Lewisian Gneiss Complex Research Articles

Temperature–time evolution of the Assynt Terrane of the Lewisian Gneiss Complex of Northwest Scotland from zircon U-Pb dating and Ti thermometry

The Lewisian Gneiss Complex of Northwest Scotland is a classic Precambrian basement gneiss complex. The Lewisian is divided into a number of terranes on the basis of structural, metamorphic and geochronological evidence. The most well-studied of these is the Assynt Terrane, which forms the central part of the Lewisian outcrop on the Scottish mainland. Field evidence shows that it has a complex tectonothermal history, the early stages of which remain poorly constrained. This paper sets out to better understand the chronology and thermal evolution of the Assynt Terrane through zircon U-Pb dating and Ti-in-zircon thermometry, the latter applied to the Lewisian for the first time. This is placed in context by integration with detailed field mapping, sample petrography, zircon cathodoluminescence (CL) imaging and rare earth element (REE) analysis.Zircons from six tonalite-trondhjemite-granodiorite (TTG) gneiss samples and two metasedimentary gneiss samples were analysed. The TTG gneisses were predominantly retrogressed to amphibolite-facies; zircons showed a range of CL zoning patterns and REE profiles were similar to those expected for magmatic zircon grains. Zircons from the metasedimentary gneisses also displayed a range of CL zoning patterns and are depleted relative to chondrite in heavy REEs due to the presence of garnet.Zircon analysis records a spread of concordant U-Pb ages from ∼2500 to 3000Ma. There is no evident correlation of ages with location in the crystal or with CL zoning pattern. A weighted average of 207Pb/206Pb ages from the oldest igneous zircon cores from the TTG gneiss samples gives an age of 2958±7Ma, interpreted to be a magmatic protolith crystallisation age. A weighted average of 207Pb/206Pb ages of the youngest metamorphic rims yields an age of 2482±6Ma, interpreted to represent the last high-grade metamorphism to affect these rocks. Ti-in-zircon thermometry records minimum temperatures of 710–834°C, interpreted to reflect magmatic crystallisation.REE profiling enabled the zircons in the metasedimentary rocks to be linked to the presence of metamorphic garnet, but resetting of U-Pb systematics precluded the determination of either protolith or metamorphic ages. Zircons from the metasedimentary gneisses generally record higher minimum temperatures (803–847°C) than the TTG gneisses, interpreted to record zircon crystallisation in an unknown protolith.

3D characterization of fracture systems using Terrestrial Laser Scanning: an example from the Lewisian basement of NW Scotland

Abstract Fractured gneiss lithologies form a basement-cored high, the Rona Ridge in the Faroe–Shetland Basin. Basement structures are known to play an important role in the petroleum system for the overlying giant Clair Field. An onshore analogue exposure in the Lewisian Gneiss Complex at Kinlochbervie in NW Scotland provides an example of a hanging-wall damage zone of a large basement-hosted normal fault. In this study, we used remote sensing (2D), outcrop line sample methods (1D) and a virtual outcrop created by terrestrial laser scanning methods (3D) to characterize spatial variations of the fracture systems. Spacing distributions from 1D line samples collected from exposures and pseudo-wells constructed through the virtual outcrop show power-law distributions. The virtual outcrop data enable us to extend the scale-invariant description from 1 to 3 orders of magnitude. We developed a novel box-counting workflow to provide an assessment of 2- and 3D variations in the fracture properties. Fracture density and fractal dimension are elevated whereas the number of intersections is decreased within a 220 m-wide volume adjacent to the fault. We discuss how the methods and results from this study can aid the development of analogue for basement reservoirs in the offshore UK continental shelf.

