Hostname: page-component-8448b6f56d-gtxcr Total loading time: 0 Render date: 2024-04-23T21:35:28.070Z Has data issue: false hasContentIssue false
  • English
  • Français

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
Get access Rights & Permissions [Opens in a new window]

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

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Bence, A. E. and Papike, J. J. (1972) Pyroxenes as recorders of lunar basalt petrogenesis. Proc. Third Lunar Sci. Conference, 431-69.Google Scholar
Cameron, M. and Papike, J. J. (1981) Structural and chemical variation in pyroxenes. Amer. Min., 66, 1–50.Google Scholar
De Klerk, W. J. (1991) Petrology and geochemistry of the upper Critical Zone in the Western Bushveld Complex. Unpubl. Ph.D. thesis, Rhodes University, 294 pp.Google Scholar
Eales, H. V., Field, M., de Klerk, W. J., and Scoon, R. N. (1988) Regional trends of chemical variation and thermal erosion in the Upper Critical Zone, Western Bushveld Complex. Mineral. Mag., 52, 63–79.Google Scholar
Eales, H. V. de Klerk, W. J., Butcher, A. R., and Kruger, F. J. (1990a) The cyclic unit beneath the UG1 chromitite at R.P.M. Union Section Mine. Ibid., 54, 23–43.Google Scholar
Eales, H. V. de Klerk, W. J., Butcher, A. R., and Kruger, F. J. and Teigler, B. (1990b) Evidence for magma mixing processes within the Critical and Lower Zones in the northwestern Bushveld Complex. Chem. Geol., 88, 261–78.Google Scholar
Grove, T. L. and Bence, A. E. (1977) Experimental study of pyroxene-liquid interaction in quartz-nor-mative basalt. Proc. Eighth Lunar Sci. Conference, 1549-79.Google Scholar
Maeda, J., Shimura, T., Arita, K., Osanai, Y., Murata, M., Bamba, M., and Suetake, S. (1991) Chemical features of orthopyroxene in peraluminous igneous rocks. Amer. Min., 76, 1674–82.Google Scholar
Maier, W. D. (1991) Geochemical and petrological trends in the UG2-Merensky Unit interval, Western Bushveld Complex. Unpubl. Ph.D. thesis, Rhodes University, 241 pp.Google Scholar
Shearer, C. K., Papike, J. J., Simon, S. B., and Shimizu, N. (1989) An ion microprobe study of the intra-crystalline behaviour of REE and selected trace elements in pyroxene. Geochim. Cosmochim. Acta, 53, 1041–54.Google Scholar
Teigler, B. (1990) Mineralogy, petrology and geochemistry of the Lower and lower Critical Zones, northwestern Bushveld Complex. Unpubl. Ph.D. thesis, Rhodes University, 247 pp.Google Scholar
Teigler, B. and Eales, H. V. (1991) Correlation between chromite composition and PGE mineralisation in the Critical Zone of the western Bushveld Complex. Internat. Congr. Appl. Mineral. ‘91, Pretoria, 2, Paper 59.Google Scholar
Teigler, B. and Eales, H. V. and Scoon, R. N. (1992) The cumulate succession in the Critical Zone of the Rustenburg Layered Suite at Brits, Western Bushveld Complex. South African J. Geol., 95, 17–28.Google Scholar
Viljoen, M. J. and Hieber, R. (1986) The Rustenburg Section of R.P.M. Ltd. In Mineral Deposits of Southern Africa (C. R. Anhaeusser and S. Maske, Editors), Geol. Soc. South Africa, 1107-34.Google Scholar
Wilson, A. H. (1982) The geology of the Great ‘Dyke', Zimbabwe. J. Petrol., 23, 249–92.Google Scholar