Unusual Textures and Structures Associated with a Magnetitite Layer in the Bushveld Complex: A Contribution to the Adcumulus Debate

Alan R. Butcher and Roland K. W. Merkle
Department of Geology, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa
Institute for Geological Research on the Bushveld Complex, University of Pretoria, Pretoria 0002, South Africa

Abstract: In the Brits area of the Bushveld Complex, the stratigraphically lowermost magnetitite layer of the Upper Zone is underlain by an anorthosite containing discrete, euhedral magnetite crystals, measuring 0.1–0.3 mm in diameter, which form thin layers (1 mm-1 cm thick) and laminae (commonly one grain in thickness). Discordant relationships between laminae and layers indicate a consistent younging direction upwards towards the contact with the overlying magnetitite. Our interpretation is that these crystals are cumulus and have escaped modification by postcumulus overgrowth. They also record, in places, the morphology of the floor onto which they accumulated. Their preservation is largely attributed to adcumulus growth of plagioclase before, during and after, accumulation, but may also indicate that there was only a limited supply of magnetite to the floor, thus precluding grain contact and prohibiting grain enlargement. The results of a study of compositional variations across individual plagioclase grains suggest that the presence of magnetite within the crystallising mush hindered the development of an ideal plagioclase adcumulate.

Keywords: Bushveld Complex • Upper Zone • magnetitite • adcumulus growth • anorthosite

Mineralogical Magazine; September 1991 v. 55; no. 380; p. 465-477; DOI: 10.1180/minmag.1991.055.380.16
© 1991, The Mineralogical Society
Mineralogical Society (www.minersoc.org)