Sedimentary provenance, age and possible correlation of the Iona Group SW Scotland

Synopsis Most clast lithologies within the basal conglomerate of the Iona Group, exposed on Iona, Inner Hebrides, SW Scotland, resemble the adjacent Archaean–Palaeoproterozoic Lewisian Gneiss Complex, supporting the hypothesis that the Iona Group unconformably overlies Lewisian gneisses on Iona. Moreover, a whole-rock Sm–Nd t DM age of 2512 Ma from one of these clasts supports its lithological correlation with granitic pegmatites in the underlying gneisses, while a second quartz-rich paragneiss clast with a Sm–Nd t DM age of 2745 Ma, is similar to Lewisian paragneisses on Coll. However, whole-rock t DM ages of 1755–2060 Ma and pronounced c . 1780–1800 Ma peaks in U–Pb detrital zircon and titanite data for Iona Group metasediments suggest additional distal sources of Laurentian affinity including the Rhinns Complex as well as Labradorian, Pinwarian and Archaean detritus. The Iona Group was deposited and deformed after c . 1490 Ma and before c . 422 Ma, constrained by the youngest detrital zircon and titanite and the intrusion of the Ross of Mull Granite, respectively. The detrital data permit correlation of the Iona Group with the Colonsay Group and the Glenshirra Subgroup of the Dalradian Grampian Group, although correlation with parts of the Torridonian cannot be excluded. Supplementary material: Full analytical results areavailable at http://www.geolsoc.org.uk/SUP18768 .

Microstructural and Metamorphic Constraints on the Thermal Evolution of the Southern Region of the Lewisian Gneiss Complex, NW Scotland

Felsic metagranitoids form a major part of the crust, but the metamorphic story they record is difficult to decipher because of a lack of index minerals. The microstructures and metamorphic assemblages of felsic gneisses and metadolerite dykes from the Lewisian Gneiss complex, NW Scotland, have been examined to estimate the pressure^temperature^time (P^T^t) history of the region. Characteristic geometries and compositions of zoned epidote and plagioclase from the gneisses and amphibole from the dykes provide key information. Bulk-rock compositions are modelled to constrain the likely metamorphic conditions experienced by the rocks. P^T^t paths are refined using a novel model for fractionation during grain-recycling of the plagioclase. In the gneisses, plagioclase grains have relatively albitic cores (An10^12) grading to more anorthitic rims (An20^30). The equant grain shapes of the plagioclase and asymmetry of the zoning across grain boundaries are consistent with the zoning having formed during coarsening, or grain-recycling, following deformation.The increase in anorthite content is due to the breakdown of epidote to release Ca and Al. Sharp boundaries between Fe-poor cores and Fe-rich rims in epidote result from the resorption of epidote whilst the plagioclase is growing followed by later regrowth.The possible P^Tconditions for the end of deformation and start of grain-recycling are restricted to those that occur along the plagioclase isopleth with the same value as the core compositions. These starting conditions are explored along with P^T^t path orientations over a range of values. The results are compared with the observed compositions and grain sizes to determine the best-fit P^T^t path. Most of the best-fit paths are dominated by decompression rather than heating (both of which result in epidote breakdown). Starting conditions are probably between 9 and 10·2 kbar at around 5808C along a quasi-linear path ending at around 7·5^8 kbar at 600^6208C.The timescale of decompression is poorly constrained owing to the uncertainty in the grain-recycling parameters. Rates of exhumation are between 0·14 and 2 mm a, which are reasonable within the range of present-day processes. Scourie dyke assemblages and mineral zoning broadly corroborate this P^T^t path. The path is similar to those recorded in Phanerozoic orogenic cycles but the significance of this work lies in our new methods for elucidating the metamorphic histories of metagranitoids.

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.

High-precision U–Pb dating of complex zircon from the Lewisian Gneiss Complex of Scotland using an incremental CA-ID-TIMS approach

A novel approach of thermally annealing and sequentially partially dissolving single zircon grains prior to high-precision Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) is presented. This technique is applied to complex zircon from the Precambrian Lewisian Gneiss Complex of Scotland. Up to six partial dissolutions were conducted at incrementally higher temperatures and analysed at each successive step. ID-TIMS analyses reveal the portions of zircon affected by the lowest temperature partial dissolution step have suffered Pb-loss. Successively higher temperature partial dissolution steps yield a series of analyses from the younger domains, followed by mixing trajectories with older components, presumably from the inner domains. Specifically, for a partially retrogressed granulite tonalite gneiss from the central block (Assynt), high-grade metamorphic zircon ages of c. 2500Ma and c. 2700Ma are resolved with a protolith age of c. 2860Ma also recognised. This unequivocally demonstrates two separate episodes of high-grade metamorphism affected rocks from this region. The c. 2700Ma age provides a minimum age constraint on the highest pressure event known from Archean crustal rocks. Using this technique of pseudo-spatial resolution coupled with high-precision analysis it is possible to recognise discrete Pb-loss and multiple stages of zircon growth or isotopic resetting within single grains to within 0.1–0.2% error (2σ) on individual 207Pb/206Pb ages. This method has relevance to U–Pb zircon geochronology where conventional micro-beam techniques are unable to resolve between separate ages within single grains.

