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R G Park - One of the best experts on this subject based on the ideXlab platform.
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the laxford shear zone an end archaean terrane boundary
Geological Society London Special Publications, 2010Co-Authors: K M Goodenough, C R L Friend, R G Park, John Wheeler, Quentin Crowley, Maarten Krabbendam, J S Myers, Susan C Loughlin, A BeachAbstract: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.
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the Lewisian terrane model a review
Scottish Journal of Geology, 2005Co-Authors: R G ParkAbstract:Synopsis The Lewisian terrane model of Friend and Kinny divides the Lewisian complex into nine separate terranes believed to have amalgamated during the Palaeoproterozoic. This paper analyses the rationale for the terrane model and suggests criteria to evaluate and refine it. Similarities and differences in Palaeoproterozoic structural, metamorphic and igneous features are used to divide the complex into 14 separate blocks of three types: type A (upper-plate) characterized by retrogressed high-pressure granulite-facies Archaean metamorphic rocks and localized Laxfordian deformation; type B (lower plate) characterized by amphibolite-facies Archaean metamorphic rocks originating at higher crustal levels, intense Laxfordian deformation, and swarms of Laxfordian granites and pegmatites; and third, Palaeoproterozoic complexes comprising oceanic and volcanic arc elements. The boundaries between the type A and type B blocks, where seen, are major shear zones believed to represent a deformed and disrupted collisional suture between two large continental plates, the central Greenland craton (CGC) to the NE and the North Atlantic craton (NAC) to the SW. The following sequence of events is proposed. The Palaeoproterozoic complexes were accreted to the upper plate (NAC) at c. 1.9 Ga. This was followed by collision with the CGC at c. 1.87 Ga, causing the intense early Laxfordian deformation and high-grade metamorphism. At c. 1.74 Ga a second collision took place with the 1.80 Ga ‘Malin’ volcanic arc to the south, resulting in reactivation and late Laxfordian deformation, producing NW-trending folds, dextral shear zones, granite sheets and pegmatites. These late Laxfordian movements are considered to have resulted in considerable disruption of the early Laxfordian terranes. They include substantial strike-slip movements that may have led to the creation of further displaced terranes, some of which may prove to be exotic.
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discussion on a terrane based nomenclature for the Lewisian gneiss complex of nw scotland journal vol 162 2005 pp 175 186
Journal of the Geological Society, 2005Co-Authors: R G Park, P D Kinny, C R L Friend, G J LoveAbstract:Graham Park writes: The terrane model of Kinny et al. (2005; see also Friend & Kinny 2001) presents a radically different picture of the Lewisian complex from the ‘traditional’ view of a single piece of Archaean crust that has been subsequently modified and augmented in various Proterozoic events. The authors are to be congratulated on their attempt to rationalize a very confused terminology that has arisen over many decades, and for forcing us to rethink our views. I have reservations about their re-definition of ‘Laxfordian’, but my comments mainly concern the validity of the terrane model as applied to the Lewisian and suggestions to refine it (see Park 2005). I agree that uncritical use of terms such as ‘Scourian’, ‘Laxfordian’ and ‘Scourie dyke suite’ can obscure genuine differences of rock, process or event (e.g. see Park et al . 2002). Most of the authors’ suggested revisions seem sensible, with the exception of their proposed restriction of ‘Laxfordian’ to a c. 1.74 Ga metamorphic event at Loch Laxford. I believe that this term is more useful in its current sense as a general name for the series of events etc. that affected the entire Lewisian complex after the incorporation of the early Palaeoproterozoic material (e.g. the Loch Maree Group and the ‘Scourie’ dykes). I have retained the traditional usage below for convenience. According to this concept, distinction is made between ‘suspect terranes’ and ‘displaced terranes’ i.e. those that are exotic or allochthonous. In order to qualify as a displaced terrane, a piece of crust should display evidence of significant displacement relative to its neighbour. Differences in geological history between two adjacent pieces of crust may not in themselves be sufficient evidence of large relative displacements between them. Transverse or oblique convergence between two continental plates inevitably involves the subduction …
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the Lewisian geology of gairloch nw scotland
2002Co-Authors: R G ParkAbstract:This Memoir presents a detailed account of one of the most critical and interesting parts of the Lewisian complex of NW Scotland; a detached fragment of a supercontinent made up of Laurentia, Siberia and Baltica, which existed at the end of the early Proterozoic period. The Gairloch area, together with neighbouring Loch Maree, is the only part of the mainland Lewisian outcrop where Palaeoproterozoic supracrustal rocks (the Loch Maree Group] and their relationships to the Archaean basement can be studied, and has been the subject of a considerable amount of research, spanning a period of more than forty years. The Loch Maree Group represents an amalgamation of oceanic, trench, and arc assemblages with continental basement, and forms part of a Palaeoproterozoic collisional orogen stretching from Labrador through South Greenland to Scandinavia. The author is Emeritus Professor of Tectonic Geology at Keele University, England, and has studied the structural geology of Precambrian terrains in many parts of the world. He has mapped the area covered by the Memoir in detail, at a scale of 1:10 000, and has an intimate knowledge of the geology, acquired over many years.
