The use of plagioclase composition as an indicator of magmatic processes in the Upper Zone of the Bushveld Complex

Magnetite layer formation in the Bushveld Complex of South Africa

“…We conclude that flattening of the upper portions of the profiles could have resulted from compositional Reactive melt infiltration. The footwall of the MML is proposed to have been permeated by interstitial melt that carried a slightly more primitive signature with it from lower parts of the UUMZ crystal pile, causing reverse zoning of cumulus plagioclase 21 and crystallization of more anorthitic interstitial grains 25 . Whereas crystallization of magnetite from interstitial melt in the magnetitite could drive compositional convection, it is not expected during deposition of the anorthositic footwall when interstitial melt has density very close to that of the cumulus phase.…”

“…The formation of these magnetite layers cannot be accommodated by a simple process of fractional crystallization from a single homogeneous body of liquid the size of the entire Upper Zone. Instead, it seems probable that a repeated cyclic process of spontaneous double-diffusive convection occurred wherein the rejected liquid complementary to the gabbroic cumulates was denser than the main body of magma, causing the system to become stratified with a dense Fe-rich residual melt separating the growing cumulate pile from the rest of the magma 17,21 . It is thought that when magnetite became saturated and a layer of magnetitite was crystallized from this dense Fe-rich layer the liquid density rapidly diminished, causing convective overturn and re-homogenization of the magnetite-depleted basal layer with the rest of the magma and terminating magnetite crystallization until the closing stage of the next such cycle.…”

Magnetite layer formation in the Bushveld Complex of South Africa

“…The footwall of the MML is proposed to have been permeated by interstitial melt that carried a slightly more primitive signature with it from lower parts of the UUMZ crystal pile, causing reverse zoning of cumulus plagioclase 21 and crystallization of more anorthitic interstitial grains 23 . This slightly more primitive interstitial melt would retain high Cr content because it had not yet equilibrated with magnetite in the footwall of the MML, further either enhanced by crystallization of Cr-free plagioclase in the footwall anorthositic mush or minimally diminished by the onset of crystallization of trace amounts of intercumulus magnetite.…”