Structural characteristics and Re–Os dating of quartz-pyrite veins in the Lewisian Gneiss Complex, NW Scotland: Evidence of an Early Paleoproterozoic hydrothermal regime during terrane amalgamation

In the Archaean basement rocks of the Assynt and Gruinard terranes of the mainland Lewisian Complex in NW Scotland, a regional suite of quartz-pyrite veins cross-cut regional Palaeoproterozoic (Badcallian, ca. 2700Ma; Inverian, ca. 2480Ma) fabrics and associated Scourie dykes. The quartz veins are overprinted by amphibolite-greenschist facies Laxfordian deformation fabrics (ca. 1760Ma) and later brittle faults. The hydrothermal mineral veins comprise a multimodal system of tensile/hybrid hydraulic fractures which are inferred to have formed during a regional phase of NW-SE extension. The almost orthogonal orientation of the quartz veins (NE-SW) to the Scourie dykes (NW-SE) are incompatible and must result from distinct paleostress regimes suggesting they are related to different tectonic events. This hypothesis is supported by Rhenium–Osmium dating of pyrite that yields an age of 2249±77Ma, placing the vein-hosted mineralisation event after the oldest published dates for the Scourie Dykes (2420Ma), but before the youngest ages (1990Ma). Sulphur isotope analysis suggests that the sulphur associated with the pyrite is isotopically indistinguishable from primitive mantle. The presence of the ca. 2250Ma quartz-pyrite veins in both the Assynt and Gruinard terranes confirms that these crustal units were amalgamated during or prior to Inverian deformation. The absence of the veins in the Rhiconich Terrane is consistent with the suggestion that it was not finally amalgamated to the Assynt Terrane until the Laxfordian.

The South Barra shear zone: A composite Inverian–Laxfordian shear zone and possible Terrane boundary in the Lewisian gneiss complex of the Isle of Barra, NW Scotland

Synopsis The Lewisian Gneiss Complex (LGC) of NW Scotland largely comprises Archaean tonalite–trondhjemite–granodiorite (TTG) gneisses variably reworked in the Palaeoproterozoic. One form of this heterogeneous reworking is the formation of major shear zones in the earliest Palaeoproterozoic which are then reactivated in the late Palaeoproterozoic. Recent remapping of the LGC of SE Barra, in the southern Outer Hebrides, has led to the interpretation of such a composite shear zone, the Southeast Barra Shear Zone (SBSZ). The SBSZ separates a block of heterogeneously overprinted pyroxene-bearing TTG gneisses from a dioritic meta-igneous complex to the west. Despite a lack of geochronological data, the juxtaposition of different protolith rocks along the SBSZ raises the possibility it may be a previously unrecognized terrane boundary.

New U-Pb age constraints for the Laxford Shear Zone, NW Scotland: Evidence for tectono-magmatic processes associated with the formation of a Paleoproterozoic supercontinent

The Lewisian Gneiss Complex in north-west Scotland is a part of the extensive network of Archaean cratonic areas around the margins of the North Atlantic. It is considered to be made up of a number of terranes with differing protolith ages, which have been affected by a range of different metamorphic events. A major shear zone, the Laxford Shear Zone, forms the boundary between two of these terranes. New dates presented here allow us to constrain the timing of terrane assembly, related to the formation of Palaeoproterozoic supercontinents. Early deformation along the Laxford Shear Zone, and primary accretion of the two terranes, occurred during the Inverian event at c. 2480Ma. This was followed by extension and the intrusion of the mafic Scourie Dykes. Subsequently, renewed silicic magmatic activity occurred at c. 1880Ma, producing major granite sheets, considered to have formed as part of a continental arc. A further collisional event began at c. 1790Ma and was followed by slow exhumation and cooling. This Laxfordian event caused widespread crustal melting, metamorphism and deformation, and is considered to represent the final assembly of the Lewisian Gneiss Complex within the major supercontinent of Columbia (or Nuna).

