Emplacement of the Cleveland Dyke: Evidence from Geochemistry, Mineralogy, and Physical Modelling

Abstract

The igneous rocks of the British Tertiary Volcanic Province (BTVP) comprise intrusive central complexes and associated lava fields in northwest Scotland and northern Ireland. These centres are associated with linear dyke swarms which are radial around the major central complexes. The most extensive dyke swarm is related to the Mull intrusive complex and includes the Cleveland dyke, which appears to extend some 430 km from Mull through the Scottish Midland Valley (SMV) to the coast of northeast England. The dyke may have been emplaced by lateral magma migration from Mull, by vertical magma migration, or by a combination of these processes associated with the emplacement of the Mull centre and the presence of a regional stress field in northern Britain. Petrographic, mineralogical, and geochemical data for samples collected across and along the Cleveland dyke have been used to evaluate its petrogenesis and emplacement mechanism. The segment of the dyke north of, and along, the Southern Uplands Fault, the southern boundary of the SMV, is not comagmatic with that to the south, which is now defined as the Cleveland dyke sensu stricto. The Cleveland dyke is an olivine-free, plagioclase- and pyroxene-phyric basaltic andesite. Plagioclase mineralogy and bulk composition indicate that it experienced a complex magmatic history involving polybaric fractional crystallization and minor crustal contamination. Despite this complex evolution, the dyke magma is relatively homogeneous and shows chemical characteristics closely similar to tholeiitic rocks from Mull. The data substantiate lateral emplacement from this BVTP centre, rather than by vertical emplacement through heterogeneous lithosphere. Numerical modelling of dyke dynamics is consistent with emplacement of the Cleveland dyke as a single pulse of magma from the Mull centre, flowing in a manner transitional between laminar and turbulent conditions. According to this model, the dyke (volume c. 85 km3 was initiated in a large magma chamber below Mull subject to a small excess magmatic pressure. Lateral migration at relatively high velocity (1–5 ms−1) caused emplacement of the dyke in 1–5 days. Following emplacement, minor vertical ascent of magma may have contributed to the local en echelon distribution of dyke segments.