Carboniferous and Early Permian Magmatism in England

Abstract The cause of widespread Carboniferous and Early Permian magmatism that occurred across northern Europe and the United Kingdom remains enigmatic. New 40Ar-39Ar radiometric dating reveals that some of the earliest manifestations of alkaline to sub-alkaline magmatism, found near the southern limit of the province in the Peak District of central England (Waterswallows Sill: 328.6 ±4.2 Ma; Calton Hill: 316.4 ±3.7 Ma), coincided with episodes of extensional and thermal subsidence. The variable geochemical compositions, changing tectonics, and geographic location of the Peak District rocks provide opportunity to understand regional magmatic processes. A combination of geodynamic and geochemical modelling is done using new whole-rock major- and trace-element data, and Sr-Nd-Pb isotope results. To aid comparison, new geochemical data is also provided for the magmatism in Scotland and for the younger Whin Complex in northern England (ca. 297 Ma). Due to different amounts of partial melting, olivine assimilation and post-magmatic alteration, a new type of data visualisation – the compatible element plot – is devised to aid interpretation. Given the shape of REE patterns, together with the Sc and V contents, it indicates melting took place within the garnet stability field. Geodynamic modelling of the Derbyshire carbonate platform and Edale Basin calculated a low β-value (~1.1) for a uniform extensional regime. However, the Caledonian and Acadian orogenies may have made the deep lithosphere beneath Britain unstable, promoting later founding and asthenospheric upwelling. To help with discussion on this scenario, a non-uniform lithospheric extension model is presented with a β-value of 2. On this basis, an alternative model is introduced, whereby warmer-than-ambient (not hot) fingers of mantle plume under-flow facilitates decompression melting of the asthenosphere during extensional processes. In locations such as Derbyshire, magma ascended to the surface utilising the deep-seated faults present in asymmetric basins. With such a model, a plume trace or significant doming may not necessarily be obvious but an interplay between tectonics, lithospheric structure and mantle processes is key.