Carboniferous-Permian magmatism in the UK

The UK preserves a diverse record of Carboniferous to Permian magmatism that occurs in a lower plate setting, relative to the southwards-dipping latest Devonian Rheic-Rhenohercynian suture zone, that is seismically imaged offshore to the south of SW England. The classical model of foreland-propagating thrust systems, immediately following the closure of the Rhenohercynian Ocean, is not consistent with the widespread occurrence of Early Carboniferous syn-rift mafic magmatism immediately north of the suture that persists to Scotland.

The SW England Permian magmatic province includes the Cornubian and Haig Fras batholiths and associated mafic igneous rocks developed over a 20 Ma period in Early Permian. These developed in a post-Variscan extensional regime that brought about exhumation and thinning of the lower plate during reactivation of the Rhenohercynian suture zone (Cornubian Batholith) and Bristol Channel Fault Zone (Haig Fras Batholith). Decompression melting of the mantle during lithospheric extension is the most likely cause but lithospheric delamination cannot be excluded.

North of the Variscan Front, latest Carboniferous to Early Permian magmatism was renewed following Variscan intraplate shortening. Most occurrences are consistent with decompressive melting during N-S extension and/or localised extension during movements on wrench faults.

The overall record of Carboniferous and Permian magmatism across the UK does can be explained by extensional tectonics and decompression melting of lithospheric ± asthenospheric mantle plus, possibly, lithospheric delamination below SW England. Whilst mantle plume involvement has been invoked for magmatism in some areas, it is not required to explain heat flow and magmatic activity.