gneisses show minimal evidence for a Caledonian Pb-loss event (Corfu et al. 1994; Kinny and Friend, 1997; Zhu et al. 1997; Whitehouse and Bridgewater, 2001; Love et al. 2004; Whitehouse and Kemp, 2010; Goodenough et al., 2013; Macdonald et al., 2013).  Some samples do show evidence for more recent Pb loss, which can be attributed to the Caledonian (e.g. Pidgeon and Bowes, 1972; Friend and Kinny, 1995; Love et al., 2004), although this is not a dominant feature of the Lewisian zircon populations.

Figure 3. Correlation chart showing some worldwide dyking events at ~2.4 Ga dated by U-Pb zircon or baddeleyite.  The dykes are arranged by craton (or gneiss complex in the case of the Lewisian), all bars correspond to 2σ errors.  In the Lewisian, the dark grey bars reflect picritic or olivine gabbroic dyke compositions, where as, the light grey bars reflect dolerite or norite dyke compositions, the dykes on the other cratons are not coloured by composition.  The horizontal bars show the timing of the Scourie dyke emplacement events.  The symbols refer to specific dykes, The dykes from the Dharwar craton are from Halls et al., (2007) and French and Heaman, (2010) Dykes from the Yilgarn Craton are from  (Doehler and Heaman, 1998Baltic shield dykes are from Kullerud et al., (2006), NAC is from Nilsson et al., (2012), and Zimbabwe dykes are from Söderlund et al., (2010). The other cratons are displayed to highlight the temporal difference between the Scourie dykes and other dyke swarms around the world at the same time. Figure from Davies & Heaman (2014).

Geochemistry of the Scourie Dykes

Major elements

The Scourie dykes are Fe-enriched and tholeiitic, classifying as picrobasalts to basaltic andesites according to TAS diagrams (Fig. 4). Major element binary diagrams (vs. MgO) highlight clear separation between the dyke groups (Fig. 4c-f). These diagrams display considerable major element variability in the dolerites, due to fractional crystallisation of minerals such as olivine, pyroxenes, spinel and magnetite/ilmenite. The clustered Al2O3 composition of the dolerites indicates little or no plagioclase fractionation took place in the magma, therefore the variation of CaO in the dolerites is a response to clinopyroxene fractionation. Importantly however, no systematic variation can be observed for the dolerite dyke group throughout the three Lewisian Terranes. Whole rock major element geochemical compositions of these samples do not vary according to the terrane in to which they were intruded.

Figure 4. Major element plots (anhydrous). (a) AFM classification of Scourie Dyke Suite. Fe2O3T where T denotes total iron. FeOT in AFM diagram was converted from analysed Fe2O3T (total iron). FeO for use in calculating Mg-number (Mg#) was calculated by assuming actual Fe2O3 (Fe2O3*) content = 1.5+TiO2. Then, FeO = (Fe2O3T – Fe2O3*)/1.1. (b) Total alkali silica (TAS) classification diagram. (c) to (f) Major elements versus MgO (wt%). Note that all major element abundances have been recalculated as anhydrous. Figure from Hughes et al. (2014).

Trace elements

All dyke suites display chondrite-normalised rare earth element (REE) patterns that are enriched in the light REE (LREE) (Fig. 5). All dyke suites are also depleted in high field strength elements (HFSE). Mantle-normalised multi-element patterns show a negative Nb-Ta anomaly, and the picrite suite also displays a trough at Ti. Rare dolerite dyke samples display very flat multi-element patterns (Fig.5) and these may represent rare ‘uncontaminated’ asthenospheric magmas. However these particular dykes are also located in Laxfordian shear zones and show evidence of alteration textures (e.g., high abundance of serpentine, talc or micas) indicating that they have been altered by fluids circulating in the shear zones.

All dyke suites are enriched in Th, LREE and LILE and have slight positive Zr-Hf anomalies. The Th and some LILE enrichment of Scourie Dykes is in stark contrast to the whole-rock geochemistry of the Lewisian granulite-facies TTG of the Assynt terrane, which is notably depleted in these elements (Fig. 5d). However, the amphibolite-facies gneisses of terranes to the north and south are less depleted in these elements.  Both amphibolite- and granulite-facies gneisses are more enriched in Ba and Sr than the dykes.

Figure 5. (a) to (c) Multi-element normalised diagrams for each dyke suite. Plots are primitive mantle normalised (McDonough & Sun, 1995). (d) Comparison plot with Rollinson (2012) average granulite- and amphibolite-facies gneiss. (e) to (g) Chondrite-normalised (McDonough & Sun, 1995) rare earth element multi-element diagrams per dyke suite. Figure from Hughes et al. (2014).

