Pyroxenes of the Dufek Intrusion, Antarctica

• Published in: Journal of Petrology
• Main Authors: HIMMELBERG, G. R., FORD, A. B.
• Format: Text
• Language: English
• Published: Oxford University Press 1976
• Subjects: Articles; Pensacola Mountains; Antarc*; Antarctica
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/17/2/219; https://doi.org/10.1093/petrology/17.2.219

The Dufek intrusion is a stratiform mafic body, 24,000 to 34,000 km2 in area and 8 to 9 km thick, in the Pensacola Mountains of Antarctica. Textures, structures, magmatic stratigraphy, and chemical variation indicate that layered gabbros and related rocks of this body developed by accumulation of crystals that settled on the floor of a magma chamber. The major cumulus phases in the exposed part of the intrusion are plagioclase, pyroxene, and iron-titanium oxides. The base of the Dufek intrusion is not exposed, and both Ca-rich and Ca-poor pyroxene coexist as cumulus phases in the lower exposed rocks. The Ca-rich pyroxenes belong to an augite-ferroaugite series (Ca 36.4 Mg 48.7 Fe 14.9 -Ca 30.0 Mg 23.5 Fe 46.5 ) that extends up through the 300 m thick capping granophyre. The Ca-poor pyroxenes belong to a bronzite-inverted pigeonite series (Ca 3.5 Mg 69.1 Fe 27.4 -Ca 11.4 Mg 34.0 Fe 54.6 ) that extends only to about 200 m below the granophyre layer. In addition to the cumulus pyroxenes some rocks contain post-cumulus green calcic augite and ferrohypersthene. The compositional change of the cumulus pyroxenes with stratigraphic height is one of general iron enrichment. Superimposed on this trend are (1) a 1 km thick section in the lower part of the body that shows slight to no iron enrichment and (2) a marked reversal in the Fe/(Fe+Mg) ratio about 1 km below the top of the body. The variations from the general trend are associated with cyclic units and are best explained by convective overturn of the magma. In general, the pyroxene compositional trends are similar to those of the Skaergaard and Bushveld intrusions. One significant difference in the Dufek intrusion is the limited iron enrichment of its Ca-rich pyroxenes, that may relate to a slower decrease of P O2 during crystallization of the Dufek magma.