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detrital zircon ages from the loch maree group Lewisian complex nw scotland confirmation of a palaeoproterozoic laurentia fennoscandia connection
Terra Nova, 1997Co-Authors: M J Whitehouse, D Bridgwater, R G ParkAbstract:Ion-microprobe U-Pb zircon ages of detrital zircons from a metasediment of the Loch Maree Group, Lewisian Complex, NW Scotland show striking similarities to those in coeval Palaeoproterozoic belts of Laurentia and Fennoscandia, and confirm previous suggestions of a connection between them. Late-Archaean zircons (3.06–2.48 Gyr old) represent derivation from Lewisian quartzofeldspathic gneisses, andlor a contemporaneous terrane. Palaeoproterozoic zircons (2.2–2.0 Gyrold) place a maximum age constraint upon deposition. An appropriate quartzofeldspathic source for these latter zircons is not presently known, either in the Lewisian or the wider Laurentia-Fennoscandia region, although its requirement would be consistent with a development of the Loch Maree Group, together with juvenile magmatic arc rocks, at an active margin which was probably removed by lateral tectonics. Comparisons are made with contemporaneous supracrustal sequences throughout the Laurentia–Fennoscandia region, which exhibit marked similarities in source region age characteristics, lithologies, and inferred depositional environment.
P D Kinny - One of the best experts on this subject based on the ideXlab platform.
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discussion on a terrane based nomenclature for the Lewisian gneiss complex of nw scotland journal vol 162 2005 pp 175 186
Journal of the Geological Society, 2005Co-Authors: R G Park, P D Kinny, C R L Friend, G J LoveAbstract:Graham Park writes: The terrane model of Kinny et al. (2005; see also Friend & Kinny 2001) presents a radically different picture of the Lewisian complex from the ‘traditional’ view of a single piece of Archaean crust that has been subsequently modified and augmented in various Proterozoic events. The authors are to be congratulated on their attempt to rationalize a very confused terminology that has arisen over many decades, and for forcing us to rethink our views. I have reservations about their re-definition of ‘Laxfordian’, but my comments mainly concern the validity of the terrane model as applied to the Lewisian and suggestions to refine it (see Park 2005). I agree that uncritical use of terms such as ‘Scourian’, ‘Laxfordian’ and ‘Scourie dyke suite’ can obscure genuine differences of rock, process or event (e.g. see Park et al . 2002). Most of the authors’ suggested revisions seem sensible, with the exception of their proposed restriction of ‘Laxfordian’ to a c. 1.74 Ga metamorphic event at Loch Laxford. I believe that this term is more useful in its current sense as a general name for the series of events etc. that affected the entire Lewisian complex after the incorporation of the early Palaeoproterozoic material (e.g. the Loch Maree Group and the ‘Scourie’ dykes). I have retained the traditional usage below for convenience. According to this concept, distinction is made between ‘suspect terranes’ and ‘displaced terranes’ i.e. those that are exotic or allochthonous. In order to qualify as a displaced terrane, a piece of crust should display evidence of significant displacement relative to its neighbour. Differences in geological history between two adjacent pieces of crust may not in themselves be sufficient evidence of large relative displacements between them. Transverse or oblique convergence between two continental plates inevitably involves the subduction …
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proposal for a terrane based nomenclature for the Lewisian gneiss complex of nw scotland
Journal of the Geological Society, 2005Co-Authors: P D Kinny, C R L Friend, G J LoveAbstract:The current nomenclature for the Lewisian Gneiss Complex has evolved from lithological and structural correlations made prior to any dating. Initial (flawed) geochronological studies gave some names an apparent chronological standing but, as work advanced, fitting events into a coherent regional framework became increasingly difficult. Modern dating studies have shown that the Lewisian Gneiss Complex was progressively assembled from disparate blocks of Archaean continental crust and juvenile Proterozoic arcs, which satisfy the definition of terranes. Each terrane had its own separate accretionary and metamorphic history followed by a common history once juxtaposed against other terranes. Based on a new compilation of modern geochronology allied to the many detailed structural and metamorphic studies, this paper proposes a new systematic terminology for the Lewisian Gneiss Complex that is more applicable to this new tectonic framework.