Overall, the geochemical compositions of the three dyke suites suggest that they are not cogenetic and cannot be related to one another by a fractional crystallization from the same magma source. Instead these indicate at least two, magma sources (dolerites vs. picrites). No systematic variation can be identified in the trace element abundances of dolerite dykes intruded into the three mainland Lewisian terranes.

Total PGE+Au concentrations range from 38.3 ppb (olivine gabbro) to 2.7 ppb (dolerite). All dyke suites display fractionated chondrite-normalised PGE profiles enriched in palladium-group PGE (PPGE) and the dolerite suite has notable iridium-group PGE (IPGE) depletion (Fig. 6). The picrite and olivine gabbro dykes have relatively flat PGE+Au patterns, although these are still slightly fractionated. The picrite and olivine gabbro suites have significantly higher concentrations of Ni, Cr and Co than the dolerites, whilst Cu concentration varies widely across and within the dyke suites. This variation in Cu is predominantly due to its fractionation as an incompatible element in silicate magma. Chalcophile element ratios, such as Cu/Pd, are typically higher than primitive mantle ratios. There is little consistent variation in Cu/Pd between dyke suites, although olivine gabbros tend to have the most primitive values (Fig. 6).

Figure 6. (a) to (d) Platinum group element multi-element plots (chondrite normalised using McDonough & Sun, 1995) for  picrite dykes, olivine gabbros, and  dolerite dykes. (e) and (f) Platinum group element bivariant plots. Hatched area in (e) indicate typical mantle ratios for Cu/Pd. (f) [Pd/Ir]N is chondrite-normalised Pd/Ir ratio. PRIMA is typical [Pd/Ir]N for primitive mantle (McDonough & Sun, 1995). Figure from Hughes et al. (2014).

Mantle Xenolith Geochemistry

The Loch Roag mantle xenoliths from beneath the Lewisian, have similar negative Nb and Ta, Zr and Hf, and Ti anomalies identified in the Scourie Dykes (Fig. 7a). PGE and Au analyses indicate that the lithospheric mantle below this region of NW Scotland is enriched in these elements (Fig. 7c). Primitive mantle estimates for Au and Pd are 1 and 3.9 ppb, respectively (McDonough & Sun, 1995), but the Loch Roag xenoliths contain 2.0 – 3.3 ppb Au, and 4.7 – 11.5 ppb Pd. Pt is also elevated (11 – 16.9 ppb, compared with 7 ppb for primitive mantle) suggesting that the PGE and Au underwent enrichment, in addition to Th, LILE and LREE. Finally the IPGE occur in concentrations comparable to primitive mantle, with Ir concentrations ranging from 2.9 to 3.6 ppb.

Previous detailed work has highlighted the metasomatised state of this region of lithospheric upper mantle (Upton et al., 1983; 2011 Menzies et al., 1987; Long et al., 1991). Sr, Nd and Pb isotopic analyses of the spinel lherzolite and pyroxenite xenoliths suggest the presence of an old, stable keel below the LGC, which has been enriched over a period of 1000-1500 Myr in incompatible elements, particularly Ba, Rb, Sr, P and the LREE (Menzies et al., 1987; Long et al., 1991). Time-integrated Nd and Sr isotope systematics indicate a geochemical enrichment event at ca. 2.5-2 Ga, involving interaction with carbonatite (Long et al., 1991). The only magmatic event known to have occurred within this time period is the intrusion of the Scourie Dykes, and thus the Loch Roag xenoliths may provide information about the source of these dykes.

Figure 7. Loch Roag multi-element diagrams: (a) primitive mantle normalised trace element multi-element diagram, (b) REE chondrite-normalised plot, and (c) chondrite-normalised PGE+Au plot. Figure from Hughes et al. (2014).

Re-Os isotopes

Two whole-rock samples were analysed for Re and Os abundances and isotopic composition, including the dolerite ‘Graveyard dyke’ (sampled X8) and a picrite dyke from the north shore of Loch an Leathaid (sample X23). X8 and X23 were analysed for comparison to earlier data collected by Frick (1998), in the light of new Re-Os methodologies and modern dating of the Scourie Dykes.

