The lower Carboniferous–late Permian dyke swarms of the Scottish Highlands and Islands comprise a mild-strongly alkaline basic series of dolerites, camptonites and monchiquites. Differentiation within the suite was largely controlled by olivine + clinopyroxene fractionation. Major and trace element data indicate that dolerites and camptonites chemically overlap, their mineralogical contrasts resulting from differential loss of an H2O, CO2-rich fluid phase during ascent. By contrast most monchiquites have high Mg-values and are relatively primitive compositions, some being near-primary magmas which have risen rapidly from mantle levels with little chemical modification.

HREE-buffered incompatible element profiles imply a garnet–lherzolite source, which must underlie the lithospheric mantle region represented by spinel lherzolite xenoliths found in some monchiquites. C. 0·5–2·0% partial melting can account for the gross incompatible element variation in the suite, but relative fluctuations in K, Ba, Rb, Sr, P and Zr imply chemical heterogeneity controlled either by refractory mantle accessory phases or by modification of magmas during ascent through variably metasomatised lithospheric mantle.