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Cryptic variations of minor elements Al, Cr, Ti and Mn in Lower and Critical Zone orthopyroxenes of the Western Bushveld Complex

Published online by Cambridge University Press:  05 July 2018

H. V. Eales ,
B. Teigler  and
W. D. Maier
H. V. Eales
Affiliation:
Department of Geology, Rhodes University, Grahamstown, South Africa
B. Teigler
Affiliation:
Department of Geology, Rhodes University, Grahamstown, South Africa
W. D. Maier
Affiliation:
Department of Geology, Rhodes University, Grahamstown, South Africa
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Abstract

Compositional variations with respect to minor elements A1, Cr, Ti and Mn, and major elements Fe and Mg, in orthopyroxenes along ca. 160 km of strike of the Lower (LZ), Lower Critical (LCZ) and Upper Critical (UCZ) Zones are reviewed on the basis of 1900 analyses by electron microprobe. AI increases with stratigraphic height and declining Mg/(Mg + Fe2) ratios (hereafter MMF ratios) through the LZ and LCZ, reaching peak values close to the base of the UCZ, where the first cumulus plagioclase appears in the succession. Above this, Al contents decline as MMF ratios decline. Through the same interval, subdued increase in Ti occurs through >1000 m of ultramafic cumulates, but this increase accelerates within the ca. 450 m UCZ sequence. Mn increases linearly with declining MMF ratios through the entire succession, Cr levels are highest in orthopyroxenes of the ultramafic LZ and LCZ, and olivine norites of the UCZ, but decline in more evolved norites and associated anorthosites of the UCZ.

This pattern of cryptic variations, displayed by a thick succession of cumulates, is consistent with the model of Bence and Papike (1972) and Grove and Bence (1977) for basaltic rocks, which links the levels of minor elements in pyroxenes with entry of plagioclase into the paragenesis.

Keywords

orthopyroxeneBushveld complexpyroxenitesnoritesanorthosites
Type
Mineralogy
Information
Mineralogical Magazine , Volume 57 , Issue 387 , June 1993 , pp. 257 - 264
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1993

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