X8 has significantly higher Re and Os abundances than X23 (3.4 vs 0.4 ppb and 1090 vs. 108 ppt, respectively). Therefore X8 has high 187Re/188Os and very radiogenic Osi and γOs, suggesting significant disturbance of the isotopic system, post-intrusion. This is comparable to previous measurements of the ‘Graveyard dyke’ by Frick (1998). In contrast picrite dyke X23 has γOs = -6.73 and Osi = 0.104 (lower than the predicted mantle range of 0.11 – 0.50 at 2400 Ma), and its 187Re/188Os and 187Os/188Os are within the range of results by Frick (1998).

The Osi (0.127) and γOs (-0.94) of xenolith LR80 has been calculated at 45.2 Ma, based on the age of the host dyke at Loch Roag (Faithfull et al., 2012). However, Long et al. (1991) used time integrated Nd-isotopes to suggest there was a major metasomatic event between 2.3 – 2.5 Ga and 1.0 – 1.5 Ga. If we recalculate Osinitial for LR80 at 2.4 Ga, this is 0.07 – a result too unradiogenic for mantle at that time, and suggesting that the Re-Os system has been disturbed.


Dyke composition in relation to Lewisian terrane

The dolerite dykes are the only member of the Scourie Dyke Swarm to occur across the three mainland Lewisian Terranes. Although the compositions of the dolerite dykes are variable, no systematic geochemical differences were observed across the Rhiconich, Assynt and Gruinard terranes. Picrite and olivine gabbro dykes are rare in the granulite facies Assynt Terrane, and absent in the amphibolite facies Gruinard and Rhiconich Terranes to the south and north.  This may be due to the depth of current erosion levels across the Lewisian terranes. The granulite-facies Assynt Terrane is considered to represent the lower crust (Park & Tarney, 1987), while the amphibolite-facies terranes represent mid-crustal levels. The absence of picrite and olivine gabbro dykes from the amphibolite-facies terranes might imply that these more MgO-rich magmas did not ascend as high into the crust, becoming trapped in lower crustal regions. Their elevated MgO contents imply their parental magmas were considerably denser than the dolerite dyke magmas, restricting their ascent. In addition, the homogeneity of the dolerite dyke suite across the three Lewisian terranes suggests that these Lewisian blocks had been accreted onto each other before dyke intrusion, confirming the conclusions of Goodenough et al. (2010), and supporting those of Davies and Heaman (2014).

Mantle melting regime – evidence of crustal contamination or SCLM melting?

Crustal contamination:

In order to test the possible mantle sources of the dykes and whether crustal contamination played a significant role in the geochemistry of the parental magmas for the dykes, trace element modelling has been carried out. For in-depth details, refer to Hughes et al. (2014).

AFC modelling using Lewisian amphibolite-facies gneiss (Rollinson, 2012) can successfully reproduce the trace element signature of the dolerite dykes from a 30% melt of a spinel lherzolite primitive mantle (PM) source. Indeed, lower degrees of partial melting of this source (10-25%) can still successfully replicate the range of dolerite dyke trace element compositions. However this modelling has a major flaw – most samples in this study were intruded into LILE- and Th-poor granulite-facies gneisses of the Assynt terrane (a geochemical feature common to deep continental crustal material – Rudnick & Fountain (1995)). The granulite-facies metamorphism predates the Scourie Dyke intrusion, and there is no evidence for post-intrusion tectono-metamorphic events that could have removed the LILE and/or Th. LILE-enriched amphibolite-facies gneiss only occurs in the Rhiconich and Gruinard terranes, and therefore was not available to contaminate the dolerite dykes in the Assynt Terrane. Additionally, a mechanism of contamination followed by prolonged lateral movement of the magmas would also provide an unsatisfactory explanation, as the same problem of the availability of a suitable contaminant would apply. There is no systematic change in composition of the Scourie Dykes across the basement terrane boundaries, indicating that local crustal contamination could not have been a major factor. Therefore this AFC model is inappropriate for most Scourie dolerite dykes. Further, assimilation of Lewisian granulite TTG (Rollinson, 2012) in AFC models cannot fully reproduce the dolerite trace element geochemistry, with a particularly poor correlation for Th.

In summary, simple partial melting of asthenospheric mantle sources cannot reproduce the continental-like signature of the Scourie Dykes. Some subtleties of the trace element signatures, particularly for dolerites, can be accounted for by inferring some degree of crustal contamination, but by components not available to the magma at the observed level of intrusion. Moreover this contamination signature is highly variable and a very minor feature of particular trace element abundances (Zr-Hf) in the dykes. Models highlight the insensitivity of any reasonable ‘crustal’ signature which would be easily out-weighed by the addition of a lithospheric mantle component.

