A Turbulent Magmatic Density Current and the Origin of the Anastomosing UG-1 Chromitites at Dwars River in the Bushveld Complex

Abstract

Abstract The origin of the enigmatic UG-1 chromitites at Dwars River in the eastern limb of the Bushveld Complex has been vigorously debated. The UG-1 chromitites form an anastomosing network of multiple layers that are hosted in poikilitic anorthosites and their formation has previously been explained by depositional, erosional, and intrusive processes. We propose that the UG-1 chromitites formed in response to the emplacement of a turbulent magmatic density current into the developing chamber. We use theoretical constraints in fluid mechanics to describe the evolution of the current and in this context, we provide an explanation for the enigmatic igneous features that are preserved at Dwars River. The current was emplaced as an instantaneous single pulse (fixed volume) of dense plagioclase-charged magma (i.e. a plagioclase slurry) that turbulently propagated along the chamber floor. Settling of the initial cargo of entrained plagioclase laths resulted in the formation of a thick sequence of feldspathic mush. The remaining melt turbulently outruns on the chamber floor ahead of the feldspathic mush. This led to the density current becoming stratified into two layers: (1) a basal granular layer, and (2) an upper melt-rich layer. Both layers evolved in a viscous-dominated regime from a Newtonian to a non-Newtonian behaviour (i.e. a power law fluid). In the current’s propagation stage, the resident magma in the chamber was entrained and efficiently admixed into the melt-dominated upper layer, producing a superheated chromite-saturated hybrid melt. Chromite precipitation from this hybrid melt led to the formation of a series of chromite-laden slurries that flowed in the turbulent current and were split (‘bifurcated’) as they passed around rising buoyant plagioclase diapirs that were developing in the back of the current. This led to the anastomosing and bifurcating nature of the UG-1 chromitites. The chromite slurries merged in the tail of the current and eventually back-injected into the deposited plagioclase cumulates on the chamber floor—forming the main ∼2-m thick UG-1 chromitite layer. The cooling and crystallization of the propagating upper layer of the current led to an increase in its viscosity that slowed its velocity until it eventually stagnated on the chamber floor. The mechanical sorting of crystals in plume structures became dominant at the stagnation stage. We argue that spatial variations in crystal packing controlled the heterogeneous distribution of brittle and ductile deformation features that are observed at Dwars River. In situ crystallization of trapped pore melts led to the nucleation of large orthopyroxene and plagioclase oikocrysts that cemented the anorthosite and the chromitite layers, respectively. The emplacement of magmatic density currents and intra-chamber magma mixing may be pertinent to the development of stratiform chromitites in incrementally constructed magma chambers.