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  • 1
    Electronic Resource
    Electronic Resource
    U-Pb zircon geochronological evidence for Neoproterozoic events in the Glenfinnan Group (Moine Supergroup): the formation of the Ardgour granite gneiss, north-west Scotland (1997)
    Springer
    Contributions to mineralogy and petrology 128 (1997), S. 101-113 
    Details
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract The age and Precambrian history of the Moine Supergroup within the Caledonide belt of north-west Scotland have long been contentious issues. The Ardgour granite gneiss is essentially an in situ anatectic granite formed during deformation and regional high-grade metamorphism from Moine metasediments. High-precision TIMS and SHRIMP U-Pb zircon dating shows that the age of the anatectic Ardgour granite gneiss and its enclosed segregation pegmatites is 873 ± 7 Ma. This demonstrates the reality of a Neoproterozoic episode of high-grade metamorphism in the Glenfinnan Group Moine and, contrary to previous evidence, the absence of Grenvillian-aged metamorphism. This conclusion places constraints on Neoproterozoic palaeogeographic reconstructions of the North Atlantic region, indicating that the Moine rocks cannot be used as a link between the Grenvillian belt of North America and the Sveconorwegian orogen in Scandinavia. SHRIMP ages of between c. 1100 and 1900 Ma were obtained from detrital, inherited zircons and reflect the provenance of the Glenfinnan Group Moine sediments which must, therefore, have been deposited between c. 1100 and 870 Ma. Potential sources are found as relatively minor, tectonically bounded basement inliers within the British Caledonides, although more widespread source areas occur outside Britain in both Laurentia and Baltica. The most important feature of the provenance is the absence of detrital Archaean grains. This suggests that the Archaean Lewisian gneiss complex, which forms the basement component of the western foreland to the Caledonides in Britain, was not a major contributor to the Glenfinnan Group basin.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004100050297
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  • 2
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    Evidence from Rb-Sr mineral ages for multiple orogenic events in the Caledonides of Shetland, Scotland (2016)
    Geological Society of London
    Details
    Publication Date: 2016-05-04
    Description: Shetland occupies a unique central location within the North Atlantic Caledonides. Thirty-three new high-precision Rb–Sr mineral ages indicate a polyorogenic history. Ages of 723–702 Ma obtained from the vicinity of the Wester Keolka Shear Zone indicate a Neoproterozoic (Knoydartian) age and preclude its correlation with the Silurian Moine Thrust. Ordovician ages of c . 480–443 Ma obtained from the Yell Sound Group and the East Mainland Succession constrain deformation fabrics and metamorphic assemblages to have formed during Grampian accretionary orogenic events, broadly contemporaneously with orogenesis of the Dalradian Supergroup in Ireland and mainland Scotland. The relative paucity of Silurian ages is attributed to a likely location at a high structural level in the Scandian nappe pile relative to mainland Scotland. Ages of c . 416 and c . 411 Ma for the Uyea Shear Zone suggest a late orogenic evolution that has more in common with East Greenland and Norway than with northern mainland Scotland. Supplementary material: Detailed appraisal of biotite petrography is available at http://www.geolsoc.org.uk/SUP18887 .
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    Topics: Geosciences
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  • 3
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    U-Pb zircon constraints on obduction initiation of the Unst Ophiolite: an oceanic core complex in the Scottish Caledonides? (2015)
    Geological Society of London
    Details
    Publication Date: 2015-05-02
    Description: The Unst Ophiolite is the best exposed of a chain of early Ordovician ophiolites in the Scottish Caledonides and is widely regarded as having formed in a supra-subduction zone setting within the Iapetus Ocean. Reinterpretation of sheeted dykes suggests that it formed as an oceanic core complex, presumably during subduction roll-back immediately prior to obduction onto the Laurentian margin. A new U–Pb zircon age of 484 ± 4 Ma for development of the metamorphic sole places a lower limit on the timing of obduction, which was subsequently followed by regional-scale crustal thickening and Barrovian metamorphism during the Grampian orogenic event. Supplementary material: Analytical methods, CL images of representative zircon grains and an LA-ICP-MS U–Th–Pb zircon data table are available at www.geolsoc.org.uk/SUP18814 .