Lithospheric mantle input:

Comparison of whole-rock geochemistry of the Scourie Dykes to that of spinel lherzolite xenoliths of Loch Roag (Isle of Lewis) can account for many significant similarities (e.g., Nb-Ta-(Ti) negative anomalies, and positive anomalies for Th, LILE and LREE). SCLM compositions underlying Archaean cratons are extremely variable, due to various metasomatic events throughout formation and cratonic keel preservation. However the position of the Loch Roag xenoliths within the undisturbed LGC at the margin of the fragmented NAC provides a rare and valuable insight into the ancient shallow mantle composition and its potential to produce later melts. In brief, by modelling the partial melting of a lithospheric source region with the same composition as the Loch Roag peridotite xenoliths, mixed with a comparatively small input of asthenospheric melting, can successful reproduce the compositions of the ultramafic and mafic Scourie Dykes.

The hypothesis that all parental magmas of the Scourie Dyke Suite involved melting of enriched and modified SCLM, with a trigger input from asthenospherically-derived melts, is supported by the Re-Os isotope data (Frick, 1998; this study). The γOsinitial values for the picrites (-1.89 to 4.30; Frick, 1998; -6.84 for X23, this study) indicate significant and prolonged isolation of the magma source from a convecting asthenospheric source, suggesting that the Scourie Dyke magmas were lithospheric. An 187Os/188Osinitial of 9.73 was obtained for the centre of the ‘Graveyard’ dolerite dyke (this study) but we suggest that the high Re content of the dyke indicates re-setting of the isotopic system for this sample. New Re-Os analyses of the xenolith LR80 yields 187Os/188Osinitial = 0.127 (using 45.2 Ma age; Faithfull et al., 2012)) or 0.07 (using 2400 Ma age of dykes). At both 2400 Ma and 45.2 Ma, the 187Os/188Osinitial is too unradiogenic for the calculated mantle ranges at those times (0.11-0.5 at 2400 Ma, 0.128 at 45.2 Ma). This suggests isolation from the convecting mantle by 2400 Ma, but there may also be some disturbance of the isotopic system.

Gd/Yb ratios and HREE spectra indicate a significantly different magma source for picrites and olivine gabbro dykes relative to the more abundant dolerite dyke suite – dolerites were derived from a source producing a flat HREE signature but olivine gabbro and picrite suites stem from a source producing a HREE-depleted signature. This is not explicable by only a change in the degree of partial melting, or by subsequent crystal fractionation. If the Scourie Dykes display a predominantly SCLM geochemical signature, then a change in the degree and depth or loci of melting, and proportions of magma inputs from the lithospheric and asthenospheric mantle, could cause the change in geochemical signature apparent between dyke groups. However we hesitate to suggest that any garnet-bearing signature in this environment would truly be at the garnet stability depth. Instead it is likely that a lithospheric source might have undergone numerous melting/freezing magmatic events, and hence a garnet-like signature could have migrated from deeper levels and solidified in shallower lithospheric zones, resulting in an inherent heterogeneity of the Scottish lithosphere. These frozen melts could have subsequently been remobilised by the Scourie magmatic event (Fig. 8). 

Lewisian SCLM geochemistry: evidence of melting mechanism – plume or no plume?

The trigger for lithospheric melting might have been the thermal anomaly associated with an impinging mantle plume. However, continental extension alone could have been sufficient to initiate partial melting both in the asthenosphere and in the fluid-rich metasomatised SCLM. Extension of the continental lithosphere generates little melt unless β (amount of crustal stretching) > 2 and Tp (mantle potential temperature) > 1380⁰C (McKenzie & Bickle, 1988) at which point melting will occur in the asthenosphere and also the hydrous lithosphere (Gallagher & Hawkesworth, 1992). Based on field evidence, it is reasonable to suggest that a very significant degree of stretching took place during the emplacement of the Scourie Dyke swarm (β > 2). In addition, numerous models (Richter, 1988; Korenaga, 2008; Davies, 2009; Herzberg et al., 2010) indicate that the mantle temperature in the Late Archaean/Palaeoproterozoic exceeded 1380⁰C. Once β > 2, alkali basalts are produced by decompression melting, and as the degree of melting increases, tholeiites are produced.