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a reappraisal of the Lewisian gneiss complex geochronological evidence for its tectonic assembly from disparate terranes in the proterozoic
Contributions to Mineralogy and Petrology, 2001Co-Authors: Clark R L Friend, P D KinnyAbstract:New U-Pb single-zircon geochronology undertaken on tonalitic gneisses, granite sheets, migmatites and metasediments from the Lewisian Gneiss Complex on the mainland and the northern part of the Outer Hebrides, NW Scotland, have been used to test the correlation of so-called Laxfordian events across the complex from the Outer Hebrides to the mainland, and the current model for the evolution of the complex as a whole. The study has revealed that the granite sheets originated in two quite different melting events. Those on the mainland at Loch Laxford are ca. 1,855 Ma old whereas those on Harris and Lewis, with which they are presently correlated, are ca. 1,675 Ma old. Grey gneisses associated with granites on the south side of Loch Laxford are confirmed to belong to the 'northern region'. A migmatitic grey gneiss on Harris has given a protolith age of ca. 3,125 Ma, the currently oldest recognised in the complex. Detrital zircons in the Leverburgh and Langavat belts range in age from 2,780 to 1,880 Ma and unequivocally demonstrate deposition in the Palaeoproterozoic. The granulite facies metamorphism in this block is dated from zircon overgrowths at ca. 1,880 Ma. The Laxford Shear Zone which separates the northern and central regions is interpreted to have evolved post-1,860 Ma, during amphibolite facies metamorphism accompanying deformation which took place at ca. 1,740 Ma in both regions. On Harris, the Langavat-Finsbay shear zone developed after 1,675 Ma when a ca. 1,880-Ma granulite facies Proterozoic arc was juxtaposed against amphibolite facies Archaean rocks to the north. Therefore, the shear zones which bound tectonic blocks in the Lewisian Complex evolved at different times and can be interpreted as terrane boundaries. The new data confirm that the Lewisian Complex was not constructed from one contiguous piece of Archaean crust reworked in the Proterozoic but was progressively assembled from several discrete terranes during the Proterozoic. Accordingly, the former regional divisions of the Lewisian Complex are here renamed as follows. On the mainland, the northern region is called the Rhiconich terrane, and the central region the Assynt terrane. On the Outer Hebrides, the Archaean gneisses of Lewis and the northern part of Harris comprise the Tarbert terrane, whereas the newly accreted Proterozoic blocks are called the Roineabhal terrane in Harris and the Niss terrane in the north on Lewis. Wider correlations show that the geology of the Outer Hebrides has more in common with East Greenland than mainland Scotland on the eastern side of the Minch Fault.