Lattice distortion in a zircon population and its effects on trace element mobility and U–Th–Pb isotope systematics: examples from the Lewisian Gneiss Complex, northwest Scotland

Zircon is a key mineral in geochemical and geochronological studies in a range of geological settings as it is mechanically and chemically robust. However, distortion of its crystal lattice can facilitate enhanced diffusion of key elements such as U and Pb. Electron backscatter diffraction (EBSD) analysis of ninety-nine zircons from the Lewisian Gneiss Complex (LGC) of northwest Scotland has revealed five zircons with lattice distortion. The distortion can take the form of gradual bending of the lattice or division of the crystal into subgrains. Zircon lattices are distorted because of either post-crystallisation plastic distortion or growth defects. Three of the five distorted zircons, along with many of the undistorted zircons in the population, were analysed by ion microprobe to measure U and Pb isotopes, Ti and REEs. Comparison of Th/U ratio, 207Pb/206Pb age, REE profile and Ti concentration between zircons with and without lattice distortion suggests that the distortion is variably affecting the concentration of these trace elements and isotopes within single crystals, within samples and between localities. REE patterns vary heterogeneously, sometimes relatively depleted in heavy REEs or lacking a Eu anomaly. Ti-in-zircon thermometry records temperatures that were either low (~700 °C) or high (>900 °C) relative to undistorted zircons. One distorted zircon records apparent 207Pb/206Pb isotopic ages (−3.0 to +0.3 % discordance) in the range of ~2,420–2,450 Ma but this does not correlate with any previously dated tectonothermal event in the LGC. Two other distorted zircons give discordant ages of 2,331 ± 22 and 2,266 ± 40 Ma, defining a discordia lower intercept within error of a late amphibolite-facies tectonothermal event. This illustrates that Pb may be mobilised in distorted zircons at lower metamorphic grade than in undistorted zircons. These differences in trace element abundances and isotope systematics in distorted zircons relative to undistorted zircons are generally interpreted to have been facilitated by subgrain walls. Trace elements and isotopes would have moved from undistorted lattice into these subgrain walls as their chemical potential is modified due to the presence of the dislocations which make up the subgrain wall. Subgrain walls provided pathways for chemical exchange between crystal and surroundings. Only five per cent of zircons in this population have lattice distortion suggesting it will not have a major impact on zircon geochronology studies, particularly as three of the five distorted zircons are from strongly deformed rocks not normally sampled in such studies. However, this does suggest there may be a case for EBSD analysis of zircons prior to geochemical analysis when zircons from highly deformed rocks are to be investigated.

Archaean Intracrustal Differentiation from Partial Melting of Metagabbro--Field and Geochemical Evidence from the Central Region of the Lewisian Complex, NW Scotland

The central region of the mainland Lewisian gneiss complex of NW Scotland is a granulite-facies migmatite terrane. With the exception of ultramafic and rare calc-silicate rocks, all other lithologies partially melted during Neoarchaean, ultrahigh-temperature (Badcallian) metamorphism. The clearest evidence is preserved within large layered mafic^ultramafic bodies that exhibit macroscopic features diagnostic of anatexis. In situ partial melting of metagabbroic rocks produced patches and sheets of coarse-grained plagioclase-rich leucosome containing euhedral peritectic clinopyroxene.These leucosomes connect with larger, laterally continuous tonalite or trondhjemite sheets that record segregation and migration of melt away from the metagabbro source rocks. This melt loss allowed wide-scale preservation of granulite-facies assemblages within the residual melanosome. Whole-rock major and trace element geochemistry is broadly consistent with the field evidence, but suggests contamination of the metagabbroic rocks by their host-rocks and a strong mineralogical control on trace element distributions, the consequence of large diffusive length scales during protracted ultrahigh-temperature metamorphism.Variations in the trace element composition of the felsic sheets reflect heterogeneities in the source rocks, the presence of material entrained from the melanosome and fractional crystallization dominated by plagioclase.The felsic sheets are largely cumulate, suggesting loss of the evolved melt fraction to higher crustal levels. Partial melting of felsic gneisses that surround the mafic^ultramafic bodies is inevitable at the implied metamorphic peak provided they contained hydrous phases, although the field evidence is largely obscured by later reworking. This study provides insights into the processes involved in intracrustal differentiation during the Neoarchaean, during which partial melting of mafic rocks is likely to have made a more significant contribution than during the Phanerozoic.

