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

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 seventeen concordant experiments with a relative age precision better than 1% (2). 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 to 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 2 to 329.2 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 to 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 300-280 Ma