Scourie Dyke Swarm Research Articles

1.99 Ga mafic magmatism in the Rona terrane of the Lewisian Gneiss Complex in Scotland

The Scourie dyke swarm has long been important to unravelling the geological history of the Lewisian Gneiss Complex (LGC) and the North Atlantic Craton. Recent dating has documented that the majority of those dykes were emplaced between c. 2418–2375 Ma. Here we show that a quartz dolerite dyke in the Rona Terrane of the LGC has a U-Pb zircon age of 1989.08 +4.3/−0.99 Ma. This is the first mafic dyke to be dated from the Rona terrane, the southernmost of those proposed for the terrane model of the LGC. Our new age also overlaps with the c. 1992 Ma age of a previously dated olivine gabbro dyke in the Northern region of the LGC and shows that the LGC contains at least three, and possibly four, temporally discrete episodes of Palaeoproterozoic mafic magmatism: The Scourie dyke swarm ‘sensu stricto’ at c. 2.4 Ga; a suite of younger dykes at c. 1.99 Ga, referred to here as the Strathan dyke swarm; and the c. 1.99–1.90 Ga mafic rocks that are part of the Palaeoproterozoic Loch Maree Group. A c. 2.04 Ga dyke in the Assynt terrain may be part of the younger suite of dykes or perhaps records a temporally separate event. Crucially, our new age data demonstrate that suites of mafic dykes emplaced across the mainland LGC are similar in age and supports the correlation of structural and metamorphic features across that Complex.

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

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

Trace-element abundances in the shallow lithospheric mantle of the North Atlantic Craton margin: Implications for melting and metasomatism beneath Northern Scotland

AbstractBulk rock geochemistry and major- and trace-element compositions of clinopyroxene have been determined for three suites of peridotitic mantle xenoliths from the North Atlantic Craton (NAC) in northern Scotland, to establish the magmatic and metasomatic history of subcontinental lithospheric mantle (SCLM) below this region. Spinel lherzolites from the southernmost locality (Streap Com'laidh) have non-NAC mantle compositions, while the two northern xenolith suites (Loch Roag and Rinibar) are derived from the thinned NAC marginal keel. Clinopyroxene compositions have characteristic trace-element signatures which show both 'primary' and 'metasomatic' origins. We use Zr and Hf abundances to identify ancient cryptic refertilization in 'primary' clinopyroxenes. We suggest that Loch Roag and Rinibar peridotite xenoliths represent an ancient Archaean-Palaeoproterozoic SCLM with original depleted cratonic signatures which were overprinted by metasomatism around the time of intrusion of the Scourie Dyke Swarm (∼2.4 Ga). This SCLM keel was preserved during Caledonian orogenesis, although some addition of material and/or metasomatism probably also occurred, as recorded by Rinibar xenoliths. Rinibar and Streap xenoliths were entrained in Permo-Carboniferous magmas and thus were isolated from the SCLM ∼200 Ma before Loch Roag xenoliths (in an Eocene dyke). Crucially, despite their geographical location, lithospheric mantle peridotite samples from Loch Roag show no evidence of recent melting or refertilization during the Palaeogene opening of the Atlantic.

Enriched lithospheric mantle keel below the Scottish margin of the North Atlantic Craton: Evidence from the Palaeoproterozoic Scourie Dyke Swarm and mantle xenoliths

