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

MacDonald, John; Goodenough, Kathryn; Wheeler, John; Crowley, Quentin; Harley, Simon L.; Mariani, Elisabetta; Tatham, Daniel J.

doi:10.1016/j.precamres.2015.01.009

Cited by 23 publications

(29 citation statements)

References 64 publications

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“…(). These methods have been used to great effect in a number of studies where the range of zircon and garnet chemistry and morphologies are relatively simple (Clark et al., , ; Gauthiez‐Putallaz, Rubatto, & Hermann, ; Hermann & Rubatto, ; Kotková & Harley, ; Kotková, Whitehouse, Schaltegger, & D'Abzac, ; Kylander‐Clark & Hacker, ; MacDonald et al., ; Štípská et al., ). The utility of these techniques in visualizing equilibrium relationships in terranes that have experienced multiple episodes or protracted durations of metamorphism becomes increasingly complex as zircon undergoes varying degrees of recrystallization, re‐equilibration and neocrystallization.…”

Section: Discussionmentioning

confidence: 99%

“…The zircon-garnet partitioning relationship is often assessed through interpretation of HREE patterns of zircon or the construction of D REE (zircon-garnet) plots that are then visually compared to the appropriate experimental data sets of Rubatto and Hermann (2007) or Taylor, Harley, et al (2015). These methods have been used to great effect in a number of studies where the range of zircon and garnet chemistry and morphologies are relatively simple (Clark et al, 2009Gauthiez-Putallaz, Rubatto, & Hermann, 2016;Kotkov a & Harley, 2010;Kotkov a, Whitehouse, Schaltegger, & D'Abzac, 2016;Kylander-Clark & Hacker, 2014;MacDonald et al, 2015; St ıpsk a et al, 2016). The utility of these techniques in visualizing equilibrium relationships in terranes that have experienced multiple episodes or protracted durations of metamorphism becomes increasingly complex as zircon undergoes varying degrees of recrystallization, re-equilibration and neocrystallization.…”

Section: An Assessment Of Zircon-garnet Equilibriummentioning

confidence: 99%

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Prolonged (>100 Ma) ultrahigh temperature metamorphism in the Napier Complex, East Antarctica: A petrochronological investigation of Earth's hottest crust

Clark

Taylor

Kylander‐Clark

et al. 2018

Journal Metamorphic Geology

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The Napier Complex in East Antarctica preserves a record of ultrahigh temperature (UHT) metamorphism during the late Archean to early Palaeoproterozoic. While there is little argument that the UHT metamorphic event began at c. 2,580 Ma, the duration over which the rocks resided at UHT has been the subject of intense debate, with estimates for the end of metamorphism ranging from 2,545 to 2,440 Ma—a discrepancy of some 105 Ma. To resolve the time‐scale of UHT metamorphism, a zircon and garnet petrochronological (U–Pb, REE and Ti) data set from a suite of rocks from the Tula Mountains region of the Napier Complex was analysed. Individual concordant populations define zircon U–Pb ages for (a) reset zircon cores of 2,502–2,439 Ma; (b) zircon rims of 2,491–2,454 Ma; and (c) neocrystallized sector‐zoned zircon from 2,492 to 2,443 Ma. Ti‐in‐zircon thermometry places a minimum estimate of 830°C for zircon crystallization, with the majority of concordant populations yielding temperatures >900°C. Zircon–garnet partitioning (DYb vs. DYb/Gd) arrays reveal that the bulk of metamorphic zircon defines an equilibrium relationship with the garnet that forms part of the peak assemblage. Combined with existing geochronological constraints, the new petrochronological data demonstrate that the Napier Complex remained at UHT from c. 2,585 Ma until at least 2,450 Ma, a residence time of 135 Ma. In the absence of evidence for contemporaneous emplacement of large volumes of igneous rocks, a number of factors likely combined to drive and maintain these extreme temperatures. We propose that the P–T conditions experienced by the Napier Complex were achieved through a combination of orogenic plateau formation, preconditioning of the crust by a high‐T magmatic and UHT metamorphic event at c. 2,850 Ma, inefficient removal of heat‐producing elements during partial melting and slow exhumation. This style of long duration, regional, extreme metamorphism is becoming more commonly identified in the rock record as larger and more robust data sets are collected (e.g. the Eastern Ghats of India and the Gondwanan East African Orogen) and is commonly associated with the amalgamation phases of supercontinents/cratons.

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Section: Discussionmentioning

confidence: 99%

Section: An Assessment Of Zircon-garnet Equilibriummentioning

confidence: 99%

Prolonged (>100 Ma) ultrahigh temperature metamorphism in the Napier Complex, East Antarctica: A petrochronological investigation of Earth's hottest crust

Clark

Taylor

Kylander‐Clark

et al. 2018

Journal Metamorphic Geology

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“…Here, Archaean granulite-facies assemblages are well-preserved in the TTG gneisses, with limited Laxfordian reworking that is focused along discrete shear zones (Sutton and Watson, 1951;Tarney and Weaver, 1987;Kinny and Friend, 2005;Wheeler et al, 2010;Zirkler et al, 2012;MacDonald et al, 2015). Although the terrane is dominated by TTG gneisses, mafic to ultramafic and metasedimentary pods are found in a number of locations within the gneisses (Davies, 1974;Cartwright et al, 1985;Kinny and Friend, 2005;Goodenough et al, 2010).…”

