Geochronology of Carboniferous-Permian magmatism in the Midland Valley of Scotland : implications for regional tectonomagmatic evolution and the numerical time scale

Four new U–Pb isotope dilution thermal ionization mass spectrometry dates from Carboniferous igneous rocks of the Midland Valley of Scotland are integrated with 16 recently published 40Ar/39Ar dates. The numerical and stratigraphical ages of the dated samples are compatible to stage level, allowing improved intrabasinal and regional correlations. Early Carboniferous extension-related volcanism in the Midland Valley was pulsed, with a date of 343.4 ± 1.0 Ma from the Garleton Hills Volcanic Formation and dates of 334.7 ± 1.7 and 335.2 ± 0.8 Ma from the Clyde Plateau Volcanic Formation. The magmatic pulses are synchronous with synrift phases in the Northumberland–Solway Basin. Volcanism continued in mid- to Late Carboniferous time contemporaneous with dextral strike-slip tectonics, in contrast to a post-rift tectonic setting in northern England. After Late Carboniferous basin inversion, tholeiitic magmatism dated at 307.6 ± 4.8 Ma provides a maximum Westphalian D age for post-Variscan magmatism and extension–dextral transtension across Scotland and northern England. In the Early Permian, as rift systems developed across NW Europe, alkaline extension-related magmatism resumed in the Midland Valley from c. 298 to 292 Ma. The improved geochronology provides important data for poorly constrained parts of the numerical Carboniferous time scale such as stages within the Viséan, and provides a well-controlled tie-point that supports a Tournaisian–Viséan boundary age of c. 345.4 Ma.