The Lewisian Gneiss Complex of NW Scotland represents the eastern margin of the North Atlantic Craton. It comprises mid-late Archaean tonalite-trondhjemite-granodiorite gneisses that were metamorphosed and deformed during the Late-Archaean and Palaeoproterozoic. A major swarm of mafic-ultramafic dykes, the Scourie Dyke Swarm, was intruded at ca. 2.4–2.3Ga during a period of extension that can be correlated across the North Atlantic Craton. The majority of dykes are doleritic, with volumetrically minor picrite and olivine gabbro suites.New major and trace element geochemical data and Re-Os isotopes indicate that the Scourie Dyke Swarm was not solely derived from a ‘typical’ asthenospheric mantle source region. The geochemical signatures of the dykes show significant negative Nb, Ta and Ti anomalies, coupled with enrichment in Th, Light Rare Earth Elements and other large ion lithophile elements. These features cannot be reproduced by simple contamination of asthenospheric sources with Lewisian granulite-facies crust. Instead they are a feature of the mantle source that produced the Scourie Dykes and may have developed during Archaean subduction episodes.Spinel lherzolite mantle xenoliths from the Isle of Lewis offer direct insight into the lithospheric mantle below this region. They display similar geochemical ‘enrichments’ and ‘depletions’ observed in the Scourie Dykes and the magma source is thus considered to reside primarily in the sub-continental lithospheric mantle (SCLM), with some potential contribution from asthenospheric melts. Platinum Group Element geochemistry and trace element modelling indicate that the dolerite dykes were formed by moderate (<15%) partial melting of the source, whilst higher degrees of partial melting led to the formation of picritic and olivine gabbro suites. Magma production was triggered by significant crustal and lithospheric extension, causing both asthenospheric and substantial lithospheric melting.

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

We report new U–Pb baddeleyite and zircon ages for doleritic and noritic dykes from the NW–SE trending Scourie dyke swarm located in the Precambrian Lewisian Gneiss Complex of northwest Scotland. These new ages indicate that the majority of the dykes were intruded over a ∼40Ma period between ∼2418 and 2375Ma and we redefine the term ‘Scourie dykes’ to refer to this earliest period of dyke intrusion, thus avoiding confusion with the nearly 400Ma of known mafic magmatism in the area.Recent tectonic interpretations of the Lewisian Gneiss Complex subdivide the area into several small terranes, which amalgamated during the ca. ∼1.9–1.7Ga Laxfordian metamorphic event. However, the majority of Scourie dykes intruded the Lewisian Gneiss Complex shortly after a ∼2.5Ga (Inverian) amphibolite grade metamorphic event, previously interpreted to be related to the collision of the Rhiconich and Assynt terranes. The timing and placement of the Scourie dykes indicates that they could have formed during a period of post-collisional extension. Scourie dykes located in the Tarbert terrane on the Isle of Lewis in the Outer Hebrides and the Assynt terrane of the Scottish mainland indicate that the two areas, along with the Rhiconich terrane, shared a common ancestry prior to dyke emplacement, rather than amalgamating during the Laxfordian orogeny.The new geochronology, along with paleomagnetism, and geochemistry of the Scourie dykes allow the Lewisian Gneiss Complex to be placed in a broad paleogeographic context. Various Neoarchean supercontinents have been proposed based on broad similarities in the timing of major cratonic events. New data on the Scourie dykes, combined with the regional geology of the NW Scottish Highlands, indicate that the Lewisian Gneiss Complex, and potentially the North Atlantic craton, were separate crustal entities from the proposed supercontinent Nunavutia during the Neoarchean and Paleoproterozoic.

The Laxford Shear Zone: an end-Archaean terrane boundary?

Abstract The Lewisian Gneiss Complex of northwestern Scotland consists of Archaean gneisses, variably reworked during the Proterozoic. It can be divided into three districts – a central granulite-facies district between districts of amphibolite-facies gneiss to the north and south. Recent work has interpreted these districts in terms of separate terranes, initiating a controversy that has implications for how Precambrian rocks are understood worldwide. The northern district of the Lewisian Gneiss Complex (the Rhiconich terrane) is separated from the central district (the Assynt terrane) by a broad ductile shear zone known as the Laxford Shear Zone. This paper reviews the geology of the Laxford Shear Zone, clarifying field relationships and discussing other evidence, to consider whether or not it does indeed represent a terrane boundary. A detailed review of field, geochemical and geochronological evidence supports the recognition of the separate Assynt and Rhiconich terranes. Mafic dykes (the Scourie Dyke Swarm) and granitoids, of Palaeoproterozoic age, occur on both sides of the Laxford Shear Zone and thus the terranes were most probably juxtaposed during the late Archaean to early Palaeoproterozoic Inverian event. Subsequently, the less-competent, more-hydrous amphibolite-facies gneisses of the Rhiconich terrane were affected by later Palaeoproterozoic (Laxfordian) deformation and partial melting, to a greater extent than the more-competent granulite-facies gneisses of the Assynt terrane.

