Formation of Bushveld anorthosite by reactive porous flow

Maier, W. D. ORCID: https://orcid.org/0000-0002-8654-6658, Barnes, S. -J., Muir, D., Savard, D., Lahaye, Y. and Smith, W. D. ORCID: https://orcid.org/0000-0002-6523-7864 2021. Formation of Bushveld anorthosite by reactive porous flow. Contributions to Mineralogy and Petrology 176 (1) , 3. 10.1007/s00410-020-01760-7

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Abstract

Bushveld anorthosites commonly contain the so-called “mottles” comprising irregular, typically centimetric domains of oikocrystic pyroxene or olivine enclosing small, embayed plagioclase grains. The mottles were traditionally interpreted to result from solidification of trapped intercumulus liquid or via in situ crystallisation at the top of the crystal mush. Here, we present microtextural and compositional data of a mottle to place further constraints on the formation of anorthosite layers. Element maps generated by scanning electron microscopy reveal that plagioclase within and around the mottle has markedly elevated An contents (up to An95) relative to the host anorthosite and is strongly reversely zoned. Other unusual features, some of which were reported previously, include a halo of sub-vertically oriented, acicular phlogopite around the mottle, elevated contents of disseminated sulfides, and relatively evolved yet Ni-rich olivine (Fo71–75, 3000 ppm Ni). These features are interpreted to result from reactive porous flow of hot, acidic fluid enriched in nickel and sulfur through proto norite. The fluids dissolved mafic minerals and leached alkalis from the outer rims of plagioclase grains. Reconnaissance studies suggest that reversed zoning of plagioclase is a common feature in Bushveld norite and anorthosite. This implies that reactive porous flow could have been far more pervasive than currently realised and that Bushveld anorthosite layers formed through recrystallisation of norites.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Earth and Environmental Sciences
Publisher:	Springer Verlag
ISSN:	0010-7999
Date of First Compliant Deposit:	28 January 2021
Date of Acceptance:	13 November 2020
Last Modified:	06 May 2023 08:27
URI:	https://orca.cardiff.ac.uk/id/eprint/138007

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