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40Ar/39Ar geochronology of Carboniferous-Permian volcanism in the Midland Valley, Scotland

By
A. A. Monaghan
A. A. Monaghan
British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK (e-mail: als@bgs.ac.uk)
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M. S. Pringle
M. S. Pringle
Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride G75 OQF, UK (e-mail: m.pringle@surrc.gla.ac.uk)
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Published:
January 01, 2004

Abstract

Twenty-one new 40Ar/39Ar step-heating experiments on mineral separates from intrusive and extrusive Carboniferous and Permian igneous rocks in the Midland Valley of Scotland yielded 17 concordant experiments with a relative age precision better than 1% (2σ). These ages resolve inconsistencies between existing K-Ar dates on the same samples and their stratigraphical constraints correlated to recently published timescales. The precise 40Ar/39Ar dates are stratigraphically constrained to stage level and can contribute to Carboniferous timescale tie points at the Tournaisian-Visean boundary, within the Visean and at the Carboniferous-Permian boundary. Situated in the extending Variscan foreland, two distinct phases of extension-related transitional-alkaline volcanism have been resolved in the Dinantian: the Garleton Hills Volcanic Formation in the eastern Midland Valley near the Tournaisian-Visean boundary, 342.1 ± 1.3 and 342.4 ± 1.1 Ma; and the Clyde Plateau Volcanic Formation in the western Midland Valley during the mid-Visean, 335 ± 2329.2 ± 1.4 Ma. Alkaline basic sills near Edinburgh, previously thought to be Namurian, appear to be coeval with the Clyde Plateau Volcanic Formation at 331.8 ± 1.3–329.3 ± 1.5 Ma. The new ages allow correlation between these short-lived Dinantian magmatic pulses and extensional and magmatic phases in the Northumberland-Solway and Tweed basins to the south. After late Westphalian, end-Variscan, compression and a regionally important tholeiitic intrusive phase at c. 301–295 Ma, alkaline magmatism related to post-Variscan extension occurred in the central and western Midland Valley during the latest Carboniferous or Permian from 298.3 ± 1.3 to 292.1 ± 1.1 Ma. This correlates well with post-Varsican extension and magmatism observed across the NW European foreland from 300 to 280 Ma.

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Contents

Geological Society, London, Special Publications

Permo-Carboniferous Magmatism and Rifting in Europe

M. Wilson
M. Wilson
Leeds University, UK
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E.-R. Neumann
E.-R. Neumann
University of Oslo, Norway
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G. R. Davies
G. R. Davies
Vrije University, The Netherlands
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M. J. Timmerman
M. J. Timmerman
Universität Potsdam, Germany
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M. Heeremans
M. Heeremans
University of Oslo, Norway
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B. T. Larsen
B. T. Larsen
Norsk Hydro ASA/Saga Petroleum ASA, Norway
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Geological Society of London
Volume
223
ISBN electronic:
9781862394711
Publication date:
January 01, 2004

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