Enigmatic textures developed along plagioclase-augite grain boundaries at the base of the Main Zone, Northern Limb, Bushveld Complex - evidence for late stage melt infiltration into a nearly solidified crystal mush

Abstract

An enigmatic texture in which orthopyroxene exsolution lamellae within clinopyroxene protrude into adjacent plagioclase crystals occurs in a gabbronorite occurring at the base of the Main Zone in the Northern Sector of the Northern Limb of the Bushveld Complex. Grain boundaries exhibiting this texture are characterised by plagioclase richer in An component and clinopyroxene with higher Mg# in comparison to the bulk of the plagioclase and clinopyroxene, respectively. The texture is interpreted as representing the products formed during late-stage infiltration of a more primitive magma into a nearly solidified crystal mush, with concomitant displacement of the last dregs of intercumulus melt. A solid-state origin for the texture appears untenable, as does derivation of the infiltrating melt from the underlying Platreef.