PYROXENES OF BUSHVELD INTRUSION, SOUTH AFRICA

New analyses are presented, for major, minor, and trace elements, of eleven Ca-rich pyroxenes, four bronzites, and two inverted pigeonites from the Bushveld layered basic intrusion. The twenty-three analyses now available are believed to represent the entire Bushveld fractionation sequence. The Ca-rich pyroxene trend is from Ca45.4Mg49.6Fe5.0 to Ca42.7Mg0.6Fe56.8, the ferrohedenbergites showing no evidence of inversion from ferriferous β-wollastonites. The Ca-poor pyroxene trend is from bronzite (Ca2.8Mg85.0Fe12.2) through pigeonites to ferropigeonites (approximately Ca3Mg27Fe55). All the pigeonitic pyroxenes have inverted to orthopyroxene.The compositional trends are remarkably similar to those of the Skaergaard pyroxene series, but the Bushveld sequence is the most complete known for a single fractionated intrusion. The compositional and other variations of the pyroxenes, considered together with those of the coexisting olivines and feldspars, leave little doubt that the Bushveld rocks originated by crystal accumulation from a slowly cooled and fractionated intrusion of tholeiitic basalt magma.The slight but significant differences between the Bushveld and Skaergaard pyroxene trend characteristics can be explained in terms of a displacement, in one intrusion as compared with the other, of the liquidus and solidus surfaces relative to the solvus and inversion surfaces in the system Wo-En-Fs. This may be due to minor differences in the initial magma compositions of the two intrusions. Differences in the Mg/Fe ratios of Bushveld and Skaergaard coexisting pyroxene pairs are believed to be due, at least in part, to the greater depth of the Bushveld magma chamber.The Bushveld trends are briefly discussed in the light of recent experimental studies on compositions within the Di-Hed-En-Fs pyroxene quadrilate. © 1969 Oxford University Press.