Hostname: page-component-cd9895bd7-dzt6s Total loading time: 0 Render date: 2024-12-23T16:32:27.916Z Has data issue: false hasContentIssue false
  • English
  • Français

Geochemistry of the Start Complex greenschists: Rhenohercynian MORB?

Published online by Cambridge University Press:  01 May 2009

P. A. Floyd ,
R. E. Holdsworth  and
S. A. Steele
P. A. Floyd
Affiliation:
Department of Geology, University of Keele, Staffordshire ST5 5BG, U.K.
R. E. Holdsworth
Affiliation:
Department of Geological Sciences, University of Durham, Durham City DH1 3LE, U.K.
S. A. Steele
Affiliation:
Department of Geological Sciences, University of Durham, Durham City DH1 3LE, U.K.
Get access Rights & Permissions [Opens in a new window]

Abstract

The meta-igneous greenschists of the Start Complex, SouthDevon, are composed of a mineralogically uniform, but texturally variable, actinolite-epidote-albite assemblage with retrogressed variants containing chlorite, muscovite, sphene, carbonate and oxidized opaque minerals. Geochemically they represent a suite of relatively primitive tholeiites, exhibiting mild differentiation, depleted incompatible element abundances, and variable light rare-earth-element-depletion patterns comparable to modern basalts from normal spreading ridge segments (N-MORB). As the Start greenschistsexhibit a number of chemical similarities to the nearby Upper Palaeozoic Lizard ophiolite, and MORB-type clasts within the Rhenohercynian Zone generally, they may also represent local Variscan ocean crust, which floored smalloceanic basins that separated the Old Red Sandstone continent from the Armorican microplate to the south. The Start Complex could thus represent a previously unrecognized oceanic component to the Variscan orogenic belt (Rhenohercynian Zone) of Northwest Europe.

Type
Articles
Information
Geological Magazine , Volume 130 , Issue 3 , May 1993 , pp. 345 - 352
Copyright
Copyright © Cambridge University Press 1993