A major high-strain zone in the Lewisian Complex in the Loch Torridon area, NW Scotland: insights into deep crustal deformation

Abstract The Lewisian Complex is an Archaean-Proterozoic high-grade gneiss region with widespread amphibolite-facies fabrics. These fabrics have, on the 100 km scale, a subhorizontal enveloping surface and are interpreted as forming in gently dipping crustal-scale shear zones and steeper lateral ramp structures. The Scourie dyke swarm, of Proterozoic age, runs NW-SE subvertically outside such zones, but is transformed into subhorizontal concordant amphibolite sheets within them. The terms ‘Scourian’ and ‘Laxfordian’ are used to describe pre-dyke and post-dyke fabrics and events. In the southern mainland Lewisian at Loch Torridon, unmodified Scourian gneisses in the north pass southwards into lozenges bounded by anastomosing zones of Laxfordian deformation, and eventually to a region consisting entirely of Laxfordian fabrics. This geometry is compatible with a major Laxfordian shear zone that dips northeastwards beneath a Scourian hanging wall. Detailed maps show that structures on the metre to kilometre scale do not relate in a simple way to the crustal shear zone model for four reasons. First, many amphibolite-facies fabrics in the quartzo-feldspathic gneisses predate the dykes (they relate to a late Scourian or ‘Inverian’ event), so the dykes cannot be used to infer overall geometry and movement sense. A few structures along the Loch Roag Line, the most northeasterly high-strain belt in the Loch Torridon inliers, may indicate syntectonic dyke emplacement. Second, local shear with opposing senses is present in both the pre-dyke and post-dyke (Laxfordian) deformation. Third, foliation deflections on a variety of scales relate more to varying shear planes than to the deflection caused by increasing simple shear strain, and cannot be used to infer shear sense. Fourth, there is pure-shear stretching within the gneisses that masks the effects of simple shear. These structures can be incorporated into a crustal-scale shear zone model, but are not diagnostic of it. The scatter of foliation planes with a relatively constant lineation is a feature of other crustal-scale high-strain zones (e.g. Canisp, Nordre Strømfjord), although different explanations are available. Other high-strain zones (e.g. the Laxford Front) show a spatially varying lineation pattern, but this could relate more to a varying shear direction than to deflections of linear features into a single movement direction. The geometry of classic simple shear is not versatile enough to explain the kilometre-scale geometry of such high-strain zone, although it may work well in local subareas.

Chapter 6 Laxfordian Structure and Metamorphism

The key to the Lewisian structural chronology in Gairloch, as elsewhere in the Lewisian complex, is the Scourie dyke swarm, which, following Peach et al. (1907) and Sutton &amp; Watson (1951), has been used to divide the complex into pre-dyke and post-dyke elements; the post-dyke structures and events are referred to the Laxfordian. However, as explained in Chapter 3, none of the dykes attributed to the Scourie dyke swarm in the Gairloch area, or in the Southern Region generally, have been dated (except for K-Ar dates interpreted as metamorphic cooling ages). Therefore they may correlate with either the 2.4 Ga or 2.0 Ga dykes of the Central Region (see Section 1.2.6), or could represent a third swarm of some other age. It is also possible that dykes of two or more different ages are represented. While the later Laxfordian structural and metamorphic elements can be correlated throughout the Lewisian complex, some of the earlier post-dyke structures assigned here to the Laxfordian may not correspond to similar structures elsewhere in the complex if the dykes themselves are not of the same age (see discussion on chronology in Section 1.3). Structures are also assigned to the Laxfordian if they affect the c. 2.0 Ga rocks of the Loch Maree Group (LMG). Although the Archaean gneisses share in the Laxfordian deformation, the following structural analysis depends critically on the evidence from the dykes and the LMG. Laxfordian deformation in the Southern Region of the mainland Lewisian is distributed through a wide belt over

