Hostname: page-component-78c5997874-lj6df Total loading time: 0 Render date: 2024-11-15T01:23:52.215Z Has data issue: false hasContentIssue false
  • English
  • Français

A U/Pb age for the Shetland Islands oceanic fragment, Scottish Caledonides: evidence from anatectic plagiogranites in ‘layer 3’ shear zones

Published online by Cambridge University Press:  01 May 2009

John G. Spray  and
Gregory R. Dunning
John G. Spray
Affiliation:
Department of Geology, University of New Brunswick, Fredericton, New Brunswick, E3B 5A3, Canada
Gregory R. Dunning
Affiliation:
Department of Earth Sciences, Memorial University of Newfoundland, St John's, Newfoundland, AIB 3X5, Canada
Get access Rights & Permissions [Opens in a new window]

Abstract

High precision U/Pb data obtained from zircons extracted from plagiogranite within the gabbro unit of the Shetland Islands oceanic fragment of northeast Scotland yield an age of 492 ± 3 Ma. Field relations indicate that the plagiogranites were generated by the partial melting of amphibolitized gabbros within high-temperature shear zones formed due to crustal deformation and fluid infiltration occurring in proximity to a spreading centre. The U/Pb data therefore constrain the crystallization age of the Shetland complex. This age is similar to U/Pb ages obtained from the Leka (497±2 Ma), Karmoy (493+7-4 Ma) and Gulfjellet (489±3 Ma) oceanic fragments of the Norwegian Caledonides, and the Pipestone Pond (4943-2 Ma) and Betts Cove (4893-2 Ma) oceanic fragments of the Canadian Appalachians.

Type
Rapid Communications
Information
Geological Magazine , Volume 128 , Issue 6 , November 1991 , pp. 667 - 671
Copyright
Copyright © Cambridge University Press 1991