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u pb isotopic evidence for the accretion of different crustal blocks to form the Lewisian complex of northwest scotland
Contributions to Mineralogy and Petrology, 1997Co-Authors: P D Kinny, Clark R L FriendAbstract:Single zircon and titanite U-Pb SHRIMP data presented for tonalite-trondhjemite-granodiorite (TTG) suite gneisses and an ultramafic rock from the northern and central regions of the Lewisian Complex of northwest Scotland, show that protolith ages of tonalitic gneisses in the northern region (2800–2840 Ma) are significantly younger than those in the central region (2960–3030 Ma). Further evidence of a major (2490–2480 Ma) metamorphic event in the central region is documented by a metamorphic zircon associated with a granulite facies ultramafic body. A dioritic gneiss from the northern region has also been dated at c. 2680 Ma. The northern region therefore does not comprise reworked central region rocks and consequently the old models for the evolution of the Lewisian which were based upon this concept need replacing. It is instead proposed that two distinct crustal blocks, now the northern and central regions, were tectonically juxtaposed along a boundary corresponding to the Laxford Front. Juxtaposition would appear to have occurred in Proterozoic times, as it must have postdated the 2490–2480 Ma (?Inverian) metamorphism recorded only in the central region, and the emplacement of granite sheets restricted to the northern side of the boundary. The first recorded event common to both regions is resetting of titanite ages associated with c. 1750 Ma Laxfordian amphibolite facies metamorphism. Zircon inheritance in rocks of both regions is scarce. Within one zircon from the northern region a c. 3550 Ma core was found. This represents the oldest known material from the region.
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new evidence for protolith ages of Lewisian granulites northwest scotland
Geology, 1995Co-Authors: Clark R L Friend, P D KinnyAbstract:The granulite facies Scourian rocks in the central region of the Lewisian complex of northwest Scotland play an important role in the understanding of the development of mid-Archean high-grade gneiss complexes. Whereas the later, largely Proterozoic history of the Scourian has been clarified in a detailed U-Pb study, unraveling the Archean ages of the protoliths using conventional isotope techniques has proved impossible. In order to advance our understanding of the deep crustal processes responsible for these granulites, it is essential to establish the age(s) of accretion of the protoliths and their subsequent tectonothermal history. A combined cathodoluminescence (CL) and sensitive high-mass resolution ion microprobe (SHRIMP) single-zircon study has revealed a hitherto unrecognized morphological complexity in zircons from the type localities of the granulites. Use of CL allowed identification of relict oscillatory igneous zoning, metamorphic overgrowths, and irregular areas of recrystallization. From the SHRIMP data, an age of ca. 2960 Ma is inferred for the gneiss protoliths, which were altered considerably during an important metamorphic event ca. 2490 Ma.
K M Goodenough - One of the best experts on this subject based on the ideXlab platform.
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Enriched lithospheric mantle keel below the Scottish margin of the North Atlantic Craton: Evidence from the Palaeoproterozoic Scourie Dyke Swarm and mantle xenoliths
'Elsevier BV', 2018Co-Authors: Hughes Hsr, K M Goodenough, Ciborowski Tjr, Ac Kerr, Davies Jhfl, Selby DAbstract:This is the final version of the article. Available from the publisher via the DOI in this record.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-Archaeanand Palaeoproterozoic.Amajor swarmofmafic-ultramafic dykes, the Scourie Dyke Swarm, was intruded at ca. 2.4–2.3 Ga 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 ofthe dykes show significant negative Nb, Ta and Ti anomalies, coupled with enrichmentin 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 directinsightinto the lithospheric mantle below this region. They display similar geochemical‘enrichments’ and ‘depletions’ observed inthe 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 (
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dykes as physical buffers to metamorphic overprinting an example from the archaean palaeoproterozoic Lewisian gneiss complex of nw scotland
Scottish Journal of Geology, 2017Co-Authors: John Macdonald, Craig Magee, K M GoodenoughAbstract: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
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petrogenesis of rare metal pegmatites in high grade metamorphic terranes a case study from the Lewisian gneiss complex of north west scotland
Precambrian Research, 2016Co-Authors: R A Shaw, K M Goodenough, Nick M W Roberts, Matthew S A Horstwood, Simon Chenery, A G GunnAbstract:Abstract Many rare metals used today are derived from granitic pegmatites, but debate continues about the origin of these rocks. It is clear that some pegmatites represent the most highly fractionated products of a parental granite body, whilst others have formed by anatexis of local crust. However, the importance of these two processes in the formation of rare-metal pegmatites is not always evident. The Lewisian Gneiss Complex of NW Scotland comprises Archaean meta-igneous gneisses which were highly reworked during accretional and collisional events in the Palaeoproterozoic (Laxfordian orogeny). Crustal thickening and subsequent decompression led to melting and the formation of abundant granitic and pegmatitic sheets in many parts of the Lewisian