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

Badham, J. P. N. 1982. Strike-slip orogens - an explanation for the Hercynides. Journal of the Geological Society, London 139, 495–506.CrossRefGoogle Scholar
Barnes, R. P. 1984. Possible Lizard-derived material in the underlying Meneage Formation. Journal of the Geological Society, London 141, 79–85.CrossRefGoogle Scholar
Barnes, R. P. & Andrews, J. R. 1986. Upper Palaeozoic ophiolite generation and obduction in south Cornwall. Journal of the Geological Society, London 143, 117–24.CrossRefGoogle Scholar
Coward, M. P. & McClay, K. R. 1983. Thrust tectonics in S Devon. Journal of the Geological Society, London 140, 215–28.CrossRefGoogle Scholar
Davies, G. R. 1984. Isotope evolution of the Lizard Complex. Journal of the Geological Society, London 141, 3–14.CrossRefGoogle Scholar
Dineley, D. L. 1986. Cornubia and the palaeogeography of a continental margin. Proceedings of the Ussher Society 2, 3–13.Google Scholar
Floyd, P. A. 1982. Chemical variation in Hercynian basalts relative to plate tectonics. Journal of the Geological Society, London 139, 505–20.CrossRefGoogle Scholar
Floyd, P. A. 1984. Geochemical characteristics and comparison of the basic rocks of the Lizard Complex and the basaltic lavas within the Hercynian troughs of SW England. Journal of the Geological Society, London 141, 61–70.CrossRefGoogle Scholar
Floyd, P. A. (in press). Rhenohercynian basalts: petrology, geochemistry and tectonic environment. In Tectonostratigraphic Evolution of the Centraland Eastern European Orogens (eds Dallmeyer, R. D., Franke, W. and Weber, K.). IGCP 233. Springer-Verlag.Google Scholar
Floyd, P. A. & Castillo, P. R. 1992. Geochemistry and petrogenesis of Jurassic ocean crust basalts, ODP Leg 129, Site 801. In Proceedings of ODP, Scientific Results, 129 (eds Larson, R., Launcelot, Y. et al.), pp. 361–88. College Station, Texas.Google Scholar
Floyd, P. A. & Winchester, J. A. 1978. Identification and discrimination of altered and metamorphosed volcanic rocks using immobile elements. Chemical Geology 21, 291–306.CrossRefGoogle Scholar
Floyd, P. A. & Winchester, J. A. 1983. Element mobility associated withmeta-shear zones within the Ben Hope amphibolite suite, Scotland. Chemical Geology 39, 1–15.CrossRefGoogle Scholar
Floyd, P. A., Exley, C. S. & Stone, M. 1983. Variscan magmatism in southwest England - discussion and synthesis. In The Variscan Foldbelt in the British Isles (ed. Hancock, P. L.), pp. 178–85. Bristol: Adam Hilger.Google Scholar
Frey, F. A. 1969. Rare-earth abundances in a high-temperature peridotite intrusion. Geochimica et Cosmochimica Acta 33, 1429–48.CrossRefGoogle Scholar
Grosser, J. & Dorr, W. 1986. MOR-typ-basalte im ostlichen Rheinischen Schiefergebirge. Neues Jahrbuch für Geologie und Paläontologie, Monatshefte 12, 705–22.Google Scholar
Hobson, D. M. 1977. Polyphase folds from the Start Complex. Proceedings of the Ussher Society 4, 102–10.Google Scholar
Holder, M. T. & Leveridge, B. E. 1986. A model for the tectonic evolution of south Cornwall. Journal of the Geological Society, London 143, 125–34.CrossRefGoogle Scholar
Holdsworth, R.E. 1989. The Start-Perranporth line: a Devonian terrane boundary in the Variscan orogen of SW England. Journal of the Geological Society, London 146, 419–22.CrossRefGoogle Scholar
Humphris, S. E. & Thompson, G. 1978. Hydrothermal alteration of oceanicbasalts by seawater. Geochimica et Cosmochimica Acta 42, 107–26.CrossRefGoogle Scholar
Kirby, G. A. 1979. The Lizard Complex as an ophiolite. Nature 282, 58–61.CrossRefGoogle Scholar
Kirby, G. A. 1984. The petrology and geochemistry of dykes of the Lizard ophiolite complex, Cornwall. Journal of the Geological Society, London 141, 53–60.CrossRefGoogle Scholar
Leveridge, B. E., Holder, M. T. & Day, G. A. 1984. Thrust nappe tectonics in the Devonian of south Cornwall and the western English Channel. In Variscan Tectonics of the North Atlantic Region (eds Hutton, D. and Sanderson, D. J.), pp. 103–12. Geological Society of London, Special Publication no. 14.Google Scholar
Phillips, F. C. 1964. Metamorphism in southwest England. Transactions ofthe Royal Geological Society of Cornwall 150, 185–200.Google Scholar
Platen, K. M., Emmermann, R. & Franke, W. 1989. Devonian MOR-type metabasalts from the Rhenohercynian Zone of Germany. Terra Abstracts, EUG V, Strasbourg 1, 365.Google Scholar
Robinson, D. 1981. Metamorphic rocks of an intermediate facies series juxtaposed at the Start boundary, south-west England. Geological Magazine 118, 297–301.CrossRefGoogle Scholar
Seago, R. D. & Chapman, T. J. 1988. The confrontation of structural styles and the evolution of a foreland basin in central SW England. Journal of the Geological Society, London 145, 789–800.CrossRefGoogle Scholar
Sun, S. S. & McDonough, W. F. 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In Magmatism in the Ocean Basins (eds Saunders, A. D. and Norry, M. J.), pp. 313–45. Geological Society of London, Special Publication no. 42.Google Scholar
Thompson, G. 1983. Basalt-seawater interaction. In Hydro-thermal Processes at Seafloor Spreading Centers (eds Rona, P. A., Bostrom, K., Laubier, L. and Smith, K. L.), pp. 225–78. Plenum Press.CrossRefGoogle Scholar
Tilley, C. E. 1923. Petrology of the metamorphosed rocks of the Start area (S Devon). Quarterly Journal of the Geological Society of London 79, 172–204.CrossRefGoogle Scholar
Vearncombe, J. R. 1979. The Lizard ophiolite and two phases of suboceanic deformation. In Proceedings of the International Ophiolite Symposium, Cyprus (ed. Panayiotou, A.), pp. 527–37.Google Scholar
Wood, D. A. 1980. The application of a Th-Hf-Ta diagram to problems of tectonmagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary Volcanic Province. Earth and Planetary Science Letters 50, 11–30.CrossRefGoogle Scholar