Unravelling dates through the ages: geochronology of the Scottish metamorphic complexes

Abstract The paper by Giletti et al. (1961) is seen as a major landmark in the evolution of dating techniques in polymetamorphic terrains. We consider certain critical issues from each of the main complexes of the Scottish Highlands studied by Giletti et al. to illustrate how subsequent developments in geochronological methodology have influenced our understanding of metamorphic belts. Lewisian examples focus on the formation of Archaean crust, and the age of the main high-grade metamorphism and the Scourie dyke swarm. The antiquity of Moinian sedimentation, its relationship to the Torridonian sandstones, and the timing of Precambrian metamorphism have been controversial issues. The timing and nature of Caledonian orogenesis, most clearly expressed in the Dalradian complex, have been the focal points for the refinement of radiometric investigation. These complexes have been subject to successive developments in methodology, with ever-tighter constraints from Rb-Sr and K-Ar mineral dating, through Rb-Sr and Pb-Pb whole-rock studies, U-Pb dating of bulk zircon fractions, and Sm-Nd whole-rock and mineral investigation, up to the latest technologies of single-grain zircon and ion microprobe analysis. The rocks have released their secrets reluctantly, and many of the questions posed in 1961 have still not been definitively answered. However, the hope of unambiguous solution leads towards greater efforts, ever more reliable data, and a clearer evolutionary picture.

Strike-slip motion in the Minches, NW Scotland, deduced from the trends of the Scourie Dyke swarm

In areas of low Laxfordian strain on the Scottish Mainland Lewisian, the strike of Scourie Dykes varies regionally; trending almost westerly in the north at Assynt and trending northwesterly in the south around Loch Torridon. These divergent trends allow an imaginary point to be constructed on the map at the intersection of these directions. In the Outer Hebrides a similarly variable arrangement of dyke trends is observed but the intersection point of the dyke trends does not coincide with that constructed for the Scottish Mainland. The non-coincidence of these intersection points is interpreted in terms of fault displacement and estimates are made of the combined slip on all faults in the Minch region (including the Minch and Outer Isles faults). Although the data are insensitive to possible dip-slip motions, they indicate a sinistral strike-slip component of between 34–145 km (90% confidence estimates) with a best estimate of 84 km. These results are in good agreement with those determined from the offset across the Minch of correlated metamorphic zones and those measured by the displacement of palaeomagnetic zones.

Unravelling dates through the ages: geochronology of the Scottish metamorphic complexes

The paper by Giletti et al. 1961 is seen as a major landmark in the evolution of dating techniques in polymetamorphic terrains. We consider certain critical issues from each of the main complexes of the Scottish Highlands studied by Giletti et al. to illustrate how subsequent developments in geochronological methodology have influenced our understanding of metamorphic belts. Lewisian examples focus on the formation of Archaean crust, and the age of the main high-grade metamorphism and the Scourie dyke swarm. The antiquity of Moinian sedimentation, its relationship to the Torridonian sandstones, and the timing of Precambrian metamorphism have been controversial issues. The timing and nature of Caledonian orogenesis, most clearly expressed in the Dalradian complex, have been the focal points for the refinement of radiometric investigation. These complexes have been subject to successive developments in methodology, with ever-tighter constraints from Rb-Sr and K-Ar mineral dating, through Rb-Sr and Pb-Pb whole-rock studies, U-Pb dating of bulk zircon fractions, and Sm-Nd whole-rock and mineral investigation, up to the latest technologies of single-grain zircon and ion microprobe analysis. The rocks have released their secrets reluctantly, and many of the questions posed in 1961 have still not been definitively answered. However, the hope of unambiguous solution leads towards greater efforts, ever more reliable data, and a clearer evolutionary picture.