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    Topics: Geosciences
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  • 4
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    Silurian-Devonian magmatism, mineralization, regional exhumation and brittle strike-slip deformation along the Loch Shin Line, NW Scotland (2015)
    Geological Society of London
    Details
    Publication Date: 2015-10-29
    Description: The Loch Shin Line is a geological–geophysical lineament associated with a zone of mantle-derived appinites, granites and strike-slip faulting that runs NW–SE across the Moine Nappe, northern Scotland. U–Pb zircon and Re–Os molybdenite dating of the Loch Shin and Grudie plutons, which lie immediately SW of the NW–SE Loch Shin–Strath Fleet fault system, yield c . 427–430 Ma ages that overlap within error. They also coincide with previously obtained U–Pb zircon ages for the Rogart pluton, which lies along-strike to the SE. Field and microstructural observations confirm the similarity and contemporaneous nature of the plutons and associated sulphide mineralization. Fluid inclusion analyses place further constraints on the P – T – X conditions during regional late Caledonian exhumation of the Moine Nappe. Synchronous to slightly younger brittle dextral strike-slip faulting along the WNW–ESE Loch Shin–Strath Fleet Fault System was probably antithetic to sinistral movements along the nearby Great Glen Fault Zone. Our findings support the hypothesis that the Loch Shin Line acted as a deep crustal channelway controlling the ascent and emplacement of Silurian magmas into the overlying Moine Nappe. We propose that this deep structure corresponds to the southeastern continuation of the Precambrian-age Laxford Front shear zone in the buried Lewisian autochthon. Supplementary Material: Field photographs, photomicrographs and fluid inclusion information are available at http://www.geolsoc.org.uk/SUP18859 .
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  • 5
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    Provenance of the Highland Border Complex: constraints on Laurentian margin accretion in the Scottish Caledonides (2012)
    Geological Society of London
    Details
    Publication Date: 2012-09-01
    Description: Contrasting tectonic models for the Highland Border Complex in the Scottish Caledonides view it either as part of the rifted Laurentian margin of the Iapetus Ocean or as an oceanic terrane. Detrital zircon data from sandstones of the complex yield age peaks at 2.8–2.6, 1.3 and 1.1–1.0 Ga and minor peaks at c . 1.7–1.9 Ga. These characteristics compare closely with those of the upper Dalradian Supergroup of the adjacent Grampian terrane, and with the record of eastern Laurentia. The data are also consistent with the Laurentian provenance indicated by palaeontological evidence from the complex, and field evidence for continuity with the Dalradian Supergroup. Detrital ages for the Cambrian Salterella Grit of the Caledonian foreland compare with those for approximately age-equivalent sandstones from the Highland Border Complex. Both were contemporaneous with the regressive Hawke Bay event, accounting for similarities in provenance, and further linking the Highland Border Complex to Laurentia. The Grampian terrane was being uplifted and shedding detritus throughout the Ordovician and Silurian. The absence of this event from the detrital zircon records of either the Midland Valley or Southern Upland terranes suggests that these blocks cannot have been in their current location relative to the Grampian terrane before the end of the Silurian. Supplementary material: The complete analytical dataset and cathodoluminescence images are available at http://www.geolsoc.org.uk/SUP18531 .
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  • 6
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    P-T constraints and timing of Barrovian metamorphism in the Shetland Islands, Scottish Caledonides: implications for the structural setting of the Unst ophiolite (2011)
    Geological Society of London
    Details
    Publication Date: 2011-10-26
    Description: : An integrated in situ monazite laser-ablation inductively coupled plasma mass spectrometry and metamorphic equilibria study is used to establish the P – T conditions and timing of Barrovian metamorphism in the Shetland Islands, Scottish Caledonides. The results have implications for the structural setting of the Unst ophiolite, which was obducted onto metasedimentary rocks of the Dalradian Supergroup. Metapelites in the footwall of the ophiolite yield U–Pb ages between 462 and 451 Ma with P – T conditions varying from c . 7.5 kbar and 550 °C directly below the ophiolite to c . 10 kbar and 775 °C at structurally deeper levels. The timing of peak metamorphism corresponds closely to that of Grampian ( c . 450–470 Ma) metamorphism in mainland Scotland and Ireland, and Taconic ( c . 450–460 Ma) metamorphism in the Appalachians, thus confirming the near-synchroneity of this important arc accretion event along the Laurentian margin. There is a significant metamorphic contrast between the low-grade rocks associated with the Unst ophiolite and the P – T conditions recorded in its footwall. If published K–Ar ages of c. 470 Ma broadly record obduction of the ophiolite, its present basal contact is probably a younger tectonic break that was associated with the excision of at least c . 10 km of crustal section. Supplementary material: Electron microporobe chemical analyses of garnet grains are available at www.geolsoc.org.uk/SUP18494
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  • 7
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    Timing of regional deformation and development of the Moine Thrust Zone in the Scottish Caledonides: constraints from the U-Pb geochronology of alkaline intrusions (2011)
    Geological Society of London
    Details
    Publication Date: 2011-01-07
    Description: : The Moine Thrust Zone in the Scottish Highlands developed during the Scandian Event of the Caledonian Orogeny, and now forms the boundary between the Caledonian orogenic belt and the undeformed foreland. The Scandian Event, and the formation of the Moine Thrust Zone, have previously been dated by a range of isotopic methods, and relatively imprecise ages on a suite of alkaline intrusions localized along the thrust zone have provided the best age constraints for deformation. Recent British Geological Survey mapping has improved our understanding of the structural relationships of some of these intrusions, and this work is combined with new U–Pb dates in this paper to provide significantly improved ages for the Moine Thrust Zone. Our work shows that a single early intrusion (the Glen Dessarry Pluton) was emplaced within the orogenic belt to the east of the Moine Thrust Zone at 447.9 ± 2.9 Ma. A more significant pulse of magmatism centred in the Assynt area, which temporally overlapped movement in the thrust zone, occurred at 430.7 ± 0.5 Ma. Movement in the thrust zone had largely ceased by the time of emplacement of the youngest intrusions, the late suite of the Loch Borralan Pluton, at 429.2 ± 0.5 Ma, and the Loch Loyal Syenite Complex.
