The MD dyke swarm is composed of four generations of large basic tholeiite dykes which cut the entire Archaean craton of southern West Greenland. The four successive generations (MD1, MD2, MD3a, MD3b) are characterized by their orientation and cross-cutting relationships and by their mineralogy, texture and progressively evolved tholeiitic chemistry. Rare-earth element (REE) abundances suggest that the dykes may have a fairly complex petrogenetic evolution. The suite varies from early (MD1) heteradcumulate norites to ophitic and sub-ophitic gabbroic and doleritic rocks (MD2 and MD3) and the youngest generation (MD3b) comprises plagioclase-phyric dolerites. The pyroxene chemistry parallels the geochemical evolution of the dykes showing an overall Fe-enrichment trend. However, the clinopyroxenes are enigmatic in that, although they occur predominantly as part of medium and coarse-grained holocrystalline textures, they are chemically highly variable and calcium-poor, many plotting in the metastable field in the system MgSiO3 (En)-CaSiO3 (Wo)-FeSiO3 (Fs). Many individual grains are extremely complex and may be regularly or irregularly zoned. Along with more typical pyroxene forms, the MD1 dykes contain pyroxene dendrites poikilitically enclosed by plagioclase. The dendrites vary compositionally from hypersthene bases to branches of pigeonite and subcalcic augite and terminate in augite branch tips. The MD2 and MD3a dyke pyroxenes are the most complex. The majority of them are sub-ophitic grains, many with successive zones of orthopyroxene, pigeonite, subcalcic augite, augite and ferroaugite. However, Ca-enrichment or Ca-depletion, Fe-enrichment or Fe-depletion and apparently opposing zoning trends can occur in neighbouring grains. Even small interstitial pyroxenes show a very wide range of compositions. Morphologically unusual and complex clinopyroxene ‘cylinders’ occur in some of the MD3a dykes. They are chemically relatively uniform and are normal tholeiitic augites. The MD3b rocks have small concentrically zoned sub-ophitic pyroxenes which show Ca-enrichment with a relatively constant Fs component (29 to 39 mol. per cent). The most extremely zoned grains have hypersthene cores with successive coronas of pigeonite and subcalcic augite and have margins of augite or ferroaugite. The present ‘coexistence’ of such compositionally widely variable pyroxenes and the extreme and often irregular nature of their chemical zoning make the determination of true original coexisting pyroxene phases and the use of a two pyroxene geothermometer very difficult and of limited significance. The presence of a wide variety of pyroxenes of apparently both stable and metastable compositions in these holocrystalline dykes suggests that these rocks have undergone a complex and rather unusual cooling history. The principal genetic factors which could have influenced their crystallization are (1) supercooling, (2) the evolution of discrete interstitial liquid cells, (3) augite-pigeonite peritectic reactions and (4) plagioclase growth and delay of pyroxene nucleation during supercooling of liquid to below the basalt liquidus.