Palaeoproterozoic terrane assembly in the Lewisian Gneiss Complex on the Scottish mainland, south of Gruinard Bay: SHRIMP U–Pb zircon evidence

New SHRIMP U–Pb zircon geochronology and fieldwork integrated with reappraisal of earlier mapping demonstrates that the so-called ‘southern region’ of the mainland Lewisian Gneiss Complex comprises a package of distinct tectono-stratigraphic units. From south to north these are the Rona (3135–2889 Ma), Ialltaig (c. 2000 Ma) and Gairloch (ca. 2200 Ma) terranes. These terranes were metamorphosed and deformed separately until ca. 1670 Ma by which time they had been juxtaposed and were integral with terranes to the north. The northern boundary of the Palaeoproterozoic Gairloch terrane is a shear zone, north of which is the Archaean Gruinard terrane with 2860–2800 Ma protoliths and ca. 2730 Ma granulite facies metamorphism. In contrast, south of the Gairloch terrane, the Archaean gneisses of the Rona terrane have older protolith ages, underwent an anatectic event at ca. 2950 Ma and show no evidence of 2730 Ma granulite facies metamorphism. In current structural interpretations the Gruinard terrane forms a structural klippe over the intervening Gairloch terrane. However, the Rona and Gruinard terranes cannot be equivalent on age grounds, and are interpreted as unrelated different entities. Contained within the southern margin of the Gairloch terrane is the Ialltaig terrane, shown here to comprise an exotic slice of granulite facies Palaeoproterozoic crust, rather than Archaean basement as previously thought. The ca. 1877 Ma granulite facies metamorphism of the Ialltaig terrane is the youngest event that is unique to a single terrane in the mainland Complex, making it an upper estimate for the timing of amalgamation with surrounding tectonic units. U–Pb titanite ages of 1670 ± 12 Ma and ca. 1660 Ma for low-strain zones at Diabaig are interpreted to be cooling through the titanite closure temperature after the amphibolite facies reworking of these southern terranes and the southern margin of the Gruinard Terrane. These new data have implications for the tectonic setting of the mainland in relation to the Outer Hebrides and in the wider evolution of the basement in the North Atlantic.

Multiple high-pressure metamorphic events and crustal telescoping in the NW Highlands of Scotland

Abstract: The Glenelg–Attadale Inlier is the largest basement inlier within the Caledonian orogen in NW Scotland. A Western Unit consists of trondhjemite–tonalite–granodiorite (TTG) gneisses and subordinate mafic intrusions with locally preserved mafic high-pressure granulite and eclogite assemblages. An Eastern Unit comprises TTG gneisses, Grenville-age ( c . 1.1 Ga) eclogite and metasediments. U–Pb zircon ages from the Western Unit TTG gneisses are highly disturbed with Neoarchaean upper intercepts and Palaeo- to Mesoproterozoic lower intercepts, suggesting strong reworking at these times. U–Pb zircon, Sm–Nd and Lu–Hf garnet–clinopyroxene dates from the Western Unit high-pressure granulite and eclogite yield Neoarchaean ( c . 2.6–2.8 Ga) and Palaeoproterozoic ( c . 1.75 Ga) ages, respectively. These ages correspond to the ages of partial resetting of the TTG gneisses. The eclogite in the Western Unit may represent the high-pressure convergent margin to the lower-pressure Laxfordian events within the Lewisian Gneiss Complex, whereas the Mesoproterozoic eclogites in the Eastern Unit may represent a farther eastward Grenville-age margin. Further east, Ordovician high-pressure granulites within the Moine Supergroup may represent another, later, Grampian convergent margin. These high-pressure belts were developed sequentially towards the east and telescoped westwards from Palaeoproterozoic to Early Palaeozoic times. Supplementary material: U–Pb analytical data and LA-ICP-MS trace element data are available at http://www.geolsoc.org.uk/SUP18389 .