U–Pb baddeleyite ages for the Scourie dyke swarm, Scotland: evidence for two distinct intrusion events

PRECISE and accurate radiometric ages for continental mafic dyke swarms are a prerequisite for global correlation of mafic magmatic events, calibration of apparent polar-wander paths and deciphering mechanisms of dyke emplacement. Precambrian dyke swarms, such as the Scourie dyke swarm in north-west Scotland, are invaluable time-markers, so that a precise and accurate knowledge of their emplacement age is critical when unravelling the complex geological evolution of many terrains. Precise dating is often difficult, however, because magma interaction with country rock and subsequent metamorphic events can severely perturb some isotopic systems. Recent advances in U-Pb geochronology1–3 combined with the discovery that some mafic dykes contain trace amounts of uranium-bearing minerals such as baddeleyite (ZrO2) and/or zircon have made it possible to obtain U–Pb ages for these rocks with a precision typically on the order of 1–2 Myr4,5. Some of the first detailed U–Pb studies of Precambrian mafic dyke swarms5,6 have shown that large volumes of mafic magma, 100,000 km3, were emplaced into the continental crust in surprisingly short periods of time (<2 Myr). Here we report the results from a U–Pb study of three members of the Scourie dyke swarm, and find at least two periods of dyke emplacement at 2,418 and 1,992 Myr BP. We speculate on a potential global correlation of early Proterozoic mafic magmatism and hence on the origin of dyke swarms.

Internal evolution of the major Precambrian shear belt at Torridon, NW Scotland

Summary The Torridon shear belt is a NW-striking region of heterogeneous reworking within the Lewisian complex. Mapping of the deformation fabric intensity reveals a geometry of low-strain, lozenge-shaped blocks surrounded by an anastomosing set of shear zones which form a linked system of flats and lateral ramps. These various orientations of shear zones share a common movement direction but show both sinistral-reverse and dextral-normal shear sense. This is true both in the Inverian, when the linked system was established, and in the Laxfordian, when shearing followed the established network but was localized along the ductile Scourie dykes. Possible explanations for the occurrence of opposed shear senses include local differential shear, and a modification of the Lister &amp; Williams model of deformation. In the latter, shear zones generally show a sense of shear opposite to the overall displacement on the belt, and this underlines the need for a comprehensive analysis of the geometry of such shear belts. The Inverian structure controlled emplacement of the Scourie dyke swarm, which in turn controlled the localization and orientation of the Laxfordian structures. Given the ductility of the dykes, this leads to a simple explanation for the coaxiality of Inverian and Laxfordian structures. A tentative model for the evolution of the Torridon shear belt is given, in which emphasis is placed on the importance of dykes as easy-slip horizons.

Mineralogy, petrology and geochemistry of the Scourie dykes: petrogenesis and crystallization processes in dykes intruded at depth

Summary The mineral chemistry, petrology and geochemistry of representative members of the Scourie dyke swarm are described, and discussed in relation to crystallization processes in deep-seated dykes and to their petrogenesis. Four types of dykes can be defined on petrological and geochemical grounds: bronzite-picrites, norites, olivine-gabbros and quartz dolerites. The latter constitute by far the most abundant dyke-type. In the central Scourian granulite zone in particular the dykes were emplaced at depth into hot country rocks; this enables the through-flow behaviour of different magma types in dyke conduits to be compared. Whereas the dolerites and norites have chilled margins, the bronzite-picrites and olivine-gabbros have coarse grained orthopyroxene-rich and augite-rich margins respectively. This probably reflects turbulent-flow conditions in the less viscous, more ultramafic magmas which permitted rapid crystallization of pyroxenes on the dyke walls during intrusion. The picrite dykes, and to a lesser extent the olivine-gabbro dykes, display additional across-dyke modal and textural variations. These are not accompanied by significant variations in mineral composition, and can largely be attributed to crystal settling in variably inclined dyke sheets. All dykes typically show a ‘continental’ trace element signature (enrichment in light rare-earth elements and large-ion-lithophile elements), but this must have been inherited from the sub-continental lithosphere because contamination by Lewisian gneisses would not generate the observed trace element characteristics. Both trace element and mineral chemical data indicate that the dykes were derived from at least two distinct mantle sources, that supplying the picrite and norite dykes being more refractory with respect to major elements, but showing a greater relative enrichment in light REE and LIL elements. The emplacement of the Scourie dyke swarm represents considerable crustal extension, but also poses a thermal problem in the generation of high temperature picritic magmas. It is suggested that the dykes and their compositional characteristics reflect the processes of growth and evolution of the sub-continental lithosphere, and that the extensive retrogression of the granulites, which occurred penecontemporaneously with dyke intrusion, may also be linked with these processes.

