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Journal of the Geological Society

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Formation of bifurcating chromitite layers of the UG1 in the Bushveld Igneous Complex, an analogy with sand volcanoes

School of Geosciences, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa (e-mail: nexp@geosciences.wits.ac.za)

Chromitite layers occur frequently within the Critical Zone of the Bushveld Complex. In particular, Dwars River in the eastern Bushveld is well known for the intimate association of chromitite and anorthosite layers. An explanation has been deduced for bifurcations of chromitite layers and other features frequently found in exposures of the UG1 and its footwall. It is proposed that the unconsolidated footwall anorthosite experienced liquefaction possibly caused by major magma influx and associated seismicity. This resulted in structures analogous to those seen in sediments that undergo liquefaction, including sand volcanoes or boils. Bifurcations of chromitite layers occur where there is ‘background sedimentation’ of chromite at the same time as periodic extrusion of plagioclase plus magma slurry at the magma–cumulate-pile interface. These extrusions form ‘waterlilies’ of anorthosite that build up on top of each other to form domes. Simultaneously, chromite accumulates and forms multiple layers, which vary in thickness from those formed by a single layer of chromite grains to those over 0.5 m thick. Subsequent post-depositional structures suggest continued liquefaction in the footwall rocks, mobility of residual anorthositic magma and brittle deformation of chromitite layers.

KEYWORDS: Bushveld Igneous Complex, chromitite, anorthosite

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