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.

Recording and analyzing geospatially accurate structural data through ‘digital mapping’ technique: A case study from the Canisp Shear Zone, NW Scotland

Spatial accuracy of structural data is an important factor for construction of a properly scaled model of crustal structures. Digital Geological Mapping methods can add the required spatial accuracy to the data, and also can improve the versatility of the geological/structural map in many ways, which are difficult to achieve through conventional mapping techniques. The present paper describes the methodology of digital mapping and discusses its applicability in structural analyses in the Canisp Shear Zone (CSZ) — a reactivated continental basement structure in the Precambrian Lewisian Gneiss complex of NW Scottish Highland. The CSZ is reinterpreted as a major dextral transpressional shear zone developed during Laxfordian deformation. It overprinted an earlier Inverian shear zone fabric, and was later reactivated and/or overprinted by brittle shearing of different phases and different scales. Spatial analyses within the ArcGISTM environment has helped bring out the geological relationships between different types of structural data in the shear zone, highlighting the partitioning of deformation into zones of high and low strain. A kinematic interpretation based on the geospatial data analysis, combined with conventional stereographic projection technique, is presented.

High spatial resolution 40 Ar/ 39 Ar dating of pseudotachylites: geochronological evidence for multiple phases of faulting within basement gneisses of the Outer Hebrides (UK)

Abstract: Pseudotachylites within the Neoarchaean to Palaeoproterozoic basement of the Lewisian Gneiss Complex have been analysed using the high spatial resolution 40 Ar/ 39 Ar UV laserprobe technique. It has been possible to measure 40 Ar/ 39 Ar age and ‘apparent' age variations that would be beyond resolution by lower spatial resolution or bulk 40 Ar/ 39 Ar dating methods. We report a range of complexities including significantly younger narrow (&lt;1 mm) margins on much older pseudotachylite veins, and heterogeneous excess argon contamination. We also report pseudotachylite veins of varying magnitudes that have been subject to later thermal reheating, whereby there is a relationship between vein thickness and extent of argon loss. In spite of these it is still possible to determine geologically meaningful 40 Ar/ 39 Ar ages for pseudotachylite-forming events. In Lewis the c . 1900 Ma event may relate to early stages of Laxfordian reworking. In Barra and South Uist a 1300–1200 Ma event is recorded, whereas in Barra, South and North Uist a second event is recorded at c . 700 Ma, which is likely to be associated with NW- and west-directed thrusting, and could represent far-field effects of tectonic events recorded at this time east of the Moine Thrust in NW Scotland. Supplementary material: 40 Ar/ 39 Ar data are available at http://www.geolsoc.org.uk/SUP18372 .

New SIMS U–Pb zircon ages from the Langavat Belt, South Harris, NW Scotland: implications for the Lewisian terrane model

Secondary ionization mass spectrometry (SIMS) U/Pb dating of zircon has been applied to rocks of the Lewisian Complex of South Harris (Outer Hebrides, NW Scotland), yielding insights into the complex geological evolution of this region and enhancing our understanding of terranes in the Lewisian Gneiss Complex. Results show that tonalite–trondhjemite–granodiorite magmatism occurred in the Tarbert Terrane between c . 2850 and 2830 Ma, equivalent to similar rocks in other parts of the Outer Hebrides. Magmatism was accompanied or closely followed by high-grade metamorphism at c . 2830 Ma, the first reported occurrence of such an event at this time. These rocks were affected by a subsequent high-grade metamorphic event at c . 2730 Ma, which has correlatives both in the Outer Hebrides and in the mainland Lewisian Gruinard Terrane. These data suggest that the ‘terranes’ forming the Outer Hebrides and parts of the mainland Lewisian were once contiguous and were fragmented by post-2730 Ma events. The c . 1700–1660 Ma ages from felsic sheets and pegmatites within the Langavat Shear Zone may record the reassembly of these crustal fragments.

