Basaltic magmatism in the North Sea and its relationship to lithospheric extension

Abstract

Mesozoic magmatism in the North Sea rift system varies from mildly to strongly alkaline in character. In the ‘triple-junction’, which occurs at the intersection of the Viking Graben-Moray Firth-Central Graben, large volumes of alkali basalt erupted to form the Forties volcanic province. These basalts are located in the most extended region of the rift which underwent lithospheric stretching by a factor of two. Much smaller volumes of distinctive undersaturated volatile-rich ultrapotassic and nephelinitic rocks were erupted and intruded in off-axis regions in which β is generally less than 1.2. The average thickness (~ 1 km) and composition of the Forties basalts is most easily explained in terms of mixing of melt components derived from the asthenosphere and the more easily fusible volatile-enriched parts of the continental lithosphere. Using recent work by McKenzie and O'Nions (1991) we show that asthenosphere of normal potential temperature will melt if it is brought rapidly to depths of less than 80 km. The asthenosphere rose to ~ 60 km during rifting in the Forties region of the North Sea. The thickness (0.66 km) and composition of melt produced from the asthenosphere is calculated using the results of a REE inversion scheme (McKenzie and O'Nions, 1991). The composition of the lithospheric melt component appears to be some mixture of nephelinite and ultrapotassic melt. Mixing asthenospheric and lithospheric melts in approximately equal proportions appears to be able to produce ~ 1.2 km of melt whose elemental and isotopic composition is very similar to that of the Forties basalts.