Geochemical Evolution and Unusual Pyroxene Chemistry of the MD Tholeiite Dyke Swarm from the

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 poikiliticall