U–Pb zircon dating of basement inliers within the Moine Supergroup, Scottish Caledonides: implications of Archaean protolith ages

Basement gneiss inliers within the Scottish Caledonides have been conventionally correlated with the Archaean Lewisian Gneiss Complex of the Caledonian foreland. Alternatively, the inliers could represent allochthonous terranes accreted to Laurentia before or during the Caledonian orogeny. Secondary ionization mass spectrometry U–Pb zircon dating indicates that the Ribigill, Borgie, Farr and Western Glenelg basement inliers are characterized by late Archaean protolith ages, and a period of isotopic disturbance in the late Palaeoproterozoic. The data are broadly consistent with correlation between the inliers and components of the Lewisian Gneiss Complex of the Caledonian foreland. The c . 2900 Ma protolith ages support correlation of the Borgie and Farr inliers with the Assynt terrane, and a younger, c . 2800 Ma age for the Ribigill inlier supports correlation with the Rhiconich terrane. None of the studied inliers shows a complete match of protolith and early metamorphic histories with any of the Lewisian basement terranes, but differences between the inliers and the foreland are no greater than those recorded within the foreland basement terranes themselves. Therefore, it remains probable that the dated inlier gneisses formed a distal part of the Laurentian margin prior to final telescoping during the Caledonian orogeny.

Timing of magmatism and metamorphism in the Gruinard Bay area of the Lewisian Gneiss Complex: comparison with the Assynt Terrane and implications for terrane accretion—reply

U-Pb zircon data from selected gneiss samples provide estimates of the timing of igneous protolith emplacement and granulite facies metamorphism in the Gruinard Bay, Badcall Bay and Lochinver areas of the mainland Lewisian Gneiss Complex of north-west Scotland. Zircons from the gneisses contain complex zoning patterns making them amenable to SHRIMP ion microprobe analysis, with inherited zircon distinguishable from magmatic growth and metamorphic recrystallisation. From Gruinard Bay, a tonalitic granulite yielded a protolith age of 2,825±8 Ma for oscillatory-zoned zircon cores, with recrystallised low-U rims preserving an age of 2,733±12 Ma. An inherited 2,905±15 Ma age component was recognised in a sample of trondhjemitic gneiss, the protolith of which is estimated to have been emplaced at 2,858±11 Ma. Low-U rims yielding an age of 2,729±20 Ma are also present, which together with the rim age from the granulite are considered to date granulite facies metamorphism in the Gruinard region. For comparison with the Gruinard Bay gneisses, a trondhjemitic partial melt and trondhjemite sheet were analysed from the granulite facies rocks at Badcall Bay in the Assynt terrane. Age data do not show definitive relationships with zoning patterns in the zircons, and show a spread along concordia typical of previous ion microprobe age data from the Scourie gneisses. Both samples do, however, show a significant isotopic disturbance at 2,480 Ma, the most recent estimate for the Badcallian metamorphism. Protolith ages for components of the Assynt terrane range between ca. 2,900 and 3,030 Ma, and appear to be older than the 2,860–2,825 Ma gneiss protoliths from the Gruinard Bay area. The timing of granulite facies metamorphism also appears to be different between the two areas, suggesting that the two granulite regions of the mainland may not be related as previously thought. Instead, they are interpreted in terms of a terrane accretion model whereby the two areas evolved as separate blocks prior to their juxtaposition during the Palaeoproterozoic. The boundary between the two terranes is thought to lie to the south of Lochinver, corresponding with the Strathan Line, because of the observed 2,480 Ma age signature to the north and 2,730 Ma signature to the south. By defining a new mainland terrane, the Gruinard Terrane, the number of mainland blocks, which includes the Assynt (central), Rhiconich (northern) and Southern terranes, has been expanded.