Gneiss Complex. This paper presents new petrological, geochemical and age data for those pegmatites and shows that, whilst the majority are barren biotite–magnetite granitic pegmatites, a few muscovite–garnet (rare-metal) pegmatites are present. These are mainly intruded into a belt of Palaeoproterozoic metasedimentary and meta-igneous rocks known as the Harris Granulite Belt. The rare-metal pegmatites are distinct in their mineralogy, containing garnet and muscovite, with local tourmaline and a range of accessory minerals including columbite and tantalite. In contrast, the biotite–magnetite pegmatites have biotite and magnetite as their main mafic components. The rare-metal pegmatites are also distinguished by their bulk-rock and mineral chemistry, including a more peraluminous character and enrichments in Rb, Li, Cs, Be, Nb and Ta. New U–Pb ages (c. 1690–1710 Ma) suggest that these rare-metal pegmatites are within the age range of nearby biotite–magnetite pegmatites, indicating that similar genetic processes could have been responsible for their formation. The peraluminous nature of the rare-metal pegmatites strongly points towards a metasedimentary source. Notably, within the Lewisian Gneiss Complex, such pegmatites are only found in areas where a metasedimentary source is available. The evidence thus points towards all the Laxfordian pegmatites being formed by a process of crustal anatexis, with the formation of rare-metal pegmatites being largely controlled by source composition rather than solely by genetic process. This is in keeping with previous studies that have also challenged the widely accepted model that all rare-metal pegmatites are formed by fractionation from a parental granite, and raises questions about the origin of other mineralised pegmatites worldwide.
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temperature time evolution of the assynt terrane of the Lewisian gneiss complex of northwest scotland from zircon u pb dating and ti thermometry
Precambrian Research, 2015Co-Authors: John Macdonald, Simon L Harley, K M Goodenough, John Wheeler, Quentin Crowley, Elisabetta Mariani, D TathamAbstract: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 3000 Ma. 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 ± 7 Ma, 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 ± 6 Ma, 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.
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the laxford shear zone an end archaean terrane boundary
Geological Society London Special Publications, 2010Co-Authors: K M Goodenough, C R L Friend, R G Park, John Wheeler, Quentin Crowley, Maarten Krabbendam, J S Myers, Susan C Loughlin, A BeachAbstract: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.
Clark R L Friend - One of the best experts on this subject based on the ideXlab platform.
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a reappraisal of the Lewisian gneiss complex geochronological evidence for its tectonic assembly from disparate terranes in the proterozoic
Contributions to Mineralogy and Petrology, 2001Co-Authors: Clark R L Friend, P D KinnyAbstract:New U-Pb single-zircon geochronology undertaken on tonalitic gneisses, granite sheets, migmatites and metasediments from the Lewisian Gneiss Complex on the mainland and the northern part of the Outer Hebrides, NW Scotland, have been used to test the correlation of so-called Laxfordian events across the complex from the Outer Hebrides to the mainland, and the current model for the evolution of the complex as a whole. The study has revealed that the granite sheets originated in two quite different melting events. Those on the mainland at Loch Laxford are ca. 1,855 Ma old whereas those on Harris and Lewis, with which they are presently correlated, are ca. 1,675 Ma old. Grey gneisses associated with granites on the south side of Loch Laxford are confirmed to belong to the 'northern region'. A migmatitic grey gneiss on Harris has given a protolith age of ca. 3,125 Ma, the currently oldest recognised in the complex. Detrital zircons in the Leverburgh and Langavat belts range in age from 2,780 to 1,880 Ma and unequivocally demonstrate deposition in the Palaeoproterozoic. The granulite facies metamorphism in this block is dated from zircon overgrowths at ca. 1,880 Ma. The Laxford Shear Zone which separates the northern and central regions is interpreted to have evolved post-1,860 Ma, during amphibolite facies metamorphism accompanying deformation which took place at ca. 1,740 Ma in both regions. On Harris, the Langavat-Finsbay shear zone developed after 1,675 Ma when a ca. 1,880-Ma granulite facies Proterozoic arc was juxtaposed against amphibolite facies Archaean rocks to the north. Therefore, the shear zones which bound tectonic blocks in the Lewisian Complex evolved at different times and can be interpreted as terrane boundaries. The new data confirm that the Lewisian Complex was not constructed from one contiguous piece of Archaean crust reworked in the Proterozoic but was progressively assembled from several discrete terranes during the Proterozoic. Accordingly, the former regional divisions of the Lewisian Complex are here renamed as follows. On the mainland, the northern region is called the Rhiconich terrane, and the central region the Assynt terrane. On the Outer Hebrides, the Archaean gneisses of Lewis and the northern part of Harris comprise the Tarbert terrane, whereas the newly accreted Proterozoic blocks are called the Roineabhal terrane in Harris and the Niss terrane in the north on Lewis. Wider correlations show that the geology of the Outer Hebrides has more in common with East Greenland than mainland Scotland on the eastern side of the Minch Fault.