LREE concentrations of the Scourie Dyke suites are very enriched, comparable to some alkaline magmas globally (e.g., McDonald et al., 1995), especially taking into consideration the high degrees of partial melting indicated for the ultramafic magmas by the IPGE data. This further highlights the substantial LREE metasomatic enrichment of the mantle source. Multi-element plots of each of the dyke suites show a comparable, but progressively exaggerated metasomatic signature, which becomes increasingly more discernible from the dolerites through to the olivine gabbro and picrite suites (i.e., from mafic to ultramafic compositions). This presents a paradox for any melting model incorporating a plume (± contamination).

Asthenospheric melts produced by progressively increasing degrees of melting should dilute any initial geochemical anomaly such as that for Nb-Ta-Ti. In addition the relative ages of the dolerite (mafic) and picrite (ultramafic) dyke groups may provide clues as to the presence of a plume. Geochronological work has highlighted the long time-period over which Scourie dykes were intruded  a spanning a period of c.40Myr between 2418-2375 Ma (Davies & Heaman, 2014) which itself has apparent temporal subdivisions. This prolonged phase of dyke intrusion is exceptional for any Large Igneous Province (flood basalt-related events are observed to last ~1-5Myr, Ernst et al., 2013), but in Scourie these appear to have been intruded in ‘pulses’ over at least 40 Myr. The uniformity of dyke azimuth and longevity of intrusion dates could instead be the result of persistent rifting, as observed in the Tarim block of NW China (Zhu et al., 2008; Davies & Heaman, 2014).

Figure 8. Model scenario for formation of Lewisian depth-zoned SCLM and Scourie Dyke Suites. (1) Archaean subduction during Lewisian TTG formation (based on Rollinson, 2010) – subduction of dense Fe-rich oceanic crust and thick depleted lithosphere, causing slab dewatering and melting and over-riding mantle wedge (SCLM) metasomatism. Slab melts (TTG magma precursors) eventually form buoyant Archaean crust and preserved buoyant SCLM. (2a) Palaeoproterozoic extension causes decompression melting in asthenosphere and subduction-enriched SCLM producing dolerite dyke parental magmas. This is followed by (2b) higher degree partial melting of subduction-enriched SCLM producing parental magmas of the olivine gabbro dykes and picrite dykes. Magmas ascend and are emplaced into Lewisian crust, although denser Mg-rich olivine gabbro and picrite dyke magmas only penetrate lower crustal levels. γOsinitial values from Frick (1998) and this study. (3) Later tectonic displacement and erosion to present-day exposure levels see the Rhiconich and Gruinard Terranes with mid-crustal amphibolite-facies Lewisian gneiss and only the dolerite dyke suite encountered. In contrast, the Assynt Terrane is a lower crust granulite-facies block with all three dyke suites exposed. See text for discussion. Figure from Hughes et al. (2014).


  1. The main phase of Scourie dyke intrusion occurred at ~2.4 Ga and lasted for ~40 Ma.  This long time span is distinct from most mafic dyke swarms worldwide.  There is a subset of dykes that intruded ~400 Ma later at ~2.0 Ga although this period of dyke emplacement seems volumetrically small, we define the term ‘Scourie dyke’ as referring to the ~2.4 Ga dyke swarm. 
  2. Geochemical modelling of the Scourie Dyke Swarm has shown that the swarm could not have been derived from an asthenospheric mantle region with variable crustal contamination using the available crustal components. Therefore we investigate where such an enriched geochemical signature could have been sourced.
  3. Negative Nb-Ta-Ti anomalies, and enrichments in Th, LREE and other LILE, particularly in the picrite suite represent an Archaean metasomatic geochemical signature in the lithospheric mantle. Whole-rock geochemical analyses, and a new measurement of the Re-Os isotopic composition of spinel lherzolite mantle xenoliths from Loch Roag confirm the presence of this signature in the shallow mantle underlying this region of Scotland, and corroborate the presence of significant metasomatic enrichment of the mantle lithospheric keel of the Lewisian portion of the NAC. 
  4. PGE can be used to further characterise mantle melting regimes and the extent of partial melting. Ir-group PGE depletion in dolerite dykes suggests up to c.15% partial melting of the SCLM source, while low Pd/Ir ratios in both the picrite and olivine gabbro dyke suites highlight a significantly higher degree of melting (>25-30%) within a fluid-rich lithospheric mantle source.
  5. Our trace element geochemical modelling suggests that the Scourie Dykes have variably sampled this enriched and fusible lithospheric mantle region through direct partial melting of the metasomatised SCLM itself, possibly coupled with, or triggered by, some degree of asthenospheric melting. This was a direct result of tectonic extension causing lithospheric thinning and decompression.


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