Geochemical Evolution and Unusual Pyroxene Chemistry of the MD Tholeiite Dyke Swarm from the Archaean Craton of Southern West Greenland

• Published in: Journal of Petrology
• Main Authors: HALL, R. P., HUGHES, D. J., FRIEND, C. R. L.
• Format: Text
• Language: English
• Published: Oxford University Press 1985
• Subjects: Articles; Greenland
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/26/2/253; https://doi.org/10.1093/petrology/26.2.253

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 MgSiO 3 (En)-CaSiO 3 (Wo)-FeSiO 3 (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 ...