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u pb isotopic evidence for the accretion of different crustal blocks to form the Lewisian complex of northwest scotland
Contributions to Mineralogy and Petrology, 1997Co-Authors: P D Kinny, Clark R L FriendAbstract:Single zircon and titanite U-Pb SHRIMP data presented for tonalite-trondhjemite-granodiorite (TTG) suite gneisses and an ultramafic rock from the northern and central regions of the Lewisian Complex of northwest Scotland, show that protolith ages of tonalitic gneisses in the northern region (2800–2840 Ma) are significantly younger than those in the central region (2960–3030 Ma). Further evidence of a major (2490–2480 Ma) metamorphic event in the central region is documented by a metamorphic zircon associated with a granulite facies ultramafic body. A dioritic gneiss from the northern region has also been dated at c. 2680 Ma. The northern region therefore does not comprise reworked central region rocks and consequently the old models for the evolution of the Lewisian which were based upon this concept need replacing. It is instead proposed that two distinct crustal blocks, now the northern and central regions, were tectonically juxtaposed along a boundary corresponding to the Laxford Front. Juxtaposition would appear to have occurred in Proterozoic times, as it must have postdated the 2490–2480 Ma (?Inverian) metamorphism recorded only in the central region, and the emplacement of granite sheets restricted to the northern side of the boundary. The first recorded event common to both regions is resetting of titanite ages associated with c. 1750 Ma Laxfordian amphibolite facies metamorphism. Zircon inheritance in rocks of both regions is scarce. Within one zircon from the northern region a c. 3550 Ma core was found. This represents the oldest known material from the region.
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new evidence for protolith ages of Lewisian granulites northwest scotland
Geology, 1995Co-Authors: Clark R L Friend, P D KinnyAbstract:The granulite facies Scourian rocks in the central region of the Lewisian complex of northwest Scotland play an important role in the understanding of the development of mid-Archean high-grade gneiss complexes. Whereas the later, largely Proterozoic history of the Scourian has been clarified in a detailed U-Pb study, unraveling the Archean ages of the protoliths using conventional isotope techniques has proved impossible. In order to advance our understanding of the deep crustal processes responsible for these granulites, it is essential to establish the age(s) of accretion of the protoliths and their subsequent tectonothermal history. A combined cathodoluminescence (CL) and sensitive high-mass resolution ion microprobe (SHRIMP) single-zircon study has revealed a hitherto unrecognized morphological complexity in zircons from the type localities of the granulites. Use of CL allowed identification of relict oscillatory igneous zoning, metamorphic overgrowths, and irregular areas of recrystallization. From the SHRIMP data, an age of ca. 2960 Ma is inferred for the gneiss protoliths, which were altered considerably during an important metamorphic event ca. 2490 Ma.