Section: Geological Setting: the Lewisian Gneiss Complexmentioning

confidence: 90%

Petrogenesis of rare-metal pegmatites in high-grade metamorphic terranes: a case study from the Lewisian Gneiss Complex of north-west Scotland

Shaw

Goodenough

Horstwood³

et al. 2016

Applied Earth Science

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a b s t r a c tMany 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 biotitemagnetite 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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“…1; Burton et al, 2000) yielding likely crystallisation dates of 2.68 ± 0.02 Ga and 3.26 Ga ± 0.21 Ga (2σ), while Sm-Nd dating (Whitehouse, 1989) of the Achiltibuie, Drumbeg and Scouriemore Complexes yielded dates of 2.85 Ga ± 0.10 Ga, 2.91 Ga ± 0.06 Ga and 2.67 Ga ± 0.11 Ga (2σ) respectively. U-Pb zircon geochronology from TTG gneisses in northern Central Region yielded similarly disparate results, with a spread of concordant ages from 3.1 to 2.5 Ga (Whitehouse and Kemp, 2010;MacDonald et al, 2015) attributed, in part, to Pb diffusion during the LGC's protracted, high-grade metamorphic evolution (MacDonald et al, 2013). Despite these disturbances to the U-Pb isotopic system, the protolith crystallisation ages for the TTG gneiss protoliths in the north of the Central Region are generally interpreted as 3.05 -2.90 Ga (e.g., Kinny and Friend, 1997;MacDonald et al, 2015).…”

Section: Ultramafic-mafic Complexes In the Central Regionmentioning

confidence: 99%

“…U-Pb zircon geochronology from TTG gneisses in northern Central Region yielded similarly disparate results, with a spread of concordant ages from 3.1 to 2.5 Ga (Whitehouse and Kemp, 2010;MacDonald et al, 2015) attributed, in part, to Pb diffusion during the LGC's protracted, high-grade metamorphic evolution (MacDonald et al, 2013). Despite these disturbances to the U-Pb isotopic system, the protolith crystallisation ages for the TTG gneiss protoliths in the north of the Central Region are generally interpreted as 3.05 -2.90 Ga (e.g., Kinny and Friend, 1997;MacDonald et al, 2015).…”

Section: Ultramafic-mafic Complexes In the Central Regionmentioning

confidence: 99%

Re-evaluating ambiguous age relationships in Archean cratons: Implications for the origin of ultramafic-mafic complexes in the Lewisian Gneiss Complex

Guice

McDonald

Hughes

et al. 2018

Precambrian Research

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Archean ultramafic-mafic complexes have been the focus of important and often contentious geological and geodynamic interpretations. However, their age relative to the other components of Archean cratons are often poorly-constrained, introducing significant ambiguity when interpreting their origin and geodynamic significance. The Lewisian Gneiss Complex (LGC) of the northwest Scottish mainland -a high-grade, tonalite-trondhjemite-granodiorite (TTG) terrane that forms part of the North Atlantic Craton (NAC) -contains a number of ultramafic-mafic complexes whose origin and geodynamic significance have remained enigmatic since they were first described. Previous studies have interpreted these complexes as representing a wide-range of geological environments, from oceanic crust, to the sagducted remnants of Archean greenstone belts. These interpretations, which are often critically dependent upon the ages of the complexes relative to the surrounding rocks, have disparate implications for Archean geodynamic regimes (in the NAC and globally). Most previous authors have inferred that the ultramafic-mafic complexes of the LGC pre-date the TTG magmas. This fundamental age relationship is re-evaluated in this investigation through re-mapping of the Geodh' nan Sgadan Complex (where tonalitic gneiss reportedly cross-cuts mafic rocks) and new mapping of the 7 km 2 Ben Strome Complex (the largest ultramafic-mafic complex in the LGC), alongside detailed petrography and spinel mineral chemistry. This new study reveals that, despite their close proximity in the LGC (12 km), the Ben Strome and Geodh' nan Sgadan Complexes are petrogenetically unrelated, indicating that the LGC (and thus NAC) records multiple temporally and/or petrogenetically distinct phases of ultramafic-mafic Archean magmatism that has been masked by subsequent high-grade metamorphism. Moreover, field observations and spinel mineral chemistry demonstrate that the Ben Strome Complex represents a layered intrusion that was emplaced into a TTG-dominated crust. Further to representing a significant re-evaluation of theLGC's magmatic evolution, these findings have important implications for the methodologies utilised in deciphering the origin of Archean ultramafic-mafic complexes globally, where material suitable for dating is often unavailable and field relationships are commonly ambiguous.

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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

Cited by 23 publications

References 64 publications

Prolonged (>100 Ma) ultrahigh temperature metamorphism in the Napier Complex, East Antarctica: A petrochronological investigation of Earth's hottest crust

Prolonged (>100 Ma) ultrahigh temperature metamorphism in the Napier Complex, East Antarctica: A petrochronological investigation of Earth's hottest crust

Petrogenesis of rare-metal pegmatites in high-grade metamorphic terranes: a case study from the Lewisian Gneiss Complex of north-west Scotland

Re-evaluating ambiguous age relationships in Archean cratons: Implications for the origin of ultramafic-mafic complexes in the Lewisian Gneiss Complex

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