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

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Abstract

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.

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Acknowledgements

Research was carried out while GJL was in receipt of a Curtin University Postgraduate Scholarship, which is gratefully acknowledged. Funding for analytical work was courtesy of a Curtin University small grant (PK) with travel assistance for fieldwork from a Tectonics Special Research Centre travel grant. U-Pb zircon analyses were performed on the sensitive high-resolution ion microprobe (SHRIMP II) located at the John de Laeter Centre for Mass Spectrometry at Curtin University, which is operated by a consortium consisting of Curtin University of Technology, the University of Western Australia and the Geological Survey of Western Australia, with support from the Australian Research Council. C.R.L.F. acknowledges financial support from Oxford Brookes University towards costs of the field and probe work. Constructive reviews by A. Crane and S. Harley were much appreciated. This is TSRC publication number 219.

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  1. Tectonics Special Research Centre, Department of Applied Geology, Curtin University of Technology, GPO Box U1987, 6845, Perth, Australia

    G. J. Love & P. D. Kinny

  2. Department of Geology, Oxford Brookes University, Oxford, OX3 0BP, UK

    C. R. L. Friend

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  1. G. J. Love
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  2. P. D. Kinny
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Correspondence to G. J. Love.

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Love, G.J., Kinny, P.D. & Friend, C.R.L. Timing of magmatism and metamorphism in the Gruinard Bay area of the Lewisian Gneiss Complex: comparisons with the Assynt Terrane and implications for terrane accretion. Contrib Mineral Petrol 146, 620–636 (2004). https://doi.org/10.1007/s00410-003-0519-1

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