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    Topics: Geosciences
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  • 8
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    Sedimentology of the early Neoproterozoic Morar Group in northern Scotland: implications for basin models and tectonic setting (2012)
    Geological Society of London
    Details
    Publication Date: 2012-01-03
    Description: The metasedimentary rocks of the Morar Group in northern Scotland form part of the early Neoproterozoic Moine Supergroup. The upper part of the group is c . 2–3 km thick and contains two large kilometre-scale facies successions: a coarsening-upwards marine-to-fluvial regression overlain by a fining-upwards fluvial-to-marine transgression. Fluvial facies make up less than a third of the total thickness; shallow-marine lithofacies make up the remainder. Combining these new findings with previously published data indicates that the Morar Group represents, overall, a transgressive stratigraphic succession c. 6–9 km thick, in which there is both an upward and eastward predominance of shallow-marine deposits, and a concomitant loss of fluvial facies. Smaller-scale (hundreds of metres thick) transgressive–regressive cycles are superimposed on this transgressive trend. Collectively, the characteristics of the succession are consistent with deposition in a foreland basin located adjacent to the Grenville orogen, and possibly linked to the peri-Rodinian ocean. Subsidence and progressive deepening of the Morar basin may have, at least in part, been driven by loading of thrust sheets emplaced during the Grenville orogeny, and aided by sediment loading. However, the relative contributions of thrust loading versus plate boundary effects and/or eustatic sea-level rise on basin evolution remain speculative.
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  • 9
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    Lu-Hf and Sm-Nd dating of metamorphic garnet: evidence for multiple accretion events during the Caledonian orogeny in Scotland (2013)
    Geological Society of London
    Details
    Publication Date: 2013-03-14
    Description: Caledonian orogenesis in Scotland is currently interpreted in terms of a Mid-Ordovician arc–continent collision (Grampian event) followed by the Silurian collision of Laurentia with Baltica (Scandian event). Lu–Hf and Sm–Nd garnet ages of c . 475–460 Ma obtained from prograde garnets in metasedimentary successions and metabasic intrusions within the Northern Highland and Grampian terranes confirm that the Mid-Ordovician Grampian orogenic event was approximately synchronous in the two terranes. Lu–Hf and Sm–Nd ages of c . 450 Ma obtained from prograde garnets within the Moine Nappe of the Northern Highland terrane provide evidence for a hitherto unrecognized Late Ordovician regional metamorphic event. The existing two-stage Grampian–Scandian model for Caledonian orogenesis in northern Scotland is thus an oversimplification, and the new ages imply a more complex structural evolution. The restriction of the Late Ordovician and Silurian events to the Northern Highland terrane reinforces the suggestion that it was far removed from the Grampian terrane until juxtaposition following major end-Caledonian (Devonian) sinistral displacement along the Great Glen Fault. A similar record of Mid- and Late Ordovician metamorphic events within the Laurentian-derived Uppermost Allochthons of Norway has been attributed to episodic accretion significantly prior to Silurian continent–continent collision and closure of the Iapetus Ocean. Supplementary materials: Results of trace element analysis of the garnets by laser ablation inductively coupled plasma mass spectrometry are available at www.geolsoc.org.uk/SUP18583 .
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  • 10
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    Identification of mantle peridotite as a possible Iapetan ophiolite sliver in south Shetland, Scottish Caledonides (2017)
    Geological Society of London
    Details
    Publication Date: 2017-02-23
    Description: The Neoproterozoic Dunrossness Spilite Subgroup of south Shetland, Scotland, has been interpreted as a series of komatiitic and mafic lava flows formed in a marginal basin in response to Laurentian continental margin rifting. We show that ultramafic rocks previously identified as komatiites are depleted mantle peridotites that experienced seafloor hydrothermal alteration. The presence of positive Bouguer gravity and aeromagnetic anomalies extending from the Dunrossness Spilite Subgroup northward to the Shetland Ophiolite Complex suggests instead that these rocks may form part of an extensive ophiolite sliver, obducted during Iapetus Ocean closure in a forearc setting. Supplementary material : Supplementary information, including methods, supplementary figures and tabulated data, is available at https://doi.org/10.6084/m9.figshare.c.3469161 .
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