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the palaeoproterozoic anatomy of the Lewisian complex nw scotland evidence for two laxfordian tectonothermal cycles
Journal of the Geological Society, 2016Co-Authors: Andrew J MasonAbstract:A new structural examination of Palaeoproterozoic high- P granulites on South Harris, NW Scotland, when integrated with previous geochronological, structural and metamorphic studies on key areas of the Lewisian Complex, suggests the existence of two distinct tectonothermal cycles within the Palaeoproterozoic ‘Laxfordian Event’, which on South Harris are separated by a >100 myr hiatus in deformation. The older cycle, from c . 1.91 to 1.85 Ga, records the development of an active continental margin on the Archaean gneisses that dominate the Complex, and the subsequent onset of continent–continent collision; this represents the continuation of the Nagssugtoqidian orogen of Greenland. Evidence for this is concentrated in allochthonous slivers of the former active continental margin displaced during the younger cycle. The younger cycle, around 1.75–1.65 Ga, began with thrust-related crustal thickening that initiated regionally extensive amphibolite-facies metamorphism and ductile deformation, which dominates the preserved ‘Laxfordian’ deformation history. This may be the peripheral expression of the accretion of the Malin block to the SW of the Lewisian, and represents the lateral continuation of the Labradorian–Ketilidian orogen of North America. Supplementary Material: Additional figures are available at http://www.geolsoc.org.uk/SUP18863.
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major early thrusting as a control on the palaeoproterozoic evolution of the Lewisian complex evidence from the outer hebrides nw scotland
Journal of the Geological Society, 2012Co-Authors: Andrew J MasonAbstract:New structural, metamorphic and geochronological data suggest that the Lewisian Complex of the Outer Hebrides can be interpreted as a Proterozoic orogen in which a thrust sheet of juvenile arc material was driven over Archaean gneisses at an early stage. Incubation and loading of the underlying Archaean gneisses triggered prograde metamorphism and extensive ductile deformation with a strong gravitational flattening component combined with top-to-the-WNW transport. Large shear zones developed at this time are of local importance, but are not fundamental structural boundaries. The recognition of early orogenic thrusting followed by extensive ductile deformation shows a close resemblance to the sequence of events recognized within the Nagssugtoqidian orogen, with which the Lewisian Complex is correlated. As in the Nagssugtoqidian orogen, younger penetrative ductile deformation often masks the major structural boundaries formed by older thrusts. Supplementary material: Detailed structural and lithological maps of the Langavat Belt, and geochronological data and analytical methods description are available at www.geolsoc.org.uk/SUP18515.
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mafic dyke remnants in the Lewisian complex of the outer hebrides nw scotland a geochemical record of continental break up and re assembly
Precambrian Research, 2004Co-Authors: Andrew J Mason, T S BrewerAbstract:Abstract In the predominantly late Archaean Lewisian Complex of NW Scotland, the Palaeoproterozoic (2.4–2.0 Ga) Scourie dykes have been used to discriminate between late Archaean events, and ca.1.7 Ga Laxfordian reworking. On the Outer Hebrides metabasite dykes intrude the Lewisian Complex, and were previously assumed to be correlatives of the Scourie dykes of the Scottish mainland. New geochemical data allow the Outer Hebrides metabasites to be divided into two groups. Dykes intruded into Archaean Gneisses north and south of the ca. 1.9 Ga South Harris Complex (SHC) are identical, and have depleted geochemical signatures. They are linked with an early episode of rifting (ca. 2 Ga) related to the fragmentation of the Lewisian crust and eventual formation of ocean crust. Later subduction of this ocean crust generated the ca. 1.9 Ga arc related rocks of the SHC, which contain a second group of subduction-related metabasites that are different from the dykes in the adjacent Archaean gneisses. Closure of this ocean and subsequent Laxfordian continent-arc-continent collision at ca. 1.7 Ga, sutured the rifted Archaean fragments back together. The northeast boundary of the SHC has previously been interpreted as a terrane boundary. However, the new dyke data demonstrates that any terrane boundary actually lies within the SHC. The northeast segment of the SHC, the Langavat Belt, contains dykes chemically identical to those intruding the Archaean gneisses to the northeast. The southwest segment of the SHC, the Harris Granulite Belt, contains arc related plutons and metabasites with arc signatures, which are distinct from mafic dykes in the adjacent Archaean gneisses and Langavat Belt. Consequently, much of the Langavat Belt is unrelated to the arc complex. Data from the Outer Hebrides dykes casts doubt on their correlation with the Scourie dykes, and demonstrate that the Outer Hebrides dykes have a closer affinity with the Palaeoproterozoic Kangâmiut dykes of Greenland, than to the Scourie dykes.