Annales de la Société Géologique de Belgique
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Type de document : Article
T.C. Pharaoh, R.J. Merriman, J.A. Evans, T.S. Brewer, P.C. Webb, N.J.P. Smith
Early Palaeozoic arc-related volcanism in the concealed caledonides of southern Britain
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Deep boreholes in a NW-SE trending belt running from the south Pennines to The Wash, to NE of the Midlands Microcraton, proved pre-Carboniferous intermediate to felsic tuffs and lavas. Associated mafic volcanic rocks are rare. A cluster of boreholes near the edge of the microcraton also proved felsic tuffs. This concealed volcanic suite is petrographically and geochemically distinct from the exposed Precambrian basement in eastern England, but the age of the individual provings is poorly constrained, and they may not be coeval. Isotopic studies suggest a probable mid to late Ordovician age, comparable to that of calc-alkaline plutonic magmatism in the East Midlands. It is proposed that a subaerial calc-alkaline volcanic arc extended from the Brabant Massif in Belgium, beneath eastern England to the Lake District in the Ordovician, developing in response to closure of the Tornquist Sea convergence zone.
In the southern part of the Midlands Microcraton, mafic lavas and intermediate tuffs of Llandovery (or earlier) age encountered in a few deep boreholes are geochemically comparable with exposed early Silurian volcanic sequences at Skomer in south Wales, the Mendips and the Tortworth Inlier. These rocks form a seismic marker horizon which can be traced over an extensive area, and are believed to represent an eastward continuation of the early Silurian volcanic suite along the northern edge of the concealed Variscan Front.
Pour citer cet article :
Pharaoh T.C., Merriman R.J., Evans J.A., Brewer T.S., Webb P.C. et Smith N.J.P., «Early Palaeozoic arc-related volcanism in the concealed caledonides of southern Britain», Ann. Soc. Geol. Belg., Volume 114 (1991) (International Meeting on the Caledonides of the Midlands and the Brabant Massif : Brussels, 20 to 23 September 1989) Fascicule 1 (Proceedings of the International Meeting on the Caledonides of the Midlands and the Brabant Massif) : 63-91
Les auteurs :
Stratigraphy & Tectonics Group, British Geological Survey, Keyworth U.K.
Mineral Sciences Group, British Geological Survey, Keyworth, U.K.
NERC Isotope Geology Laboratory, Keyworth, U.K.
Department of Mining Engineering, The University, Nottingham, U.K.
Department of Earth Sciences, Open University, Milton Keynes, U.K.
Hydrocarbons Research Group, British Geological Survey, Edinburgh, U.K.