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Anonymous. 1972. Penrose field conference on ophiolites. Geotimes 17, 24–5.Google Scholar
Bédard, J. H. 1991. Cumulate recycling and crustal evolution in the Bay of Islands ophiolite. Journal of Geology 99, 225–49.CrossRefGoogle Scholar
Bluck, B. J., Halliday, A. N., Aftalion, M. & MacIntyre, R. M. 1980. Age and origin of the Ballantrae ophiolite and its significance to the Caledonian orogeny and Ordovician time scale. Geology 8, 492–5.2.0.CO;2>CrossRefGoogle Scholar
Cannat, M. 1990. Late Caledonian northeastward ophiolite thrusting in the Shetland Islands, U.K. Tectonophysics 169, 257–70.CrossRefGoogle Scholar
Coleman, R. G. & Peterman, Z. E. 1975. Oceanic plagiogranite. Journal of Geophysical Research 80, 1099–108.CrossRefGoogle Scholar
Dunning, G. R. & Krogh, T. E. 1985. Geochronology of ophiolites of the Newfoundland Appalachians. Canadian Journal of Earth Sciences 22, 1659–70.CrossRefGoogle Scholar
Dunning, G. R. & Pedersen, R. B. 1988. U/Pb ages of ophiolites and arc-related plutons of the Norwegian Caledonides: implications for the development of Iapetus. Contributions to Mineralogy and Petrology 98, 13–23.CrossRefGoogle Scholar
Flagler, P. A. & Spray, J. G. 1991. Generation of plagiogranite by amphibolite anatexis in oceanic shear zones. Geology 19, 70–3.2.3.CO;2>CrossRefGoogle Scholar
Flinn, D. 1985. On the nappe structure of North East Shetland. Quarterly Journal of the Geological Society, London 144, 107–36.Google Scholar
Garson, M. S. & Plant, J. A. 1973. Alpine-type ultramafic rocks and episodic mountain building in the Scottish Highlands. Nature 242, 34–8.Google Scholar
Harland, W. B., Armstrong, R. L., Cox, A. V., Craig, L. E., Smith, A. G. & Smith, D. G. 1990. A Geologic Time Scale 1989. Cambridge University Press, 263 pp.Google Scholar
Hutton, D. H. W., Aftalion, M. & Halliday, A. N. 1985. An Ordovician ophiolite in County Tyrone, Ireland. Nature 315, 210–2.Google Scholar
Krogh, T. E. 1973. A low contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations. Geochimica el Cosmochimica Acta 37, 485–94.Google Scholar
Krogh, T. E. 1982. Improved accuracy of U–Pb zircon ages by creation of a more concordant system using an air abrasion technique. Geochimica el Cosmochimica Acta 46, 637–49.Google Scholar
Lippard, S. J., Shelton, A. W. & Gass, I. G. 1986. The Ophiolite of Northern Oman. Geological Society of London Memoir no. 11, 178 pp.Google Scholar
Ludwig, K. R. 1980. Calculation of uncertainties of U–Pb isotope data. Earth and Planetary Science Letters 26, 207–21.Google Scholar
Malpas, J., Brace, T. & Dunsworth, S. M. 1989. Structural and petrologic relationships of the CY-4 drill hole of the Cyprus Crustal Study Project. In Cyprus Crustal Study Project: Initial Report, Hole CY-4 (eds Gibson, I. L., Malpas, J., Robinson, P. T. and Xenophontos, C.), pp. 39–67. Geological Survey of Canada Paper no. 88–9.Google Scholar
Mevel, C. 1988. Metamorphism in ocean layer 3, Gorringe Bank, eastern Atlantic. Contributions to Mineralogy and Petrology 100, 496–509Google Scholar
Miller, J. A. & Flinn, D. 1966. A survey of the age relations of Shetland rocks. Geological Journal 5, 95–116Google Scholar
Pallister, J. S. & Knight, R. Y. 1981. Rare-earth element geochemistry of the Samail Ophiolite near Ibra, Oman. Journal of Geophysical Research 86, 2673–97.Google Scholar
Pedersen, R. B. & Malpas, J. 1984. The origin of oceanic plagiogranite from the Karmoy ophiolite. Contributions to Mineralogy and Petrology 88, 36–52Google Scholar
Pedersen, R. B., Furnes, H. & Dunning, G. R. 1991. A U/Pb age for the Sulitjelma Gabbro, North Norway: further evidence for the development of a Caledonian marginal basin in Ashgill-Llandovery time. Geological Magazine 128, 141–53.Google Scholar
Prichard, H. M. 1985. The Shetland ophiolite. In The Caledonide Orogen – Scandinavia and Related Areas (eds Gee, D. G. and Sturt, B. A.), pp. 1173–84. U.S.A., Chichester: Wiley-Inter-science.Google Scholar
Read, H. H. 1934. The metamorphic geology of Unst in the Shetland Islands. Quarterly Journal of the Geological Society, London 90, 637–88.Google Scholar
Rogers, G. & Dunning, G. R. 1991. Geochronology of appinitic and related granitic magmatism in the W. Highlands of Scotland: constraints on the timing of transcurrent fault movement. Journal of the Geological Society, London 148, 17–27.Google Scholar
Sigmarsson, O., Hémond, C., Condomines, M., Fourcade, S. & Oskarsson, N. 1991. Origin of silicic magma in Iceland revealed by Th isotopes. Geology 19, 621–4.Google Scholar
Spray, J. G. 1988. Thrust-related metamorphism beneath the Shetland Islands oceanic fragment, northeast Scotland. Canadian Journal of Earth Sciences 25, 1760–76.Google Scholar
Spray, J. G. 1991 a. Structure of oceanic crust as deduced from ophiolites. In Oceanic Basalts (ed. Floyd, P. A.), pp. 49–62. Glasgow: Blackie & Sons Limited.Google Scholar
Spray, J. G. 1991 b. Late Caledonian northeastward ophiolite thrusting in the Shetland Islands, U.K. – Discussion of the obduction process. Tectonophysics 188, 393–6.CrossRefGoogle Scholar
Spray, J. G., Flagler, P. A. & Dunning, G. R. 1990. Crystallization and emplacement chronology of the Fournier oceanic fragment, Canadian Appalachians. Nature 344, 232–5.Google Scholar
Stacey, J. S. & Kramers, J. D. 1975. Approximation of terrestrial lead isotopic evolution by a two-stage model. Earth and Planetary Science Letters 26, 207–21.Google Scholar