The Scourie dyke suite: Petrogenesis and geochemical nature of the Proterozoic sub-continental mantle

The petrogenesis of bronzite-picrite, olivine-gabbro, norite and quartz-tholeiite dykes, which make up the 2.39 b.y. Scourie dyke swarm cutting the Archaean Lewisian gneisses of N.W. Scotland, is interpreted on the basis of their major and trace element geochemistry. Most of the dykes bear primary amphibole and/or phlogopite and, with one exception, are all hypersthene- or quartz-normative. Apart from one tholeiite dyke which shows relative light rare-earth element depletion, all the dykes show enrichment in light rare-earths and large-ion lithophile elements. They do not however show an equivalent enrichment in other incompatible high field strength ions such as Nb and Ta, and in this respect resemble island arc and calc-alkaline basalts. The different dyke types have distinctive rare-earth patterns and other trace element ratios which are maintained over a range of major element compositions. Petrogenetic modelling of the major and trace element compositions of the various dykes demonstrates that very few can be related by fractional crystallisation. Indeed, even with partial melting mechanisms at least two different mantle sources, with different major and trace element compositons, are required to explain the compositional differences between the major dyke types. The high degrees of mantle melting implied for the generation of the magnesium-rich bronzite-picrites suggests that their rare earth and other trace element patterns closely reflect those of their mantle source. Similar arguments, though less well constrained, can be advanced for the other dyke types. The results suggest that the sub-continental mantle source feeding the dykes was heterogeneous with respect to both major and trace elements, and that their mantle sources must have been enriched in lithophile elements. Enrichment at the time the Lewisian gneisses were generated (i.e. 2.92 b.y. ago) would be compatible with the initial 87Sr/86Sr ratios of the dykes and the inferred Rb/Sr ratios of their mantle sources. The sub-continental mantle sources have thus retained the geochemical signature of the crustgenerating processes some 500 m.y. earlier.

2,390 Myr Rb–Sr whole-rock for the Scourie dykes of north-west Scotland

THE Scourie dyke swarm intrudes the Lewisian Complex in north-west Scotland. The structural state of these basic dykes is used to subdivide the Lewisian into the pre-dyke ‘Scourian’ and post-dyke ‘Laxfordian’ complexes1. The ∼2,700 Myr (refs 2–4) pyroxene-granulite facies Scourian gneisses of the Assynt area contain undeformed NW-trending dykes. Gneisses and dykes, which underwent the ∼1,900 Myr (refs 5, 6) Laxfordian event, are now deformed and metamorphosed at amphibolite facies. Previous K–Ar measurements for the Scourie dykes in the Assynt area of the Lewisian gave a minimum age for dyke injection of 2,200 Myr (ref. 7). Younger K–Ar ages were thought to reflect later Laxfordian overprinting. I report here a Rb–Sr whole-rock isochron age of 2,390±20 Myr (2σ) and an initial 87Sr/86Sr ratio of 0.7022±0.0001 (2σ) for the Scourie dykes.

Variations in crustal level and geothermal gradient during the evolution of the Lewisian complex of northwest Scotland

The metamorphic reactions observed in the polycyclic Precambrian gneisses of parts of western Sutherland and the Outer Hebrides are discussed in the light of other evidence bearing on the tectonic and metamorphic evolution of the Lewisian complex. The evidence that early granulite facies assemblages were transformed into successively lower-pressure assemblages seems consistent with the views expressed by several authors that the complex remained continuously at deep crustal levels for nearly a thousand million years after the earliest dated metamorphic event and that it began to rise towards the earth's surface during the Laxfordian metamorphic stage. The Scourie dyke swarm appears to have been emplaced under deep-seated conditions. The Laxfordian tectonic patterns record the effects of regional uplift and the associated metamorphic assemblages suggest local steepening of the geothermal gradient.