Origin of chromitite-anorthosite interlayering in the Bushveld Complex

Maier, Wolfgang ORCID: https://orcid.org/0000-0002-8654-6658 and Barnes, Sarah-Jane 2024. Origin of chromitite-anorthosite interlayering in the Bushveld Complex. The Canadian Journal of Mineralogy and Petrology 62 (5) , pp. 731-745. 10.3749/2300069

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Abstract

An idealized stratigraphic section of the five main chromitite-bearing cyclic units of the Upper Critical Zone of the Bushveld Complex typically consists of basal chromitite overlain by pyroxenite, norite, and anorthosite. Such units can be explained by classical fractional crystallization from a Mg-rich basalt followed by injection of new magma. However, in some cases both the underlying and overlying rocks of the chromitite are anorthosite. Furthermore, most of the interstitial material in the chromitites is not pyroxene, which would be the mineral expected to crystallize with chromite, but rather it is oikocrystic plagioclase. Finally, recent age dating suggests that the cyclic units may not have been emplaced in stratigraphic order, with the upper group chromitites being older than the underlying middle group chromitites. In order to reconcile some of these data we compiled field and drill core observations from the Upper Critical Zone. Of particular interest is the MG2–MG3 interval, showing pronounced layering, abundant autoliths of various shapes and compositions, as well as flame structures and boudinage. Many of these features are typical of sedimentary mass flows, suggesting that the MG2–MG3 interval formed via granular flow of intruding chromite-orthopyroxene crystal slurries that deposited chromitite and orthopyroxenite. The heat of the new magma influx caused partial melting of the host leuconorite to produce restitic anorthosite and buoyant noritic melt. The latter infiltrated the slurry, resulting in a disequilibrium assemblage of cumulus chromite and oikocrysts of plagioclase.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Earth and Environmental Sciences
Publisher:	Mineralogical Association of Canada
ISSN:	0008-4476
Date of First Compliant Deposit:	7 June 2024
Date of Acceptance:	2 May 2024
Last Modified:	01 Oct 2024 14:01
URI:	https://orca.cardiff.ac.uk/id/eprint/169595

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