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Early Palaeozoic arc-related volcanism in the concealed caledonides of southern Britain
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.
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A propos de : T.C. Pharaoh
Stratigraphy & Tectonics Group, British Geological Survey, Keyworth U.K.
A propos de : R.J. Merriman
Mineral Sciences Group, British Geological Survey, Keyworth, U.K.
A propos de : J.A. Evans
NERC Isotope Geology Laboratory, Keyworth, U.K.
A propos de : T.S. Brewer
Department of Mining Engineering, The University, Nottingham, U.K.
A propos de : P.C. Webb
Department of Earth Sciences, Open University, Milton Keynes, U.K.
A propos de : N.J.P. Smith
Hydrocarbons Research Group, British Geological Survey, Edinburgh, U.K.